Aviation medicine’s role in safeguarding aviation safety

Abstract

Aviation medicine safeguards flight safety by addressing three critical areas: managing physiological challenges of the aviation environment, preventing in-flight medical incapacitation and ensuring psychological fitness for flight. The field adopts occupational medicine’s hierarchy of risk control to mitigate physiological risks in the operating environment, while employing systematic medical screening with tailored standards based on operational requirements to reduce the likelihood of in-flight incapacitation. A comprehensive approach incorporating mental health education, support systems and regular monitoring helps prevent psychological incapacitation. Recent data from the Singapore Changi Aeromedical Centre reveal that ophthalmological, otolaryngological and respiratory conditions are the primary causes of medical disqualification during air force pilot screening, reflecting the unique physiological demands of military aviation. This review emphasises the ongoing challenge of balancing rigorous medical standards with maintaining an adequate pilot recruitment pool, while highlighting the need for evidence-based approaches to aeromedical assessment and certification.

INTRODUCTION

For an aircraft to take off, fly, reach its intended destination and land safely, multiple components both on board and off-board the aircraft must function as a system. These components include the airframe, engines, fuel, electrical, flight control, environmental control, navigation and communication systems. One system that is not typically considered in the same vein is the pilot, who is also a critical component essential for flight operation. A failure of the human system, especially during critical phases of flight, can potentially result in an accident.

The human system can fail because of physiological, physical and psychological causes. Physiological incapacitation occurs when the pilot’s function is impaired by environmental conditions associated with aviation. Physical (or medical) incapacitation results in disruption of the pilot’s function by an underlying medical condition. Finally, psychological incapacitation occurs when the mental state of the pilot compromises their ability to perform flying tasks optimally. Thus, aviation medicine plays a key contributory role to overall flight safety by mitigating this risk of physiological, physical and psychological incapacitation of the human system.

Epidemiology of pilot incapacitation

Pilot incapacitation, a rare occurrence, still happens, with a study of in-flight medical incapacitation and impairment of US airline pilots finding 39 incapacitations and 11 impairments of US airline pilots on board 47 flights between 1993 and 1998. The rate of in-fight incapacitations and impairments was 0.04549 per 100,000 h and 0.01283 per 100,000 h, respectively. Two non-fatal aircraft accidents were also attributed to the in-flight medical impairment of pilots. This equates to a probability of 4%, or two out of 50 in-flight medical events being associated with a commercial airline flying accident.[1] According to the Australia Transportation Safety Bureau (ATSB), 113 occurrences of flight crew incapacitation were reported to the ATSB between 2010 and 2014; 86 of these incidents occurred on board high-capacity passenger transportation. This translates to about 29 incapacitation occurrences per million departures (or about one incident every 34,000 flights), with the main cause being gastrointestinal illness (40%), followed by laser strikes (13%). With multi-pilot crews in high-capacity operations, these occurrences usually had minimal effect on the safety of the flight. Less than 10% of the flights had to return to the originating airport or were diverted, and no accidents resulted from these events.[2]

The same ATSB study showed that low-capacity air transport and general aviation had fewer events but a wider range of causes that ended in incapacitation. These ranged from environmental causes, such as hypoxia, to medical conditions, such as a heart attack. However, 70% of pilot incapacitation occurrences in general aviation affected flight operations, resulting in either a return to the departure aerodrome or collision with terrain. Data from the US National Transportation Safety Bureau corroborate the higher risk of pilot incapacitation having an impact on flight operations, with 92 general aviation accidents between 1975 and 1982 recognising pilot incapacitation as a causal factor.[3] Both sets of data showed a higher risk of pilot incapacitation resulting in a negative flight safety outcome.

Physiological incapacitation

The aviation environment, with its hypobaria, hypothermia and hypoxia, is considered physically incompatible with normal human physiology, which has adapted to living at relatively low altitudes and motion in only two dimensions. Despite a history of over two centuries of manned aviation, the physiological hazards of the aviation environment [Box 1] remain a persistent hurdle that humans have yet to overcome.

Box 1.

Physiological hazards of aviation.

Hypobaric hypoxia

Pressure change effect on gas-containing body cavities

Sub-atmospheric decompression illness

Long-duration acceleration

Short-duration acceleration and crash dynamics

Spatial disorientation

Thermal stresses

Noise

Vibration

Motion sickness

Cosmic radiation

Prolonged wakefulness and fatigue

Occupational medicine’s hierarchy of occupational hazard and risk control model can be adopted to eliminate or mitigate physiological risk in aviation [Box 2]. While Elimination and Substitution are not entirely feasible owing to the persistent need for manned aviation, Engineering safeguards, Administrative controls and the use of Personal Protective Equipment (PPE) are extensively used to mitigate the physiological hazards of flying. Aviation medicine specialists are heavily involved in every level of the risk control model. A well-designed and thoroughly evaluated aircraft cabin pressurisation and environmental control system is an Engineering control measure that provides the human operator with a safe local operating environment with reduced hypoxia, decompression illness and pressure change-related risks. Examples of Administrative controls include oxygen prebreathing and limiting the time of altitude exposure to mitigate the risk of decompression illness, limitations on crew duty and flight duty periods to counter fatigue, and aviation physiology training for pilots to understand, recognise and counter physiological hazards. Finally, PPEs are extensively used to mitigate residual risks of the aviation environment. Examples include the use of full-body pressure suits to protect pilots operating at extremely high altitudes against hypoxia and decompression illness, a G-suit to counter the effects of long-duration acceleration on circulatory dynamics and noise-attenuating headsets with active noise reduction to protect pilots against aviation noise. Such equipment are engineered with inputs from aviation medicine specialists to ensure robust aircrew protection.

Box 2.

Examples for application of risk control model to hypoxia risk mitigation.

Elimination	Utilising unmanned platforms for high-altitude operations
Substitution	Utilising unmanned platforms for high-altitude operations
Engineering	Cabin pressurisation and environmental control, primary oxygen source, delivery regulator
Administration	Flight altitude limitation, duration-at-altitude limitation, aeromedical fitness screening, hypoxia recognition and response training, emergency response procedures
Personal protective equipment	Back-up oxygen system, oxygen masks, pressure breathing systems

Physical incapacitation

Physical pilot incapacitation owing to medical causes can occur in various forms and either overtly or subtly. Overt incapacitation, such as an acute coronary event resulting in cardiovascular collapse and loss of consciousness, is obvious to other aircrew or observers. However, subtle incapacitation, such as that of a pilot with obstructive sleep apnoea who experiences microsleep while performing flying duties, can go undetected by the affected pilot or the crew. The rate of onset of incapacitation also has a significant bearing on the impact on flight safety. A sudden incapacitation event during critical phases of flight, typically during takeoff or landing, can rapidly lead to an accident. Incapacitation that occurs slowly, such as the gradual cognitive decline of an aged pilot, is difficult to detect, and the pilot’s function can appear normal on routine flights until challenged during a stressing aircraft emergency.

As with all aircraft systems, the risk of failure needs to be quantified and managed. The acceptable risk of pilot incapacitation varies depending on the type of aviation operation. Commercial air operations involve greater numbers of passengers on board. Accidents can, thus, potentially result in many casualties, both on the aircraft and on the ground. The acceptable risk is, therefore, very low. For commercial passenger operations, the ‘1% Rule’ is frequently used, which states that a pilot is fit for dual-pilot operations if the likelihood of medical incapacitation is less than 1% per annum.[4] While the assumptions made in the derivation of the ‘1% Rule’ [Box 3] are increasingly considered overconservative without accounting for advancements in aircraft automation, crew training, operational procedures and flight duration, this rule still provides a useful basis for aeromedical risk assessment as it errs on the side of safety.[5]

Box 3.

Derivation of ‘1% Rule’.

• The derivation of the ‘1% Rule’ starts with a target of one fatal accident in 107 flying hours (1/107), half that of the prevailing fatal accident rate in large passenger aircraft transport in that period in the 1980s.

• Of these accident rates, the proportion of fatal accidents resulting from human system failure instead of other nonhuman causes should not exceed 10% (1/108), and medical incapacitation being the cause of this human system failure should also not exceed 10% (1/109).

• Therefore, the target risk of fatal air accident resulting from crew medical incapacitation should not exceed one per 109 flying hours.

Assumptions:

1. The average length of each flight sector is 1 h;

2. Only 10% of the time (1/108 h) in each flight sector is critical (i.e. take-off, climb out, approach, landing);

3. Based on simulator studies, in less than 1 in 100 events during the critical periods (1/106) would a second pilot not be able to take control in time to avoid a fatal accident.[5]

• Therefore, to achieve the target medical incapacitation accident rate of less than 1/109, neither pilot in a twopilot aircraft should have a medical incapacitation risk in excess of 1 in 106.

• This translates to an acceptable pilot incapacitation risk of 1% (0.01) per annum, as each year has approximately 10,000 (104) h (1% per annum = 0.01 medical incapacitation per 104 h = 1 medical incapacitation per 106 h).

In single-pilot flight operations, the increased risk of fatal air accidents resulting from the pilot’s medical incapacitation is generally balanced between full risk acceptance and having additional measures in place to ensure that the accepted risk of medical incapacitation is kept significantly lower than 1% per year. Private light aircraft aviation typically involves smaller and slower aircraft, which, in the worst-case scenario, is less likely to result in significant damage to built-up areas or mass casualties. This form of aviation needs to remain accessible to maintain transport links in austere areas, or for agricultural and industrial use. Correspondingly, the accepted risk of accident is higher than that of commercial airline passenger transport operations. Military aviation has to consider single-seat aircraft operations with additional and often unique physiological and operational demands imposed by high-performance aircraft. As such, the ‘1% Rule’ would be of inadequate risk mitigation against human system failure.

Just as other aircraft components are subject to quality control, inspections, repair or replacement, the human system is also put through a similar process. Pilot candidates first undergo a medical selection screen before commencement of flight training. This process entails an aeromedical fitness examination typically comprising a self-declaration questionnaire to elicit new symptoms across all organ systems, a review of medical records, physical examination and targeted investigations. Unique to aeromedical fitness assessment is the comprehensiveness of the ophthalmological, otolaryngological, anthropometrical and cardiorespiratory examinations owing to their increased relevance to flight safety. The medical selection standards these pilot applicants are assessed against are customised based on several considerations — characteristics of the aircraft platform the applicant will be operating, the nature of flying operations, the size and health characteristics of the pilot recruitment pool and the cost of flying training to the organisation. For example, standards for assessing a military single-seat fast jet pilot applicant who would need to routinely operate under high acceleration forces, in darkness, at low altitudes and with limited automation will differ from those used for civil airline pilot applicants operating on board a modern jetliner with high degrees of automation and the presence of another trained co-pilot. Organisations may layer on additional medical requirements to guard against the financial and opportunity costs arising from the subsequent medical disqualification of trained pilots. As such, historical or mild conditions with a potential for recurrence or deterioration may also be considered disqualifying during initial pilot selection.

Once trained, pilots are subjected to medical examinations at regular intervals to ensure their continued fitness for flying duties and to detect the onset of any new conditions early. These periodic medical examinations are like initial selection examinations, but may have a reduced scope, and are tailored to focus on detecting the development of new medical conditions. Pilots are also educated on the importance and requirement to obtain medical advice once they are unwell and the need to notify aviation medical authorities of any new medical conditions that may be diagnosed or managed by other healthcare practitioners. Pilots with minor ailments can be grounded for a short period of time to allow for recovery. However, those who develop more significant or chronic medical conditions are subjected to holistic aeromedical assessment and management, in which both the medical condition and occupational demands are taken into consideration to guide treatment requirements and to determine their fitness to resume flying duties. This process has an approach different from that of medical selection, with the focus being on safely returning the pilot with medical condition(s) to flying, instead of the elimination focus of selection medical examinations. This differing focus not only reduces the loss of valuable trained pilots, but also provides pilots with the reassurance that they can surface new medical issues for aeromedical management, rather than bear the risk of concealment due to concerns over job security. For pilots who do not need grounding from all flying duties but remain at an increased risk of in-flight incapacitation, risk mitigation restrictions can be imposed to reduce the aeromedical risk down to an acceptable level. Examples of such restrictions include limitation to dual-pilot-only operations or restrictions from high-G flying.

Psychological incapacitation

Mental health disorders have multiple detrimental effects on pilot performance. Incomplete or insidious psychological incapacitation increases the risk of errors, violations and hurried or biased decision-making.[6] Complete incapacitation can not only result in the loss of a crew member during the critical phases of flight, but also impose additional workload on the remaining pilot to secure the incapacitated pilot. There is also the rare[7,8] but present risk of pilot aircraft-assisted suicide, which came into recent focus again with the Germanwings Flight 9525 accident.

A multipronged approach is utilised to reduce the risk of psychological pilot incapacitation. Initial medical examinations screen applicants for pre-existing psychiatric conditions, childhood behavioural issues and abnormal coping mechanisms under psychological stresses such as a history of self-harm behaviour. Medical examiners also work to establish close rapport with the pilots returning for annual medical examinations to facilitate self-declaration of psychological symptoms. These self-declarations are also enabled by organisational policies that provide financial security in the event the pilot must be grounded from flying duties to undergo treatment and close aeromedical management to return the pilot to flying duties once well. Regular mental health education is conducted to provide pilots knowledge and skills on how to maintain mental health and to seek help early in the event of psychological duress. Aviation medicine physicians also work with airline management and national regulatory bodies to establish peer support groups, access to counselling services, mechanisms to provide pilots a safe environment to declare psychological issues and educate non-aviation doctors on their obligation to disclose aeromedically significant psychiatric conditions and treatment.[9]

AUDIT OF SELECTION MEDICAL EXAMINATION OUTCOMES

The selection medical examination is the most comprehensive medical examination a pilot undergoes in their career. Apart from helping to ensure that the candidate entering the training pipeline is fit for pilot training and subsequent flying duties, the examination also reduces the risk of subsequent aeromedical disqualification and the financial loss of the initial training cost for relevant organisations. A review of selection medicals performed for pilots will provide the reader with an understanding of the leading conditions that result in selection failure and their aeromedical significance, and highlight the tailoring of selection standards to match operational requirements.

A retrospective observational study was conducted to identify the causes of medical selection failures among pilot applicants We performed an audit (unpublished data) to identify the causes of medical selection failures among pilot applicants undergoing initial military pilot medical selection at Singapore Changi Aeromedical Centre (SCAC) from January 2022 to October 2024. The number of unique candidates who received a final assessment outcome of ‘unfit’ was established. The causes resulting in an unfit outcome were identified and categorised by organ systems. For applicants who failed medical selection owing to conditions affecting multiple organ systems, unfit outcomes in each organ system were counted individually. In total, 1213 pilot applicants who underwent selection medical examination were deemed medically unfit. Of the 1509 individual organ system unfit grades, the main organ systems leading to failure were ophthalmological, otolaryngological, respiratory, musculoskeletal and haematological [Table 1]. We further elaborate on some of these organ systems below.

Table 1.

Causes of pilot selection failure.

Organ system and condition	n (%)
Ophthalmological	541 (35.9)
Refractive error	415 (76.7)
Strabismus	69 (12.8)
Colour vision deficiency	34 (6.3)
Others	23 (4.3)
Otolaryngological	303 (20.1)
Allergic rhinitis	218 (71.9)
Eustachian tube dysfunction	58 (19.1)
Hearing loss	15 (5.0)
Others	12 (4.0)
Respiratory	235 (15.6)
Asthma/childhood asthma	226 (96.1)
History of pneumothorax	7 (3.0)
Others	2 (0.9)
Musculoskeletal	181 (12.0)
Scoliosis	100 (55.2)
Anthropometry issues	46 (25.4)
Others	35 (19.3)
Haematological	84 (5.6)
Anaemia including thalassemia	82 (97.6)
Others	2 (2.4)

Ophthalmological system

Excellent distant visual acuity, contrast sensitivity and visual fields are required for pilots to detect and track airborne objects or terrain features at distance in challenging visual conditions. Near vision is important for the reading of cockpit instruments, navigational charts and checklists. Colour vision is important for the correct discernment and interpretation of runway approach lighting, aircraft lighting, airport taxiway markings and signs. Modern multifunctional displays in cockpits make heavy use of colour symbology. While there are inbuilt non-colour cues in the symbology as a redundancy, colour discrimination provides the most intuitive interpretation, especially under time-sensitive conditions. Good stereovision and depth perception are important during ground taxi to judge distances between aircraft wing tips and other aircraft or ground structures. This is also applicable during landing, aerial refuelling operations or formation flying. With these considerations in mind, the aeromedical examination typically includes distant and near visual acuity tests, colour vision and contrast sensitivity assessments, strabismus examination, stereovision tests and direct ophthalmoscope or slit-lamp evaluation.

Civilian pilot selection standards generally accept applicants who can achieve normal visual acuity with corrective lenses. The potential interference of spectacles with helmets, visors and night vision goggles, as well as the risk of spectacle displacement during high-G manoeuvres or ejection means that high-performance jet pilots should achieve a safe visual acuity without corrective lenses. To this end, corneal refractive surgery can be used to increase the potential recruitment pool for military pilots, and several correction techniques have been deemed to be aeromedically safe.[10,11] High degrees of myopia and astigmatism before surgical correction remain disqualifying for fast jet operations owing to the risk of future corneal ectasia that occurs following the loss of large amounts of stromal tissue during corrective surgery, as well as the increased underlying risk of retinal detachment.[12] This is an example of selection standards being used to reduce the risk of future medical disqualification.

Otolaryngological system

Otolaryngological assessment includes obtaining a detailed history of sinus or ear pathologies, examination of the upper airway tracts, auditory canals and tympanic membranes, as well as audiology testing. Eustachian tube function is also assessed by visual inspection of tympanic membrane mobility during the Valsalva manoeuvre, or via tympanogram.

Allergic and vasomotor rhinitis are common conditions among pilot applicants[13] and can compromise consistent Eustachian tube function, which is essential for middle-ear and sinus pressure equalisation during rapid altitude changes encountered in both fast jet and helicopter aviation. Inability to rapidly equalise middle-ear and sinus pressures not only results in tympanic membrane trauma, but can also cause incapacitating pain in-flight or pressure vertigo with its attendant flight safety risks. Civilian passenger aircraft cabin pressurisation systems provide gentle pressure change profiles to reduce untrained passenger discomfort during altitude changes. The maximum rate of cabin pressure increase adopted for most civil passenger aircraft is 1 kPa (0.15 lb/in2)/min or about 300 feet per minute. In contrast, high-performance aircraft expose aircrew to far greater changes in pressure that can exceed 10,000 feet per minute. Any impediment to normal Eustachian tube or sinus ostia function will lead to barotrauma, which can be both acutely incapacitating and take a pilot off flying duties for a prolonged period. With this consideration, functionally significant allergic or vasomotor rhinitis and abnormal Eustachian tube function account for the majority of attrition in military pilot applicants, whereas these conditions are not typically considered disqualifying for civilian pilot applicants.

Hearing deficit is disqualifying for military and civilian applicants as it impedes safe and effective communications between the aircraft and with ground controllers. Sensitivity to air or motion sickness is not medically disqualifying as it is a normal physiological phenomenon that does not predict subsequent training success and can generally resolve with progressive acclimatisation and desensitisation.

Respiratory system

Normal respiratory function is crucial in the aviation environment. A review of chest symptoms, history of respiratory conditions, pulmonary examination and plain chest radiography are utilised for initial pilot screening. Of aeromedical interest is the presence of active asthma. An acute asthma attack can be acutely incapacitating. Asthma triggers such as cool, dry air; airborne irritants such as smoke, fumes or dust; and intense exertion are found in the aviation environment. Positive pressure breathing for hypoxia or G-protection involves administration of high flow rates of cool, dry air and can trigger an attack in an asthmatic pilot. As such, most leading air forces do not accept pilot applicants with active asthma. Civilian aviation medical certification agencies, however, generally accept applicants with active asthma if their condition is stable with satisfactory pulmonary function tests and is treated with medication compatible with flight safety. A history of spontaneous pneumothorax that has not been definitively treated with pleurodesis or pleural blebs is also disqualifying owing to the risk of pneumothoraxes occurring as a result of pressure change.[14]

Other conditions

Significant scoliosis increases the risk of back pain during exposure to sustained acceleration as well as spinal injury during ejection. The latter occurs owing to the abnormal transfer of force along the misaligned spine.[15] Anthropometric issues encompass excessive or inadequate sitting height and arm, leg and thigh lengths. These factors can affect the pilot’s ability to reach controls when strapped into the seat and maintain proper vision over the instrument panel, or can lead to unintended interactions between parts of the body and cockpit structures during movement or ejection. Both scoliosis and anthropometric failures are disqualifying only for fast jet operations owing to the unique challenges posed by G-forces, limited seat adjustments, secure restraint systems and the use of ejection seats in military aircraft. Anaemia increases the risk of hypoxia during altitude exposure, as well as G-induced loss of consciousness due to reduced blood oxygen-carrying capacity. Congenital anaemia with baseline haemoglobin concentration not satisfying the criteria is disqualifying, while acquired anaemia has to be treated before selection.

CONCLUSION

Aviation medicine plays a crucial role in safeguarding aviation safety by mitigating the risk of physiological, physical and psychological incapacitation of the pilot. Ophthalmological, otolaryngological and respiratory pathologies are the leading causes of medical disqualification in military pilot applicants. The specific conditions leading to failure highlight the customisation of medical selection standards to the expected physiological demands and operational environments of the intended aircraft platform. The findings of this study underscore the importance of continued research and development in aviation medicine to define selection protocols that ensure flight safety while maintaining an adequate pilot recruitment pool. Future studies could focus on the long-term follow-up of pilots meeting selection standards to validate the effectiveness of the current selection criteria and explore potential areas for further refinement of medical standards.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.