What aircraft was involved and where did it happen?

The accident on null involved a aircraft, registration 9M-SSZ, at MY.

What was the probable cause of the accident?

The accident was caused by the pilot unintentionally pressing the ACCU accumulator pushbutton instead of the servo pushbutton, which induced a momentary hydraulic lock and a sudden loss of collective control during a hover check.

Hard Landing During Hover Check Destroys Eurocopter AS355 F2

A Eurocopter AS355 F2 sustained heavy damage after a pilot accidentally activated the wrong switch during a maintenance hover check at Kota Kinabalu International Airport.

What happened

On 15 November 2017, a Eurocopter AS355 F2, registration 9M-SSZ, was performing a hover check at the Terminal 2 dispersal area of Kota Kinabalu International Airport in Sabah, Malaysia. The flight was intended to verify the functionality of the Automatic Flight Control System (AFCS) due to intermittent autopilot issues. The aircraft was occupied by one pilot and three maintenance crew members.

During the procedure, the pilot attempted to test the servo light functionality. However, instead of pressing the intended SERVO pushbutton, the pilot inadvertently activated the ACCU (Accumulator) momentary pushbutton. This action caused the aircraft to descend vertically and strike the tarmac with significant force. Despite the violence of the impact, there were no injuries among the 4 persons on board.

The investigation

AAIB Malaysia investigators examined the wreckage and consulted with type-rated engineers to understand the mechanical consequences of the button activation. The investigation focused on how pressing the ACCU button affected the hydraulic systems.

Technical analysis revealed that activating the ACCU momentary pushbutton opens a solenoid valve, which pressurizes the return line to the tail servo unit. This creates a momentary hydraulic lock where both the supply and return lines are pressurized. This phenomenon causes a split-second period of stiffness and a loss of control in the tail rotor. Because the return line is interconnected with the lower servo units for the left, right, and front controls, this effect also manifests in the collective. The investigation established that the pilot's hand was at the overhead panel during the error, leaving him no time to compensate for the sudden drop in the collective.

Findings

The aircraft's starboard skid broke upon impact, and the tail rotor struck the ground, causing buckling and tearing to the tail boom skin.
The Eurocopter AS355 F2 was declared a total write-off.
The primary cause of the accident was pilot error involving the accidental activation of the wrong pushbutton during a maintenance check.
The use of the ACCU momentary pushbutton should only occur in a specific sequence following the actuation of the Solenoid Valve Control Switch on the collective.