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Optical Illusions

Optical Illusion or Distraction?

On August 10, 1997, a float-equipped Cessna 180, accompanied by an identical Cessna on a fishing trip, crashed on the south slope of the Rivière aux Mélèzes valley in northern Quebec. The pilot and passenger were fatally injured, and the aircraft was destroyed by the post-crash fire. This summary is largely based on Final Report A97Q0168 by the Transportation Safety Board of Canada (TSB).

Both aircraft had taken off around 12:30 p.m. and were flying west over the Rivière aux Mélèzes. The elevation of the valley floor is 300 ft. ASL, and the terrain on both sides of the river rises to at least 1200 ft. ASL. The pilots used a common VHF frequency during the flight. After finding a landing site that appeared suitable, the pilot of the accident aircraft radioed to the other pilot to inspect the site. The latter did a reconnaissance and made a water landing without difficulty. A short time later, the pilot of the accident aircraft initiated a final approach to land at the same spot, but he executed a missed water landing procedure before touching down. He then told the other pilot he was going to examine the landing area more carefully and fly another circuit because he was unsure about the strength of the river current. He climbed above the river to about 450 ft. AGL, then turned 90 degrees left onto the crosswind leg. The aircraft levelled off and proceeded toward the south slope of the valley.

About 30 seconds later, the second pilot saw that the accident aircraft was getting close to the rising terrain and still hadn’t turned parallel to the river and valley wall for the downwind leg. The second pilot suggested he watch out for the mountain. A few seconds later, the aircraft pitched up without changing heading, then made two turns; the first was a right turn at a low bank angle to upwind, and the second was steep and to the left to downwind. The aircraft started to lose altitude and pitched up gradually until it struck some trees, then the ground, in a nose-down attitude. Four or five seconds after the impact, a fire started on the right side of the cabin. Thirty to sixty seconds elapsed between the go-around and the accident.

The accident occurred 100 NM southwest of Kuujjuaq, Quebec, in a partially wooded area approximately one-half mile south of the Rivière aux Mélèzes. The area is somewhat hilly and mainly covered with larch trees of average size. The south side of the valley slopes upward at an angle of about 20 degrees. The aircraft came to rest about 450 ft. above the valley floor. The aircraft cut a swath 50 ft. long through the trees before striking the ground. The impact damage and the nose-down attitude of the wreck are consistent with a loss of control following a stall.

Examination of all recovered components revealed no evidence of pre-impact failure or malfunction, no signs of airframe failure, flight control problems, electrical problems, loss of power, or in-flight fire. No messages were received from the pilot.

According to the TSB report, some situations can severely hinder a pilot's ability to estimate size, distance, speed, or the direction to a slope, or even to identify objects. Pilots can be misled by an optical illusion when approaching rising terrain at right angles. When approaching a ridge, the pilot may tend to maintain a constant angle between the extended cowl and the summit; this causes the pitch attitude of the aircraft to increase while speed decreases. Consequently, aircraft performance decreases and vertical separation with the terrain decreases. The pilot tends to focus on the proximity of the ground, sometimes to the point where flying performance is affected. There is a vivid impression that speed is increasing in relation to the ground, and the pilot may be tempted to reduce speed.

The pilot's decision to fly a left-hand circuit was sound, since he was in the left seat and visibility was better on that side. As the purpose of the reconnaissance was to examine the surface of the water, the pilot had to fly at low level and low speed.

With the aircraft in approximately level flight, it must have closed rapidly with the rising terrain. The pilot did not fly close to the north side of the valley before turning onto the crosswind leg, and as a result, he did not take advantage of all the available airspace in case he needed it. Consequently, not all the space available for the circuit was used to minimize roll attitude in the turns and maximize aircraft performance. The type of circuit selected indicated the pilot intended to do a landing area reconnaissance at low altitude.

Flying in mountainous terrain demands heightened vigilance. Pilots must constantly confirm their impressions with instrument readings. The TSB was unable to determine why the pilot continued flying towards the slope at right angles instead of trying to avoid the ridge until the second pilot warned him. Two hypotheses might explain the pilot's delay in turning onto the downwind leg: he may have been distracted, and/or he may have been misled by optical illusions.

One hypothesis is that the pilot’s attention may have been focussed on planning the water landing or on an untimely event in the cabin. It is possible that, after the go-around, most of his attention was focussed on a continuous examination of the landing area to his left and slightly behind him, and not on flying the circuit. Also, a distraction caused by the passenger feeling the effects of motion sickness or being otherwise indisposed could have produced the same result.

The other hypothesis is that, while flying towards the rising slope, the pilot may have been tricked by an optical illusion, which can be treacherous at low altitude and at near-stall speeds.

After the second pilot called to tell him to watch out for the mountain, the accident aircraft pilot seemed to react, but he did not have much time and his room to manoeuvre may have been reduced to the point where turning around would cause a stall in the turn. The pilot may have assessed the situation and decided to terminate the flight immediately with a forced landing in the best available conditions on the slope of the valley.

The pilot apparently did not have time to cut electrical power and fuel and prepare the cabin for rapid evacuation. Based on the evidence and witness statements, the cause of the accident could not be determined.

Damage to the right wing caused a fuel leak, and, on contact with an ignition source, the fuel ignited and sustained the fire. The witness statements and fire damage suggested that the fire started on the right side of the cabin and the most likely ignition source was electrical; however, the investigation could not identify the source of ignition with certainty.

As a result, the TSB determined that an unknown distraction and/or an optical illusion may have contributed to diverting the pilot's attention from flying the circuit. Although the cause of this accident was not determined, the conditions were conducive to optical illusions associated with flying over rising terrain at low altitude.