72-002 - Rolls-Royce/Allison 250-C20 series engines fitted with a 3rd stage turbine wheel P/N 23065818, or 4th stage turbine wheel P/N 23055944.

Applicability:           

All Rolls-Royce/Allison 250-C20 series engines fitted with a 3 ^rd stage turbine wheel P/N 23065818, or 4 ^th stage turbine wheel P/N 23055944. 

Purpose:      

To advise affected helicopter operators and maintainers of a recent uncontained failure of a 4 ^th stage turbine wheel P/N 23055944 on a 250-C20R/2 engine installed on a MD520N helicopter. In the most current revision of Rolls-Royce CEB A-1400 (CEB A-72-4095) the OEM advises that testing and analysis shows that operation in the N2 speed avoidance range is the most possible cause.

Background:          

This Continuing Airworthiness Notice (CAN) is prompted by a recent loss of engine power on a MD520N helicopter.

The aircraft was on the ground at 100% N2/Nr preparing for a maintenance test flight after replacement of a NOTAR fan blade. The pilot increased collective for take-off and the engines failed. Some debris exited the engine. An inspection of the aircraft did not reveal any damage to the airframe. The engine was subsequently removed and taken to a maintenance facility for investigation. An engine tear down revealed that a blade had separated from the 4 ^th stage turbine wheel P/N 23055944.

Rolls-Royce has advised that engines fitted with affected P/N 3 ^rd and 4 ^th stage turbine wheels are susceptible to failure which could result in engine failure and damage to the aircraft. In the most current revision of Rolls-Royce CEB A-1400 (CEB A-72-4095) the OEM advises that testing and analysis shows that operation in the N2 speed avoidance range is the most possible cause.

In March 2015 the FAA issued AD 2015-02-22 applicable to 3 ^rd stage turbine wheel P/N 23065818 and 4 ^th stage turbine wheel P/N 23055944. The AD mandates a visual inspection and a fluorescent-penetrant inspection (FPI) on the affected turbine wheels to check for cracks at the trailing edge of the turbine blades near the fillet at the rim. The inspection is required within 1750 hours since last inspection, and thereafter re-inspect every 1750 hours since last inspection, and any time the power turbine is disassembled. If any cracks are detected the turbine wheel must be replaced.

Rolls-Royce advised that a redesigned 4 ^th stage turbine wheel was released in December 2015 and from March 2016 these turbine wheels are installed in new engines at manufacture.

Rolls-Royce is in the process of developing a service bulletin which will introduce the redesigned 4 ^th stage turbine wheel as a replacement part.

Please Turn Over

Recommendation:

The CAA strongly recommends the following:

Note:

FAA 2015-02-22 pertains to the subject of this Continuing Airworthiness Notice. Compliance with the FAA AD is mandatory. Any time the power turbine is disassembled, comply with the requirements of FAA AD 2015-02-22.

Further CAA Airworthiness Directive action may follow when Rolls-Royce issues the SB which introduces the re-designed 4 ^th stage turbine wheel.

Enquiries:

Please send enquiries regarding this CAN to:

Owen Olls

Airworthiness Specialist

Email: ollso@caa.govt.nz

Phone: 04 560 9569