Spatial Disorientation Event During Flight Due to Proposed Reverse-Dip Visual Illusion
BACKGROUND: Spatial disorientation is a leading nontechnical cause of fatal military aviation accidents, triggered by insufficient or misleading cues mainly from the visual and vestibular systems. Spatial disorientation accounts for 20–38% of fatal military aviation accidents, but case reports describing specific illusions during actual flight are rare.
CASE REPORT: On a clear night, an aerial refueling tanker was cruising at 16,000 ft (4877 m), while an F-15I fighter aircraft was 3 mi behind and 971 ft (296 m) below. About 100 s before a near collision, the fighter pilot switched to Air-to-Air mode, displaying the distance and speed difference to the tanker via a Target Designator box on the Head-Up Display. However, the crew did not realize they were accelerating toward the tanker [from 597–663 ft · s−1 (182–202 m · s−1)] or climbing by 738 ft (225 m). The two aircraft came within 49 ft (15 m) of each other before the navigator noticed the tanker and initiated a roll to the left.
DISCUSSION: The dip illusion occurs when a pilot flying in trail attempts to maintain the image of the lead aircraft in a fixed position on the windscreen as separation increases, which can lead to unintentional descent. In this case, we propose a “reverse-dip” illusion. The fighter crew was unaware they were closing in on the tanker, causing the Target Designator box to rise in the Head-Up Display. The pilot instinctively pulled back on the stick to maintain the box’s position, resulting in an unintentional climb. Recognizing how such illusions develop in flight is essential to reducing future risks.
Nakdimon I, Gordon B. Spatial disorientation event during flight due to proposed reverse-dip visual illusion. Aerosp Med Hum Perform. 2025; 96(11):1015–1018.
Situational awareness for military aircrew involves a correct understanding of various conditions, including the tactical environment, location, weather, weapon capabilities, and is affected by the aircrew member’s mental state and spatial orientation. When a pilot loses awareness of their position, attitude and motion relative to the Earth’s surface, this leads to spatial disorientation (SD), a specific form of situational awareness loss.1 During flight, the spatial orientation of an aircrew is primarily influenced by the interplay between the visual, vestibular, and proprioceptive systems, with the visual and vestibular systems being the dominant contributors.2,3 Insufficient or misleading cues perceived by these two systems can create illusions that may lead to SD events during flight.4
SD events can be classified into several types. First, based on the physiological mechanism that causes the event, whether it results from a visual illusion, a vestibular illusion, or insufficiently perceived orientational cues due to the aircrew’s attention being focused on other aspects of the flight rather than the aircraft’s spatial position.1 Second, based on the component of the aircraft’s spatial location that is misperceived by the aircrew, whether it is the aircraft’s attitude, its position (including altitude), or its change in motion.1 Third, the recognition status of the event during its occurrence. Type 1 is an unrecognized event, where there is a discrepancy between the aircrew’s perceived spatial location and the actual location of the aircraft. This is the most common type of recognition status classification, accounting for 80–85% of cases, and it has the greatest impact on the severity of the event.5 Type 2 refers to a recognized status, in which the aircrew notices a conflict between their perception of the aircraft’s spatial state and what the flight instruments indicate. Type 3 is incapacitation, in which the illusion is so severe that the aircrew is unable to correct the aircraft’s spatial position due to overwhelming sensory sensations.1
SD is the main nontechnical cause of fatal military aviation accidents. Several studies and reports have examined this issue in the U.S. Air Force, U.S. Navy, and Royal Air Force. Between 1983–2017, SD was identified as the primary cause in 20–38% of all fatal military aviation accidents.2,3 This data correlates with the numbers from the Israeli Air Force (IAF), where SD was responsible for 27% of fatal accidents between 2000–2024. In addition to fatal aviation accidents, there have also been several severe, nonfatal incidents attributed to SD.
Most studies investigating SD have been surveys that assess the incidence of accidents and the rates at which pilots report experiencing disorientation, or experiments using flight simulators to induce various illusions. However, there are very few case reports that describe the occurrence of a specific illusion during actual flight, explain the flight conditions that triggered it, and analyze the underlying mechanisms. One such example, which took place in a simulator rather than in a real flight, is “The Giant Hand Illusion experienced on a simulator”.6 In the present case report, we describe an unusual visual illusion that occurred during flight.
CASE REPORT
At 19:30 on a clear night in May 2019, a Boeing 707 acting as an aerial refueling tanker took off and established a cruising altitude of 16,000 ft (4877 m) above mean sea level. The crew, following standard training protocols, was positioned to await contact from multiple fighter aircraft, which were required to approach and establish communication before the refueling procedure could begin.
At 20:13, approximately 7 min before the near collision, an F-15I took off with a highly experienced crew for a refueling training mission. The F-15I, operated by the IAF, is a dual-seat ground attack aircraft based on the F-15E, a U.S. multirole strike fighter derived from the McDonnell Douglas F-15 Eagle. The IAF operates the F-15I with unique avionics systems.
Starting 2 min after the F-15I took off and continuing for the following 3 min, the crew attempted to repair a failure in the navigation pod. During this time, the crew experienced communication overload on the radio channel and had difficulty determining the spatial location of the refueling aircraft. Despite several attempts to locate it using the Global Positioning System, the lack of clarity created an unusual workload that impaired the crew’s ability to perform necessary pre-refueling tasks. As a result, the pilot failed to switch the avionics master mode from Navigation to Air-to-Air mode, as required prior to refueling. At 20:18, the fighter aircraft’s crew noticed the refueling tanker crossing in front of them in the opposite direction and subsequently performed a turn to follow it at the required distance.
According to IAF standard practice for refueling missions, the fighter aircraft is required to maintain a horizontal distance of 3 mi and a vertical distance of 1000 ft (305 m) behind and below the refueling tanker. The separation must be maintained until radio communication contact is established and the tanker crew gives approval to initiate the refueling procedure. At 20:19, approximately 100 s before the near collision, the fighter aircraft was 3 mi behind the refueling tanker, at an altitude of 15,030 ft (4581 m), which is 970 ft (296 m) below the refueling aircraft, and traveling at a speed of 350 kn (180 m · s−1).
At that point, the fighter pilot switched the master mode to Air-to-Air mode, which enabled the display of the distance to the refueling tanker and the speed difference between them. This data was simultaneously presented on the Head-Up Display using the Target Designator (TD) box feature. The TD-box is a graphical display that shows target information, including not only the location of the target but also its relative motion. As a result, the TD box moves along the pilot’s Head-Up Display field of view, representing the location and motion of the refueling tanker.
At this stage, the fighter aircraft’s crew was attempting to communicate with the refueling tanker’s crew. However, the refueling tanker crew was listening to a different radio channel than the one briefed to the fighter aircraft crew before the flight. As a result, they did not receive any response to their transmissions. Both the pilot and the navigator were therefore occupied with trying to find the correct radio channel. During this time, the fighter aircraft crew did not notice the trend of their aircraft closing in on the refueling tanker. In the 60 s before the near collision, the fighter aircraft accelerated from 354–393 kn (182–202 m · s−1) without the crew’s awareness. This acceleration caused the aircraft to close the distance to the tanker and triggered an unrecognized ascent, with its altitude increasing from 14,930 ft (4551 m) to 15,670 ft (4776 m) in the same 60-s window, an unintentional climb of 740 ft (225 m). The combination of both the accelerating closure and altitude climb caused the two aircraft to come within 197 ft (60 m) horizontally 2 s before the near collision, while the pilot estimated the distance to the tanker to be 1.5 mi (2414 m) and the navigator believed it was 3 mi (4828 m). At that point, the tanker crew gave the fighter aircraft permission to initiate the refueling procedure. The fighter aircraft’s navigator saw the lights of the refueling tanker 1 s later and initiated a roll to the left, which occurred while the aircraft were only 49 ft (15 m) apart, as shown in Fig. 1.
Citation: Aerospace Medicine and Human Performance 96, 11; 10.3357/AMHP.6656.2025
DISCUSSION
Dip illusion is a relatively common visual illusion in aviation, particularly during formation flying at night. In two separate surveys conducted by the U.S. Air Force and the Royal Netherlands Air Force, 22–38% of the pilots reported experiencing this illusion during flight.7,8
In their book Fundamentals of Aerospace Medicine, Davis et al. define this illusion as follows:
It occurs during formation flying at night, when one aircraft is in trail behind another. The pilot in trail places the image of the lead aircraft in a particular position on the windscreen and keeps it there. If the pilot is told to ‘take spacing’ (separate) to 10 km (5 nautical miles), for every 1 degree below the lead, the pilot is lower by 1.7% of the distance behind the lead. Therefore, if the pilot is 2 degrees below lead and keeps the image of the lead aircraft at the same spot on the windscreen all the way back to 10 km, the trailing aircraft will descend to 350 m (1100 ft) below the lead aircraft. In the absence of ambient visual orientation cues, the pilot cannot detect this large loss of altitude unless he or she monitors the flight instruments and may inadvertently ‘dip’ far below the intended flight path.1
The near collision began with the aircraft crew not realizing they were getting closer to the refueling tanker, which likely resulted from the cumulative effects of high workload throughout the flight. This led the fighter crew to be unaware of their increasing velocity, indicative of degraded situational awareness. As a result, the crew did not know they were closing in on the refueling tanker. To keep the TD box in the same position relative to the windscreen, the pilot may have instinctively and unintentionally pulled back on the stick. While this action keeps the TD box steady, it also results in the aircraft climbing, leading the crew to believe they were maintaining a vertical distance of 1000 ft (305 m) from the tanker, when, in reality, they were closing the gap, as illustrated in Fig. 2.
Citation: Aerospace Medicine and Human Performance 96, 11; 10.3357/AMHP.6656.2025
We propose a subtype of the dip illusion, which will be referred to as the “reverse-dip” illusion. In the classical dip illusion, the aircraft descends due to the intended fixation of the position of the leading aircraft on the windscreen during increased distance between the aircraft. In “reverse dip", due to the same intention to fix the position of the leading aircraft on the windscreen (in our case the TD box) during decreasing distance between the aircraft, the trailing aircraft ascends (Fig. 3).
Citation: Aerospace Medicine and Human Performance 96, 11; 10.3357/AMHP.6656.2025
When examining the various classifications of SD events in this specific case, it can be categorized as a visual illusion that caused SD in the crew’s perception of their position. The crew misperceived their own absolute altitude, inadvertently climbing from their assigned 15,000 ft (4572 m). This unrecognized climb led to a significant reduction in the intended vertical separation of 1000 ft (305 m) between the two aircraft. As the crew remained unaware of the deviation, the event fits the criteria for a Type 1: Unrecognized Event. This status held until 2 s before the near collision, when the navigator finally saw the refueling tanker and maneuvered the aircraft to avoid the collision by moving the stick.
It is crucial to understand the factors that contributed to the occurrence of this event: A) refueling during flight is a highly complex task that demands intense concentration; B) nighttime flight reduces the availability of visual cues necessary for proper orientation; C) work overload decreases the ability to follow the proper procedures for the primary flight tasks, thus increasing the likelihood of mistakes that can affect spatial orientation; and D) undefined and overlapping crew roles led both aircrew members to focus on resolving the radio channel issue, rather than having one of them monitor the flight conditions. By understanding these factors as risk elements for SD events, we can better mitigate the risk of recurrence in the future. To that end, clearer procedural guidelines may be warranted for night aerial refueling. In dual-seat aircraft, one crewmember should be explicitly responsible for monitoring key flight parameters and calling them out at regular intervals. In single-seat aircraft, pilots should be trained to integrate routine instrument cross-checks even under high workload. These practices could reduce the likelihood of SD events and help counteract the conditions that lead to them.
The distance between the aircraft at the near collision event. The F-15 is below the refueling tanker. The distance between the two aircraft was 15 m.
Comparison of the TD-box’s location on the Head-Up Display 40 s vs. 15 s before the near-collision. The TD-box is marked with a dotted arrow. A) Left image: the box is located near the lower altitude indicator; B) Right image: the box is located above the higher altitude indicator. The two altitude indicators are fixed in the Head-Up Display image.
Illustration of the Reverse-Dip Illusion. A) The F-15 is 3 mi behind the tanker, at an altitude of 15,000 ft (4572 m). The TD-box is located in the middle of the F15’s windscreen. B) As the F-15 approaches the tanker, the TD-box moves higher in the windscreen. C) To return the TD-box to its original position on the windscreen, the F-15 pilot makes small adjustments to the stick, which brings the TD-box back to its original spot, but also causes the aircraft to ascend.
Contributor Notes
