Accident Overview
On a fire-patrol mission for the British Columbia Forest Service, a Cessna 337 departed Nelson, British Columbia at 1313 PDT. The aircraft was crewed by a pilot and an observer tasked with detecting and reporting smoke or forest fires. The flight was uneventful for the first two hours. At 1511, the pilot reported by radio that they were five nautical miles east of Salmo, heading north.
Approximately seven minutes later, witnesses observed the aircraft flying at low level up a valley. It commenced a steep left turn at the end of the valley and crashed into the mountainside. The aircraft was destroyed upon impact, and both occupants sustained fatal injuries. There was no post-crash fire.
Terrain and Impact
The accident site was a steeply rising mountain bowl at the end of a rising valley. The valley makes a 60-degree bend around a knoll. From the pilot's perspective, the valley appeared to continue around the knoll, but beyond it lay the mountain bowl, not visible until the turn was completed. The aircraft struck the mountain at the 5,800-foot level in a steep, nose-down attitude and flipped onto its back. The wreckage trail was short, consistent with a steep impact angle. Witnesses reported that the aircraft did not hit any trees before impact, and no evidence of tree strikes was found.
Pilot and Aircraft Information
The pilot began flying training in Castlegar in 1989 and had been employed by the company since 1996 as a flight instructor and charter pilot. One of his duties included teaching mountain flying courses. He had accumulated approximately 1,200 total flying hours, including about 34 hours of multi-engine experience, of which 21 hours were on the Cessna 337. He received ground and flight training on the Cessna 337 in April 1997 and successfully completed a Transport Canada pilot proficiency check on 29 April 1997. The pilot was certificated and qualified for the flight in accordance with existing regulations.
The aircraft, serial number 337-1107, was certificated, equipped, and maintained in accordance with regulations and approved procedures. No evidence was found suggesting any airframe failure or system malfunction before or during the flight.
Performance Considerations
According to the aircraft manufacturer's performance data, the stall speed of a Cessna 337 at maximum gross weight (4,400 pounds) with landing gear and flaps retracted and wings level is 80 miles per hour indicated airspeed. In a 60-degree bank turn, the stall speed increases to 114 mph. The maximum rate of climb achievable under the environmental conditions at the time was calculated to be about 600 feet per minute at 105 mph indicated airspeed.
Witness Accounts
Four witnesses provided observations. Two hikers on a ridge about 0.75 nautical miles from the crash site heard the aircraft and saw it flying up the valley. It came around the knoll, continued into the bowl, and then initiated a steep left turn with wings nearly vertical. They reported that the aircraft had almost completed the turn when it struck the mountainside. Two other witnesses, motorcyclists stopped on a dirt road in the valley, saw the aircraft at 300 to 400 feet above the valley floor, proceed overhead, and crash about 0.25 nautical miles away. All witnesses described the weather as sunny, clear, and hot, with temperatures possibly reaching 30°C. Winds at the mountain crest were estimated at about 15 knots across the tops, but conditions in the bowl were calm. The aircraft was seen in a climbing attitude with wings level while coming up the valley, engines at high power, and with landing gear and flaps retracted. No difficulties were observed.
Operational Context
The BC Forest Service typically submits flight requests to the aircraft company by facsimile, with routes assigned based on areas of potential fire risk. Pilots and observers dispatch themselves following the company's operations manual. En route flight-following is provided by the Southeast Fire Centre, with crews communicating at specific points or times or upon spotting a fire. A four-hour training course was provided to pilots and observers at the start of the fire season to familiarize them with fire detection and reporting procedures. The Forest Service's Air Patrol Course manual notes that "rock slides, pollen flights and low hanging wispy clouds can give the illusion of smoke," requiring pilots to check such sightings. The manual instructs pilots to perform high-level reconnaissance of smoke or fire before any low-level reconnaissance and to ensure the aircraft can always exit an area without climbing; up-valley flight toward a dead-end must allow room for a safe 180-degree turn to downhill.
On a previous flight, the accident pilot had made several low passes at what initially appeared to be smoke but turned out to be dissipating cloud.
Investigation Findings
The investigation found it unlikely that a mechanical defect contributed to the accident. The pilot had called the Fire Centre seven minutes before the accident with no indication of problems. It is possible that the crew observed dust raised by the motorcycles in the valley and, suspecting it was smoke from a forest fire, decided to conduct low-level reconnaissance before reporting.
The pilot flew up a valley that likely gave the illusion of a suitable exit route. High outside air temperature and operating altitude reduced aircraft climb performance, such that the available rate of climb was only sufficient to maintain near-constant height above the rising valley floor. It is probable that when the aircraft turned around the knoll, the pilot saw the mountain bowl and realized the aircraft could not outclimb the steep walls. The pilot likely decided to turn around, entering a steep left turn. The combination of flight path angle (parallel to the rising valley floor) and steep impact angle is characteristic of an aircraft that stalled and struck the ground. The stall speed increased in the steep turn, exceeding the suspected climb speed of about 105 mph, causing the aircraft to stall. With the aircraft 300 to 400 feet above the ground, there was insufficient height for recovery.
Probable Cause
The probable cause, as stated in the investigation, is: "While the pilot was manoeuvring to avoid rising terrain, the aircraft stalled at a height which was insufficient to allow him to effect a recovery. Contributing to the accident was the reduced aircraft climb performance and the illusion created by the features of the surrounding terrain."