Piper 28-161, VH-XCB, Mt McKeahnie, Australian Capital Territory, on 24 February 1991

Circumstances:

The aircraft was operating on a pleasure flight in a mountainous area. The aircraft had been flown to the area at 6000 ft and was planned to operate below 5000 ft for 20 min before returning to Canberra. The aircraft was observed flying at low level following the floor of a valley. After completing a pass in a northerly direction, the aircraft continued tracking north-west to climb over a range. While over rising terrain, the aircraft struck a tree, the impact tearing the right wing from the airframe. The right wing remained lodged in the tree approximately 27 m above the ground while the aircraft descended through the trees and impacted the ground, on its right side, approximately 35 m beyond the first tree impact. The aircraft was destroyed by fire and the occupants received fatal injuries. Examination of the wreckage at the accident site and a subsequent strip examination of the engine did not reveal any technical defects that would have caused the accident. Examination of the propeller indicated that it was rotating as the aircraft descended through the trees but was stationary at the time of impact with the forest floor. Onsite examination of the engine revealed the throttle in the closed position and the mixture selected to 'Idle Cutoff'.

However, damage to the right wing was consistent with a high-speed impact, indicating that the aircraft possessed the necessary kinetic energy to maintain terrain clearance. The flap lever was found in the fully retracted position, a further indication that the initial impact with the tree was unexpected and did not result from the pilot attempting a forced landing in the treetops. The pilot was medically fit and endorsed on the aircraft type. The aircraft was serviceable at the time of the occurrence, but its gross weight was estimated to have been approximately 37 kg above the permissible maximum. The weather was clear and mild, but the combination of high terrain and temperature produced density altitudes between 3900 and 5500 ft which would have adversely affected aircraft performance. Reduction of available engine power, coupled with high aircraft gross weight, would have degraded climb performance and aircraft manoeuvrability, both critical factors for low level operations in mountainous terrain. The pilot had not been formally trained in low level operations, his training having been limited to low level.

Significant Factors:

The following factors were considered relevant to the development of this accident:

1. The aircraft was flown at low level in mountainous terrain.

2. The pilot lacked the necessary experience for safe low-level operations.

3. The aircraft gross weight was above the permissible maximum.

4. Density altitude degraded aircraft engine performance.

5. The combination of density altitude and high aircraft gross weight degraded aircraft manoeuvrability.

6. The pilot either misjudged clearance from treetops or under-estimated the effect of aircraft inertia on aircraft manoeuvrability.