Uncontained PW1524G Failure During CS100 Certification Testing
The Transportation Safety Board of Canada (TSB) has recently issued its investigation report, into an uncontained engine failure and fire during certification ground testing of Bombardier CS100 (now the Airbus A220-100) C-FBCS on 29 May 2014 at Montréal International (Mirabel) Airport.
The Aircraft and Engine
Bombardier announced the development of the C Series aircraft in July 2004. Two variants of the C Series aircraft are offered: the 108–125 passenger CS100 and the 130–160 passenger CS300.
The PW1524G is a twin-spool, axial-flow, ultra-high–bypass ratio geared turbofan engine that includes core-mounted angle and main gearboxes. It is part of the PW1000G family of geared turbofan engines that were first run in 2008; the family comprises variants ranging from 15 000 pounds (66.75 kilonewtons [kN]) to 33 000 pounds (146.85 kN) of static thrust. The PW1524G variant, specifically developed for the C Series aircraft, delivers 24 400 pounds (108.5 kN) of thrust.
TSB describe the failure mode as follows:
The investigation determined that “heat soaking,” as a result of insufficient cooldown, caused the seal of a bearing oil feed tube to fail. It was determined that the engine had been shut down after high power operation, without sufficient time for its internal temperatures to reduce at lower power. As a result, when the seal failed, it allowed engine oil to mix with the turbine rotor’s cooling air flow.
The resulting air/oil mixture ignited due to high ambient temperatures…
…and the ensuing combustion caused the entire [Low Pressure] turbine rotor stage to fail.
This resulted in major damage to the engine, nacelle and wing.
This damage included:
The [failed engine's] left side thrust link was severed in line with the LPT [Low Pressure Turbine] plane; some engine-air and -oil lines, as well as some electrical wires and fire detection loops, were damaged or severed in the vicinity of the LPT case breach.
The left-wing structure sustained major debris impact damage when a segment of the first-stage LPT rotor disk 28 inches long penetrated the wing’s centre fuel tank.
The impact created a span-wise gash 33 inches long and 3 inches wide in the carbon composite lower skin plank, inboard of the engine…with a total delamination area of 16 × 37 inches.
The turbine disk segment then partially exited through the upper wing plank, where it remained stuck, creating a hole approximately 13 × 7 inches, with a total delamination area of 21 × 10 inches. Signs of burning were found around the hole, although the fuel contained in the tank did not ignite.
The TSB explain that:
From its initial release until the issuance of the engine’s type certificate by TC [Transport Canada], the Pratt & Whitney PW1500G installation and operating manual (IOM) specified a 10-minute cooling period before shutdown in order to minimize the potential for oil coking in the main engine bearing compartments and to mitigate a bowed rotor start condition. This cooling period was determined based on the results of testing carried out on development engines and demonstrated compliance with the PW1500G’s certification basis.
[PWC] issued a Restriction and/or Special Instruction (RSI) with cooling procedures for their engines before shutdown, with alternate solutions for hot shutdowns.
Bombardier interpreted the alternate solutions in the RSI as an alternative equal to the other shutdown options contained in the RSI. This resulted in the engine being exposed to one or more hot shutdowns, which led to heat soaking beyond the design criteria of the bearing oil feed tube’s seal.
TSB Findings as to Causes and Contributing Factors
The TSB report that:
Following the occurrence, Bombardier grounded the C Series test aircraft fleet until the cause of the occurrence could be clearly established. For its part, Pratt & Whitney proposed a plan to return to flight which included an enhanced seal, a revised cool-down procedure, and other measures to monitor engine temperatures and prevent hot shutdowns. Further, production engines will feature an enhanced oil supply tube and a cooling airflow configuration that will physically separate the turbine rotor airflow from the bearing compartment to eliminate the possibility of recurrence.
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