Spatial Disorientation at Night

On December 15, 1998, a pilot and passenger departed Shearwater, Nova Scotia, at 18:43 local time in a Cessna 172 on a night visual flight rules (NVFR) flight to the Liverpool airport for a touch-and-go before a return to Shearwater. About 2 hours after departure, an emergency locator transmitter (ELT) signal was reported and a search was initiated. The wreckage was found the next day. The aircraft had crashed in heavily wooded terrain 2 NM west of the Liverpool airport. The 2 occupants were fatally injured, and the aircraft was destroyed. This synopsis is based on the Transportation Safety Board of Canada (TSB) Final Report A98A0184.

This was a time-building flight in preparation for the pilot's upcoming commercial flight test. The aircraft was equipped with an altitude reporting transponder, and a review of the radar data indicated that the aircraft approached the Liverpool airport from the east, turned south across Runway 25/07 and joined the circuit for Runway 25. The aircraft disappeared from radar at 1100 ft above sea level (ASL) while on final to Runway 25 and reappeared on radar at the same altitude just west of the airport 1 min and 27 seconds later; radar coverage continued for another 47 seconds. During this phase, the aircraft climbed to 1300 ft, levelled off, and then descended to 1100 ft ASL before disappearing from radar.

The pilot was issued his night endorsement in July 1998 and, at the time of the occurrence, had about 187 hr. total flight time. He had recently flown to the Liverpool airport on four occasions. Three of the flights were conducted at night with either an instructor or another pilot on board; the accident flight was the first night flight without another pilot on board. The pilot flew with his instructor on the morning of the occurrence, and slept several hours in the afternoon before returning to the Shearwater airport for the night flight.

At the time of the occurrence, the Liverpool area was under clear skies with no restrictions to visibility and no possibility of icing at lower levels. The moon was below the horizon at the time of the accident, and pilot reports indicated that dark-sky conditions existed; there would have been fewer visual cues than there would have been during his previous flights to Liverpool. A local resident, who had frequency-scanning equipment for recreational purposes, heard the pilot transmit his intentions and said that there was no inflection in the pilot's voice to suggest he was experiencing difficulty.

The aircraft descended into trees about 2 NM beyond the departure end of Runway 25, on a magnetic heading of 270°. The aircraft was in a wings-level, 30° descent angle when it struck the trees. Propeller strike marks on trees along the wreckage trail were consistent with the propeller being powered at impact. The flaps were in the retracted position. The elevator trim tab position was consistent with a slight nose-down trim setting, normal for final approach for a touch-and-go landing. The engine was examined and there was no indication of a pre-impact mechanical failure.

All undamaged light bulbs were examined by the TSB. The light bulbs for the aircraft’s overhead instrument flood light, cabin dome light, compass light, and tail navigation light were retrieved from the wreckage. With the exception of those for the dome light, these bulbs would normally be illuminated during a night flight. The analysis determined that the instrument flood light bulb was illuminated at impact. The remaining lamps were either off at impact or had not received sufficient force to distort the filament.

The TSB conducted a representative night flight to the Liverpool airport in a rented Cessna 172 at a time when light and sky conditions were similar to those of the occurrence night. The purpose was to identify the visual references available to a pilot when flying a Runway 25 approach and departure/go-around. The airport is located in a sparsely populated area where there is little peripheral lighting. The runway lights were observed clearly on approach and during the go-around phase, and the aircraft passed over a road about 1.5 mi. west of the airport, where there was some street lighting in an area of houses. Beyond the road there were few external visual cues, and the horizon was not easily discernable. The TSB flight was recorded on radar, allowing a comparison between the radar data for the occurrence flight and the TSB flight. The TSB flight included four approaches to Runway 25, with two touch-and-go landings and two go-arounds. A comparison of elapsed time during a touch-and-go versus a go-around indicated that the pilot of the occurrence flight had conducted a go-around.

The most accurate sensory information available to a pilot about aircraft attitude and motion are the visual cues provided by the earth's horizon, the aircraft's flight instruments, or both. When this information is not available, such as when the horizon is obscured by darkness or weather, or when the pilot's attention is distracted from the attitude instruments for a short time, the pilot's sense of orientation may be taken over by the inner ear, a very inaccurate source of sensory information during flight. Spatial disorientation occurs when a pilot's sense or "orientation percept" of the position, motion, or attitude of his/her aircraft or himself/herself with respect to the earth's surface and the gravitational vertical is based on incorrect or misinterpreted sensory information. Pilots with limited instrument flight time are most susceptible to spatial disorientation.

One form of spatial disorientation is the false climb illusion. This illusion can occur during acceleration when a pilot loses or is uncertain of visual references and relies on the inner ear rather than on the instruments. Because the inner ear cannot distinguish between gravity and horizontal acceleration, forward acceleration can generate the same perception as backward tilt (i.e., a climbing aircraft ). This illusion can be experienced by pilots operating low- or high-performance aircraft.

In low visibility, a pilot may attempt to counteract a perceived climb by lowering the aircraft's nose until the downward pitch of the aircraft counterbalances the apparent backward tilt caused by the acceleration, often resulting in flight into terrain. Furthermore, if the false climb illusion is reinforced by the presence of a false visual horizon (such as a shoreline or other extended cluster of lights with ocean or unlighted terrain beyond) receding under the aircraft, the pilot's compulsion to push the nose down can become overwhelming.

Knowledge and experience are the key determinants of a pilot's susceptibility to disorientation. A pilot's only defence against spatial disorientation is to develop the ability to suppress natural vestibular responses through training and practice (vestibular suppression), and to always use visual information from the instruments to maintain spatial orientation (instrument discipline) and, consequently, his/her situational awareness. The environmental conditions on the night of the occurrence and the limited outside visual ground references in the vicinity of the Liverpool airport were elements conducive to spatial disorientation. During the go-around, false horizon and false climb illusions were both possible. At low altitude there is minimal time for a pilot to recognize an illusion and take the appropriate corrective action. The impact angle of the aircraft appeared to be more consistent with the nose-down pitch attitude associated with the false climb illusion.

The complex skill set that a pilot requires to recognize and counter the effects of spatial disorientation are developed through flight instrument training, experience, and practice. In the end, the TSB determined that, during the overshoot from the approach to the airport, the pilot probably lost situational awareness as a result of spatial disorientation and unintentionally flew the aircraft into the ground.