While current helicopter health usage and monitoring systems (HUMS) are valuable tools, they are not always fail-safe. That's the takeaway from a report released earlier this year by the UK's Air Accidents Investigation Branch (AAIB) detailing a potentially catastrophic hard landing on a North Sea oil platform by a CHC Sikorsky S-92 on Dec. 28, 2016, and using it to make the case for real-time HUMS.
The accident resulted from a failure of the tail rotor pitch shaft bearing (TRPCS), a misreading of the downloaded HUMS data the night before the accident flight by the maintenance crew, and the flight crew's misinterpretation of the helicopter's flight behavior before losing yaw control and gouging Total's West Franklin wellhead platform helideck during landing. The helicopter sustained left outer main wheel rim distortion and a seized tail rotor. There were no injuries to the two crewmembers and nine passengers.
The investigation determined that the TRPCS bearing failed for undetermined reason precipitating damage to the tail rotor pitch control servo. During a flight a day before the accident the helicopter's HUMS unit recorded vibrations that included exceedences related to the TRPCS bearing. This data was downloaded and an anomaly for tail rotor gearbox (TGB) bearing energy was detected by the maintenance engineer; however, the exceedences were not identified due in part to the way they were presented on the analysis tool, and the helicopter was released back into service. HUMS recorded further exceedences on the day of the accident but there was no way that either the flight or maintenance crew could know of these until the end of the flying day when the data could be downloaded.
Lifting off for West Franklin the day of the accident, the helicopter suffered an uncommanded right yaw through 45 degrees. After immediately landing, lifting into hover, and running a control check, the crew attributed this to wind effect and departed en route for the five-minute flight to West Franklin where, in the later stages of landing, yaw control was completely lost, the helicopter yawed right, and the crew landed immediately and heavy on deck, rotating through 180 degrees before closing the throttles.
The investigation found that while the impending failure of the TRPCS bearing was detected by the HUMS, routine maintenance missed it due to a combination of human factors and the design of the HUMS ground station man-machine interface. The HUMS GS software in use at the time also has a previously unidentified “anomaly” in the way data was displayed to maintenance personnel and the recommended method for viewing data per the manufacturer was not always used by maintenance personnel.
In the wake of the accident, Sikorsky introduced HUMS software with enhanced diagnostic capabilities and improved user interfaces and “took tighter control” of bearing manufacturing and assembly tolerances, lubrication quality and application, and in-service temperature monitoring.
The accident followed the issuance of an FAA emergency airworthiness directive on Nov. 18, 2016 after a report of another S-92A losing tail-rotor authority. That AD called for inspecting helicopters with recently installed TRPCS assemblies before further or significant flight. The December 28 accident prompted an Alert Service Bulletin (ASB 92-64-011) from the manufacturer that called for a special one-time inspection of the tail rotor and bearing assemblies and a data check of the aircraft's HUMS. It mandated an off-the-aircraft check of the tail-rotor pitch-change-shaft (PCS) bearing.
The AAIB found that the operator's practice of downloading HUMS data every five flight hours, which surpassed the UK regulatory standard of 25 hours, still failed to provide timely warning of impending TRPCS failure; that the time between detectable degradation of the bearing by HUMS and bearing failure was four flight hours; and by the time HUMS data was reviewed by a second organization the TRPCS bearing had already failed. The investigators concluded that if the loss of yaw control had occurred earlier in the flight, the helicopter likely would have made an uncontrolled descent into the North Sea.
The AAIB noted that the current vibration health monitoring (VHM) regulatory requirements for intervals between data downloads are ineffective for the detection of imminent in-service component failures and that opportunities to detect problems from low frequency data capture from individual VHM sensors are missed. It recommended that the European Aviation Safety Agency (EASA) conduct research into VHM data with the aim of enhancing its usefulness to predict failures and amend regulations to require VHM analysis in near real-time with that information made available to the flight crew.