Flight Information
A Hughes 269C helicopter, registration HB-XPL, was involved in an accident on 22 January 1977. The pilot held a valid commercial pilot license and had 418 hours total flight time, with 279 hours on type. The helicopter was privately owned and operated.
Accident Sequence
On the day of the accident, the pilot flew with two passengers from Schindellegi to the Lauberhorn ski race, landing at a designated helicopter site near Kleine Scheidegg. The return flight departed at 1535 hours. Another pilot flying a similar helicopter departed ahead. After a full circle over Hotel Männlichen, both helicopters crossed the Lütschental valley toward the pass west of Faulhorn. The lead pilot reported initially seeing the accident helicopter to his left at the same altitude, then ahead and to the right, slightly lower. After crossing the valley, the lead pilot turned left, climbing with full power (22 inches manifold pressure) at 60 knots, gaining about 700 feet per minute in updraft, then encountered turbulence and strong downdrafts after passing the saddle. The distance from the slope was about 150 meters.
The accident pilot reported that the other helicopter was to his right at the same level but climbed faster. He felt his helicopter was weaker. His manifold pressure showed 22 inches, speed 55 knots. He turned away from the slope to gain height but immediately began sinking. He reduced to best climb speed of 30 knots, but manifold pressure dropped to 19 inches. Despite turning away from the slope, sink increased. He lowered the nose to gain speed, but sink worsened. He decided to land and lowered the collective. The helicopter slid downhill on skis, leaving 35-meter skid marks on a 19-degree slope, and overturned forward before returning to upright, ending facing south.
Investigation Findings
Technical examination of the engine found no evidence of sudden power loss. A fuel sample showed aviation gasoline with slight rust contamination. The fuel control unit had a lean mixture tendency at high power but was mechanically in order. The report noted that a manifold pressure drop from 22 to 19 inches could occur if the pilot inadvertently lowered the collective without adjusting throttle, causing the throttle valve to close at roughly constant RPM.
Operationally, the helicopter's weight of 1890 pounds allowed hover-in-ground-effect up to 6300 feet, while the accident site was at 7808 feet. The prescribed minimum flight altitude was 150 meters above ground. The pilot's decision to turn away from the slope came when altitude was already below the minimum. The pilot selected a climb speed of 30 MPH, which is 17 MPH less than the manual's recommended best climb speed, bringing the helicopter near translational lift speeds. Meteorologists reported possible strong foehn gusts, which could have temporarily reduced the helicopter to hover flight conditions, requiring power not available at that altitude, leading to descent. At low speed with high rotor disk loading, a vortex ring state could develop, causing uncontrollable sink. The pilot lacked sufficient height for recovery by autorotation.
Probable Causes
The accident was likely due to failure to maintain minimum flight altitude, unsuitable flight tactics, and insufficient consideration of wind effects.