Yesterday aircraft manufacturer Sikorsky released an Alert Service Bulletin (ASB) detailing a series of mandatory inspection requirements that had to be completed on all S92 airframes prior to their next flight.

The ASB was issued as a result of an investigation into the West Franklin incident, where a CHC S92 helicopter span on the helideck while attempting to land on the platform. The incident damaged the aircrafts wheels and took gouges out of the installations helideck, with the aircraft finally ocming to a stop a mere foot or two from the edge.

The ASB from Sikorsky required operators to remove the Tail Rotor Pitch Change Shaft (TRPCS) and bearing assembly and inspect them for signs of wear and damage. The document made specific reference to a component called a Gask-O-Seal (used to prevent oil leaking from the part) and it was suspected by Oil and Gas People that this was the primary cause of the incident.

Today the UK’s Air Accident Investigation Branch (AAIB) released it’s preliminary findings on the West Franklin incident producing the following explanation:

The technical investigation focussed on the tail rotor and associated components. Once the panels were removed it was immediately apparent that the tail rotor servo piston was damaged. The servo was removed and revealed that the tail rotor pitch change shaft (TRPCS) double row angular contact bearing was in a severely distressed condition (Figure 1).

TRPCS double row angular contact bearing from G-WNSR

“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 (0.5 in), such that the tail rotor driveshaft imparted a torsional load to the tail rotor servo. This torsional load caused the primary piston rod to fracture inside the servo. Due to the failure of the primary piston, the secondary piston sleeve separated axially from the primary piston adjacent to the link  tting, with the consequential total loss of control of the tail rotor.

The components were shipped to the helicopter manufacturer for forensic analysis. Initial  ndings indicate that the failure of this speci c bearing was rapid; a period of 4.5 hours had elapsed from the  rst exceedance of the relevant bearing condition indicator recorded on the operator’s Health and Usage Monitoring System (HUMS) to the point of failure.

The West Framklin incident bears similarities to a separate incident where an S92 lost tail rotor control during a hover and an Airworthiness Directive was released by the American Federal Aviation Authority (FAA) in November 2011 in response to the earlier incident.”