Analysis of mission and aircraft factors in G-induced loss of consciousness in the USAF: 1982-2002

INTRODUCTION: Previous studies have used questionnaires to evaluate G-LOC incident rates in different aircraft types, but no studies of G-LOC-related incident, crash, and fatality rates in different aircraft types were found in a search of the literature. METHODS: G-LOC events (including both incidents and crashes) for the fiscal years 1982 to 2002 were obtained from the USAF Safety Center. Aircraft sortie numbers were obtained for all aircraft that had a G-LOC event reported. Contingency table analysis and Chi-squared tests were used to evaluate differences in G-LOC rates. RESULTS: Overall 559 G-LOC events occurred for a rate of 25.9 per million sorties (PMS), but event rates differed by almost two orders of magnitude between aircraft categories, being highest in basic training aircraft, intermediate in single crewmember fighters, and lowest in other aircraft types (p < 0.001). The proportion of events resulting in crashes was 30% in single-crewmember aircraft compared with 0.6% in trainers and other two-crewmember aircraft (p < 0.001). All of the crashes and fatalities occurred in aircraft occupied at the time by only a single crewmember. The crash fatality rate was 100% in attack aircraft, 73% in single-crewmember fighters, and zero in basic trainers (p < 0.05). The F-15 and F-16 aircraft did not have higher G-LOC rates than other single-seat fighters. DISCUSSION: The data suggest that both crew complement and mission play a role in determining G-LOC rates. The data also suggest that G-LOC in an aircraft with a ground attack mission is more likely to result in a fatality.     

+Gz-induced loss of consciousness and aircraft recovery

Aircrew incapacitation resulting from very high onset sustained +Gz stress has resulted in significant losses of aircraft and aircrew. Enhanced protection and training toward prevention of +Gz-induced loss of consciousness (G-LOC) will continue to be vital. Techniques for reduction of the time of incapacitation, should G-LOC occur, must also be explored and developed. Current capability of aircraft autorecovery as demonstrated by the Advanced Fighter Technology Integration F-16 (AFTI/F-16) promises to enhance safety from the acute incapacitation resulting from G-LOC (and spatial disorientation). Physiologic monitoring for determining G-LOC has certain advantages especially in the aerial combat arena. The optimum physiologic monitoring technique would be direct determination of failure of brain cell function at the cellular or subcellular level. Complete investigation of G-LOC is necessary to understand the phenomenon and to develop methods for enhancing recognition and recovery. This paper discusses aircraft autorecovery technology and potential methods for physiologic monitoring of G-LOC. Integration of physiologic monitoring techniques into aircraft autorecovery systems requires a broad approach for optimal development.     

G-induced loss of consciousness: definition, history, current status

G-induced loss of consciousness (G-LOC) is defined as "a state of altered perception wherein (one's) awareness of reality is absent as a result of sudden, critical reduction of cerebral blood circulation caused by increased G force." This phenomenon was first identified in Great Britain in World War I (circa 1918-1919) as "fainting in the air." In the United States G-LOC was first encountered in 1922 during the Pulitzer Trophy Air Race. Although recognized during World War II as an operational hazard for fighter aircraft, the invention of the pneumatic anti-G suit reduced concern about G-LOC at that time. A 1984 survey of pilots of high performance aircraft has shown G-LOC to be an operational problem--probably one that has caused aircraft mishaps for several years. The concern of this panel was to focus on various approaches in reducing the G-LOC hazard.     

Visual symptoms and G-LOC in the operational environment and during centrifuge training of Turkish jet pilots.

BACKGROUND: High-performance fighter aircraft produce high-sustained +Gz forces with rapid onset rates. Because of this G-producing capability, military jet pilots are subjected to physiological stress, which may lead to visual disturbances and G-induced loss of consciousness (G-LOC). Although visual disturbances are very common in jet flights, G-LOC is relatively rare but more dangerous. The frequency and causes of G-LOC need to be determined in the interest of flight safety. METHODS: Part I. A survey was conducted on Turkish jet pilots to reveal the incidence of symptoms due to +Gz acceleration. Anonymous questionnaires were given to F-16, F-4, and F-5 pilots. They consisted of inquiries about the occurrence of visual symptoms and/or G-LOC during +Gz acceleration in the operational environment. Part II. During the years 1992-1996, 486 F-16, 801 F-4, and 256 F-5 fighter pilots underwent high "G" training at Turkish Aerospace Medical Center and they were assessed in terms of G-LOC rates. RESULTS: Part I. A total of 325 pilots who flew T-37 in undergraduate pilot training (UPT) answered the questionnaire. The pilots were divided into 3 groups according to the types of aircraft, which they fly now: 116 F-16, 182 F-4, and 27 F-5 pilots. A total of 311 pilots (95.7%) reported having experienced greyouts and/or blackouts. With 25 pilots (7.7%) experiencing G-LOC, the G-LOC frequency according to the type of aircraft was: 5.2% (T-37) [in UPT]; 4.3% (F-16), 1.6% (F-4), and 0% (F-5). Part II. In centrifuge training, the incidence of G-LOC in pilots of the various types of aircraft were: 12% (F-16), 6.4% (F-4), and 8.6% (F-5). CONCLUSIONS: Centrifuge training reduces G-LOC rates of subsequent centrifuge training; and it is hoped might reduce the G-LOC rate in the operational environment. Almost all jet pilots reported having experienced +Gz related visual symptoms, but G-LOC seems to be a more common problem for pilots who fly rapid onset rate aircraft than pilots who fly high "G" capable but lower G onset rate aircraft.     

Preventing G-induced loss of consciousness: 20 years of operational experience

INTRODUCTION: Although anecdotal reports of G-induced loss of consciousness (G-LOC) in military aviation date back to before 1920, regular reporting did not begin until 1982. The effectiveness in the operational setting of G-LOC preventive measures, such as positive pressure breathing for G protection (PBG), has not been studied. METHODS: We use the term "crash" to represent an event where the aircraft was destroyed and "incident" to define those events where the crew reported a G-LOC episode and the aircraft was not damaged. Data on G-LOC crashes, incidents, and aircraft sorties (number of take-offs) were obtained from the USAF Safety Center database for FY 82-01. RESULTS: During FY 82-01, there were 29 G-LOC crashes while those aircraft at risk of G-LOC crashes flew a total of 13,959,816 sorties. Poisson regression showed a non-significant decrease in crashes with an incidence rate ratio (IRR) of 0.096 (CI 0.89-1.03) (4% per yr). G-LOC crashes decreased from 4.4 per million flight sorties (PMFS) to 1.6 after the implementation of anti-G-LOC training programs beginning in 1985. However, G-LOC crashes remained unchanged after implementation of PBG in 1995. In contrast, incidents showed an IRR of 1.04 (CI 1.02-1.06) for G-LOC incidents, an estimated increase of 5% per yr. DISCUSSION: The physical/mechanical limitations of PBG, risk homeostasis, and the possibility that G-LOC rates have reached their asymptotic minimum are all discussed as possible explanations for the failure of PBG to decrease G-LOC crashes.     

G-induced loss of consciousness: retrospective survey results from 2259 military aircrew

INTRODUCTION: Prevalence of G-induced loss of consciousness (G-LOC) in the United Kingdom Royal Air Force (RAF) was found to be 19.3% in 1987. With the introduction of the Typhoon, a fourth generation aircraft, the prevalence of G-LOC has been re-assessed to determine the effectiveness of current G tolerance training. METHOD: A survey was sent to 4018 RAF aircrew, irrespective of their current role. Information was requested on G-LOC, role and aircraft type, experience, and attitudes toward G-LOC prevention. RESULTS: Responses were received from 2259 (56.2%) individuals, 882 (39%) of whom were current fast jet aircrew. At least one episode of G-LOC was reported by 20.1% of all respondents. In front line aircraft, prevalence of G-LOC among the 882 fast jet aircrew who responded was 6%. In the whole group, G-LOC was reported most commonly in aircrew under training (70.9%), and was most prevalent in training aircraft (77.4% of G-LOC events). At the time of the G-LOC, 64% of aircrew had less than 100 h total flying time. G-LOC was reported most frequently between +5 to +5.9 Gz, and "push-pull" maneuvers were associated with 31.3% of G-LOC events. Pulling G was not considered a problem by 50.6% of respondents, although over 80% recognized the value of flying currency, use of an anti-G suit, and physical fitness, and 55.6% felt that centrifuge training would be valuable. DISCUSSION: The prevalence of G-LOC in the RAF has changed little since 1987, and there remains considerable scope for aircrew education, particularly with the introduction of the Typhoon.     

G-induced loss of consciousness: case-control study of 78 G-Locs in the F-15, F-16, and A-10

INTRODUCTION: This study determined the trends of reported G-induced loss of consciousness (G-LOC) mishaps from 1980--1999, and determined potential risk factors in pilot characteristics; specifically, 30/60/ 90-h and sortie history, total flight hours, total hours in the aircraft, age, height, weight, and BMI. METHODS: Using aircraft malfunction mishaps to reflect a cross-section of USAF pilots, potential risk factors were determined using a case-control method; cases were all G-LOC mishaps and controls were aircraft malfunction mishaps. The data consisted of 2002 mishap pilots in the history of the F-16, F-15, F-15E, and A-10 from 1980-1999. RESULTS: During this time, G-LOCs represented only 2.5% of all mishaps. The mean engagement number for G-LOC mishaps was three at an average of 8 Gs. A poor anti-G straining maneuver was cited in 72% of the mishaps, fatigue and G-suit malfunction in 19%, low G-tolerance at 14%, and 37% were student pilots. Within pilot characteristics, only two factors were found to be statistically significant: the time in the aircraft and pilot age. In the F-16, there was a 3.5 times greater chance of experiencing a G-LOC mishap if the pilot had less than 600 h in the aircraft [3.5 (1.7-7.2, 95%CI)], and a 9.5 times greater chance in the F-15 [9.5 (2.2-41.9, 95%CI)]. There was a 4.5 times greater chance of experiencing a G-LOC mishap if under the age of 30 in the F-16 [4.5 (2.3-8.5, 95% CI)] and a 3 times greater chance in the F-15 [2.8 (1.2-6.8, 95% CI)]. DISCUSSION: Though it is difficult to predict who will experience G-LOC, emphasis on prevention must be concentrated in training and in pilots new to the aircraft.     

A new look at the loss of consciousness experience within the U.S. Naval forces

Two surveys on the incidence of G-induced loss of consciousness (G-LOC) in U.S. Navy aircraft have been completed. Questionnaires returned (981) indicated an incidence rate of 12.2% in the first survey. A slightly higher incidence rate was found in the second survey based on the 2,459 questionnaires returned. Results indicated that G-LOC is a significant problem in naval aviation in older as well as newer generation aircraft. Age, height, and weight of respondents did not appear to be related to incidence of G-LOC. Results indicated a need for improvement in the anti-G protective system and its use. Different forms of physical fitness training may differentially influence G-tolerance.     

Converging research on +Gz-induced loss of consciousness

The G-induced loss of consciousness (G-LOC) hazard can be reduced either by preventing its occurrence or shortening the period of incapacitation. The latter requires an understanding of this period of incapacitation. Two types of G-LOC occur: Type I is short duration and without convulsive type movements; and Type II is longer and with convulsions. Psychological suppression (denial) by pilots that G-LOC had occurred appears to be an important problem in reporting surveys and flying safety. Auditory and visual types of sensory stimuli to reduce the period of incapacitation are discussed. Recognition by the pilot that G-LOC has occurred appears to decrease incapacitation times and should be considered part of G training. Methods of developing an aircraft recovery system after G-LOC has occurred in pilots is considered a viable approach and is examined. Converging on the G-LOC problem by both, reducing its incidence as well as its duration appears to offer an additional dimension in the approach towards solving this important operational problem.     

+Gz-induced loss of consciousness: a case for training exposure to unconsciousness

+Gz-induced loss of consciousness (G-LOC) continues to be a threat to aircrew flying high-performance fighter aircraft. All avenues to prevent G-LOC, and to reduce the resulting incapacitation should G-LOC occur, must be explored. Research has begun to accurately quantify all aspects of the G-LOC phenomenon. The emerging pattern from this research indicates that, theoretically, G-LOC incapacitation could be significantly reduced. Comparison of G-LOC with LOC induced by acute arrest of cerebral circulation reveals that the G-LOC incapacitation could be reduced by as much as 17 s. Results also indicate that the relative incapacitation period (confusion and disorientation) following unconsciousness could be reduced by at least 9 s for an individual who has previously experienced G-LOC. This suggests that exposure to G-LOC during centrifuge training could provide this orientation to G-LOC and potentially reduce the incapacitation period should it occur inflight. This exposure may be likened to the current altitude-hypoxia training requirement for aircrew. Experience to date supports the contention that such training may be accomplished with an acceptable safety margin.     

Current and emerging technology in G-LOC detection: noninvasive monitoring of cerebral microcirculation using near infrared

G-induced loss of consciousness (G-LOC) has emerged as an important operational problem of high-performance aircraft. Since it appears that G-LOC will continue to be a problem, a requirement exists to detect its occurrence in pilots so that the aircraft may be placed on autopilot. One excellent method of detecting G-LOC physiologically, one would assume, would be based on the oxidative status of the brain. This determination can be made noninvasively with an Oxidative Metabolism Near-Infrared monitor using 4 wave lengths (OMNI-4). The OMNI-4 is capable of measuring the relative quantities in the brain of hemoglobin (Hb), oxygenated hemoglobin (HbO2), blood volume (BV), and oxidative status of cytochrome c oxidase. This instrument was tested on subjects in the USAFSAM human-use centrifuge at +3, 4, and 5 Gz with onset rates of 1 G.s-1. Results showed changes within the brain, as expected, during increased G with reductions in Hb, BV, and HbO2. Cytochrome c oxidase measurements were inconclusive. Immediately following G exposure, Hb, BV, and HbO2 "overshoots" occurred suggesting vasodilation of the cerebral microcirculation. The use of OMNI-4 in the laboratory and its possible role as a detector of G-LOC in pilots are discussed including suggestions for future developments.     

Defining risk in aerospace medical unconsciousness research

The maneuverability envelopes of current and future fighter/attack aircraft exceed unprotected human tolerance to environmental stress. Human exposure to unconsciousness therefore can result not only inflight, but in research and training laboratories which endeavor to provide methods of enhanced protection for aircrew. Solving the problem of unconsciousness requires a thorough understanding of the phenomenon itself. This can only be accomplished by defining the psychophysiologic aspects of unconsciousness and techniques to prevent its occurrence or enhance recovery, should it occur. The safety of experimental human exposure to G-LOC however, remains of some concern. A framework for discussing the relative insult to the central nervous system may be constructed from what is currently known about G-LOC. The results of animal experimentation allow an estimation of the central nervous system tolerance to hypoxia without permanent alteration of tissue integrity. Clinical medicine documentation of syncope and fainting episodes, coupled with a long history of uncomplicated G-LOC episodes suggests that a certain window of safe exposure exists. Utilization of G-LOC as an endpoint included exposure of very large numbers of humans to unconsciousness without significant complication. Animal experimentation suggests that 180 s of central nervous system hypoxia is associated with uncomplicated recovery. Human exposure as long as 100 s has also been safely accomplished. Centrifuge G-LOC exposure typically results in only 15-20 s of central nervous system hypoxia. As long as G-LOC experimentation using humans is performed within well defined limits, it may be accomplished within an acceptable risk envelope.(ABSTRACT TRUNCATED AT 250 WORDS)     

G-induced visual and cognitive disturbances in a survey of 65 operational fighter pilots

INTRODUCTION: Only one previous study has assessed almost loss of consciousness (A-LOC) in operational fighter pilots, reporting an incidence rate of 14%. Research also indicates that 8-13% of pilots have experienced G-induced loss of consciousness (G-LOC). A-LOC can be as insidious as G-LOC due to the associated altered state of awareness and relative incapacitation time, making it a significant risk factor in the high +Gz environment. Royal Australian Air Force (RAAF) pilots currently fly the F/A-18 and Hawk 127, producing +Gz accelerations up to +7.5 Gz, which places these pilots at risk of both A-LOC and G-LOC. METHODS: A survey was administered to 100 active RAAF fighter pilots requesting information on G-induced visual and cognitive disturbances, A-LOC symptoms, and G-LOC. Details regarding type of aircraft, flying maneuvers performed and mission outcome were also sought. RESULTS: There were 65 RAAF fighter pilots who completed the survey (age 20-53 yr, height 168-193 cm, weight 64-110 kg, jet hours 30-5700 h). Of these pilots, 98% indicated they had experienced at least one visual or cognitive disturbance in the high G environment: gray-out 98%; black-out 29%; and A-LOC symptoms 52%, including abnormal sensation in limbs, disorientation, and confusion. There were 9% who indicated they had experienced G-LOC (50% were the pilot flying the aircraft). DISCUSSION: These findings indicate that RAAF fighter pilots are experiencing a similar rate of visual disturbances and G-LOC when compared with other air forces. However, RAAF pilots reported a much higher incidence of A-LOC compared with the only other study of operational fighter pilots.     

Panel on deliberate G-induced loss of consciousness: introduction

Over the last decade, G-induced loss of consciousness (G-LOC) has been recognized as a significant operational problem for pilots of high performance aircraft in both the U.S. Air Force and U.S. Navy. Consequently, government laboratories have initiated research studies to learn more about the G-LOC phenomenon in an attempt to reduce its hazards. Many of these studies require the occurrence of LOC during their conduct. For this reason, animal models have been developed to use in these studies. However, even though sophisticated animal models such as baboons can be taught to perform tasks before and after G-LOC has occurred, these models have deficiencies that can be overcome only by the use of human volunteers who willingly submit to G-LOC studies on the centrifuge. Such human G-LOC studies began in World War II and continue today. These studies are presently conducted without guidelines regarding subject selection, numbers of G-LOCs allowed per subject per unit time, restraint systems, or specific medical examinations required before, during, and after each episode of LOC, or after each completed study. This panel will discuss what is known about the pathophysiology of G-LOC, the limitations of animal models in these types of studies, the possible medical and psychological sequelae, and the legal implications of conducting deliberate G-LOC research. We hope that the information developed by this panel will be useful to laboratory human-use review committees in determining the requirements and the nature of guidelines for conducting such studies.     

A review of central nervous system effects of G-induced loss of consciousness on volunteer subjects

A review of the literature on the central nervous system (CNS) effects of repeated centrifuge acceleration studies involving G-induced loss of consciousness (G-LOC) reveals that remarkably few adverse effects have been reported, aside from the G-LOC itself, even in subjects with numerous exposures. However, most of the followup studies were performed before the availability of sophisticated neuropsychological tests and non-invasive means of imaging the CNS, such as computerized tomography (CT), magnetic resonance imaging (MRI), brain electrical activity measurement (BEAM), and positron emission tomography (PET). Further, only a handful of long-term followup studies have been done. Thus, although repeated G-LOC may have induced some long-term adverse CNS effects, either organic or functional, little has been done to detect them. Even granted that some damage may be done, this risk must be weighed against the risk of aviators of similar damage from high +Gz exposures in flight, and even more against the risk of fatal G-LOC aircraft accidents. Volunteers for centrifuge acceleration studies should be fully informed about what is known and not known in this regard. As an ancillary measure, head restraints may be useful in avoiding postural trauma to the cervical spine, or impact injury to the head when G-LOC occurs and the head snaps forward and down.     

Legal and ethical aspects of deliberate G-induced loss of consciousness experiments

Informed consent is both a legal and accepted ethical prerequisite to nontherapeutic human experimentation. The informed consent obtained from the subject in G-LOC experimentation is in the same form as the risk disclosures used in high-G acceleration experiments. However, in high acceleration protocols G-LOC is a potential risk while in G-LOC experiments it is the result. The case law embodies three modern evidentiary standards (the "professional," "material fact," and "possible risks" tests) employed by common law courts when deciding whether the risk disclosures are sufficient to elicit the informed consent of the subject. Each standard is applied against the disclosures in the G-LOC protocol to determine if the elements of the requirement are met. The risk disclosures are wanting in specific identification under the three tests. The deficiency is the failure to inform the subject that G-LOC may result in a pathologic state of unconsciousness about which little is known. Without complete disclosure of this lacking state of medical knowledge, it is questionable whether informed consent can be given. If subjected to judicial scrutiny, the disclosures stated in the G-LOC protocol used in government sponsored research will probably be found deficient.     

Medical considerations for human exposure to acceleration-induced loss of consciousness

Unconsciousness in humans has probably been occurring since before recorded history. Acceleration-induced loss of consciousness (G-LOC) in flight has been occurring since 1919. Loss of consciousness and syncope are common occurrences in clinical medicine with G-LOC, occurring in a large number of aircrew and research subjects during centrifuge exposures. Although the major risk to humans exposed to centrifuge-induced G-LOC is related directly to the central nervous, cardiac, and musculoskeletal (neck and back) systems, other risks are also present. Human exposure to G-LOC is required to help solve the G-LOC problem in aviators. To perform such human research, the benefits must clearly outweigh the risks to the human. Even if the risk-benefit ratio is considered favorably balanced, continued monitoring of individuals exposed to G-LOC is mandatory. To facilitate monitoring of humans exposed to G-LOC, a central nervous system (CNS) insult classification system would be of significant value. A suggested classification scheme which considers the type of CNS insult, the history of exposure to G-LOC, and the temporal evolution of potential CNS insult is developed. To date there is no indication that G-LOC episodes have any associated long term or persistent psychophysiological sequelae. Improved acute and long term evaluation of humans exposed to G-LOC are, however, important aspects of conducting G-LOC research with humans. Such research and careful monitoring are necessary to understand and eventually solve the G-LOC problem in aviators.