Introduction
A single-engine airplane was involved in an accident after departing from an airport located in a mountain valley. The aircraft collided with trees during an attempted return to the airport.
Sequence of Events
The pilot initially radioed intentions to perform a downwind departure from runway 18. Approximately three minutes after takeoff, the pilot made a final radio transmission stating he was turning left crosswind. Witnesses observed the airplane in a right turn approximately 20 feet above treetops, with a slightly nose-low attitude. The airplane then rolled to a level attitude heading north and began to descend just before colliding with trees. Witnesses reported hearing the engine at a high power setting initially, which then dropped to a lower sounding power setting just before impact.
Environmental Conditions
The airport is situated in a mountain valley with rising terrain immediately to the west, south, and east, and a lake to the north. The elevation is 6,264 feet. At the time of the accident, winds were from the south at 16 knots, gusting to 31 knots. The density altitude was calculated to be 8,445 feet. According to the airplane's pilot operating handbook (POH), the expected rate of climb at 89 miles per hour for this density altitude was 220 feet per minute. The POH also listed a stall speed of 55 miles per hour for zero angle of bank with zero flaps. Advisory Circular AC 00-57 notes that aircraft conducting low-level flight over mountainous terrain in strong winds (20 knots or greater at ridge level) can expect moderate or greater turbulence, strong up- and downdrafts, and strong rotor and shear zones. Information available at the airport indicates that downdrafts are often encountered near the runway abeam the terminal building and west of the golf course, which are along the crosswind and downwind routes for runway 18 departures.
Postaccident Inspection
A postaccident inspection of the airframe and engine revealed no evidence of a mechanical failure or malfunction.
Probable Cause
The source states: "The pilot probably encountered turbulence and downdrafts, a reduced rate of climb, and rising terrain to his left shortly after takeoff. It is likely he elected to return to the airport for landing, but was unable to maintain altitude during the turn due to the turbulence/downdrafts, and collided with trees."