Aircraft maintenance operations are broadly distributed within and across nations and are performed by both military and civilian mechanics. Mechanics work at airports, maintenance bases, private fields, military installations and aboard aircraft carriers. Mechanics are employed by passenger and freight carriers, by maintenance contractors, by operators of private fields, by agricultural operations and by public and private fleet owners. Small airports may provide employment for a few mechanics, while major hub airports and maintenance bases may employ thousands. Maintenance work is divided between that which is necessary to maintain ongoing daily operations (line maintenance) and those procedures that periodically check, maintain and refurbish the aircraft (base maintenance). Line maintenance comprises en route (between landing and takeoff) and overnight maintenance. En route maintenance consists of operational checks and flight-essential repairs to address discrepancies noted during flight. These repairs are typically minor, such as replacing warning lights, tyres and avionic components, but may be as extensive as replacing an engine. Overnight maintenance is more extensive and includes making any repairs deferred during the day’s flights.

The timing, distribution and nature of aircraft maintenance is controlled by each airline company and is documented in its maintenance manual, which in most jurisdictions must be submitted for approval to the appropriate aviation authority. Maintenance is performed during regular checks, designated as A through D checks, specified by the maintenance manual. These scheduled maintenance activities ensure that the entire aircraft has been inspected, maintained and refurbished at appropriate intervals. Lower level maintenance checks may be incorporated into line maintenance work, but more extensive work is performed at a maintenance base. Aircraft damage and component failures are repaired as required.

Line Maintenance Operations and Hazards

En route maintenance is typically performed under a great time constraint at active and crowded flight lines. Mechanics are exposed to prevailing conditions of noise, weather and vehicular and aircraft traffic, each of which may amplify the hazards intrinsic to maintenance work. Climatic conditions may include extremes of cold and heat, high winds, rain, snow and ice. Lightning is a significant hazard in some areas.

Although the current generation of commercial aircraft engines are significantly quieter than previous models, they can still produce sound levels well above those set by regulatory authorities, particularly if the aircraft are required to use engine power in order to exit gate positions. Older jet and turboprop engines can produce sound level exposures in excess of 115 dBA. Aircraft auxiliary-power units (APUs), ground-based power and air-conditioning equipment, tugs, fuel trucks and cargo-handling equipment add to the background noise. Noise levels in the ramp or aircraft parking area are seldom below 80 dBA, thus necessitating the careful selection and routine use of hearing protectors. Protectors must be selected that provide excellent noise attenuation while being reasonably comfortable and permitting essential communication. Dual systems (ear plugs plus ear muffs) provide enhanced protection and allow accom-modation for higher and lower noise levels.

Mobile equipment, in addition to aircraft, may include baggage carts, personnel buses, catering vehicles, ground support equipment and jetways. To maintain departure schedules and customer satisfaction, this equipment must move quickly within often congested ramp areas, even under adverse ambient conditions. Aircraft engines pose the danger of ramp personel being ingested into jet engines or being struck by a propeller or exhaust blasts. Reduced visibility during night and inclement weather increase the risk that mechanics and other ramp personnel might be struck by mobile equipment. Reflective materials on work clothing help to improve visibility, but it is essential that all ramp personnel be well trained in ramp traffic rules, which must be rigorously enforced. Falls, the most frequent cause of serious injuries among mechanics, are discussed elsewhere in this Encyclopaedia.

Chemical exposures in the ramp area include de-icing fluids (usually containing ethylene or propylene glycol), oils and lubricants. Kerosene is the standard commercial jet fuel (Jet A). Hydraulic fluids containing tributyl phosphate cause severe but transient eye irritation. Fuel tank entry, while relatively rare on the ramp, must be included in a comprehensive confined- space-entry programme. Exposure to resin systems used for patching composite areas such as cargo hold panelling may also occur.

Overnight maintenance is typically performed under more controlled circumstances, either in line-service hangers or on inactive flight lines. Lighting, work stands and traction are far better than on the flight line but are likely to be inferior to those found in maintenance bases. Several mechanics may be working on an aircraft simultaneously, necessitating careful planning and coordination to control personnel movement, aircraft component activation (drives, flight control surfaces and so on) and chemical usage. Good housekeeping is essential to prevent clutter from air lines, parts and tools, and to clean spills and drips. These requirements are of even greater importance during base maintenance.

Base Maintenance Operations and Hazards

Maintenance hangars are very large structures capable of accommodating numerous aircraft. The largest hangars can simultaneously accommodate several wide-body aircraft, such as the Boeing 747. Separate work areas, or bays, are assigned to each aircraft undergoing maintenance. Specialized shops for the repair and refitting of components are associated with the hangars. Shop areas typically include sheet metal, interiors, hydraulics, plastics, wheels and brakes, electrical and avionics and emergency equipment. Separate welding areas, paint shops and non-destructive testing areas may be established. Parts-cleaning operations are likely to be found throughout the facility.

Paint hangars with high ventilation rates for workplace air contaminant controls and environmental pollution protection should be available if painting or paint stripping is to be performed. Paint strippers often contain methylene chloride and corrosives, including hydrofluoric acid. Aircraft primers typically contain a chromate component for corrosion protection. Top coats may be epoxy or polyurethane based. Toluene diisocyanate (TDI) is now seldom used in these paints, having been replaced with higher molecular weight isocyanates such as 4,4-diphenylmethane diisocyanate (MDI) or by prepolymers. These still present a risk of asthma if inhaled.

Engine maintenance may be performed within the maintenance base, at a specialized engine overhaul facility or by a sub-contractor. Engine overhaul requires the use of metalworking techniques including grinding, blasting, chemical cleaning, plating and plasma spray. Silica has in most cases been replaced with less hazardous materials in parts cleaners, but the base materials or coatings may create toxic dusts when blasted or ground. Numerous materials of worker health and environmental concern are used in metal cleaning and plating. These include corrosives, organic solvents and heavy metals. Cyanide is generally of the greatest immediate concern, requiring special emphasis in emergency preparedness planning. Plasma spray operations also merit particular attention. Finely divided metals are fed into a plasma stream generated using high-voltage electrical sources and plated onto parts with the concomitant generation of very high noise levels and light energies. Physical hazards include work at height, lifting and work in uncomfortable positions. Precautions include local exhaust ventilation, PPE, fall protection, training in proper lifting and use of mechanized lifting equipment when possible and ergonomic redesign. For example, repetitive motions involved in tasks such as wire tying may be reduced by use of specialized tools.

Military and Agricultural Applications

Military aircraft operations may present unique hazards. JP4, a more volatile jet fuel that Jet A, may be contaminated with n-hexane. Aviation gasoline, used in some propeller-driven aircraft, is highly flammable. Military aircraft engines, including those on transport aircraft, may use less noise abatement than those on commercial aircraft and may be augmented by afterburners. Aboard aircraft carriers the many hazards are significantly increased. Engine noise is augmented by steam catapults and afterburners, flight deck space is extremely limited, and the deck itself is in motion. Because of combat demands, asbestos insulation is present in some cockpits and around hot areas.

The need for lowered radar visibility (stealth) has resulted in the increased use of composite materials on fuselage, wings and flight control structures. These areas may be damaged in combat or from exposure to extremes of climate, requiring extensive repair. Repairs performed under field conditions may result in heavy exposures to resins and composite dusts. Beryllium is also common in military applications. Hydrazide may be present as part of auxiliary-power units, and anti-tank armament may include radioactive depleted uranium rounds. Precautions include appropriate PPE, including respiratory protection. Where possible, portable exhaust systems should be used.

Maintenance work on agricultural aircraft (crop dusters) may result in exposures to pesticides either as a single product or, more likely, as a mixture of products contaminating a single or multiple aircraft. Degradation products of some pesticides are more hazardous than the parent product. Dermal routes of exposure may be significant and may be enhanced by perspiration. Agricultural aircraft and external parts should be thoroughly cleaned before repair, and/or PPE, including skin and respiratory protection, should be used.

Back

Adapted from the 3rd edition Encyclopaedia article “Aviation - flying personnel” authored by H. Gartmann.

This article deals with the occupational safety and health of the crew members of civil aviation aircraft; see also the articles “Airport and flight control operations”, “Aircraft maintenance operations” and “Helicopters” for additional information.

Technical Crew Members

The technical personnel, or flight crew members, are responsible for the operation of the aircraft. Depending on aircraft type, the technical crew includes the pilot-in-command (PIC), the co-pilot (or first officer), and the flight engineer or a second officer (a pilot).

The PIC (or captain) has the responsibility for the safety of the aircraft, the passengers and the other crew members. The captain is the legal representative of the air carrier and is vested by the air carrier and the national aviation authority with the authority to carry out all actions necessary to fulfil this mandate. The PIC directs all duties on the flight deck and is in command of the entire aircraft.

The co-pilot takes his or her orders directly from the PIC and acts as the captain’s deputy upon delegation or in the latter’s absence. The co-pilot is the primary assistant to the PIC in a flight crew; in newer generation, two-person flight deck operations and in older two-engine aircraft, he or she is the only assistant.

Many older generation aircraft carry a third technical crew member. This person may be a flight engineer or a third pilot (usually called the second officer). The flight engineer, when present, is responsible for the mechanical condition of the aircraft and its equipment. New generation aircraft have automated many of the functions of the flight engineer; in these two-person operations, the pilots perform such duties as a flight engineer might otherwise perform that have not been automated by design.

On certain long-distance flights, the crew may be supplemented by a pilot with the qualifications of the PIC, an additional first officer and, when required, an additional flight engineer.

National and international laws stipulate that aircraft technical personnel may operate aircraft only when in possession of a valid licence issued by the national authority. In order to maintain their licences, technical crew members are given ground school training once every year; they are also tested in a flight simulator (a device that simulates real flight and flight emergency conditions) twice a year and in actual operations at least once a year.

Another condition for the receipt and renewal of a valid licence is a medical examination every 6 months for airline transport and commercial pilots over 40 years old, or every 12 months for commercial pilots under 40 years old and for flight engineers. The minimum requirements for these examinations are specified by the ICAO and by national regulations. A certain number of physicians experienced in aviation medicine may be authorized to provide such examinations by the national authorities concerned. These may include air ministry physicians, airforce flight surgeons, airline medical officers or private practitioners designated by the national authority.

Cabin Crew Members

The cabin crew (or flight attendants) are primarily responsible for passenger safety. Flight attendants perform routine safety duties; in addition, they are responsible for monitoring the aircraft cabin for security and safety hazards. In the event of an emergency, the cabin crew members are responsible for the organization of emergency procedures and for the safe evacuation of the passengers. In flight, cabin crew may need to respond to emergencies such as smoke and fire in the cabin, turbulence, medical trauma, aircraft decompressions, and hijackings or other terrorist threats. In addition to their emergency responsibilities, flight attendants also provide passenger service.

The minimum cabin crew ranges from 1 to 14 flight attendants, depending on the type of aircraft, the aircraft’s passenger capacity and national regulations. Additional staffing requirements may be determined by labour agreements. The cabin crew may be supplemented by a purser or service manager. The cabin crew is usually under the supervision of a lead or “in-charge” flight attendant, who, in turn, is responsible and reports directly to the PIC.

National regulations do not usually stipulate that the cabin crew should hold licences in the same way as the technical crew; however, cabin crew are required by all national regulations to have received appropriate instruction and training in emergency procedures. Periodic medical examinations are not usually required by law, but some air carriers require medical examinations for the purposes of health maintenance.

Hazards and Their Prevention

All air crew members are exposed to a wide variety of stress factors, both physical and psychological, to the hazards of an aircraft accident or other flight incident and to the possible contraction of a number of diseases.

Physical stress

Lack of oxygen, one of the main concerns of aviation medicine in the early days of flying, had until recently become a minor consideration in modern air transport. In the case of a jet aircraft flying at 12,000 m altitude, the equivalent altitude in the pressurized cabin is only 2,300 m and, consequently, symptoms of oxygen deficiency or hypoxia will not normally be encountered in healthy persons. Oxygen deficiency tolerance varies from individual to individual, but for a healthy, non-trained subject the presumed altitude threshold at which the first symptoms of hypoxia occur is 3,000 m.

With the advent of new generation aircraft, however, concerns about cabin air quality have resurfaced. Aircraft cabin air consists of air drawn from compressors in the engine and often also contains recirculated air from within the cabin. The flow rate of outside air within an aircraft cabin can vary from as little as 0.2 m³ per minute per person to 1.42 m³ per minute per person, depending upon aircraft type and age, and depending on location within the cabin. New aircraft use recirculated cabin air to a much greater degree than do older models. This air quality issue is specific to the cabin environment. The flight deck compartment air flow rates are often as high as 4.25 m³ per minute per crew member. These higher air flow rates are provided on the flight deck to meet the cooling requirements of the avionic and electronic equipment.

Complaints of poor cabin air quality from cabin crew and passengers have increased in recent years, prompting some national authorities to investigate. Minimal ventilation rates for aircraft cabins are not defined in national regulations. Actual cabin airflow is seldom measured once an aircraft is put into service, since there is no requirement to do so. Minimal air flow and the use of recirculated air, combined with other issues of air quality, such as the presence of chemical contaminants, micro-organisms, other allergens, tobacco smoke and ozone, require further evaluation and study.

Maintaining a comfortable air temperature in the cabin does not represent a problem in modern aircraft; however, the humidity of this air cannot be raised to a comfortable level, due to the large temperature difference between the aircraft interior and exterior. Consequently, both crew and passengers are exposed to extremely dry air, especially on long-distance flights. Cabin humidity depends on the cabin ventilation rate, passenger load, temperature and pressure. The relative humidity found on aircraft today varies from about 25% to less than 2%. Some passengers and crew members experience discomfort, such as dryness of the eyes, nose and throat, on flights that exceed 3 or 4 hours. There is no conclusive evidence of extensive or serious adverse health effects of low relative humidity on flight personnel. However, precautions should be taken to avoid dehydration; adequate intake of liquids such as water and juices should be sufficient to prevent discomfort.

Motion sickness (dizziness, malaise and vomiting due to the abnormal movements and altitudes of the aircraft) was a problem for civil aviation crews and passengers for many decades; the problem still exists today in the case of small sports aircraft, military aircraft and aerial acrobatics. In modern jet transport aircraft, it is much less serious and occurs less frequently due to higher aircraft speeds and take-off weights, higher cruising altitudes (which take the aircraft above the turbulence zones) and the use of airborne radar (which enables squalls and storms to be located and circumnavigated). Additionally, the lack of motion sickness also may be attributed to the more spacious, open design of today’s aircraft cabin, which provides a greater feeling of security, stability and comfort.

Other physical and chemical hazards

Aircraft noise, while a significant problem for ground personnel, is less serious for the crew members of a modern jet aircraft than was the case with the piston-engined plane. The efficiency of noise control measures such as insulation in modern aircraft have helped to eliminate this hazard in most flight environments. Additionally, improvements in communications equipment have minimized background noise levels from these sources.

Ozone exposure is a known but poorly monitored hazard for air crew and passengers. Ozone is present in the upper atmosphere as a result of the photochemical conversion of oxygen by solar ultraviolet radiation at altitudes used by commercial jet aircraft. The mean ambient ozone concentration increases with increasing latitude and is most prevalent during spring. It can also vary with weather systems, with the result of high ozone plumes descending down to lower altitudes.

Symptoms of ozone exposure include cough, upper airway irritation, tickle in the throat, chest discomfort, substantial pain or soreness, difficulty or pain in taking a deep breath, shortness of breath, wheezing, headache, fatigue, nasal congestion and eye irritation. Most people can detect ozone at 0.02 ppm, and studies have shown that ozone exposure at 0.5 ppm or more causes significant decrements in pulmonary function. The effects of ozone contamination are felt more readily by persons engaged in moderate to heavy activity than those who are at rest or engaged in light activity. Thus flight attendants (who are physically active in flight) have experienced the effects of ozone earlier and more frequently than technical crew or passengers on the same flight when ozone contamination was present.

In one study conducted in the late 1970s by the aviation authority in the United States (Rogers 1980), several flights (mostly at 9,150 to 12,200 m) were monitored for ozone contamination. Eleven per cent of the flights monitored were found to exceed that authority’s permissible ozone concentration limits. Methods of minimizing ozone exposure include choice of routes and altitudes that avoid areas of high ozone concentration and the use of air treatment equipment (usually a catalytic converter). The catalytic converters, however, are subject to contamination and loss of efficiency. Regulations (when they exist) do not require their periodic removal for efficiency testing, nor do they require monitoring of ozone levels in actual flight operations. Crew members, especially cabin crew, have requested that better monitoring and control of ozone contamination be implemented.

Another serious concern for technical and cabin crew members is cosmic radiation, which includes radiation forms that are transmitted through space from the sun and other sources in the universe. Most cosmic radiation that travels through space is absorbed by the earth’s atmosphere; however, the higher the altitude, the less the protection. The earth’s magnetic field also provides some shielding, which is greatest near the equator and decreases at the higher latitudes. Air crew members are exposed to cosmic radiation levels inflight that are higher than those received on the ground.

The amount of radiation exposure depends on the type and the amount of flying; for example, a crew member who flies many hours at high altitudes and high latitudes (e.g., polar routes) will receive the greatest amount of radiation exposure. The civil aviation authority in the United States (the FAA) has estimated that the long-term average cosmic radiation dose for air crew members ranges from 0.025 to 0.93 millisieverts (mSv) per 100 block hours (Friedberg et al. 1992). Based on FAA estimates, a crew member flying 960 block hours per year (or an average of 80 hours/month) would receive an estimated annual radiation dose of between 0.24 and 8.928 mSv. These levels of exposure are lower than the recommended occupational limit of 20 millisieverts per year (5-year average) established by the International Commission on Radiological Protection (ICRP).

The ICRP, however, recommends that occupational exposure to ionizing radiation should not exceed 2 mSv during pregnancy. In addition, the US National Council on Radiation Protection and Measurements (NCRP) recommends that exposure not exceed 0.5 mSv in any month once a pregnancy is known. If a crew member worked an entire month on flights with the highest exposures, the monthly dose rate could exceed the recommended limit. Such a pattern of flying over 5 or 6 months could result in an exposure which also would exceed the recommended pregnancy limit of 2 mSv.

The health effects of low-level radiation exposure over a period of years include cancer, genetic defects and birth defects to a child exposed in the womb. The FAA estimates that the added risk of fatal cancer resulting from exposure to inflight radiation would range from 1 in 1,500 to 1 in 94, depending on the type of routes and number of hours flown; the level of added risk of a serious genetic defect resulting from one parent’s exposure to cosmic radiation ranges from 1 in 220,000 live births to 1 in 4,600 live births; and the risk of mental retardation and childhood cancer in a child exposed in utero to cosmic radiation would range between 1 in 20,000 to 1 in 680, depending upon the type and amount of flying the mother did while pregnant.

The FAA report concludes that “radiation exposure is not likely to be a factor that would limit flying for a non-pregnant crew member” because even the largest amount of radiation received annually by a crew member working as much as 1,000 block hours a year is less than half the ICRP recommended average annual limit. However, for a pregnant crew member, the situation is different. The FAA calculates that a pregnant crew member working 70 block hours per month would exceed the recommended 5-month limit on about one-third of the flights they studied (Friedberg et al. 1992).

It should be stressed that these exposure and risk estimates are not universally accepted. Estimates are dependent upon assumptions about the types and mix of radioactive particles encountered at altitude and the weight or quality factor used to determine dose estimates for some of these forms of radiation. Some scientists believe that the actual radiation hazard to air crew members may be greater than described above. Additional monitoring of the flight environment with reliable instrumentation is needed to more clearly determine the extent of inflight radiation exposure.

Until more is known about exposure levels, air crew members should keep their exposure to all types of radiation as low as possible. With respect to inflight radiation exposure, minimizing the amount of flight time and maximizing the distance from the source of radiation can have a direct effect on the dose received. Reducing monthly and yearly flight time and/or selecting flights which fly at lower altitudes and latitudes will reduce exposure. An air crew member who has the ability to control his or her flight assignments might choose to fly fewer hours per month, to bid for a mix of domestic and international flights or to request leaves periodically. A pregnant air crew member might choose to take a leave for the duration of the pregnancy. Since the first trimester is the most crucial time to guard against radiation exposure, an air crew member planning a pregnancy also may want to consider a leave especially if she is flying long-distance polar routes on a regular basis and has no control over her flight assignments.

Ergonomic problems

The main ergonomic problem for technical crew is the need to work for many hours in a sitting but unsettled position and in a very limited working area. In this position (restrained by lap and shoulder harness), it is necessary to carry out a variety of tasks such as movements of the arms, legs and head in different directions, consulting instruments at a distance of about 1 m above, below, to the front and to the side, scanning the far distance, reading a map or manual at close distance (30 cm), listening through earphones or talking through a microphone. Seating, instrumentation, lighting, cockpit microclimate and radio communications equipment comfort have been and still remain the object of continuous improvement. Today’s modern flight deck, often referred to as the “glass cockpit”, has created yet another challenge with its use of leading-edge technology and automation; maintaining vigilance and situational awareness under these conditions has created new concerns for both the designers of aircraft and the technical personnel who fly them.

Cabin crew have an entirely different set of ergonomic problems. One main problem is that of standing and moving around during flight. During climb and descent, and in turbulence, the cabin crew is required to walk on an inclined floor; in some aircraft the cabin incline may remain at approximately 3% during cruise as well. Also, many cabin floors are designed in a manner that creates a rebound effect while walking, putting an additional stress on the flight attendants who are constantly moving about during a flight. Another important ergonomic problem for flight attendants has been the use of mobile carts. These carts can weigh up to 100 to 140 kg and must be pushed and pulled up and down the length of the cabin. Additionally, the poor design and maintenance of the braking mechanisms on many of these carts have caused an increase in repetitive-strain injuries (RSIs) among flight attendants. Air carriers and cart manufacturers are now taking a more serious look at this equipment, and new designs have resulted in ergonomic improvements. Additional ergonomic problems result from the need to lift and carry heavy or bulky items in restricted spaces or while maintaining uncomfortable body posture.

Workload

The workload for air crew members depends on the task, the ergonomic layout, the hours of work/duty and many other factors. The additional factors affecting the technical crew include:

• duration of rest time between present and last flight and the duration of sleep time during the rest period

• the pre-flight briefing and problems encountered during the pre-flight briefing

• delays preceding departure

• timing of flights

• meteorological conditions at the point of departure, en route and at the destination

• number of flight segments

• type of equipment being flown

• quality and quantity of radio communications

• visibility during descent, glare and protection from the sun

• turbulence

• technical problems with the aircraft

• experience of other crew members

• air traffic (especially at point of departure and destination)

• presence of air carrier or national authority personnel for purposes of checking crew competency.

Certain of these factors may be equally important for the cabin crew. In addition, the latter are subject to the following specific factors:

• pressure of time due to short duration of flight, high number of passengers and extensive service requirements

• extra services demanded by passengers, the character of certain passengers and, occasionally, verbal or physical abuse by passengers

• passengers requiring special care and attention (e.g., children, the disabled, the elderly, a medical emergency)

• extent of preparatory work

• lack of necessary service items (e.g., insufficient meals, beverages and so on) and equipment.

The measures taken by air carrier managements and government administrations to keep crew workload within reasonable limits include: improvement and extension of air-traffic control; reasonable limits on hours of duty and requirements for minimum rest provisions; execution of preparatory work by dispatchers, maintenance, catering and cleaning personnel; automation of cockpit equipment and tasks; the standardization of service procedures; adequate staffing; and the provision of efficient and easy-to-handle equipment.

Hours of work

One of the most important factors affecting both technical and cabin crew member occupational health and safety (and certainly the most widely discussed and controversial) is the issue of flight fatigue and recovery. This issue covers the broad spectrum of activity encompassing crew scheduling practices—length of duty periods, amount of flight time (daily, monthly and yearly), reserve or standby duty periods and availability of time for rest both while on flight assignment and at domicile. Circadian rhythms, especially sleep intervals and duration, with all their physiological and psychological implications, are especially significant for air crew members. Time shifts due either to night flights or to east/west or west/east travel across a number of time zones create the greatest problems. Newer generation aircraft, which have the capability of remaining aloft for up to 15 to 16 hours at a time, have exacerbated the conflict between airline schedules and human limitations.

National regulations to limit duty and flight periods and to provide minimum rest limitations exist on a nation by nation basis. In some instances, these regulations have not kept pace with technology or science, nor do they necessarily guarantee flight safety. Until recently there has been little attempt to standardize these regulations. Current attempts at harmonization have given rise to concerns among air crew members that those countries with more protective regulations may be required to accept lower and less adequate standards. In addition to national regulations, many air crew members have been able to negotiate more protective hours of service requirements in their labour agreements. While these negotiated agreements are important, most crew members feel that hours of service standards are essential to their health and safety (and to that of the flying public), and thus minimum standards should be adequately regulated by the national authorities.

Psychological stress

In recent years, aircraft crew have been confronted with a serious mental stress factor: the likelihood of hijacking, bombs and armed attacks on aircraft. Although security measures in civil aviation worldwide have been considerably increased and upgraded, the sophistication of terrorists has likewise increased. Air piracy, terrorism and other criminal acts remain a real threat to all air crew members. The commitment and cooperation of all national authorities as well as the force of worldwide public opinion are needed to prevent these acts. Additionally, air crew members must continue to receive special training and information on security measures and must be informed on a timely basis of suspected threats of air piracy and terrorism.

Air crew members understand the importance of starting flight duty in a sufficiently good mental and physical state to ensure that the fatigue and stresses occasioned by the flight itself will not affect safety. Fitness for flight duty may occasionally be impaired by psychological and physical stress, and it is the responsibility of the crew member to recognize whether or not he or she is fit for duty. Sometimes, however, these effects may not be readily apparent to the person under duress. For this reason, most airlines and air crew member associations and labour unions have professional standards committees to assist crew members in this area.

Accidents

Fortunately, catastrophic aircraft accidents are rare events; nonetheless, they do represent a hazard for air crew members. An aircraft accident is practically never a hazard resulting from a single, well-defined cause; in almost every instance, a number of technical and human factors coincide in the causal process.

Defective equipment design or equipment failure, especially as a result of inadequate maintenance, are two mechanical causes of aircraft accidents. One important, although relatively rare, type of human failure is sudden death due, for example, to myocardial infarction; other failures include sudden loss of consciousness (e.g., epileptic fit, cardiac syncope and fainting due to food poisoning or other intoxication). Human failure may also result from the slow deterioration of certain functions such as hearing or vision, although no major aircraft accident has been attributed to such a cause. Preventing accidents from medical causes is one of the most important tasks of aviation medicine. Careful personnel selection, regular medical examinations, surveys of absence due to illness and accidents, continuous medical contact with working conditions and industrial hygiene surveys can considerably decrease the danger of sudden incapacitation or slow deterioration in technical crew. Medical personnel should also routinely monitor flight scheduling practices to prevent fatigue-related incidents and accidents. A well-operated, modern airline of significant size should have its own medical service for these purposes.

Advances in aircraft accident prevention are often made as a result of careful investigation of accidents and incidents. Systematic screening of all, even minor, accidents and incidents by an accident investigation board comprising technical, operational, structural, medical and other experts is essential to determine all causal factors in an accident or incident and to make recommendations for preventing future occurrences.

A number of strict regulations exist in aviation to prevent accidents caused by use of alcohol or other drugs. Crew members should not consume quantities of alcohol in excess of what is compatible with professional requirements, and no alcohol at all should be consumed during and for at least 8 hours prior to flight duty. Illegal drug use is strictly prohibited. Drug use for medicinal purposes is strictly controlled; such drugs are generally not allowed during or immediately preceding flight, although exceptions may be allowed by a recognized flight physician.

The transport of hazardous materials by air is yet another cause of aircraft accident and incidents. A recent survey covering a 2-year period (1992 to 1993) identified over 1,000 aircraft incidents involving hazardous materials on passenger and cargo air carriers in one nation alone. More recently, an accident in the United States which resulted in the deaths of 110 passengers and crew involved the carriage of hazardous cargo. Hazardous materials incidents in air transportation occur for a number of reasons. Shippers and passengers may be unaware of the dangers presented by the materials they bring aboard aircraft in their baggage or offer for transport. Occasionally, unscrupulous persons may choose to illegally ship forbidden hazardous materials. Additional restrictions on the carriage of hazardous materials by air and improved training for air crew members, passengers, shippers and loaders may help to prevent future incidents. Other accident prevention regulations deal with oxygen supply, crew meals and procedures in case of illness.

Diseases

Specific occupational disease of crew members are not known or documented. However, certain diseases may be more prevalent among crew members than among persons in other occupations. Common colds and upper respiratory system infections are frequent; this may be due in part to the low humidity during flight, irregularities of schedules, exposure to att large number of people in a confined space and so on. A common cold, especially with upper respiratory congestion, that is not significant for an office worker may incapacitate a crew member if it prevents the clearing of pressure on the middle ear during ascent and, particularly, during descent. Additionally, illnesses that require some form of drug therapy may also preclude the crew member from engaging in work for a period of time. Frequent travel to tropical areas may also entail increased exposure to infectious diseases, the most important being malaria and infections of the digestive system.

The close confines of an aircraft for extended periods of time also carry an excess risk of airborne infectious diseases like tuberculosis, if a passenger or crew member has such a disease in its contagious stage.

Back

Since the first sustained flight of a powered aircraft at Kitty Hawk, North Carolina (United States), in 1903, aviation has become a major international activity. It is estimated that from 1960 to 1989, the annual number of air passengers of regularly scheduled flights increased from 20 million to over 900 million (Poitrast and deTreville 1994). Military aircraft have become indispensable weapons systems for the armed forces of many nations. Advances in aviation technology, in particular the design of life support systems, have contributed to the rapid development of space programmes with human crews. Orbital space flights occur relatively frequently, and astronauts and cosmonauts work in space vehicles and space stations for extended periods of time.

In the aerospace environment, physical stressors that may affect the health of aircrew, passengers and astronauts to some degree include reduced concentrations of oxygen in the air, decreased barometric pressure, thermal stress, acceleration, weightlessness and a variety of other potential hazards (DeHart 1992). This article describes aeromedical implications of exposure to gravity and acceleration during flight in the atmosphere and the effects of microgravity experienced in space.

Gravity and Acceleration

The combination of gravity and acceleration encountered during flight in the atmosphere produces a variety of physiological effects experienced by aircrew and passengers. At the surface of the earth, the forces of gravity affect virtually all forms of human physical activity. The weight of a person corresponds to the force exerted upon the mass of the human body by the earth’s gravitational field. The symbol used to express the magnitude of the acceleration of an object in free fall when it is dropped near the earth’s surface is referred to as g, which corresponds to an acceleration of approximately 9.8 m/s² (Glaister 1988a; Leverett and Whinnery 1985).

Acceleration occurs whenever an object in motion increases its velocity. Velocity describes the rate of movement (speed) and direction of motion of an object. Deceleration refers to acceleration that involves a reduction in established velocity. Acceleration (as well as deceleration) is a vector quantity (it has magnitude and direction). There are three types of acceleration: linear acceleration, a change of speed without change in direction; radial acceleration, a change in direction without a change of speed; and angular acceleration, a change in speed and direction. During flight, aircraft are capable of manoeuvring in all three directions, and crew and passengers may experience linear, radial and angular accelerations. In aviation, applied accelerations are commonly expressed as multiples of the acceleration due to gravity. By convention, G is the unit expressing the ratio of an applied acceleration to the gravitational constant (Glaister 1988a; Leverett and Whinnery 1985).

Biodynamics

Biodynamics is the science dealing with the force or energy of living matter and is a major area of interest within the field of aerospace medicine. Modern aircraft are highly manoeuvrable and capable of flying at very high speeds, causing accelerative forces upon the occupants. The influence of acceleration upon the human body depends upon the intensity, rate of onset and direction of acceleration. The direction of acceleration is generally described by the use of a three-axis coordinate system (x, y, z) in which the vertical (z) axis is parallel to the long axis of the body, the x axis is oriented from front to back, and the y axis oriented side to side (Glaister 1988a). These accelerations can be categorized into two general types: sustained and transitory.

Sustained acceleration

The occupants of aircraft (and spacecraft operating in the atmosphere under the influence of gravity during launch and re-entry) commonly experience accelerations in response to aerodynamic forces of flight. Prolonged changes in velocity involving accelerations lasting longer than 2 seconds may result from changes in an aircraft’s speed or direction of flight. The physiological effects of sustained acceleration result from the sustained distortion of tissues and organs of the body and changes in the flow of blood and distribution of body fluids (Glaister 1988a).

Positive or headward acceleration along the z axis (+G_z) represents the major physiological concern. In civil air transportation, G_z accelerations are infrequent, but may occasionally occur to a mild degree during some take-offs and landings, and while flying in conditions of air turbulence. Passengers may experience brief sensations of weightlessness when subject to sudden drops (negative G_z accelerations), if unrestrained in their seats. An unexpected abrupt acceleration may cause unrestrained aircrew or passengers to be thrown against internal surfaces of the aircraft cabin, resulting in injuries.

In contrast to civil transport aviation, the operation of high- performance military aircraft and stunt and aerial spray planes may generate significantly higher linear, radial and angular accelerations. Substantial positive accelerations may be generated as a high-performance aircraft changes its flight path during a turn or a pull-up manoeuvre from a steep dive. The +G_z performance characteristics of current combat aircraft may expose occupants to positive accelerations of 5 to 7 G for 10 to 40 seconds (Glaister 1988a). Aircrew may experience an increase in the weight of tissues and of the extremities at relatively low levels of acceleration of only +2 G_z. As an example, a pilot weighing 70 kg who performed an aircraft manoeuvre which generated +2 G_z would experience an increase of body weight from 70 kg to 140 kg.

The cardiovascular system is the most important organ system for determining the overall tolerance and response to +G_z stress (Glaister 1988a). The effects of positive acceleration on vision and mental performance are due to decreases in blood flow and delivery of oxygen to eye and brain. The capability of the heart to pump blood to the eyes and brain is dependent upon its capability to exceed the hydrostatic pressure of blood at any point along the circulatory system and the inertial forces generated by the positive G_z acceleration. The situation may be likened to that of pulling upward a balloon partially full of water and observing the downward distension of the balloon because of the resultant inertial force acting upon the mass of water. Exposure to positive accelerations may cause temporary loss of peripheral vision or complete loss of consciousness. Military pilots of high- performance aircraft may risk development of G-induced blackouts when exposed to rapid onset or extended periods of positive acceleration in the +G_z axis. Benign cardiac arrhythmias frequently occur following exposure to high sustained levels of +G_z acceleration, but usually are of minimal clinical significance unless pre-existing disease is present; –G_z acceleration seldom occurs because of limitations in aircraft design and performance, but may occur during inverted flight, outside loops and spins and other similar manoeuvres. The physiological effects associated with exposure to –G_z acceleration primarily involve increased vascular pressures in the upper body, head and neck (Glaister 1988a).

Accelerations of sustained duration which act at right angles to the long axis of the body are termed transverse accelerations and are relatively uncommon in most aviation situations, with the exception of catapult and jet- or rocket-assisted take-offs from aircraft carriers, and during launch of rocket systems such as the space shuttle. The accelerations encountered in such military operations are relatively small, and usually do not affect the body in a major fashion because the inertial forces act at right angles to the long axis of the body. In general, the effects are less pronounced than in G_z accelerations. Lateral acceleration in ±G_y axis are uncommon, except with experimental aircraft.

Transitory acceleration

The physiological responses of individuals to transitory accelerations of short duration are a major consideration in the science of aircraft accident prevention and crew and passenger protection. Transitory accelerations are of such brief duration (considerably less than 1 second) that the body is unable to attain a steady-state status. The most common cause of injury in aircraft accidents results from the abrupt deceleration that occurs when an aircraft impacts the ground or water (Anton 1988).

When an aircraft impacts the ground, a tremendous amount of kinetic energy applies damaging forces to the aircraft and its occupants. The human body responds to these applied forces by a combination of acceleration and strain. Injuries result from deformation of tissues and organs and trauma to anatomic parts caused by collision with structural components of the aircraft cockpit and/or cabin.

Human tolerance to abrupt deceleration is variable. The nature of injuries will depend on the nature of the applied force (whether it primarily involves penetrating or blunt impact). At impact, the forces which are generated are dependent on the longitudinal and horizontal decelerations which are generally applied to an occupant. Abrupt decelerative forces are often categorized into tolerable, injurious and fatal. Tolerable forces produce traumatic injuries such as abrasions and bruises; injurious forces produce moderate to severe trauma which may not be incapacitating. It is estimated that an acceleration pulse of approximately 25 G maintained for 0.1 second is the limit of tolerability along the +G_z axis, and that about 15 G for 0.1 sec is the limit for the –G_z axis (Anton 1988).

Multiple factors affect human tolerance to short-duration acceleration. These factors include the magnitude and duration of the applied force, the rate of onset of the applied force, its direction and the site of application. It should be noted that people can withstand much greater forces perpendicular to the long axis of the body.

Protective Countermeasures

Physical screening of crew members to identify serious pre- existing diseases which might put them at increased risk in the aerospace environment is a key function of aeromedical programmes. In addition, countermeasures are available to crew of high-performance aircraft to protect against the adverse effects of extreme accelerations during flight. Crew members must be trained to recognize that multiple physiological factors may decrease their tolerance to G stress. These risk factors include fatigue, dehydration, heat stress, hypoglycemia and hypoxia (Glaister 1988b).

Three types of manoeuvres which crew members of high- performance aircraft employ to minimize adverse effects of sustained acceleration during flight are muscle tensing, forced expiration against a closed or partially closed glottis (back of the tongue) and positive-pressure breathing (Glaister 1988b; DeHart 1992). Forced muscle contractions exert increased pressure on blood vessels to decrease venous pooling and increase venous return and cardiac output, resulting in increased blood flow to the heart and upper body. While effective, the procedure requires extreme, active effort and may rapidly result in fatigue. Expiration against a closed glottis, termed the Valsalva manoeuver (or M-1 procedure) can increase pressure in the upper body and raise the intrathoracic pressure (inside the chest); however, the result is short lived and may be detrimental if prolonged, because it reduces venous blood return and cardiac output. Forcibly exhaling against a partially closed glottis is a more effective anti-G straining manoeuver. Breathing under positive pressure represents another method to increase intrathoracic pressure. Positive pressures are transmitted to the small artery system, resulting in increased blood flow to the eyes and brain. Positive-pressure breathing must be combined with the use of anti-G suits to prevent excessive pooling in the lower body and limbs.

Military aircrew practise a variety of training methods to enhance G tolerance. Crews frequently train in a centrifuge consisting of a gondola attached to a rotating arm which spins and generates +G_z acceleration. Aircrew become familiar with the spectrum of physiological symptoms which may develop and learn the proper procedures to control them. Physical fitness training, particularly whole-body strength training, also has been found to be effective. One of the most common mechanical devices used as protective equipment to reduce the effects of +G exposure consists of pneumatically inflated anti-G suits (Glaister 1988b). The typical trouser-like garment consists of bladders over the abdomen, thighs and calves which automatically inflate by means of an anti-G valve in the aircraft. The anti-G valve inflates in reaction to an applied acceleration upon the aircraft. Upon inflation, the anti-G suit produces a rise in the tissue pressures of the lower extremities. This maintains peripheral vascular resistance, reduces the pooling of blood in the abdomen and lower limbs and minimizes downward displacement of the diaphragm to prevent the increase in the vertical distance between the heart and brain that may be caused by positive acceleration (Glaister 1988b).

Surviving transitory accelerations associated with aircraft crashes is dependent on effective restraint systems and the maintenance of the cockpit/cabin integrity to minimize intrusion of damaged aircraft components into the living space (Anton 1988). The function of lap belts, harnesses and other types of restraint systems are to limit the movement of the aircrew or passengers and to attenuate the effects of sudden deceleration during impact. The effectiveness of the restraint system depends on how well it transmits loads between the body and the seat or vehicle structure. Energy-attenuating seating and rearward facing seats are other features in aircraft design which limit injury. Other accident-protection technology includes the design of airframe components to absorb energy and improvements in seat structures to reduce mechanical failure (DeHart 1992; DeHart and Beers 1985).

Microgravity

Since the 1960s, astronauts and cosmonauts have flown numerous missions into space, including 6 lunar landings by Americans. Mission duration has been from several days to a number of months, with a few Russian cosmonauts logging approximately 1-year flights. Subsequent to these space flights, a large body of literature has been written by physicians and scientists describing in-flight and post-flight physiological aberrations. For the most part, these aberrations have been attributed to exposure to weightlessness or microgravity. Although these changes are transient, with total recovery within several days to several months after returning to Earth, nobody can say with complete certitude whether astronauts would be so fortunate after missions lasting 2 to 3 years, as envisioned for a round trip to Mars. The major physiological aberrations (and countermeasures) can be categorized as cardiovascular, musculoskeletal, neurovestibular, haematological and endocrinological (Nicogossian, Huntoon and Pool 1994).

Cardiovascular hazards

Thus far, there have been no serious cardiac problems in space, such as heart attacks or heart failure, although several astronauts have developed abnormal heart rhythms of a transient nature, particularly during extra-vehicular activity (EVA). In one case, a Russian cosmonaut had to return to Earth earlier than planned, as a precautionary measure.

On the other hand, microgravity seems to induce a lability of blood pressure and pulse. Although this does not cause impaired health or crew performance during flight, approximately half of astronauts immediately post-flight do become extremely dizzy and giddy, with some experiencing fainting (syncope) or near fainting (pre-syncope). The cause of this intolerance to being vertical is thought to be a drop in blood pressure upon re-entering the earth’s gravitational field, combined with the dysfunction of the body’s compensatory mechanisms. Hence, a low blood pressure and decreasing pulse unopposed by the body’s normal response to such physiological aberrations results in these symptoms.

Although these pre-syncopal and syncopal episodes are transient and without sequelae, there remains great concern for several reasons. First, in the event that a returning space vehicle were to have an emergency, such as a fire, upon landing, it would be extremely difficult for astronauts to rapidly escape. Second, astronauts landing on the moon after periods of time in space would be prone to some extent to pre-fainting and fainting, even though the moon’s gravitational field is one-sixth that of Earth. And finally, these cardiovascular symptoms might be far worse or even lethal after very long missions.

It is for these reasons that there has been an aggressive search for countermeasures to prevent or at least ameliorate the microgravity effects upon the cardiovascular system. Although there are a number of countermeasures now being studied that show some promise, none so far has been proven truly effective. Research has focused on in-flight exercise utilizing a treadmill, bicycle ergometer and rowing machine. In addition, studies are also being conducted with lower body negative pressure (LBNP). There is some evidence that lowering the pressure around the lower body (using compact special equipment) will enhance the body’s ability to compensate (i.e., raise blood pressure and pulse when they fall too low). The LBNP countermeasure might be even more effective if the astronaut drinks moderate amounts of specially constituted salt water simultaneously.

If the cardiovascular problem is to be solved, not only is more work needed on these countermeasures, but also new ones must be found.

Musculoskeletal hazards

All astronauts returning from space have some degree of muscle wasting or atrophy, regardless of mission duration. Muscles at particular risk are those of the arms and legs, resulting in decreased size as well as strength, endurance and work capacity. Although the mechanism for these muscle changes is still ill-defined, a partial explanation is prolonged disuse; work, activity and movement in microgravity are almost effortless, since nothing has any weight. This may be a boon for astronauts working in space, but is clearly a liability when returning to a gravitational field, whether it be that of the moon or Earth. Not only could a weakened condition impede post-flight activities (including work on the lunar surface), it could also compromise rapid ground emergency escape, if required upon landing. Another factor is the possible requirement during EVA to do space vehicle repairs, which can be very strenuous. Countermeasures under study include in-flight exercises, electrical stimulation and anabolic medication (testosterone or testosterone-like steroids). Unfortunately, these modalities at best only retard muscle dysfunction.

In addition to muscle wasting, there is also a slow but inexorable loss of bone in space (about 300 mg per day, or 0.5% of total bone calcium per month) experienced by all astronauts. This has been documented by post-flight x rays of bones, particularly of those that bear weight (i.e., the axial skeleton). This is due to a slow but unremitting loss of calcium into the urine and faeces. Of great concern is the continuing loss of calcium, regardless of flight duration. Consequently, this calcium loss and bone erosion could be a limiting factor of flight, unless an effective countermeasure can be found. Although the precise mechanism of this very significant physiological aberration is not fully understood, it undoubtedly is due in part to the absence of gravitational forces on bone, as well as disuse, similar to muscle wasting. If bone loss were to continue indefinitely, particularly over long missions, bones would become so brittle that eventually there would be risk of fractures with even low levels of stress. Furthermore, with a constant flow of calcium into the urine via the kidneys, a possibility of renal stone formation exists, with accompanying severe pain, bleeding and infection. Clearly, any of these complications would be a very serious matter were they to occur in space.

Unfortunately, there are no known countermeasures that effectively prevent calcium loss during space flight. A number of modalities are being tested, including exercise (treadmill, bicycle ergometer and rowing machine), the theory being that such voluntary physical stresses would normalize bone metabolism, thereby preventing or at least ameliorating bone loss. Other countermeasures under investigation are calcium supplements, vitamins and various medications (such as diphosphonates—a class of medications that has been shown to prevent bone loss in patients with osteoporosis). If none of these simpler countermeasures prove to be effective, it is possible that the solution lies in artificial gravity that could be produced by continuous or intermittent rotation of the space vehicle. Although such motion could generate gravitational forces similar to that of the earth, it would represent an engineering “nightmare”, in addition to major add-on costs.

Neurovestibular hazards

More than half of the astronauts and cosmonauts suffer from space motion sickness (SMS). Although the symptoms vary somewhat from individual to individual, most suffer from stomach awareness, nausea, vomiting, headache and drowsiness. Often there is an exacerbation of symptoms with rapid head movement. If an astronaut develops SMS, it usually occurs within a few minutes to a few hours after launch, with complete remission within 72 hours. Interestingly, the symptoms sometimes recur after returning to the earth.

SMS, particularly vomiting, can not only be disconcerting to the crew members, it also has the potential to cause performance decrement in an astronaut who is ill. Furthermore, the risk of vomiting while in a pressure suit doing EVA cannot be ignored, as the vomitus could cause the life-support system to malfunction. It is for these reasons that no EVA activities are ever scheduled during the first 3 days of a space mission. If an EVA becomes necessary, for example, to do emergency repairs on the space vehicle, the crew would have to take that risk.

Much neurovestibular research has been directed toward finding a way to prevent as well as to treat SMS. Various modalities, including anti-motion-sickness pills and patches, as well as using pre-flight adaptation trainers such as rotating chairs to habituate astronauts, have been attempted with very limited success. However, in recent years it has been discovered that the antihistamine phenergan, given by injection, is an extremely effective treatment. Hence, it is carried onboard all flights and given as required. Its efficacy as a preventive has yet to be demonstrated.

Other neurovestibular symptoms reported by astronauts include dizziness, vertigo, dysequilibrium and illusions of self-motion and motion of the surrounding environment, sometimes making walking difficult for a short time post-flight. The mechanisms for these phenomena are very complex and are not completely understood. They could be problematical, particularly after a lunar landing following several days or weeks in space. As of now, there are no known effective countermeasures.

Neurovestibular phenomena are most likely caused by dysfunction of the inner ear (the semicircular canals and utricle-saccule), because of microgravity. Either erroneous signals are sent to the central nervous system or signals are misinterpreted. In any event, the results are the aforementioned symptoms. Once the mechanism is better understood, effective countermeasures can be identified.

Haematological hazards

Microgravity has an effect upon the body’s red and white blood cells. The former serve as a conveyor of oxygen to the tissues, and the latter as an immunological system to protect the body from invading organisms. Hence, any dysfunction could cause deleterious effects. For reasons not understood, astronauts lose approximately 7 to 17% of their red blood cell mass early in flight. This loss appears to plateau within a few months, returning to normal 4 to 8 weeks post-flight.

So far, this phenomenon has not been clinically significant, but, rather, a curious laboratory finding. However, there is clear potential for this loss of red blood cell mass to be a very serious aberration. Of concern is the possibility that with very long missions envisioned for the twenty-first century, red blood cells could be lost at an accelerated rate and in far greater quantities. If this were to occur, anaemia could develop to the point that an astronaut could become seriously ill. It is hoped that this will not be the case, and that the red blood cell loss will remain very small, regardless of mission duration.

In addition, several components of the white blood cell system are affected by microgravity. For example, there is an overall increase in the white blood cells, mainly neutrophils, but a decrease in lymphocytes. There is also evidence that some white blood cells do not function normally.

As of now, in spite of these changes, no illness has been attributed to these white blood cell changes. It is unknown whether or not a long mission will cause further decrease in numbers as well as further dysfunction. Should this occur, the body’s immune system would be compromised, making astronauts very susceptible to infectious disease, and possibly incapacitated by even minor illness that would otherwise easily be fended off by a normally functioning immunological system.

As with the red blood cell changes, the white blood cell changes, at least on missions of approximately one year, are not of clinical significance. Because of the potential risk of serious illness in-flight or post-flight, it is critical that research continue on the effects of microgravity on the haematological system.

Endocrinological hazards

During space flight, it has been noted that there are a number of fluid and mineral changes within the body due in part to changes in the endocrine system. In general, there is a loss of total body fluids, as well as calcium, potassium and calcium. A precise mechanism for these phenomena has eluded definition, although changes in various hormonal levels offer a partial explanation. To further confound matters, laboratory findings are often inconsistent among the astronauts who have been studied, making it impossible to discern a unitary hypothesis as to the cause of these physiological aberrations. In spite of this confusion, these changes have caused no known impairment of health of astronauts and no performance decrement in flight. What the significance of these endocrine changes are for very long flight, as well as the possibility that they may be harbingers of very serious sequelae, is unknown.

Acknowledgements: The authors would like to recognize the work of the Aerospace Medical Association in this area.

Back

The helicopter is a very special type of aircraft. It is used in every part of the world and serves a variety of purposes and industries. Helicopters vary in size from the smallest single-seat helicopters to giant heavy-lift machines with gross weights in excess of 100,000 kg, which is about the same size as a Boeing 757. The purpose of this article is to discuss some of the safety and health challenges of the machine itself, the different missions it are used for, both civilian and military, and the helicopter’s operating environment.

The helicopter itself presents some very unique safety and health challenges. All helicopters use a main rotor system. This is the lifting body for the machine and serves the same purpose as the wings on a conventional airplane. Rotor blades are a significant hazard to people and property because of their size, mass and rotational speed, which also makes them difficult to see from certain angles and in different lighting conditions.

The tail rotor is also a hazard. It is usually much smaller than the main rotor and turns at a very high rate, so it too is very difficult to see. Unlike the main rotor system, which sits atop the helicopter’s mast, the tail rotor is often near ground level. People should approach a helicopter from the front, in view of the pilot, to avoid coming into contact with the tail rotor. Extra care should be taken to identify or remove obstacles (such as bushes or fences) in a temporary or unimproved helicopter landing area. Contact with the tail rotor can cause injury or death as well as serious damage to the property or helicopter.

Many people recognize the characteristic slap sound of a helicopter’s rotor system. This noise is encountered only when the helicopter is in forward flight, and is not considered a health problem. The compressor section of the engine produces extremely loud noise, often in excess of 140 dBA, and unprotected exposure must be avoided. Hearing protection (ear plugs and a noise attenuating headset or helmet) should be worn when working in and around helicopters.

There are several other hazards to consider when working with helicopters. One is flammable or combustible liquids. All helicopters require fuel to run the engine(s). The engine and the main and tail rotor transmissions use oil for lubrication and cooling. Some helicopters have one or more hydraulic systems and use hydraulic fluid.

Helicopters build a static electric charge when the rotor system is turning and/or the helicopter is flying. The static charge will dissipate when the helicopter touches the ground. If an individual is required to grab a line from a hovering helicopter, as during logging, external lifts or rescue efforts, that person should let the load or line touch the ground before grabbing it in order to avoid a shock.

Rescue/air ambulance. The helicopter was originally designed with rescue in mind, and one of its most widespread uses is as an ambulance. These are often found at the scene of an accident or disaster (see figure 2). They can land in confined areas with qualified medical teams on board who care for the injured at the scene while en route to a medical facility. Helicopters are also used for non-emergency flights when speed of transport or patient comfort is required.

Offshore oil support. Helicopters are used to help supply offshore oil operations. They transport people and supplies between land and platform and between platforms.

Executive/personal transport. The helicopter is used for point-to-point transportation. This is usually done over short distances where geography or sluggish traffic conditions prevent rapid ground transportation. Corporations build helipads on company property to allow easy access to airports or to facilitate transportation between facilities.

Sightseeing. The use of helicopters in the tourist industry has seen continuous growth. The excellent view from the helicopter combined with its ability to access remote areas make it a popular attraction.

Law enforcement. Many police departments and governmental agencies use helicopters for this type of work. The helicopter’s mobility in crowded urban areas and remote rural areas makes it invaluable. The largest rooftop helipad in the world is at the Los Angeles Police Department.

Film operations. Helicopters are a staple in action movies. Other types of movies and film-based entertainment are filmed from helicopters.

News gathering. Television and radio stations employ helicopters for traffic spotting and news gathering. Their ability to land at the place where the news is happening makes them a valuable asset. Many of them are also equipped with microwave transceivers so they can send their stories, live, over fairly long distances, while en route.

Heavy lift. Some helicopters are designed to carry heavy loads at the end of external lines. Aerial logging is one application of this concept. Construction and oil exploration crews make extensive use of the helicopter’s capacity for lifting large or bulky objects into place.

Aerial application. Helicopters can be fitted with spray booms and loaded to dispense herbicides, pesticides and fertilizers. Other devices can be added that allow helicopters to fight fires. They can drop either water or chemical retardants.
 

Military

Rescue/aerial ambulance. The helicopter is used widely in humanitarian efforts. Many nations around the world have coast guards that engage in maritime rescue work. Helicopters are used to transport the sick and wounded from battle areas. Still others are sent to rescue or recover people from behind enemy lines.

Attack. Helicopters can be armed and used as attack platforms over land or sea. Weapon systems include machine guns, rockets and torpedoes. Sophisticated targeting and guidance systems are used to lock on to and destroy targets at longe range.

Transport. Helicopters of all sizes are used to transport people and supplies over land or sea. Many ships are equipped with helipads to facilitate offshore operations.

The Helicopter Operating Environment

The helicopter is used all over the world in a variety of ways (see, for example, figure 1 and figure 2). In addition, it is often working very near the ground and other obstructions. This requires constant vigilance from the pilots and those who work with or ride on the aircraft. By contrast, the fixed-wing aircraft environment is more predictable, since they fly (especially the commercial airplanes) primarily from airports whose airspace is tightly controlled.

Figure 1. H-46 helicopter landing in the Arizona, US, desert.

Figure 2.  5-76A Cougar helicopter landing in field at accident site.

The combat environment presents special dangers. The military helicopter also operates in a low-level environment and is subject to the same hazards. The proliferation of inexpensive, hand-carried, heat-seeking missiles represents another danger to rotorcraft. The military helicopter can use the terrain to hide itself or to mask its telltale signature, but when in the open it is vulnerable to small-arms fire and missiles.

Military forces also use night vision goggles (NVG) to enhance the pilot’s view of the area in low-light conditions. While the NVGs do increase the pilot’s ability to see, they have severe operating limitations. One major drawback is the lack of peripheral vision, which has contributed to mid-air collisions.

Accident Prevention Measures

Preventive measures can be grouped into several categories. Any one prevention category or item will not, in and of itself, prevent accidents. All of them must be used in concert to maximize their effectiveness.

Operational policies

Operational policies are formulated in advance of any operations. They are usually provided by the company with the operating certificate. They are crafted from governmental regulations, manufacturer’s recommended guidelines, industry standards, best practices and common sense. In general, they have proven to be effective in preventing incidents and accidents and include:

• Establishment of best practices and procedures. Procedures are essential for accident prevention. When not used, such as in early helicopter ambulance operations, there were extremely high accident rates. In the absence of regulatory guidance, pilots attempted to support humanitarian missions at night and/or in poor weather conditions with minimal training and helicopters that were ill equipped for such flights, leading to accidents.

• Crew resource management (CRM). CRM began as “cockpit resource management” but has since progressed to crew resource management. CRM is based on the idea that people in the crew should be free to discuss any situation among themselves to assure the successful completion of the flight. While many helicopters are flown by a single pilot, they are often working with other people who are either in the helicopter or on the ground. These people can provide information about the operation if consulted or allowed to speak. When such interaction occurs, CRM then becomes company resource management. Such collaboration is an acquired skill and should be taught to crews, company employees and others that work with and around helicopters.

• Provision of a threat-free company environment. Helicopter operations can be seasonal. This means long, tiring days. Crews should be able to end their duty day without fear of recrimination. If there are other, similar, operational deficiencies, crews should be permitted to openly identify, discuss and correct them.

• Physical hazards awareness. The helicopter presents an array of hazards. The aircraft’s dynamic components, its main and tail rotors, must be avoided. All passengers and crew members should be briefed on their location and on how to avoid coming into contact with them. The component’s surfaces should be painted to enhance their visibility. The helicopter should be positioned so that it is difficult for people to get to the tail rotor. Noise protection must be provided, especially to those with continuous exposure.

• Training for abnormal conditions. Training is often limited, if available at all, to practising autorotations for engine-out conditions. Simulators can provide exposure to a much wider range of atypical conditions without exposing the crew or machine to the real condition.

Crew practices

• Published procedures. One study of accidents has shown that, in more than half the cases, the accident would have been prevented had the pilot followed known, published procedures.

• Crew resource management. CRM should be used.

• Anticipating and avoiding known problems. Most helicopters are not equipped to fly in icing conditions and are prohibited from flying in moderate or severe turbulence, yet numerous accidents result from these circumstances. Pilots should anticipate and avoid these and other equally compromising conditions.

• Special or non-standard operations. Pilots must be thoroughly briefed for such circumstances.

Support operations

The following are crucial support operations for the safe use of helicopters:

• following published procedures

• briefing all passengers prior to boarding the helicopter

• keeping facilities free of obstructions

• keeping facilities well lit for night operations.

Back