The US Federal Aviation Administration (FAA) on 23 March will propose that operators of 954 Pratt & Whitney PW4000-series turbofan engines on US-registered widebody aircraft retire the engines' first stage high pressure turbine (HPT) hubs prematurely, costing an estimated $23 million in "lost life value" for the fleet.
The action stems from the PW2000 "field event" that led Pratt & Whitney to re-evaluate the low-cycle fatigue analysis of the Boeing 757 engine, as well as for other "similar" engine designs including the PW4000, says the FAA in the proposed airworthiness directive (AD).
"Pratt & Whitney's updated analysis indicated that the original grain size requirement specified on the HPT stage 1 front hub design drawing was too large, and may not be sufficient to meet published life limits," says the FAA. "Although we have not received any reports of cracks, parts with the larger grain size may initiate a crack prior to the published life limits."
If cracks were to develop, the FAA says the hub could fail, leading to an uncontained engine failure and "damage to the aircraft".
The affected parts, which have an as-delivered lifetime of either 15,000 cycles since new (CSN) or 20,000 CSN, must now be retired at 13,700 or 18,000 CSN, respectively, assuming the AD is approved as proposed.
The FAA says there are 605 PW4000 engines on the US registry that use a 20,000 CSN life limit for the part, and 349 engines with a 15,000 CSN limit.