Page:CAB Accident Report, 1965 Carmel mid-air collision.pdf/13

-12 - The three altimeters recovered from the wreckage of EA 853 were set at 29.56. The setting in TW 421s altimeters was 29.65. This would result in a difference of 65 feet assuming that both aircraft were at the same point in space.

1.16 Optical Illusions

Aircraft pilots are susceptible to many types of flight conditions which may result in spatial disorientation and optical illusion. These illusions or disorientations result from reliance on the physiological sensing elements of the body which can give false or conflicting information to the senses.

The primary device used to provide orientation with respect to the horizontal and vertical planes, depth, and distance is the eye. Experiments have been conducted to determine the effect of pilot warning indicators on the ability of pilots to discriminate between aircraft observed on collision and non-collision courses. These experiments revealed that as the miss vector decreased, the decision that a collision course existed increased.

The evaluation of the threat a target offers may depend on the observed angular velocity (sight-line rate) and the observed rate of change in range-rate of the target. If the sight-line rate of a target is well above the motion threshold, the pilot can be fairly certain the target is on a non-collision course. If the sight-line rate is below the motion threshold (no perceptible motion) and there is perceptible increase in target size, the threat may be evaluated as a collision course. For vertical misses, a sight-line rate of about six minutes of arc per second was judged to be a collision course regardless of the background structure or miss vector. With a sight-line rate of about nine minutes of arc per second the courses were judged as misses. The fact that the sight-line rate for miss decisions was about three minutes of arc per second higher than for collision judgment may indicate that perceived movement may sometimes have been used as a cue to help decide that the target was on a non-collision course. However, in those instances a horizon line was observable and the pilots reported using it in addition to a perceived motion. The presence of the horizon may have provided a structured reference for misses in the vertical plane when other structure was lacking. One criterion frequently used by the pilots was the amount of separation between the target and the horizon. Targets that appeared stationary but were clearly above the horizon were immediately judged to be misses.

2.1

There were no structural, powerplant, system, or navigation component failures that contributed to this accident. The investigation, including the