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S-92 spin blamed on tail rotor piston damage
Preliminary investigation points to cause of recent incident that left Franklin helideck damaged
A Sikorsky S-92 helicopter suffered a “total loss of control” due to the failure of a tail rotor piston when landing on the West Franklin platform off the UK, according to preliminary findings from an investigation into the incident.
The probe by the UK’s Air Accident Investigation Branch (AAIB) has traced the cause of the 28 December incident, which resulted in gouge marks on the platform’s helideck, to the seizure of a tail rotor pitch change shaft bearing on the CHC Helicopter-operated aircraft.
In a bulletin issued on Wednesday, the AAIB stated the chopper, with two crew and nine passengers onboard, had yawed unexpectedly 45 degrees to the right on take-off from Total’s Elgin platform but the pilot managed to compensate the movement using the full left yaw pedal and continued with the brief flight.
(Click here to download the AAIB's bulletin).
However, on descent to West Franklin, the aircraft again yawed rapidly to the right about four feet above the helideck and at the same time rolled 20 degrees to the left, causing the main landing gear to come in contact with the deck.
The helicopter continued to yaw to the right on its left main wheels and nose wheels before the right wheels settled on the deck and it finally came to rest, having rotated 187 degrees.
The investigation, focused on the tail rotor, found signs of “severe overheating with extreme wear” inside the pitch change shaft bearing and that a torsional load had led to the fracture of a primary piston in the servo of the mechanism.
The piston failure in turn caused the “total loss of control of the tail rotor”, according to the bulletin.
“The initial findings suggest that the damage to the servo in this case is such that it could have imparted extreme or erratic inputs to the tail rotor at any time after the failure of the primary piston,” the AAIB stated.
It “considers that this failure mode would seriously affect the ability of the flight crews to maintain control of the helicopter”.
The board highlighted two previous incidents involving degradation of the same bearing housing that resulted in immediate landings, though it has not yet established whether there is a connection with the latest incident that it will continue to investigate.
The incident led to all North Sea helicopters operated by CHC and Bristow being grounded on Tuesday for safety inspections to be carried out on the tail rotor as part of a global recall of S-92s, with flights off the UK and Norway gradually resuming on Wednesday.