What happened
On 29 September 1999, a Schweizer 330, registration N433CK, was conducting a private demonstration flight at Redhill Aerodrome in Surrey. Following a period of flight where a passenger was primarily controlling the aircraft, the pilot took command to perform an engine-off landing procedure.
While flying on a left-hand downwind leg at approximately 800 feet, the pilot lowered the collective to enter an autorotation at an airspeed of roughly 50 knots. As the aircraft transitioned to the final approach, the airspeed suddenly dropped. Despite the pilot attempting to compensate by pushing the cyclic forward, the rate of descent increased instead of the airspeed. The aircraft's speed fell to approximately 10 knots, and the rotor RPM (RRPM) became critically low. Due to the position of the sun on the horizon, the pilot found it difficult to monitor the RRPE gauge. The helicopter struck the ground with a slight nose-down attitude and rolled onto its right side, causing extensive damage to the airframe. All three occupants escaped through the canopy, with only one minor injury reported.
The investigation
Investigators examined the flight dynamics and the mechanical state of the aircraft's warning systems. It was established that during the initial left-hand descending turn, the RRPM had increased as anticipated. However, the pilot's attempt to manage the descent was complicated by the specific configuration of the aircraft's warning systems.
On this particular model, the low RRPM warning horn is deactivated when the throttle is moved to the idle position. Because the pilot had closed the throttle for the engine-off demonstration, the audible warning did not trigger. This lack of auditory feedback, combined with the visual difficulty of reading the gauges due to sun glare, meant the pilot was unaware of the declining rotor speed until the landing became unavoidable.
Findings
- The primary cause of the heavy landing was the combination of low airspeed and low RRPM during the transition from a turn to the final approach.
- A significant wind shear factor contributed to the sudden loss of airspeed.
- The pilot was unable to visually monitor the RRPM gauge due to bright sunlight on the horizon.
- The low RRPM warning horn failed to provide an audible alert because the throttle had been closed to the idle position, which suppresses the warning system on this aircraft type.