FAA, Dutch Investigators Disagree On PW4000 Issue Risk

The FAA’s review of a rare Pratt & Whitney PW4000 failure scenario linked to a 2021 incident that prompted a related safety recommendation from Dutch experts reaffirmed the U.S. agency’s earlier conclusion that the risks fall short of requiring a mandatory fix.

The Dutch safety board in a May 2023 report urged the FAA to re-examine its decision not to mandate a 1993 Pratt service bulletin that addresses a known issue on PW4000 engines . Prolonged exposure to high temperatures can cause PW4000 high pressure turbine (HPT) stage 2 blade outer air seals to deteriorate. This can cause related parts, notably a transition duct, to deteriorate and break apart.

Pratt recommended two related fixes: the March 1993 bulletin to help direct more cool air into the ducts and a November 1993 one recommending installation of improved parts to prevent seal failure. The FAA mandated the second Pratt bulletin via an airworthiness directive (AD), but not the first.

A 2023 Dutch safety board report on a 2021 Longtail Aviation Boeing 747-400 PW4056 engine failure shortly after takeoff from Maastricht Aachen Airport found the problem engine had not undergone the recommended cooling modifications.

While the flight crew shut the damaged engine down and made an uneventful return to Maastricht, debris from the engine rained down on a nearby village, causing injuries and property damage. Dutch investigators expressed concern that regulators may not consider collateral damage risks when evaluating the seriousness of equipment failures near populated areas.

The FAA during the probe told the board that the PW4000 seal duct deterioration issue did not meet the risk threshold of an AD. The Dutch board urged the FAA to “reconsider ... in light of third-party risk” whether the cooling modifications should be mandated.

The FAA’s review prompted a late July non-mandatory safety airworthiness information bulletin (SAIB).

“Although the FAA determined that the failure mode ... does not rise to the level of a safety concern warranting an AD ... it is recognized that incorporating the [cooling] modifications ... plays an important role” in protecting the HPT Stage 2 seals, the July 22 SAIB said. “Therefore, the FAA recommends that operators who have not already done so accomplish the [recommended] actions.”

The European Union Aviation Safety Agency quickly adopted the SAIB.

The FAA declined to detail what led to its conclusion.

“The FAA uses its continued operational safety process to determine the appropriate way to address safety issues,” an agency statement said.