Corrosion From Inactivity Flagged On Pratt-powered 777 Engine Parts

United Airlines
Pratt & Whitney PW4000-powered United Airlines 777 in short-term storage.
Credit: Sean Broderick / AWST

Inactivity during the fleet’s year-plus grounding is leading to corrosion on Pratt & Whitey PW4000-series air/oil heat exchangers, prompting the FAA to mandate inspections before affected Boeing 777s return to service.  

A direct final rule made public April 14 outlines the issue and orders inspections at least 30 days before the planned first flight of affected aircraft. The Pratt-powered 777 fleet has been grounded since February 2021, when a fractured fan blade triggered an inflight engine shutdown (IFSD)—the third such incident in three years. Pratt, Boeing, and the FAA have been developing new fan blade inspections and nacelle modifications that address risks spotlighted in the occurrences, including the shedding of nacelle parts due to damage caused by the blade failure.  

The latest issue came to light following an IFSD during a November 2021 ferry flight, the FAA directive said. The agency did not identify the operator involved, but Aviation Week’s Tracked Aircraft Utilization (TAU) shows United Airlines flew the only Pratt-powered 777 cycles that month.  

Pratt traced the failure to an oil leak caused by corrosion, the FAA said. More digging turned up 19 instances of air/oil heat exchanger oil leaks in the previous year—which included 10 months of no activity due to the grounding—compared to an historical average of about four annually, the agency added. 

“The lack of operation may have exacerbated corrosion on the air/oil heat exchanger,” the FAA directive explained. “Both engines installed on the airplane may develop leaks on the air/oil heat exchangers.” 

Pratt on Feb. 15 issued a service bulletin with recommended procedures for inspecting the units and storing them, if necessary. The FAA directive is based on Pratt’s bulletin.  

United operated the most Pratt-powered 777s when the grounding went into effect, with 52. It plans to return them to service as soon as the aircraft meet FAA’s requirements.  

The Chicago-based carrier was the only U.S.-based operator of the aircraft when the fleet was grounded, but Eastern Airlines has said it plans to add some to its fleet.  

Several other operators, including Japan Airlines, have indicated they will not bring their aircraft back into service. Fleet Discovery shows 115 Pratt-powered 777s in the global fleet. 

The agency in February issued three airworthiness directives outlining the steps for getting the widebody twins back into service, including a new fan-blade inspection protocol. Maintenance instructions on two issues—installation of thrust reverser debris shields and inlet modifications—still needed to be finalized when the directives were issued. United recently pushed back the phase-in of the first modified aircraft until the second half of May. CNBC was first to report the update, which United shared in an internal memo.  

Many of United’s Pratt-powered 777s have been active in the last six months, TAU shows. Airframes have been ferried in and out of storage locations, and at least seven are in China or Hong Kong, where long-time United heavy maintenance provider Haeco has facilities. 


Sean Broderick

Senior Air Transport & Safety Editor Sean Broderick covers aviation safety, MRO, and the airline business from Aviation Week Network's Washington, D.C. office.