US regulators have disclosed that the most recent incident involving failure of a Swiss Airbus A220 engine occurred with a powerplant which had accumulated more than 300 cycles.
Two earlier Swiss A220-300 incidents involved low-cycle Pratt & Whitney PW1500G engines – the failures occurring at 154 and 230 cycles.
Checks on PW1500G engines subsequently focused on engines with fewer than 300 cycles.
But the US Federal Aviation Administration states that a third Swiss A220 engine failure, on 15 October, occurred to an engine with 1,654 cycles since new – although within 300 cycles following installation of a certain version of electronic engine-control software.
The regulator adds that, following the previous checks order, cracks were discovered on the stage-one rotor of the low-pressure compressor on two other affected engines.
These cracks were found on rotors fitted on 'zero-time' spare engines, fitted to aircraft which were already in service, with fewer than 50 cycles since new.
Owing to these findings the FAA is continuing to require inspection of stage-one rotors within 50 cycles for certain engines but reduce the compliance time to 15 cycles for zero-time spare engines.
The engine manufacturer has recommended that checks should be extended to engines which have accumulated up to 300 cycles since installation of the affected engine-control software – known as version 2.11.7 or 2.11.8.
Under the prior check regime operators had to inspect the low-pressure compressor's inlet guide vane stem for misalignment.
But these checks have not detected any abnormalities. No further inspections of the inlet guide vane stem will be required in the FAA's revised directive, because the regulator has accepted the manufacturer's determination that the stem alignment is "not linked" to the condition associated with the rotor failures.
Similarly-designed Pratt & Whitney PW1900G engines, used on the Embraer E2 family, remain subject to checks under the new directive.