Hovering the AS350 Squirrel helicopter with hydraulics assistance disabled can lead to a rapid loss of control with catastrophic consequences even for highly experienced pilots, an ATSB investigation highlights.
AS350BA Squirrel helicopter registration VH-BAA was conducting an emergency procedures training flight from Hobart Airport on 7 November 2017 with a pilot under instruction (undergoing type rating endorsement training on the type) with the helicopter operator’s chief flying instructor (CFI), when it collided with the ground during a simulated hydraulic system failure exercise. The CFI, who was seated on the left, was fatally injured in the accident, while the pilot under instruction sustained serious injuries.
The helicopter was not fitted with, nor was it required to be, cockpit voice and flight data recorders, however analysis of airport CCTV footage, photographs and air traffic control data allowed the ATSB to determine the accident’s sequence of events.
That analysis showed that the helicopter entered a high hover without hydraulics with a crosswind, however, the AS350 flight manual stipulates that in order to ensure control following a hydraulic system failure (or simulated failure), a shallow approach should be made into wind and the helicopter should not enter a hover.
Entering a high hover with a crosswind rendered the helicopter uncontrollable, ATSB Director Transport Safety Mr Stuart Macleod said, with a number of elements having the potential to cause a delay in restoring hydraulics and preventing the pilots from regaining control.
”The AS350 flight manual notes that without hydraulics the helicopter is subject to rapid changes in control direction and force,” Mr Macleod said.
As detailed in the flight manual, the safe practice of a hydraulic failure sequence in the AS350 requires a flat final approach into wind and a no-hover or slow run‑on landing at a speed of around 10 knots. This is a compromise to minimise the speed of the run-on landing and avoid hovering.
“Compliance with the AS350 flight manual requirements following a real or simulated hydraulic failure ensures that the helicopter remains controllable during all phases of flight.”
The report notes that flight test evaluation of the AS350 by the Royal Australian Air Force’s Aircraft Research and Development Unit in 1997 – all three arms of the Australian Defence Force have operated the AS350 for helicopter pilot training – found that while hovering without hydraulics that the AS350 is subject to random perturbations and a reduction in control authority, Mr Macleod noted.
“This and many other similar accidents illustrate that hovering an AS350 without hydraulic assistance can lead to a rapid, catastrophic loss of control even for highly experienced pilots,” he said.
The ATSB reviewed 34 investigations of accidents involving AS350 helicopters’ hydraulic systems worldwide, Mr Macleod noted, with data indicating that loss of control accidents during simulated hydraulic failure training do occur to even highly experienced pilots.
“The average recorded experience of flight instructors involved in these events was over 9,000 hours total time, with over 1,000 hours of those on the AS350. These are very experienced helicopter pilots,” Mr Macleod said
The investigation also details that a preflight briefing was not conducted before the flight, which may have led to confusion over roles in controlling the helicopter as the emergency progressed.
“This accident’s rapid development reinforces the need for a clear understanding and coordination between instructor and student when conducting hazardous activities such as simulated system failures,” Mr Macleod said.
Subsequent to the accident the operator has undertaken a number of safety actions, including updating the training school’s operations manual with stricter controls on performing AS350 sequences in line with the flight manual requirements.
Separately, the ATSB notes that in 2019 the AS350’s manufacturer, Airbus Helicopters, released a Safety Information Notice (No. 3246-S-29) and accompanying educational video covering hydraulic failure training in the AS350. The video can be viewed on the Airbus website here.Last update 22 July 2020