On 23rd March 2012, the US Federal Aviation Administration (FAA) adopted a new airworthiness directive (AD) that requires operators of 954 Pratt & Whitney PW4000-series turbofan engines on US-registered wide body aircraft to retire the engines' first stage high pressure turbine (HPT) hubs prematurely.
This comes after Pratt & Whitney re-evaluated the low-cycle fatigue analysis of the Boeing 757 engine, as well as for other "similar" engine designs including the PW4000.
FAA said, "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." FAA farther said, "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."
According to FAA, if cracks develop, this will lead to failure of the hub, causing engine failure and aircraft damage.
If the FAA directive is approved, the affected parts that had lifetimes of either 15,000 cycles since new (CSN) or 20,000 CSN will need to be retired at 13,700 or 18,000 CSN, respectively. This will cost about $23 million in "lost life value" for the fleet.
FAA said 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.