Broken fan blade on United engine was last inspected in 2016

The fan blade that broke on a United Airlines plane over suburban Denver last month failed well before it was required to receive its next routine inspection, investigators said Friday.

The blade that fractured, triggering the Feb. 20 failure, had 2,979 cycles—each of which represents a flight or any time the engine was started—since its last inspection in 2016, the National Transportation Safety Board said in an investigative update.

The Federal Aviation Administration, after a similar failure in 2018, had ordered that blades on the engine be inspected after every 6,500 cycles. Investigators found multiple secondary cracks and are examining the composition of the metal for signs of weakness, they said.

The area at the front of the Pratt & Whitney PW4000 engine on the Boeing Co. 777-200, known as the inlet and cowling, fractured into pieces after the fan blade broke. The failure occurred shortly after takeoff on the Hawaii-bound flight. Metal shards rained down on neighborhoods and caused minor damage to the plane.

The plane returned to Denver International Airport for a safe landing and no injuries were reported.

After the earlier fan-blade failure on another United flight near Hawaii in 2018, technicians at Pratt & Whitney, a division of Raytheon Technologies Corp., reexamined the images that were created during the 2016 inspection, NTSB said.

Investigators didn’t reveal whether they’ve examined the earlier inspection data to determine whether a crack could have been present at the time.

The NTSB will attempt to determine whether the crack expanded faster than anticipated or was missed during the previous inspection.

Images on social media showed the heavily damaged engine on fire, but investigators said the damage from the flames was primarily contained to components around the turbine.

A valve that blocks fuel flow to the engine during such emergencies was found to have been closed and there was no evidence that the fire had been fed by jet fuel, the investigators said. Engines contain hydraulic fluid and other flammable materials.

The cockpit engine fire warning continued until the plane was lined up on the runway to make an emergency landing. Shortly after touchdown, the engine reignited briefly until it was doused by airport firefighters, NTSB said.

The FAA in an emergency safety order three days after the incident directed United to immediately inspect all fan blades on its 777s, effectively grounding the planes. Japan and South Korea, the other nations in which airlines operate 777s with those engines, took similar action.

After the incident, Pratt & Whitney told operators of PW4000 engines with 112-inch fan sections to conduct inspections within 1,000 cycles, NTSB said.

The Denver failure has raised questions about whether previous orders by the FAA and Pratt & Whitney to inspect blades on this engine model were adequate, and about the design of the front of the turbine that broke apart after the failure.