What happened
On August 25, 2010, a Robinson R44 II, registration HB-ZJG, was conducting a VFR training flight near Giirebad, Switzerland. The flight, led by an instructor and a student, was intended to practice emergency procedures, specifically autorotations, to prepare the student for a private pilot license skill test.
During the exercise, the instructor reduced the throttle to initiate an autorotation. Shortly after the maneuver began, the engine failed. The instructor took control of the aircraft at approximately 1,000 feet AGL. As the helicopter descended toward a field, the rear of the landing skids made contact with the ground, causing the aircraft to pitch forward. During this sliding motion, a main rotor blade struck the tail boom, causing significant structural damage. Both occupants escaped the aircraft without injury.
The investigation
SUST examined the mechanical state of the engine and the flight controls. The investigation focused on why the engine failed during a routine maneuver and why the subsequent landing resulted in a rotor strike. Investigators also reviewed the flight instructor's background, noting his extensive experience in fixed-wing aircraft compared to his relatively limited experience with the Robinson R44 II.
Findings
- The engine failure was primarily caused by a technical issue within the fuel injection unit, where fluctuations in fuel flow led to an overly rich mixture at idle, contributing to the engine stoppage.
- The instructor's use of the throttle did not strictly follow the manufacturer's recommendation to avoid rolling to full idle, a practice common in some flight schools but one that increased the risk of engine stoppage.
- The collision between the main rotor and the tail boom was caused by inappropriate control inputs during the landing phase. Specifically, the instructor applied aft cyclic pressure while the skids were sliding, an instinctive reaction likely influenced by his extensive fixed-wing flying experience, which differs from the forward cyclic input required for a level helicopter landing.
- Fatigue may have played a role, as the instructor had been awake for over 12 hours and had completed a nine-hour duty day prior to the training flight.