Pratt & Whitney Canada (PWC) PT6A-114A engines fitted with compressor turbine vane rings that have been repaired in accordance with the United States Federal Aviation Administration‑approved scheme STI 72-50-254 have a significantly increased likelihood of CMSX-6 compressor turbine blade fracture and subsequent failure of the engine compared to those engines fitted with PWC‑manufactured compressor turbine vane rings.
On 29 March 2017, Pratt & Whitney Canada (PWC) released Service Information Letter (SIL) PT6A-252 ‘Effect on CT Blades of CT Vanes Repaired Using Non-P&WC Approved Processes.’ The SIL advised operators of the heightened risk of high cycle fatigue fracture of single-crystal compressor turbine (CT) blades when used in conjunction with CT vanes that had been repaired using ‘non-P&WC approved processes.’
The ATSB is satisfied that, with adherence to the advice contained within SIL PT6A-252, the likelihood of single-crystal compressor turbine blade failure involving PWC PT6A 114A engines will be reduced.
As a result of this occurrence and other similar incidents, the national aviation administrator, Transport Canada, has taken the following proactive safety action:
Airworthiness Directive CF-2019-30 was released on 2 September 2019, linking the low-time fatigue-fracture of CMSX-6 single crystal compressor turbine blade failures and the use of compressor turbine vane rings that had been repaired in accordance with United States Federal Aviation Administration major repair specification STI 72-50-254. The AD required:
On 17 December 2019, Revision 1 to AD CF-2019-30 was subsequently released by Transport Canada. The new revision contained some minor amendments to the context, however the corrective actions and compliance times remained unchanged.
The ATSB is satisfied that compliance with AD CF-2019-30 will reduce the likelihood of CMSX-6 single-crystal compressor turbine blade failure within PT6A-114A engines to an acceptable level.
The operator advised that by October 2017, they had adopted the recommendations contained in Pratt & Whitney Service Information Letter, SIL PT6A-252. Their Cessna 208B fleet was reviewed, and any STI 72-50-254 compressor turbine vane rings that had been fitted to their PT6A-114A engines, were removed and replaced with compressor turbine vanes manufactured by Pratt & Whitney Canada.
The ATSB notes that adherence to the advice contained within SIL PT6A-252 reduces the likelihood of single-crystal compressor turbine blade failure involving PWC PT6A 114A engines.
The United States Federal Aviation Administration released a Notice of Proposed Rulemaking (NPRM) FAA‑2020-0692 on 17 August 2020, as an advisement of the regulatory agency’s consideration to legislate Transport Canada’s AD CF-2019-30. Industry comment to the proposed NPRM closed on 1 October 2020.
The ATSB welcomes the initiation of safety action by the Federal Aviation Administration to consider legislation of the Transport Canada AD (detailed in the section titled Proactive safety action taken by Transport Canada). However, with closure of the NPRM on 1 October 2020, at the time of writing, no decision has been made whether the AD will be adopted. As such, the safety issue will remain open, pending safety action.
The ATSB also notes that proactive safety action taken by the Civil Aviation Safety Authority (detailed in the section titled Proactive safety action taken by the Civil Aviation Safety Authority) in adopting the Transport Canada AD, reduces the likelihood of CMSX-6 single-crystal compressor turbine blade failure involving Australian-operated PT6A-114A engines to an acceptable level.
On 16 March 2017, concerned by reports of CMSX-6 single-crystal compressor turbine blade fractures, the holder of the major repair specification for the compressor turbine vane ring, Southwest Turbine Inc., ceased conducting repairs on vane rings for fitment into Pratt and Whitney Canada (PWC) PT6A-114A engines.
Following meetings with the United States Federal Aviation Administration (FAA), Southwest Turbine Inc. completed and subsequently submitted to the US FAA a detailed engineering analysis of their compressor turbine vane ring repair, STI 72-50-254, that compared the geometry of their repair against the vane rings produced by the engine manufacturer (PWC).
The ATSB notes that the action taken by Southwest Turbine Inc. to cease conducting repairs on vane rings fitted to PT6A‑114A engines, reduces the risk of STI 72-50-254‑related CMSX-6 compressor turbine blade fracture.
Transport Canada’s AD CF-2019-30 was published on the Civil Aviation Safety Authority website as part of AD bi‑weekly 17/2019. As the AD has been issued by the national aviation authority of the state of design for the affected aeronautical product (Transport Canada) it was automatically accepted under the Civil Aviation Safety Regulations (CASR) Part 39.
Australian aircraft covered by the AD are subject to the requirement of CASR 39, sub-section 003, and are therefore required to comply with the AD.
The ATSB is satisfied that compliance with AD CF-2019-30 will reduce the likelihood of CMSX-6 single-crystal compressor turbine blade failure involving Australian-operated PT6A-114A engines to an acceptable level.
