Investigators Find Metal Fatigue in UAL 777’s Failed Engine

 - February 23, 2021, 10:08 AM
NTSB investigators record the damage to the PW4000-112 engine that failed on a United Airlines Boeing 777-200. (Photo: NTSB)

National Transportation Safety Board investigators have found evidence of fan blade metal fatigue in the Pratt & Whitney PW4000-112 engine that failed in flight on February 20, raining nacelle debris over a mile-long area of a Denver suburb and leading to the grounding of 69 Boeing 777-200s still in service.

Briefing reporters Monday night from Washington, D.C., NTSB chairman Robert Sumwalt said that one of the blades that fractured at its root contained what he called beach marks or crack arrest marks, visible to the naked eye, that can indicate how many cycles the airplane flew before the fatal crack finally propagated to the point of the blade breaking off. The NTSB has flown the piece to Pratt & Whitney headquarters in Connecticut, where engineers began a more detailed inspection on Tuesday.

“Once you put it under a scanning electron microscope, you can actually count the number of cycles since the initiation of that fatigue,” said Sumwalt.

The NTSB’s on-site inspection found one blade fractured at the root, an adjacent blade fractured at about mid-span, and a portion of one embedded in the containment ring. The remainder of the blades showed damage to the tips and leading edges.

Sumwalt said that although investigators found one piece of a fan blade on a soccer field in Broomfield, Colorado, the NTSB does not consider this a case of uncontained failure because the pieces did not breach the containment ring surrounding the engine.

“From a practical point of view from the flying public, it really doesn’t matter,” he commented. “It doesn’t change anything as to whether it was contained or not. It was still an event that we don’t like to see.”

Sumwalt also reported no structural damage to the airframe, even though images showed a large hole in a composite faring adjacent to the engine. Sumwalt wouldn’t speculate about what created the hole, noting only that a piece of metal from the nacelle could fairly easily puncture the fairing.

NTSB officials have sent the airplane’s flight data recorder and cockpit voice recorder to Washington, D.C., for analysis, which Sumwalt said would aid in determining why the fire that broke out did not extinguish after pilots cut the fuel supply. “We do have indications that the fuel to the engine was turned off,” said Sumwalt. “So we will be looking to see exactly what may have continued to propagate a fire.”

As to any possible relationship between this latest PW4000 failure and previous events, such as the 2018 incident in which another of the engines in a United Airlines 777-200 suffered a fan blade failure on approach to Honolulu, Sumwalt indicated that investigators will certainly examine the possibility.

“I think what's important is that we really truly understand the facts, circumstances, and conditions around this particular event before we can compare it to any other events,” he noted. “But certainly we will want to know if there's a similarity.”

In the 2018 incident, investigators determined that a fractured fan blade caused the failure. Last year the NTSB determined that insufficient training for a thermal acoustic imaging (TAI) inspection process developed by Pratt & Whitney led to technicians misdiagnosing a problem with the fan blade that ultimately failed in the 2018 incident. Since then Pratt developed a formal training curriculum for the inspections and the FAA issued an airworthiness directive in March 2019 requiring repetitive inspections of all PW4000s in service. 

This past December yet another case of a PW4000 failure involved a Japan Airlines 777-200 traveling from Naha Airport in Okinawa to Tokyo Narita Airport. That airplane also landed safely after returning to Naha Airport. The investigation into that incident continues as well.