AAIB Details Cause of West Franklin Helicopter Incident

Published in Oil Industry News on Wednesday, 11 January 2017

Graphic for News Item: AAIB Details Cause of West Franklin Helicopter Incident

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 coming to a stop a mere foot or two from the edge.

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 (See picture below).

https://www.instagram.com/p/BPIOpbxhm6u/embed/captioned/?cr=1&v=7“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.”

The AAIB investigation will continue to examine all the operational aspects of this accident and conduct a detailed engineering investigation of the relevant helicopter components. The AAIB will report any signi cant developments as the investigation progresses.

CHC gave the following statement:

“CHC welcome the Air Accident Investigation Branch’s (AAIB) Special Bulletin S1/2017 published today (January 11, 2017) into the offshore event on December 28, 2016.

“CHC has worked closely with the AAIB and aircraft manufacturer Sikorsky. We will continue to support ongoing investigations by the AAIB and the NTSB, as well as Sikorsky into the root cause of the suspected TRPCS Bearing failure.

“We are confident that the measures we have taken and the actions directed by the Sikorsky in their ASB to all operators worldwide will continue to underpin the safety and airworthiness of the S92. The industry has acted swiftly to comply fully with the requirements of the ASB and to return the S92 to safe operations in both O&G and SAR roles worldwide, as soon as it was possible to do so.

“CHC’s top priority is the safety of everyone we carry in our aircraft. We have well established procedures and have been at the heart of developing industry guidelines for HUMS best practice.  We will learn from any lessons to be drawn from this event and have already taken steps to further strengthen procedures and our compliance monitoring, as well as sharing this information with other operators.”

“The offshore helicopter industry has committed to sharing best practice and driving offshore transportation safety ever higher and will continue to do so through HeliOffshore.”



Offshore Helicopter  Health & Safety is No Accident

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