What happened
On 5 May 2017, an AS350 B3e helicopter, registration G-MATH, was conducting type conversion training at Wycombe Air Park. The flight involved an instructor in the left seat, a trainee pilot in the right seat, and a second trainee acting as a passenger. The objective of the maneuver was to practice a hydraulics-off approach, simulating a loss of hydraulic pressure to practice manual control.
During the first attempt at a simulated hydraulic failure approach, the instructor intervened to perform a go-around after becoming dissatisfied with the aircraft's parameters. Following this, a second attempt was initiated. During a left turn on the downwind leg of this second attempt, the aircraft experienced a significant increase in bank angle. The instructor was unable to arrest the roll, and the helicopter rolled beyond 9 and descended rapidly, striking the ground on its left side. The accident resulted in one fatality and two serious injuries.
The investigation
The investigation examined flight data from an onboard video and flight recording system, which captured cockpit audio and aircraft attitude. Investigators analyzed the mechanical state of the aircraft and the control forces required during hydraulics-off flight. The aircraft was found to be in good technical condition, with no mechanical failures identified.
Investigators focused on the control dynamics of the AS350 B3e when the hydraulic system is depressurized. They noted that while the aircraft can be flown mechanically, the control forces change significantly. The investigation also looked into the possibility of a "startle response" in the instructor, as the sudden steep bank and proximity to the ground may have caused a psychomotor impairment that prevented effective recovery.
Findings
- The primary cause of the accident was the loss of control during a high-angle-of-bank maneuver while the hydraulic system was depressurized.
- The instructor was unable to apply sufficient right cyclic force to counter the left roll.
- The flight manual instructions for hydraulics-off flight were found to be open to interpretation, specifically regarding what constitutes a "level attitude."
- There was a lack of specific guidance regarding how to safely execute a go-around during a simulated hydraulic failure.
Safety action
Following the investigation, Airbus Helicopters implemented several changes to the AS350 B3e flight manual to prevent similar occurrences, including:
- Establishing a strict 30° angle of bank limit for all hydraulics-off flight.
- Adding explicit warnings regarding the danger of low-speed maneuvers without hydraulic assistance.
- Recommending that pilots abort training and restore hydraulic pressure if a go-around becomes necessary during a simulated failure.
- Providing warnings about the sudden decrease in control loads when resetting the hydraulic cut-off switch to the 'ON' position.