Background
The accident involved an Airbus Helicopters AS350 B3e Ecureuil, registration G-MATH, built in 2016. The helicopter was equipped with a single Turbomeca Arriel 2D turboshaft engine and a six-seat configuration. It had a factory-installed cockpit video and flight data recording system.
History of the Flight
The flight was a revision session for two pilots undergoing type conversion training, supervised by an instructor from an Approved Training Organisation based at Wycombe Air Park. The helicopter departed at 0805 hrs with the instructor in the left seat, one pilot in the right seat, and the second pilot as a passenger in the rear centre-left seat. The first part of the training included autorotations, practice engine failures, hover exercises, and low-level circuits, all completed without incident. The helicopter then returned to Wycombe Air Park.
The Accident
After returning, the helicopter positioned south-west for a base leg approach to the grass area north of Runway 06 to begin hydraulic failure training. The instructor simulated a hydraulic failure by selecting the ACCU TST switch on and then off, following the AS350 flight manual procedure. The right-seat pilot then selected HYD CUT OFF to depressurise the hydraulics and began a hydraulics-off approach for a run-on landing. During the latter stages, the instructor deemed the approach parameters unsatisfactory—specifically yaw and speed—and took control to perform a go-around, followed by a left-hand circuit with up to 32° angle of bank (AOB). The right-seat pilot kept his hand on the cyclic despite a briefing not to do so. The instructor handed control back for a second attempt, but again took control late in the approach and initiated another go-around. This time, a left turn onto downwind was flown with a higher AOB. The roll rate was initially similar to the first go-around, but the AOB increased. At 48° AOB, the instructor changed his grip on the cyclic and leaned right, apparently trying to apply more right control force. The AOB stabilised briefly, then the helicopter rolled further left, descended, and struck the ground on its left side with little forward speed. The main rotor blades impacted the ground first at near 90° left bank, and the tail boom failed due to impact inertia, but the tail rotor was undamaged. The fuselage came to rest on a heading of 019° magnetic.
All three occupants sustained serious injuries. The right-seat pilot died weeks later from injuries sustained in the accident. The instructor stated he could not move the cyclic to the right to stop the roll.
Aircraft Information
The AS350 B3e has a single hydraulic system; without hydraulics, control forces increase. The cyclic and collective operate three main servo actuators mechanically when hydraulics are off. A yaw load compensator remains pressurised when HYD CUT OFF is selected, aiding yaw control. The recommended safe airspeed for hydraulics-off flight is 40–60 kt, requiring about 4–6 kgf left cyclic force and 5 kgf forward to maintain level flight. The helicopter was also fitted with a 2-axis HeliSAS autopilot system, which when off disconnects via electromagnetic clutches.
Investigation
The AAIB investigation found no technical issues and could not determine why the instructor could not roll the helicopter level. The investigation concluded that clearer instructions in the AS350 flight manual for hydraulics-off flight could help prevent similar accidents. In response, Airbus Helicopters amended the flight manual to limit AOB to 30° during hydraulics-off flight and added warnings against low-speed manoeuvres with hydraulics off. The manufacturer also produced a safety video on hydraulics-off training.