Journal of Medical Sciences

: 2021  |  Volume : 41  |  Issue : 1  |  Page : 22--28

Analysis of in-flight spatial disorientation among military pilots in Taiwan

Min-Yu Tu1, Chao-Chien Cheng2, Yi-Hsiang Hsin2, Wun-Wei Huang2, Fang-Ling Li3, Je-Ming Hu4, Kwo-Tsao Chiang5, Chung-Yu Lai6,  
1 Aviation Physiology Research Laboratory, Kaohsiung Armed Forces General Hospital Gangshan Branch, Kaohsiung City; Department of Health Business Administration, Meiho University, Pingtung County; Department of Life Sciences and PhD Program in Translational Medicine, National Chung Hsing University, Taichung City; Institute of Medical Science and Technology, National Sun Yat-sen University, Kaohsiung City, Taiwan
2 Aviation Physiology Research Laboratory, Kaohsiung Armed Forces General Hospital Gangshan Branch, Kaohsiung City, Taiwan
3 Department of Psychiatry, Tri-Service General Hospital Beitou Branch, National Defense Medical Center, Taipei City, Taiwan
4 Division of Colorectal Surgery, Department of Surgery, Tri-Service General Hospital; Graduate Institute of Medical Sciences, National Defense Medical Center, Taipei City, Taiwan
5 Aviation Physiology Research Laboratory, Kaohsiung Armed Forces General Hospital Gangshan Branch; Institute of Medical Science and Technology, National Sun Yat-sen University, Kaohsiung City, Taiwan
6 Graduate Institute of Aerospace and Undersea Medicine, National Defense Medical Center, Taipei City, Taiwan

Correspondence Address:
Dr. Chung-Yu Lai
Rm. 8347, No. 161, Sec. 6, Minquan E. Rd., Neihu Dist., Taipei City, 11490


Background: In-flight spatial disorientation (SD) is a predominant threat to flight safety in aviation. This study was conducted to understand the prevalence, severity, and frequency of in-flight SD among military pilots in Taiwan. Methods: A survey was conducted to collect tri-service pilots' experiences of SD during flight. Participants completed anonymous SD questionnaires during refresher physiology training. There were 486 questionnaires delivered to trainees and the completion rate was 97.1% (n = 472). All data were processed using SPSS version 24 software (IBM, Armonk, NY, USA). Results: Of the 472 participants, the average age of the pilots was 36.7 so 7.3 years and 97.7% were male. About 80% of participants experienced in-flight SD events. There was a significant difference between prevalence of SD in fighter (87.0%), trainer (89.8%), transporter (70.6%), and helicopter (66.7%) pilots (P < 0.001). Less than 10% of the events were severe, and there was no obvious variation between aircraft types (P = 0.126). Pilots were sensitive to SD in clouds and under low visibility. Over 70% of pilots experienced visual illusions, especially loss of horizon during bad weather (45.1%), followed by leans (44.5%), false horizon (44.1%), false sense in clouds (39.6%), Coriolis illusion (25.0%), and confusion on entry to instrument meteorological conditions (25.0%). Conclusions: Our survey showed that SD is a common physiological problem among military pilots, who were easily disoriented by in-flight SD without visual cues. Visual illusion was relatively more frequent, especially for trainer and fighter pilots.

How to cite this article:
Tu MY, Cheng CC, Hsin YH, Huang WW, Li FL, Hu JM, Chiang KT, Lai CY. Analysis of in-flight spatial disorientation among military pilots in Taiwan.J Med Sci 2021;41:22-28

How to cite this URL:
Tu MY, Cheng CC, Hsin YH, Huang WW, Li FL, Hu JM, Chiang KT, Lai CY. Analysis of in-flight spatial disorientation among military pilots in Taiwan. J Med Sci [serial online] 2021 [cited 2021 Mar 3 ];41:22-28
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Full Text


Human visual, vestibular, and proprioceptive systems provide crucial cues for the orientation of the ground during movement. Of these, vision is the most reliable and provides the predominant contribution to spatial orientation in a dynamic flight environment. However, the efficiency of visual function is decreased by low-visibility weather conditions, and the vestibular and proprioceptive senses are insensitive to prolonged and varied acceleration exposure.[1] Pilots might experience different kinds of illusions and spatial disorientation (SD) without correct visual references.

SD is the sensory error defined by Benson as failure to correctly sense the position, motion, or altitude of the aircraft or themselves, within the fixed coordinate system provided by the surface of the earth and the gravitational vertical.[2] In earlier times, navy surgeons in England noted that aviators were almost completely unable to detect deviations during a flight in the fog or clouds; some were not even able to detect changes in attitude.[3] Past studies have shown that the majority of pilots have faced challenges from SD in their flight careers.[4],[5],[6]

Ogden et al. reported that orientation errors accounted for only 3.4% of major accidents but almost one-third of all fatalities in the US Army (USA), consistent with the results of other published studies.[7] Of 601 Class A mishaps in the US Air Force (USAF) during the period 1993–2013, 72 (12.0%) were SD related mishaps. In particular, 61.1% of the SD-related mishaps resulted in a fatality.[8] These mishaps also resulted in astonishing level of property damage. According to US Navy statistics, there were an average of 2.04 deaths per SD mishap compared to only 0.78 deaths per non-SD mishap between 1990 and 1998.[9] As a whole, only a minority of accidents involving SD, but causing significant losses and consequences.

Around the world, pilots are continuously trained to be aware of SD hazards through lectures, ground demonstrations, and flight training. In the USAF, the total rate of Class A mishaps significantly decreased during 1972–2000.[10] Nevertheless, the rate of Class A mishaps related to SD has been static over the past three decades. During mishap investigations, SDs are classified into Type I (unrecognized), Type II (recognized), and Type III (incapacitating). In Type I SD, pilots do not recognize or have no perception of SD onset.[11] These usually lead to controlled flight into terrain tragedy. Experts in aviation have desperately sought the mechanism of SD, studied the hazards of SD, and developed appropriate countermeasures to mitigate the risks.

Previous studies have revealed that the prevalence rate of in-flight SD is more than 90% in the military pilot population. The leading types of illusions were “the leans,” “loss of horizon due to atmospheric conditions,” and “misleading altitude cues.”[4],[12],[13] Various researches have quantified the frequency of SD experiences by flight hours or sorties.[3],[4] In terms of severity level, Durnford calculated the number of in-flight SD events that were dangerous for flight safety.[14] However, other statistical methods could be used to better understand the characteristics of SD during flight.

There are scant data on the prevalence of in-flight SD among military pilots in Taiwan. In this study, we aimed to investigate the prevalence, severity, and frequency of SD in flight with a subgroup analysis of aircraft types. These data will be applied to the development of SD training materials for the initial, refresher physiological training, and instrument flight courses.


Study design and procedure

We organized this retrospective recall survey, conducted in 2018, to gain information on in-flight SD in military aircraft. Particiants were active military pilots in the Army, Navy, or Air Force of Taiwan.

In accordance with the regulations of aviation physiology training, they regularly attended refresher training at the Aviation Physiology Research Laboratory every 4 years. During the training, they must undertake the following three courses: the hypobaric chamber flight, SD threat and countermeasures, and the ejection seat training.

A well-designed questionnaire was used to gather valuable information from the trainees. A total of 486 questionnaires were circulated to the study participants. There were 472 questionnaires completed and returned, with a response rate of 97.1%. These data were coded as an electronic Microsoft Excel file for storage.

Questionnaire development

A questionnaire comprised of four sections was constructed to obtain anonymous data from military pilots to ascertain the prevalence, severity, and frequency of in-flight SD experiences.

Initially, we collected the details of personal characteristics such as age, sex, aircraft type, and flight time (years and hours). Aircraft types were classified as fighter (F-16, IDF, and M-2000-5), trainer (F-5, AT-3, T-34), transporter (B-737, BH-1900, C-130, E-2K, FK-50, and P-3C), or helicopter (500MD, AH-1W, AH-64E, CH-47SD, EC-225, OH-58D, S-70C, TH-67, UH-1H, and UH-60M).

In the second and third sections, participants were required to report the total counts of in-flight SD occurring in <5 sorties, 5–10 sorties, 5%–25% of all sorties, and >25% of all sorties, stratified by aircraft type.[4] Then they were asked to specify the severity of their worst SD incidents, rated as “minor,” “mild,” or “severe.” “Minor” was defined as flight safety was not at risk; the definition of “mild” was that flight safety was not at risk, but could have been jeopardized under different conditions; and “severe” was defined as flight safety at risk.[14] Meanwhile, the weather conditions, time, and visibility during that particular flight were recorded.

Finally, we investigated the occurrence of 22 common illusions: (1) visual illusions: loss of horizon during bad weather, false horizon, black hole, autokinesis, confusion on entry to instrument meteorological conditions (IMC), brownout/whiteout, flicker vertigo, and nystagmus after the maneuver; (2) body sense illusions: the leans, false sense in clouds, Coriolis illusion, undetected drift, SD due to instrument malfunction, pitch-up illusion during takeoff, G-excess illusion, graveyard spin, and graveyard spiral; (3) display illusions: SD due to information on head-up display, SD caused by night vision goggles (NVG), SD caused by forward-looking infrared (FLIR), and SD caused by helmet-mounted display (HMD); (4) other illusion: giant hand. In the questionnaire, we provided a brief description to each illusion to assist the pilots' understanding.

Statistical analyses

We converted the Microsoft Excel file into an alternative format for statistical analyses. SPSS version 24 (IBM, Armonk, NY, USA) was used for the management and analysis of all data. The characteristics of the study participants were described as mean ± standard deviation and percentage. Prevalence, severity, and frequency of in-flight SD were also described using percentages. For comparisons, either Chi-squared test or Fisher's exact test was used to examine the difference in data distributions between the four aircraft-type groups. Statistical significance was considered at a two-sided P < 0.05.

Institutional review board approval

This study was performed in accordance with the principles of the Declaration of Helsinki. The Institutional Review Board of Kaohsiung Armed Forces General Hospital approved the protocol (No. KAFGH 107-017) prior to the start of this study.


[Table 1] shows the characteristics of the 472 participants who completed the questionnaire. The average age of the pilots was 36.7 ± 7.3 years and a majority were male (97.7%). The aircraft types were categorized as helicopter (34.4%), transporter (14.4%), trainer (10.4%), and fighter (40.9%). Nearly two-thirds of them had more than 10 years of flight experience. The proportion of pilots with <500, 500–999, 1000–1999, and ≥2000 in flight hours were 16.3%, 17.2%, 30.5%, and 36.0%, respectively. About 80% of the participants (n = 368) had encountered in-flight SD.{Table 1}

The occurrence of in-flight SD was calculated as shown in [Table 2]. The prevalence rate was significantly different by pilots from aircraft types (P < 0.001). The two highest places were occupied by trainer (89.8%) and fighter (87.0%) pilots, followed by transporter (70.6%) and helicopter (66.7%) pilots. The frequency of in-flight SD was also higher for trainer and fighter pilots (P = 0.003).{Table 2}

The severity of the worst SD incidents during the flight was shown in [Table 3]. At the mild level, the highest rate was seen in trainer (34.2%); 18.8% appeared in transporter, close to that of fighter. Overall, <10% of severe in-flight SD events threatened flight safety, and there was no dramatic variation among aircraft types (P = 0.126). Most cases occurred when pilots flew into clouds, during the daytime, and at low visibility [Table 4].{Table 3}{Table 4}

[Table 5] illustrates that more than 70% of subjects experienced different visual illusions. However, transporter pilots had a lower portion of body sense illusion during flight compared to other pilots. In [Table 6], the most common SD episodes during flight were loss of horizon in bad weather (45.1%), followed by leans (44.5%), false horizon (44.1%), false senses in clouds (39.6%), Coriolis illusion (25.0%), and confusion on entry to IMC (25.0%). The findings described above repeatedly appeared in trainer and fighter; the rank orders were only slightly different. In transporter aircraft, however, pilots were much more easily affected by the black hole illusion (25.0%) during the night flight landing phase. One-fourth of helicopter pilots had experienced the specific illusion of undetected drift. Due to aircraft characteristics, undetected drift and display illusions caused by NVG, FLIR, and HMD were only seen in helicopters; G-excess illusion and pitch-up illusion were not reported by pilots flying transporters.{Table 5}{Table 6}


In this survey, we used a well-designed questionnaire to obtain details of in-flight SD from tri-service pilots in Taiwan. Most respondents had experienced in-flight SD during their flight careers. Less than 10% of them experienced severe SD that could endanger flight safety. In-flight SD often occur in clouds and during limited visibility. Overall, military pilots most frequently experienced loss of horizon in bad weather, followed by leans and false horizon.

Previous to the 1970s, “vertigo” was the term used by pilots to refer to getting lost in the air. The term “SD” appeared in textbooks in 1971, and was first used in reference to mishap investigations in the USAF in 1989.[15] Previc and Ercoline searched different databases for SD-related papers using three keywords: “SD,” “spatial orientation,” and “attitude display.” The findings uncovered that over half of the references were published after 1999. This might be because of an increase in the reporting of SD-related mishaps due to developments in this field.[16] Our study has bridged the gap and presents a picture of pilots' in-flight SD experiences in Taiwan.

In the Netherlands Air Force, more than 75% of pilots reported at least one SD experience.[12] Holmes et al. calculated the prevalence of SD experiences among 752 military pilots and navigators. The findings showed that over 90% of fast-jet and rotary pilots had experienced SD episodes, a level higher than that seen in trainer pilots.[4] Previc and Matthews noted that fast jet pilots (83.8%) had a higher probability of experiencing SD than other aircraft pilots (75%).[18] Takada et al. found that 95.5% of Japan Air Self-Defense Force Fighter pilots have experienced SD incidents.[17] In flight, pilots receive different sensory inputs based on their aircraft types. Fighter or fast-jet pilots usually perform acrobatic combat maneuvers, which are likely to generate high G stress, stimulating vestibular illusions. Similar to previous studies, our results also showed that the majority of pilots have experienced SD events during flight, but prevalence rates were significantly different between aircraft.

The current literature shows that the proportion of severe SD in recent worst events ranged from 5% to 20%.[4],[18] Chance occurrence and severity levels were higher during night flying and in clouds.[13],[17] Our results showed the same phenomenon in that 7% of pilots reported experiencing a severe SD incident that could potentially jeopardize flight safety. However, we found no disparity in in-flight SD severity between the different aircraft groups. There were more occurrences of in-flight SD in clouds and low-visibility environments, but not during nighttime. Potential explanations are that, first, our survey obtained information regarding the “worst” incidents during the flight career, not the most “recent.” Second, some previous studies simply focused on the SD experiences of fixed and rotary-wing pilots, but we further stratified pilots into fighter, trainer, transporter, and helicopter groups. Finally, there are restrictions and regulations concerning flight maneuvers to minimize SD during night flight in Taiwan. As mentioned above, our discovery that SD during flight frequently occurred alongside limited visual references is in line with previous conclusions.

Overall, we found that the top five illusions were loss of horizon in bad weather, leans, false horizon, false sense in clouds, and Coriolis illusion. Compared with previous findings, there was only a small difference in rank order.[4],[13],[18] The descriptions of those frequent illusions were very similar from military pilots flying different types of aircraft. The results also showed that helicopter pilots had more undetected drift illusions due to the characteristics of the helicopter; transporter pilots had a higher probability of experiencing black hole illusion, possibly because of longer duty times during night flight.

Night vision systems have known limitations, such as narrow field of view, degraded visual acuity, two-dimensional and monochrome images. Compared with daytime flight conditions, pilots will experience a two-fold increase in stress and fatigue levels using night vision devices (NVD).[19] Crowley reported that pilots who used NVD also experienced visual, static, and dynamic illusion problems.[20] The SD accident risk rate during flight using NVD was significantly higher than that without using NVD.[21] In Taiwan, at the time of the present study, only Army helicopter pilots use (or are equipped with) NVD. The frequency of illusions related to NVD would be underestimated and diluted by Navy and Air Force helicopter pilots. However, consistent with previous conclusions, our survey also showed a higher rank order for SD caused by NVG.[4],[18] Future studies should focus on Army pilots to investigate the association between NVD employment and SD prevalence.

Our study has several limitations. First, underreporting of in-flight SD experiences could not be ruled out because military pilots tend to protect themselves from human errors. Second, previous studies showed that physiological and psychological factors were associated with SD incidents.[8],[13] Participants found it difficult to declare pre-SD biological conditions in this study. Third, Army helicopter pilots are only equipped with NVG, FLIR, or HMD during night flight in Taiwan. Therefore, we were not able to compare in-flight SD related to aided-vision devices between different pilot groups. Finally, SD is a contributor to severe damage and high fatality mishaps.[21],[22],[23] Due to data unavailability, we could not assess the impact of aircraft crashes induced by SD in this study.


We showed that in-flight SD events were common among military pilots, and in a few cases, they had to carefully use their skills to regain correct orientation and prevent a mishap. The main contributor of in-flight SD was limited visual reference. Trainer and fighter pilots experienced higher frequencies of visual illusions during flight.

Financial support and sponsorship

This study was supported by Medical Affairs Bureau Ministry of National Defense, Taiwan, ROC (MAB-107-107).

Conflicts of interest

There are no conflicts of interest.


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