Search and Detect

On November 20, 1999, an ERCO Aircoupe 415C and a Cessna 152 collided on nearly opposing headings while flying in visual meteorological conditions in designated flying training area CYA 125(T), near Vancouver, B.C. Both aircraft broke up in flight and plunged to the ground, killing the four occupants. This synopsis is based on the Transportation Safety Board of Canada (TSB) Final Report A99P0168.

The Aircoupe had done touch-and-go circuits at Pitt Meadows Regional Airport and then flew into training area CYA 125(T) from the Northeast. The instructor and the student in the Cessna 152 left Boundary Bay Airport to review basic flying exercises in the same area. Radar data showed that the Cessna maintained a track of about 025º magnetic (M) and was not seen to deviate from its flight path or take evasive action before the collision.

One minute before the collision, the Aircoupe orbited an ultralight, and then flew on a southwesterly track for about 40 seconds. It then turned right to the northwest, rapidly closing on the Cessna’s flight path. The Aircoupe appears to have then turned to the left, nearly into a head-on situation with the Cessna, and about 10 seconds later the two aircraft collided. During the last five or so seconds before impact, the Aircoupe was reportedly in a nearly straight and level flight attitude.

Collision damage patterns suggest that the Aircoupe’s pilot may have attempted an evasive action to the right immediately before impact. The two pilots in the ultralight indicated that the Cessna had its landing lights on, but they could not see those of the Aircoupe, even though the Aircoupe flew around them before the collision. The Aircoupe pilot was very familiar with the local area, and the Cessna 152 instructor was properly qualified for the flight. The student pilot was considered a capable and eager young man, and his flight training progress had been normal.

In Canada, the see-and-avoid principle is used as the primary means of maintaining spacing between aircraft in visual meteorological conditions. Research shows that this principle is the least effective of the available mechanisms to keep aircraft apart because of the physiological limitations of the human eye and the motor-response systems. Because those limitations challenge pilots to employ assiduous scanning techniques, “search-and-detect” has to take place before the see-and-avoid concept becomes effective.

Constant relative bearing — When two aircraft are on a collision course with constant headings and constant speed, they have a constant relative bearing to each other. Each aircraft, if detected, would appear to be motionless to the other pilot. This illusion increases the difficulty for each pilot to visually acquire the other aircraft. Even if one aircraft is travelling faster than the other, as long as their relative bearings remain constant, the aircraft will collide. From a pilot's perspective, if the approaching aircraft has no apparent relative motion and stays at the same point on the windshield, a collision will likely occur unless evasive action is taken.

Constant relative bearing.

The flight profiles of both aircraft indicate that neither pilot saw the other aircraft in sufficient time to initiate effective and timely evasive action. The Aircoupe pilot's attention was probably focused on manoeuvring around the ultralight, whereas the attention of the Cessna pilots was likely focused on the training environment. Although this cannot be proven, it is a scenario to which most pilots can relate and empahsizes why constant vigilance is so important, especially in uncontrolled airspace.

The Aircoupe is a low-wing aircraft with the pilot's seat over the wing. During the right bank in the turn, the left wing would have restricted the pilot's field of view to his left, the direction from which the Cessna approached. It may therefore have been physically impossible for the occupants of the Aircoupe to see the Cessna until just before the aircraft collided. From the perspective of the Cessna pilots, the Aircoupe would have been approaching from the front right quadrant. The target image would have been a small profile view. The yellow Aircoupe may have blended with the variegated background, and no indication was found that the Aircoupe's landing light was on.

Research indicates that a pilot is eight times more likely to acquire a target if alerted to its presence. Without a warning, the Cessna pilots may not have detected the Aircoupe. The Aircoupe would have appeared motionless to them for about ten seconds because of their constant relative bearing, which commenced when the aircraft speeds and headings combined to establish the collision course. We all need to maintain a more assiduous lookout in training areas because training aircraft generally follow erratic flight paths and perform unpredictable manoeuvres, and lookout is degraded by the focus on training.

Improved aircraft conspicuousness reduces the risk of collision in high traffic-density areas. Strobe lighting, pulsing landing lights, and electronic surveillance technology help pilots search for and detect conflicting traffic in a timely manner. Relatively inexpensive strobe lights are available on the market and light aircraft owners may want to consider them.

This short accident synopsis does not give justice to the full TSB Final Report, which expands considerably on important issues such as physiological limitations of the human eye, vision limitations of aircraft design, design eye-reference point, the see-and-avoid principle, recognition and reaction times, mid-air collision defences and more. Readers are encouraged to log on to the TSB Web site at http://www.tsb.gc.ca and read the full report.