DENVER – Thirty months after a United Airlines flight out of Denver suffered engine failure and debris fell in the Broomfield area, the National Transportation Safety Board released its final report Friday, finding that the right engine fan blade failed. Why came and safety recommendations and design changes after the February 21, 2021 incident.
The final report of the emergency of United Airlines Flight 328 in Colorado also found “inadequate inspection of the blades”, adding that “low-level signs of cracking” were found during an overhaul inspection of the fan blades five years before the incident flight. .
UA Flight 328, a twin-engine Boeing 777 that had just departed DIA for Honolulu, had a blade separated on its right engine, causing multiple failures, resulting in an engine fire while the aircraft was nearing takeoff. After only 5 minutes was passing at an altitude of 12,500 feet.
According to the NTSB, none of the 239 passengers or crew were injured, but a vehicle and home were damaged when part of the engine’s inlet lip skin and fan cowl support beam fell to the ground. No one on the ground was injured.
Investigators collected several parts from United Engine to determine the cause.
In its findings, the NTSB stated that the engine’s fan case contained the separate fan blade, as it is designed to do, but that the explosive impact resulted in deflection of the fan case and contact with the nacelle doors and hinges, “which subsequently resulted in the failure of the inlet aft bulkhead and fan cowl support beam,” the report revealed. “The failure of the bulkhead, along with damage to the inner and outer barrels, allowed these structures, as well as the inlet lip skin, to separate from the engine.”
Under federal aircraft certification standards, “separation of the inlet and fan cowl due to a fan belt out (FBO) event is prohibited, which caused Boeing to implement modifications to the inlet to ensure that during an FBO event the inlet and Fan cowls remain in place.” “Damage the rear bulkhead, inner barrel, or outer barrel and make modifications to add strength and ductility to the inlet by incorporating additional metal structure,” the report said.
Updated inspection procedures were developed to adequately ensure that fan cowls are inspected for moisture damage, which the FAA said can, over time, reduce the cowl’s strength.
According to the report, those inspection procedures were mandated in the summer of 2022 and additional modifications are expected.
The February 2021 failure in Colorado was the fourth time an FBO incident occurred for some Boeing 777 aircraft equipped with Pratt & Whitney 4000 engines, with the Denver incident described as the most damaging.
Responding to the emergency in the Denver metro skies, the NTSB report says the UA 328 flight crew followed emergency checklist procedures, including activating the engine’s dual fire extinguishers, but as seen in video shared by passengers , The flames continued during the flight. This was due to the extent of engine damage caused by the fan blade out incident.
Broomfield PD
Debris from the plane fell across a wide area of Broomfield, affecting homes and property in the Northmoor and Red Leaf neighborhoods. Parts were seen scattered in parks, lawns and rooftops. An engine intake cowl fell onto a truck, crushing the roof.
The NTSB report outlines a series of events that resulted in the engine fire, including damage that began with the initial loss of a fan blade, which created more energy rubbing against the case, “which caused the engine to fire.” Rotational torsion loads are generated through the core structure; And the fan shaft imbalance continued during engine run-down, causing rotational bending loads through the core structure,” it said.
The NTSB revealed that the fire was exacerbated by a small amount of hydraulic fluid leaking into the engine area, which caused the fire to erupt. When the pilots activated the engine fire handle, the engine-driven hydraulic pump shutoff valve should have closed, but the investigation found that it failed as a result of “silicone lubricant contamination of electrical contact components in the valve’s DC motor”. . Said in the final report.
In identifying other causes behind the fire, the report also pointed to the failure of the flange bolt that connects the main gearbox to the engine. The flange bolts attaching the engine, which had not initially failed, eventually failed, resulting in complete separation of the “K” flange, which allowed hot, compressed gases to escape out of the engine core and into the engine nacelle. Provided an ignition source,” the report said.
Engine manufacturer Pratt & Whitney is expected to make improvements to strengthen the flanges with components available in 2025, following the United Airlines Flight 328 incident.
Broomfield police continue to remove debris from the United Airlines flight Sunday
Fan blade inspection
In 2016, during a United Airlines inspection of the same fan blade, thermal acoustic imaging (TAI) found “several low-level signals, two of which were in the fatigue crack origin area,” the NTSB found.
During that inspection, it was concluded that what was indicated on the TAI was either camera noise or loose contamination in the fan blade cavity.
The report said the failure of a fan blade during a United Airlines flight in February 2021 was the result of a fatigue crack that started “on the surface of the inner radius in a hollow cavity within the blade.”
Then in 2018, following the failure of a separate fan blade in the same model of Pratt & Whitney engine, UA reviewed the 2016 data from the incident blade and again determined that “the signals were not identified as unusual ” The NTSB report said the fan blade failed during the February 2021 flight due to fatigue built up over the years.
NTSB
“Viewed from the front, the fan blade appears to have two broken blades and damage to the remaining intact blade,” the NTSB report said.
Citing a possible cause, the NTSB pointed to engine manufacturer Pratt & Whitney’s recommended schedule of blade inspections by TAI, which was increased from 6,500 to 1,000 cycles in April 2022.
Additionally, the fan blades are inspected by ultrasonic testing (UT) every 275 cycles.
Contributing to the engine damage was a failure in the engine inlet design that should have included a fan blade as it broke according to the NTSB report that “failed to ensure that the inlet could dissipate energy adequately, and therefore further “Can limit the damage caused by an in-flight fan blade out incident.”
The NTSB ultimately pointed to a failure of the engine’s flange, which allowed flammable liquids to fuel the fire, which “could not be extinguished,” the report said.
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