United Airlines Flight 328

NTSB narrative

United Airlines flight 328 was climbing through 12,500 ft mean sea level about 5 minutes after departure from Denver International Airport (DEN), Denver, Colorado, when the right engine, a Pratt & Whitney PW4077, sustained a full-length fan blade separation, or fan blade out (FBO) event. This resulted in the subsequent separation of the engine inlet lip skin, fan cowl support beam, and components of the inlet, fan cowls, and thrust reversers (TRs), as well as an engine fire. The flight crew declared an emergency and landed the airplane without incident at the departure airport about 24 minutes after takeoff. There were no injuries to the passengers or crew, and no ground injuries due to debris; however, a vehicle and a residence sustained damage when impacted by the inlet lip skin and fan cowl support beam, respectively. Fan Blade Impact Damage Examination of the engine revealed that the separated fan blade and other fan debris impacted the fan case, which successfully contained the fan blade fragments. Damage to the nacelle inner and outer barrels was observed, and a postaccident evaluation indicated that the displacement wave of the impact resulted in a 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 the fan cowl support beam. The failure of the bulkhead, along with the damage to the inner and outer barrels, allowed these structures, as well as the inlet lip skin, to separate from the engine. Following the separation of the inlet, air loads resulted in the separation of the fan cowls and the fan cowl support beam. Simulation studies indicated that the carbon fiber reinforced plastic (CFRP) honeycomb structure of the event engine inlet and inlet aft bulkhead was unable to dissipate and redistribute the energy of the loads imposed by the FBO event in the same manner as the aluminum structure inlet that was used during certification tests. Separation of the inlet and fan cowls due to an FBO event is not allowed under certification standards, and following this event, Boeing developed modifications to the inlet to ensure that inlets and fan cowls remain in place during an FBO event that may damage the aft bulkhead, inner barrel, or outer barrel and modifications to add strength and ductility to the inlet by incorporating additional metallic structure. Boeing also developed procedures for inspection and repair for moisture ingression damage to the fan cowls, which can degrade the strength of the cowls. These modifications were subsequently mandated by Federal Aviation Administration (FAA) Airworthiness Directives (AD) 2022-06-10 and 2022-06-11, effective April 15, 2022. Additional modifications are expected to the fan cowl. This event was the fourth in-service FBO event due to fatigue cracking recorded for PW4000-powered 777 airplanes and resulted in the most nacelle damage of the four events. In the first event in 2010, approximately 50 percent of the blade airfoil was released. Full-span separations occurred in 2018, 2021, and during this event. Engine Fire Propagation Seconds after the FBO event, the flight crew received a right engine fire warning. The crew completed the engine fire checklist, which included activating the fire switch and discharging both engine fire extinguishing bottles; however, the fire was not arrested and continued to propagate through the engine for the remainder of the flight due to damage the engine sustained during the fan blade out event. Although the cockpit fire warning light extinguished shortly before landing, this was likely the result of thermal damage to the engine fire detection system. The engine fire propagated as the result of several cascading failures following the FBO event. The engine core was subjected to high dynamic loads due to the energy of the initial blade release; the fan blade rubbing against the case, which created rotating torsion loads through the engine core structure; and the continued fan shaft imbalance during the engine run-down, which created rotating bending loads through the core structure. The loading associated with the high dynamic activity of the attached main gearbox (MGB) ultimately resulted in the failure of the “K” flange bolts that attached the MGB to the engine. The remaining “K” flange bolts then fractured, resulting in the total separation of the “K” flange, which allowed hot, compressed gases to escape the engine core and provided an ignition source in the engine nacelle. As the “K” flange was part of the MGB support structure, the failure of the flange also allowed the MGB to rotate and the MGB-mounted servo fuel heater to contact the engine core-mounted fuel oil cooler. As a result of this contact, a high-pressure fuel cavity within the servo fuel heater was fractured open, releasing high-pressure fuel into the nacelle, where it was ignited by the hot, compressed gases that escaped through the “K” flange separation. Pratt & Whitney is evaluating actions to improve the strength of the “K” flange and expects hardware to be available in 2025. The fire spread to the TR lower bifurcation area, burned away the support structure for the nacelle drain access door, and exited the lower aft TR area. The undercowl fire melted the aluminum latch beams at the lower end of each TR and through the TR inner wall and translating sleeves. One of the last components to separate from the airplane was a section of the outboard TR translating sleeve, which was located about 30 miles southeast of the debris associated with the initial FBO event. The burn-through of the TR lower bifurcation area likely occurred within about six to nine minutes of the initial FBO event, though certification standards required that materials in this area withstand fire for a minimum of 15 minutes. Examination of the engine’s fire suppression system revealed that the engine driven hydraulic pump supply shutoff valve failed to close as designed upon the crew’s activation of the engine fire handle due to silicone lubricant contamination of electrical contact components in the valve’s DC motor. The failure of the valve to close allowed a limited amount of hydraulic fluid to leak into the engine compartment and feed the undercowl fire. FAA AD 2022-06-10 and 2022-06-11 required installation of debris shields on the TR inner wall lower bifurcation area, as well as repeated functional checks of the engine driven hydraulic pump supply shutoff valves to ensure proper operation in response to fire switch activation. Fan Blade Fatigue Failure and Inspection Process The separated fan blade was fractured transversely across the chord of the airfoil near the fan hub fairing as the result of a fatigue crack, which originated at the surface of an internal radius in a hollow cavity within the blade. The event blade had accumulated 2,979 cycles since overhaul; at the time of the event, overhaul inspection was required every 6,500 cycles. As part of the overhaul, blades were inspected for both external and internal cracks using a proprietary thermal acoustic imaging (TAI) process. The most recent TAI inspection of the event fan blade occurred about five years before the event, in 2016. Inspection imagery revealed multiple low-level indications, two of which were in the fatigue crack origin area, that were reviewed further and interpreted as being generated by camera sensor noise or loose contamination within the cavity. Given the observed indications and the inspection criteria in place at the time, the blade should have received a second TAI inspection, or the images should have undergone a team review; however, there was no record that either of these occurred, and the blade was approved for continued service. Following an FBO event in 2018 involving another PW4077 engine, the data from the 2016 inspection of the blade involved in this event were reviewed again; once more, the indications were not identified as anomalous and the blade continued in service. Two of the low-level indications identified during the 2016 TAI inspection were likely associated with the fatigue crack that grew to result in the blade failure. The accident blade had accumulated 15,262 cycles since new, which was less than one quarter of the expected life for a nominal blade, and only 2,979 cycles since its last overhaul, less than half the prescribed inspection interval at the time. Metallurgical examination identified two conditions which contributed to the reduced fatigue life of the accident blade: a surface carbon contamination; and a geometric discontinuity that occurred during manufacturing. In assessing fatigue life of this blade relative to the nominal expectation, the reduced fatigue capability from the surface carbon contamination accounted for approximately 2/3 of the difference, and the increased stress from the geometric discontinuity accounted for approximately 1/3 of the difference. Following this event, Pratt & Whitney performed an immediate TAI inspection of the entire fleet before the next flight and issued a service bulletin introducing ultrasonic testing (UT) blade inspections to occur both immediately and at regular intervals. Additionally, the frequency of required TAI inspections was increased from every 6,500 cycles to every 1,000 cycles. The increased inspection interval and the immediate TAI inspection were made mandatory on April 15, 2022, when the FAA issued AD 2022-06-09. Additionally, the new UT inspection that was developed by Pratt & Whitney for the flowpath and midspan areas has shown a capability to detect small cracks that are below the threshold of detectability for the TAI inspection. The blades are now inspected by UT every 275 cycles. Examination of the crack in this event and previous fan blades failure events have shown the growth rates of the fatigue crack, from detectable size to full-wall penetration, are relatively stable and predictable in each case, since the sources for premature fatigue initiation are surface related and do not have a significant impact on growth through the thickness of the blade. The increased TAI inspection interval and the new UT inspections should provide multiple opportunities to detect cracks in the high-stress areas.

Verbatim NTSB analysis and probable cause from the NTSB dataset