The US Federal Aviation Administration (FAA) has issued an Emergency Airworthiness Directive for Boeing 777 aircraft powered by the Pratt & Whitney PW4000 engine, in the wake of the incident on United Airlines UA328 on 20th February.
Following the announcement, United Airlines has said it will voluntarily ground its fleet of 24 PW4000 powered Boeing 777s as “an abundance of caution”. The airline is the only operator of PW4000-powered 777 in the United States. Furthermore, ANA and Japan Airlines have removed the affected aircraft from their schedules. A total of 32 Boeing 777 are now grounded in the east-Asian nation.
In a statement issued by the FAA, the airworthiness directive is to “require immediate or stepped-up inspections of Boeing 777 airplanes equipped with certain Pratt & Whitney PW4000 engines”. At present, an investigation into the catastrophic engine failure on flight UA328 is ongoing and the cause is undetermined.
The Emergency Airworthiness Directive comes as a result of the incident involving UA328 not being an isolated incident. United Airlines flight UA328 is the fourth catastrophic failure of a Pratt & Whitney PW4000 engine fitted to a Boeing 777 in five years, three of which have occurred to aircraft operated by United Airlines.
In June 2016, United UA328 (coincidentally the same flight number as the February 2021 incident) experienced a bird strike upon take off, which resulted in the grass along the runway catching fire. In 2018, the number 2 engine of UA1175 had an uncontained fan blade failure, whilst en-route to Honolulu. On 4th December 2020, a Japan Airlines Boeing 777 (flight JL904) suffered a similar uncontained engine failure in one of its PW4000 engines, whilst travelling to Tokyo from Okinawa.
The engines on elderly Boeing 777 airliners are blowing up with worrisome frequency.
In 2018, the No. 11 fan blade on the right engine of a United Airlines 777 broke as the plane approached its destination, Honolulu. Its pieces, traveling at high speed, caused a cascade of failures within the engine’s intricate machinery such that within less than a second, the engine’s cover had blown off, leaving the naked core wobbling as it spun. Flying debris caused two punctures in the fuselage but the plane was able to land safely under the power of its remaining engine.
Last December, a Japan Airlines 777 suffered a similar engine failure after the No. 16 fan blade of its No. 1 engine broke en route between Naha, Japan, and Tokyo. That flight, too, landed safely.
And then on Saturday, another United Airlines 777 suffered an uncontained engine failure as it climbed out of Denver en route to Honolulu. Witnesses on the ground reported hearing an explosion before debris rained down on the town of Broomfield, Colorado. The pilot declared mayday and returned to Denver without further incident.
In all three cases, the aircraft were among the oldest in the worldwide 777 fleet, having been delivered in the first two years after the model was introduced in 1995. While the Pratt & Whitney PW-4000 engines in each case were likely not original to each plane — engines are regularly removed from planes for routine maintenance, then installed on different aircraft — the engines are generally of similar vintage to the aircraft on which they fly. In the 2018 incident, the engine that failed had been built in 1996 and had accumulated 77,593 hours flight hours and 13,921 cycles (combined takeoffs and landings). The blade that failed in the 2020 incident had experienced 43,060 flight hours and 33,518 cycles.
There was a fourth incident involving a PW-4000 engine and a superannuated Boeing on Sunday. A 30-year old Boeing 747 operating as a freighter suffered an uncontained failure of one of its Pratt & Whitney engines shortly after takeoff from Maastricht Airport in the Netherlands at 4:11 p.m. local time. The flight was en route to New York and dumped fuel before landing safely land in Liege, Belgium. Falling debris injured one person on the ground and damaged cars.
In the wake of Saturday’s incident in Colorado, Japan’s transportation ministry grounded the nation’s 777 fleet until further safety measures can be considered. A total of 32 planes operated by Japan Airlines and All Nippon Airways will be affected. Soon after, the FAA issued a directive requiring “immediate or stepped up” inspections of 777s equipped with PW-4000s — and United Airlines then followed suit by grounding the 24 777s in its fleet with those engines:
Patterns in aircraft accidents can be a sign of trouble. While one-off failures might be attributable to a freak coincidence or just bad luck, patterns suggest that a previously unsuspected danger is lurking. When a brand-new 737 MAX airliner crashed in October 2018, it raised eyebrows. When another one crashed four months later, it led to the realization that the plane’s autopilot had been dangerously mis-engineered, and the MAX was grounded worldwide for the next year and a half.
In the case of the exploding 777 engines, the recurring problem does not come out of the blue. It’s well known that as aircraft and engines age, their mechanical parts are subjected to repeated stresses and strains that can lead to microscopic cracks that grow over time. To prevent these from turning into fractures that could destroy a part and endanger an aircraft, the FAA mandates periodic inspection. Pratt & Whitney operates a facility in East Hartford, Connecticut, that examines PW-4000 fan blades using what’s known as non-destructive testing technology.
As expertly described in a July 2020 article by Aerossurance, the NTSB investigation into the 2018 incident found numerous faults with the inspection process. From its final report:
P&W developed [a thermal acoustic imaging] inspection process in about 2005 to be able to inspect the interior surfaces of the hollow core PW4000 fan blade. The records for the TAI inspection in July 2015 as well as an earlier TAI accomplished in March 2010 revealed a thermal indication in the same location as where the LCF crack occurred. The records for the fractured fan blade’s July 2015 TAI inspection was annotated “paint” that, according to the inspector, was consistent with him accepting the indication because he thought it was an issue with the paint.
That is to say, the individual who examined the blade that later failed saw the flaw that would later destroy the engine, but failed to recognize its significance. An earlier NTSB report noted that at the time of the inspection the workshop had a backlog of fan blades and inspectors were being asked to work overtime to deal with it.
The report observed that the facility did not have a formalized training procedure for the inspection process:
[P&W] classified the TAI as a new and emerging technology and therefore did not have to develop a formal program for initial and recurrent training … The 1st shift inspector was trained by the engineers who developed the process and the 2nd shift inspector, who was the one who last inspected the United Airlines fan blade that fractured, was trained by the 1st shift inspector. Both inspectors stated that their training on the TAI was about 40 hours of on-the-job training. In comparison, the certification requirements for the commonly used eddy current and ultrasonic inspections are 40 hours of classroom training and 1,200 and 1,600 hours of practical experience, respectively.
The NTSB report found other shortcomings in the process as well that go beyond the scope of the present article; for those interested, it’s worth a read.
At this point, it’s not definitively understood what was the root cause of the two most recent events. But in both cases, photographs of the damaged engines appear to show one whole fan blade missing, with an adjacent blade partially gone, suggesting at least a strong possibility that they had suffered a fatigue fracture similar to the one that occurred in 2018.
The 777 is not the only Boeing airliner to suffer multiple uncontained engine failures in recent years, by the way. In 2016 and again in 2018, 737s powered by CFM56 engines suffered fatigue cracks that resulted in catastrophic failure. In the 2018 case involving Southwest Airlines Flight 1380, a fragment of the engine hit and destroyed a window, causing a passenger to get partially sucked out of the aircraft and die.
Air-crash investigators are trained to carefully and methodically assess the available evidence before reaching a conclusion. The process can take a year or more. For the public, whose lives are at stake when they board an airplane, a greater sense of urgency is not only understandable but advisable.
This post has been updated to include Japan and the FAA’s orders regarding the 777, as well as Sunday’s 747 incident in the Netherlands.