What happened
On 21 July 2008, a Robinson R44 Astro, registration G-YIIK, was performing a private flight near Delph, Lancashire. The pilot was attempting to land at a private site located in a valley. During the approach, the pilot observed via a windsock that the wind was blowing from the north-north-west at approximately 20 knots.
As the helicopter entered a hover at roughly 30 feet above the ground, the wind direction shifted abruptly, with the windsock pointing vertically upwards. Simultaneously, the aircraft began to rotate clockwise. The pilot attempted to counter this rotation by applying full left pedal, but the rotation persisted. The helicopter drifted downwind toward a group of trees. In an effort to regain altitude, the pilot raised the collective lever, reaching approximately 2 and 200 feet above ground level. However, this action caused the rotation to cease, but triggered a low main rotor rpm (Nr) warning.
Faced with a rapid descent and insufficient height to recover, the pilot lowered the collective to increase rotor speed and chose to land in a flat field nearby. Just before the aircraft struck the ground, the pilot raised the collective again to cushion the impact. The helicopter landed heavily in an upright position before rolling onto its left side. The impact resulted in one serious injury to the pilot and the aircraft was damaged beyond economic repair.
The investigation
Investigators examined the flight sequence and the environmental conditions at the time of the accident. The investigation established that the helicopter entered a hover out of ground effect and was subjected to a significant wind shift. This shift likely led to a loss of directional control, potentially due to loss of tail rotor effectiveness (LTE).
Analysis of the pilot's actions revealed that while attempting to gain height, the pilot over-pitched the main rotor blades, which caused the rotor rpm to decay into the avoid curve. The investigation also noted that the pilot did not check the rotor rpm after the warning sounded. Had the rpm been near the normal operating range, the pilot might have had the option to partially lower the collective to manage the pitch and maintain flight stability.
Findings
- A sudden and large change in wind direction occurred during the hover.
- The aircraft experienced a loss of directional control, possibly due to loss of tail rotor effectiveness.
- The pilot's attempt to increase altitude by raising the collective caused the main rotor rpm to drop below the required threshold.
- The pilot did not verify the rotor rpm status following the low Nr warning.