United Airlines Flight 328 was a scheduled domestic passenger flight from Denver to Honolulu, Hawaii on February 20, 2021. The Boeing 777-200 aircraft operating the route suffered engine failure shortly after takeoff
The failure was “contained” but resulted in a debris field at least one mile wide over the Commons Park suburb of Broomfield, Colorado. This debris included the detached cowling. Fortunately, no one on the ground or in the aircraft was injured.
The failed engine was a Pratt & Whitney model 4000-112 turbofan. The Boeing 777-200 has two of these engines. The engine model has 22 fan blades which are hollow and made from titanium.
National Transportation Safety Board Chairman Robert Sumwalt reported:
“Regarding the fan blade that was fractured at the root: A preliminary, on-scene exam indicates damage consistent with metal fatigue.”
The other blade, which broke in the middle, likely did so because of the impact of that first blade. “Basically, it probably got hit as the other piece was separating,” Sumwalt said.
The PW4000-112 fan blade is a wide chord airfoil made of a titanium alloy, about 40.5 inches long and about 22.25 inches wide at the blade tip. A PW4000-112 fan blade can weigh a maximum of 34.85 pounds.
As a result, the FAA issued an Emergency Airworthiness Directive (AD) that requires U.S. operators of airplanes equipped with certain Pratt & Whitney PW4000 engines to inspect these engines before further flight. This includes thermal acoustic image (TAI) inspection of the large titanium fan blades located at the front of each engine. TAI technology can detect cracks on the interior surfaces of the hollow fan blades, or in areas that cannot be seen during a visual inspection.
The previous inspection interval for this engine was 6,500 flight cycles. A flight cycle is defined as one takeoff and landing.