Norwegian investigators have identified a fault in a pitch-trim servo circuit which might explain the unexpected pitch-up of a Sikorsky S-92 just before it fatally crashed during a North Sea search-and-rescue training exercise.
This exercise involved locating a radio beacon, after which the crew activated an automatic flight-control system mode – known as ‘Mark on Top’ – which brings the helicopter to a 150ft hover, 50m aft and left of the point of activation.
While the S-92 would normally pitch nose-up to about 10°, in order to slow for the hover, the aircraft instead continued pitching at about 2°/s until it reached a 30° nose-up attitude – enough to begin flying backwards – and it descended into the sea.
Analysis of information from the flight-data recorder, two flight-control computers, and the health-and-usage monitoring system did not clearly indicate why the pitch-up manoeuvre continued to the point of being hazardous.
Norwegian safety investigation authority SHK says it took relevant parts of the autopilot – including the pitch stability augmentation system and the four trim servos – to manufacturers in the USA for examination.
While the stability augmentation system showed “no evidence” of functional anomaly that would contribute to the accident, says SHK, in-depth examination of the pitch-trim servo identified a fault on one of the three circuit boards.
“This fault appears to have a potential of explaining the helicopter movement seen in the recorded data,” the inquiry adds.
Twelve circuit boards were tested – three from each of the four trim servos – of which two had physical damage and failed during the testing. No other faults were found on the remaining nine boards.
“The fault in the pitch trim servo is still under investigation,” says SHK.
Six occupants were on board the Bristow Norway S-92, one of whom did not survive the 28 February crash, which occurred at night some 20km west of Bergen airport.
Sikorsky has told the inquiry that the helicopter should not normally exceed 12-13° nose-up attitude during the ‘Mark on Top’ descent and hover manoeuvre.
But SHK says this information does not feature in the rotorcraft flight manual describing search-and-rescue modes.
Knowledge of this point, it says, could help S-92 crews recognise unusual behaviour from the automatic flight-control system. The inquiry is recommending that Sikorsky immediately ensures expected attitude parameters are made known to operators and included in relevant manuals.
