The United Airlines 777 that suffered an uncontained engine failure this afternoon over Broomfield, Colorado, was the third oldest 777 in operation. The aircraft, tail number N772UA, first flew in 1994 and was delivered to United in September, 1995, three months after the 777 made its first commercial flight for the airline that June.
United Airlines Flight 328 took off from Denver International Airport at 1 p.m. bound for Honolulu and was climbing through 12,000 feet altitude when its right engine suffered an uncontained engine failure, with internal parts breaking through the external casing and sending pieces of it flying.
One Broomfield homeowner got a close call, however, when the ten-foot-wide circular section of engine cowling came within feet of crushing either the house or the pickup truck and RV parked next to it:
Other large pieces landed on an athletic field in a nearby park:
UA328’s engines were Pratt & Whitney PW4000s, each 16 feet long, weighing 16,000 pounds, and capable of generating more than 90,000 foot-pounds of thrust. Turbofan engines rarely fail in flight, and even when they do, they are designed such that pieces of the engine will be contained within the surrounding cowling. But the stacks of fan blades that heat and compress air to generate thrust must handle extremely high energies, and if one comes apart due to accumulated mechanical stress it can shatter, spewing a shotgun-like blast of metal fragments that in turn can destroy neighboring blades in a cascading fashion.
Such uncontained engine failures can be extremely dangerous, as flying pieces can hit fuel tanks, shred control surfaces, sever hydraulic lines, pierce an aircraft’s pressure hull, or hit passengers or crew. In 2018, a passenger aboard a Southwest 737 was killed when one of the plane’s engines exploded and a piece of debris shattered her window; the resulting depressurization caused the top half of her body to be sucked out through the breach.
Investigators from the National Transportation Safety Board will collect as many pieces as possible from the ground as well as examine the portion of the engine that remained attached and study flight data recorders in order to determine what caused the catastrophic failure. It’s not currently known if the engine that failed was the original delivered with the plane to United in 1995, but if so its maintenance history will receive special scrutiny. Repeated stresses over time can cause microscopic fractures within metal that will eventually propagate and ultimately break if not detected in time.
Three years ago, an eerily similar event happened to a sister aircraft of the 777 involved in Saturday’s accident. An aircraft with the tail number N773UA, operating as United flight 1175, was also en route to Honolulu when it suffered an uncontained failure of its right engine that resulted in its cowling getting ripped off:
In that accident, the plane was 40 minutes from the end of its flight, and landed safely in Honolulu. The NTSB later determined that one of the fan blades had suffered a fatigue crack that had grown over time despite repeated inspections. Built in 1996, the engine had been installed on the plane in 2015 and had 77,593 hours in operation since new.
That plane was the fourth 777 off Boeing’s production line; N772UA was the fifth. Both aircraft were delivered to United on the same day: September 29, 1995.
Update: Saturday’s incident was actually the third uncontained engine failure aboard a 777 since 2018, and the issue has prompted both the FAA and Japan to scrutinize the aircraft, as explained in this followup post.
This post has been updated.