N772UA

HISTORY OF FLIGHTOn February 20, 2021, about 1309 mountain standard time, a Boeing 777-222, N772UA, operated by United Airlines (UAL) as flight 328, experienced a right engine fan blade separation and subsequent engine fire shortly after takeoff from Denver International Airport (DEN), Denver, Colorado. The two pilots, eight crew members, and 229 passengers onboard were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 121 scheduled passenger flight.

The airplane departed DEN about 1304 enroute to Daniel K. Inouye International Airport (HNL), Honolulu, Hawaii. The captain was the pilot flying, and the first officer was the pilot monitoring. The pilots reported that preflight weather forecasts indicated moderate turbulence from about 13,000 ft mean sea level (msl) to 23,000 ft msl, and as the airplane climbed through about 12,500 ft msl at an airspeed about 280 knots (kts), they increased engine power in order to minimize the time spent climbing through the altitudes where turbulence was forecast. About 5 to 7 seconds after advancing the throttles, the cockpit voice recorder (CVR) captured a loud “bang,” and the flight data recorder (FDR) showed an uncommanded shutdown of the No. 2 (right) engine. Shortly thereafter, an engine fire warning activated on the engine indicating and crew alerting system (EICAS).

The flight crew declared an emergency with air traffic control (ATC) and completed multiple checklists, including the engine fire checklist. As part of the engine fire checklist, the crew discharged both right engine fire extinguishing bottles; however, the engine fire warning continued to display on the EICAS until shortly before landing. The crew landed the airplane on runway 26 at DEN at 1328 and the airplane was met by aircraft rescue and firefighting (ARFF), which applied water and foaming agent to the right engine for about 40 minutes. The airplane was then towed off the runway, where the passengers disembarked via air stairs and were bussed to the terminal. Figure 1 below, a still image captured from in-flight video recorded by a passenger, shows the damage to the engine nacelle as well as the under-cowl fire about 11 minutes after the fan blade separation.

Figure 1. Still image from passenger in-flight video showing engine nacelle damage and under-cowl fire about 11 minutes after fan blade separation (Courtesy Boeing via YouTube).

At the time of the event, the airplane was over Broomfield, Colorado; multiple pieces of the engine inlet, fan cowls, and thrust reversers separated from the airplane and were found scattered over an area of about 40 acres, including a public park and residential areas. There were no ground injuries reported. PERSONNEL INFORMATIONCaptain

The captain, age 60, held an airline transport pilot certificate with a rating for airplane multiengine land and multiple type ratings, including the B-777. His most recent first-class Federal Aviation Administration (FAA) medical certificate was issued on February 23, 2021. Operator records indicated that the captain had 28,062 total hours of flight experience, including 414 hours on the B-777 in the previous 12 months. His most recent proficiency check was completed on February 5, 2021.

First Officer

The first officer, age 54, held an airline transport pilot certificate with a rating for airplane multiengine land and multiple type ratings, including the B-777. Operator records indicated that the first officer had 18,612 total hours of flight experience, including 355 hours on the B-777 in the previous 12 months. His most recent proficiency check was completed on November 27, 2020. AIRCRAFT INFORMATIONOverview

The Boeing 777-222 is a long range, twin-engine, transport category airplane. The primary wing and fuselage structure is of all metal construction, primarily aluminum alloys. The control surfaces and engine cowlings are of composite construction, which comprises graphite epoxy carbon fiber reinforced plastic (CFRP), fiberglass, or honeycomb sandwich. The incident airplane was manufactured in September 1995.

Engines

The airplane was powered by two Pratt & Whitney (P&W) PW4077 turbofan engines. The right engine was manufactured in 1995 and installed on the accident airplane in August 2016. At the time of the event, the engine had accumulated 12,384 hours and 2,979 cycles since overhaul and 81,768 hours and 15,262 cycles since new.

The PW4077 is a dual-spool, axial flow, high-bypass turbofan engine that features a single-stage, 112-inch diameter fan (low pressure compressor [LPC] 1st stage), a 6-stage LPC, 11-stage high pressure compressor (HPC), annular combustor, a 2-stage high pressure turbine (HPT) that drives the HPC, and a 7-stage low pressure turbine (LPT) that drives the fan and LPC. Each engine is attached to a pylon on its respective wing. The engine inlet is attached to the forward end of the engine, the fan cowls are attached around the center portion of the engine, and the thrust reversers are attached around the aft portion of the engine. (see figure 2.) Engine flanges are identified alphabetically from the front of the engine aft, with the A-flange located where the inlet attaches to the fan case and the T-flange at the aft end of the exhaust case. (see figure 3.)

Figure 2. Engine installation drawing for 777-200 (Source: Boeing).

Figure 3. PW4000 112-inch significant engine flanges (Source: Pratt & Whitney)

Inlet

The engine inlet is a cantilevered structure attached to the forward flange of the engine fan case through the inlet attach ring with 52 bolts. The inlet consists of two concentric cylindrical structures joined together by forward and aft bulkheads (see figure 4). The hollow aluminum lip skin is attached to the forward bulkhead and provides an aerodynamic surface for the leading edge of the inlet and a passage for engine anti-ice air. The inlet aft bulkhead consists of the aluminum inlet attach ring and aluminum outer ring chord with a CFRP honeycomb sandwich composite web. The inlet forward bulkhead consists of the aluminum inner and outer ring chords with a stiffened aluminum web.

The inlet outer barrel comprises three CFRP honeycomb sandwich panels. A section of the outer barrel in the lower right quadrant is comprised of a titanium skin, where the anti-ice exhaust duct is located. The inlet inner barrel is comprised of two CFRP honeycomb sandwich panels. The inner face sheet of the inner barrel is perforated for noise suppression and the outer face sheet is solid.

Figure 4. Inlet cross-section drawing for 777-200 (Source: Boeing).

Fan Cowl

The fan cowl provides an aerodynamic closure around the engine fan cases and the doors open to allow maintenance access to the engine. The CFRP honeycomb sandwich construction cowls are semi-cylindrical doors fastened to four hinges at the upper ends; two on the cowl support beam, one floating hinge, and one hinge on the engine. The fan cowl support beam is a CFRP honeycomb sandwich panel attached at the forward end to the inlet attach ring and to the fan case at the aft end through aluminum fittings. The fan cowls interface with the inlet at the forward edge through a v-blade on the fan cowls that seats in a v-groove on the inlet aft bulkhead. The fan cowls interface with the thrust reversers at the aft edge through a sliding contact seal.

Thrust Reversers

The thrust reversers (TRs) provide an aerodynamic enclosure around the core of the engine, direct the fan exhaust, and actuate to provide reverse thrust during landing. The two semi-cylindrical TR halves comprise three main components; the translating sleeve, the fan duct cowl, and the aft cowl. The CFRP honeycomb sandwich inner wall of the fan duct cowl and the titanium aft cowl enclose the engine core and comprise the fire zone in the TR. The TRs are hinged at the upper end to the pylon and open to provide maintenance access.

The main structural skeleton of the TR consists of the aluminum hinge beam at the upper end, the aluminum torque box at the forward end, the aluminum latch beam at the lower end, and the aluminum aft support ring and titanium aft cowl at the aft end. The CFRP honeycomb sandwich inner wall is connected to the TR at the upper and lower bifurcations. The CFRP honeycomb sandwich translating sleeve forms the outer surface of the TR and the outer wall of the fan duct cowl in the closed position. The translating sleeve slides aft along a mechanism attached to the torque box when actuated for reverse thrust.

Rubber fire seals are installed in each TR half to help contain an undercowl fire within the interior of the fan duct inner wall and aft cowl. The fabric-reinforced silicone rubber seals are installed along the upper and lower bifurcation walls and down the upper aft edge of the aft cowl. Kapton-faced thermal insulation blankets are installed on the upper and lower bifurcations and on the inside of the inner wall in the fire zone to protect the composite structure from radiant engine heat and fire.

Engine Fire Protection and Extinguishing Systems

The B-777 engine fire protection comprised two systems: an engine fire and overheat detection system, and an engine fire extinguishing system.

The engine fire and overheat detection system comprised two detector loops in each engine nacelle. Normally, both loops must detect a fire or overheat condition to cause an engine fire warning or overheat caution message to display on the EICAS. If a fault was detected in one loop, the system automatically switched to single-loop operation. If there were faults in both detector loops, no fire or overheat detection would be provided. The EICAS advisory message DET FIRE ENG (L or R) would be displayed if the engine fire detection system failed.

An engine fire warning would be accompanied by several indications, including an aural fire bell, the illumination of master WARNING lights, an EICAS warning message (FIRE ENG [L or R]), the illumination of the affected engine ...