The December 28 hard landing of a Sikorsky S-92A on a North Sea oil platform has prompted yesterday's Alert Service Bulletin (ASB 92-64-011) from the manufacturer that calls for a special one-time inspection of the tail rotor and bearing assemblies and a data check of the aircraft's health usage and monitoring system (HUMS).
It mandates an off-the-aircraft check of the tail-rotor pitch-change-shaft (PCS) bearing. Inspections are to be carried out immediately as aircraft are returned to base, and Sikorsky said it expected that the majority of inspections would be finished this week.
The new ASB comes after the December 28 incident, in which a CHC S-92A operating from Aberdeen spun upon landing on the Total West Franklin rig, damaging the helideck and the aircraft's landing gear. The operator CHC said the flight crew reported "unexpected control responses" on final approach and executed an emergency landing.
Additionally, the ASB follows an FAA emergency Airworthiness Directive on November 18 after a report of another S-92A losing tail-rotor authority. That AD called for inspecting helicopters with recently installed tail-rotor PCS assemblies before further or significant flight. Preliminary investigation of the incident helicopter that triggered the Emergency AD found that binding in the assembly's double-row angular contact bearing reduced tail-rotor control. It also found evidence of excessive heat, an indicator of a binding bearing.
Another Sikorsky S-92A accident involving loss of antitorque control is renewing scrutiny of the type's tail rotor bearing system. The December 28 hard landing on a North Sea oil platform prompted the manufacturer to issue an Alert Service Bulletin (ASB 92-64-011) on January 10 that calls for a special one-time inspection of the tail rotor and bearing assemblies and a data check of the aircraft's health usage and monitoring system (Hums). The new ASB mandates an off-the-aircraft check of the tail-rotor pitch change shaft (PCS) bearing. The inspections are to be carried out as soon as aircraft return to base.
The latest ASB follows the issuance of an emergency Airworthiness Directive by the FAA on November 18 (2016-24-51) after the report of another S-92A losing tail-rotor authority. That AD called for inspecting helicopters with recently installed tail-rotor PCS assemblies before further or significant flight. Preliminary inspection of the incident helicopter that triggered the emergency AD found that binding in the assembly's double-row angular contact bearing had compromised tail-rotor control. It also found evidence of excessive heat, an indicator of a binding bearing.
The December 28 accident involved a CHC S-92 operating from Aberdeen that spun through 180 degrees while landing on the Total West Franklin rig, damaging the helideck and the aircraft's landing gear. CHC said the flight crew reported "unexpected control responses" on final approach and executed an emergency landing.
There were no injuries among the two crew and nine passengers aboard.
Sudden Component Failure
Details of the December 28 incident are provided in a UK Air Accidents Investigation Branch (AAIB) Bulletin released last month. It paints a vexing and harrowing mosaic of a sudden component failure that occurred with very little warning.
As the helicopter lifted off from the Elgin Process Utilities Quarters (PUQ) helideck it "yawed unexpectedly to the right through 45 degrees," according to the AAIB report. "The commander applied full left yaw pedal, checked the rotation and landed back onto the deck. The flight crew discussed the likely cause, which they thought to have been the result of local turbulence or wind effects created by the platform structures which, anecdotally, is not uncommon for this helideck. They decided to continue and during the subsequent lift-off into the hover the commander applied left yaw pedal, the helicopter responded and turned to the left; all control responses appeared normal.
"The commander then climbed to 500 feet for the brief transit to the West Franklin wellhead platform, 3.3 nm to the south. The helicopter made a normal approach and deceleration to the West Franklin and crossed over the helideck. During the descent to land, at four feet above the helideck, it yawed rapidly to the right, reaching a maximum rate of 30 degrees per second. At the same time it rolled 20 degrees to the left, at which point the main landing gear contacted the helideck. It continued to yaw to the right on its left mainwheels and nosewheels before the right mainwheels contacted the surface. The helicopter came to rest on a heading of 041, having rotated through 187 degrees."
Initial post-incident inspection revealed that the tail-rotor servo piston was damaged and the tail-rotor pitch change shaft (TRPCS) double row angular contact bearing was "severely distressed."
The AAIB went on to note, "Further disassembly and examination of the components found signs of severe overheating, with extreme wear on the inner and outer thrust races and barrel-shaped rollers of the bearing. It was found that the roller bearings seized to the inner member. The outer race roller had excessive axial play (half an inch) such that the tail-rotor drive shaft imparted a torsional load to the tail-rotor servo. This torsional load caused the primary piston rod to fracture inside the servo. Failure of the primary piston caused the secondary piston sleeve to separate axially from the primary piston adjacent to the link fitting, with the consequential total loss of control of the tail rotor."
The AAIB notes that the bearing failure was rapid, occurring just 4.5 hours after the first exceedance of the relevant bearing condition indicator had been recorded on the operator's Hums. The operator's Hums was downloaded the night before the accident and the helicopter was released back into service. A detailed analysis of the Hums data conducted after the accident shows that the tail gearbox bearing energy analysis limit had in fact been exceeded on December 27. The AAIB report also notes that there have been previous events involving S-92 TRPCS bearings dating back to 2007 resulting in "reduced" tail-rotor control in flight, immediate landings and subsequent cure by the manufacturer. The AAIB points out that it "is not clear whether this bearing degradation is the result of a new root cause or a previously unidentified failure mode."
Immediately after the accident CHC instituted fleet-wide inspection of all Hums data and conducted new borescope inspections, reduced the time between Hums download and analysis to five hours, and added another assurance check. Sikorsky is encouraging all operators to use the Hums Tail Gearbox Bearing Energy Tool at the ground station, which will detect TRPCS bearing degradation, as often as possible.