Aviation Safety Letter 4/2004

VFR Flight Into Adverse Weather — Collision With Terrain

Collision With Terrain

On June 6, 2002, a Cessna 182P was on an afternoon flight from Abbotsford, B.C., to Calgary, Alta. It failed to arrive at its destination. Emergency locator transmitter (ELT) signals were detected and the wreckage was located 17 NM northeast of Hope, B.C., at an elevation of 4 048 ft above sea level (ASL). The aircraft was destroyed. The four persons on board were fatally injured. This synopsis is based on the Transportation Safety Board of Canada (TSB) Final Report A02P0109.

Before departure, the pilot received a pre flight weather briefing in person from the Abbotsford flight service station (FSS) specialist, who advised that the weather appeared to be suitable for flight in accordance with visual flight rules (VFR). It was suggested that the pilot contact Abbotsford FSS, in the vicinity of Hope for a weather update, since the weather in the vicinity of Hope is known to be subject to rapid changes. No such call was received from the aircraft. The pilot filed a VFR flight plan to Springbank Airport via Revelstoke, B.C., and indicated he would proceed direct to Revelstoke. The aircraft departed Abbotsford at 14:05 Pacific daylight time (PDT) and was observed on radar to fly directly to Hope at an altitude of 5 000 ft ASL at a ground speed of 150 kt. At Hope, at approximately 14:30, the radar returns ceased because of the mountainous terrain.

The aircraft was equipped for instrument flight, including a transponder, dual VORs, DME, ADF, and a GPS. It was not equipped with any anti-icing or de icing equipment. It could not be determined if the pilot considered the aircraft weight and balance; no calculations were found. Prior to arriving in Abbotsford, the aircraft had left Boeing Field in Seattle, Washington, and the TSB determined that the aircraft's weight on departure from Boeing Field was 164 lbs above the maximum allowable take off weight of 2 950 lbs. The aircraft was not refuelled in Abbotsford. Its weight at the time of the accident was calculated to be within limits, at 2 949 lbs.

The wreckage was found on a heavily wooded, west facing 45° slope. This site is about 1 NM south of the direct track from Abbotsford to Revelstoke. Tree damage and contact markings showed that the aircraft's flight path was mainly vertical at the time of impact. The wreckage was examined for pre impact defects and none were found.

The pilot had 3 370 flying hours on light, single engine aircraft, including the Cessna 182P, and was qualified to operate the aircraft under IFR. There were no medical condition that could have led or contributed to the accident.

The graphical area forecast (GFA) for the area between Abbotsford and Calgary for 11:00 (3 h 37 min before the accident) and that for 17:00 (2 h 23 min after the accident) are almost identical. They called for broken clouds based at 6 000 ft ASL topped at 16 000 ft ASL; scattered towering cumulus clouds topped at 20 000 ft ASL; prevailing visibility more than 6 SM in light rain showers; isolated cumulonimbus clouds topped at 25 000 ft developing after 13:00; and the prevailing visibility more than 6 SM in light thunderstorms with hail along the mountains. The freezing level was forecast to be around 6 200 ft ASL.

Additional weather data was obtained from three British Columbia Ministry of Transportation (BC MOT) weather observation stations, all located within a few miles of the accident site. These stations record data for the BC MOT snow avalanche and weather system. This data showed that, at the time of the accident, the winds were from the southwest at 24 kt, the temperature was close to the freezing mark, and some precipitation in the form of snow occurred.

A surveillance video, taken at the Coquihalla highway toll booth, approximately 5 NM northwest of the accident site, showed low cloud, rain, and gusty winds at that location around the time of the accident.

Analysis — Information from the three BC MOT avalanche weather stations and reference to the Coquihalla toll booth surveillance video indicate weather conditions at the time and place of the accident were probably much worse than forecast. The ceiling was probably lower than the forecast 6 000 ft ASL and the freezing level very close to the surface, around 4 000 ft ASL. In the area of the accident site, the pilot would have encountered rising terrain. He would also probably have encountered a lowering ceiling, likely forcing him to descend below his cruising altitude of 5 000 ft ASL in order to maintain VFR flight. Near the base of the cloud, he may have encountered turbulence, snow, and airframe icing. But he would have had very little room to descend, as the terrain in that area is relatively high, with no less than five mountain peaks ranging in elevation from 6 009 to 7 088 ft ASL, located within a 10 NM radius of the accident site.

While the pilot held a valid instrument rating and had considerable experience in instrument flight, he was not in contact with air traffic control (ATC) and had no IFR clearance. To contact ATC, he would have had to climb several thousand feet because of the high terrain. A climb through cloud from his location would have been risky because of the low performance of the aircraft at its high weight and high elevation, and the close proximity of numerous mountain peaks. Had the pilot abandoned visual flight, made a transition to instrument flight, and attempted to climb to a safe altitude, he would likely have encountered icing and possibly thunderstorms. It is likely that he elected to manoeuvre his way around visually, taking the risk of encountering instrument meteorological conditions (IMC).

The aircraft's flight path was mainly vertical at the time of impact, indicating the aircraft was not under control. The severity of the damage and the angle at which the aircraft contacted the terrain indicates the aircraft was likely in a spiral dive at impact, not in a stalled condition. As indicated by the last GPS-recorded aircraft position and the accident position, the aircraft was travelling southward prior to impact. The fact that the fuselage was pointing 330° may be indicative of the aircraft being in a spiral dive. The most likely scenario to account for this accident involves a known phenomenon encountered by pilots flying in mountainous terrain. The high ground obscures the natural horizon and, in this occurrence, the difficulty in seeing the horizon would be exacerbated by the low cloud.

When he encountered rising terrain and lowering cloud, the pilot probably lowered the aircraft's nose to avoid entering cloud and started a turn to reverse his course. Because no horizon would be visible when looking outside the aircraft, the only way to maintain control during this turn would be by reference to flight instruments. For unknown reasons, the pilot lost control of the aircraft, and because of the relative proximity of the terrain, the aircraft struck a tree before the pilot was able to recover control.

This accident should serve as a good reminder to all that weather in mountainous terrain can vary significantly from forecast. Although the pilot does not appear to have updated his weather enroute, he did attend a weather briefing in person before departure and was advised that the weather appeared to be suitable for flight in accordance with VFR. Keep this in mind next time you fly near or over mountains. Keep your options open, particularly in being able to remain VFR at all times. The full report is available on the TSB website. — Ed.

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