Debris from a United Airlines flight that made an emergency landing in Denver. (Broomfield Police Department)

United Airlines said Sunday that it will “voluntarily and temporarily” ground 24 aircraft powered by a certain type of engine after one of its planes suffered a catastrophic engine failure over Denver on Saturday.

The decision comes as federal aviation regulators appear to be zeroing in on the jet’s fan blades as a culprit for the drama that played out in Colorado’s skies and in the northwest Denver metro area.

The aircraft that are being removed from United’s service have Pratt & Whitney 4000 series engines. The line of engines is used on many popular, large aircraft types, including the Boeing 747-400, Boeing 777, Boeing 767 and Airbus A330.

United, which has a hub at Denver International Airport, said the aircraft that are being grounded are Boeing 777’s.

A Boeing 777-200 powered by Pratt & Whitney 4077 engines was the kind of aircraft involved in Saturday’s engine failure.

United also operates the Boeing 777-300ER and Boeing 777-200ER, or extended range, which use General Electric GE90 turbofan engines.

United operates dozens of Boeing 777 aircraft on high-demand domestic routes and many international routes, too. The plane is one of the world’s safest and most technologically advanced and as become a popular commercial and freight workhorse.

“We will continue to work closely with regulators to determine any additional steps and expect only a small number of customers to be inconvenienced,” United said in a written statement. “Safety remains our highest priority, which is why our crews take part in extensive training to prepare and manage incidents like UA328. We remain proud of our employees’ professionalism and steadfast dedication to safety every day.”

United Airlines Fight 328, en route from Denver to Honolulu carrying 229 passengers and 10 crew, had just taken off when its right engine failed over Broomfield. Videos of the incident showed the engine ablaze, shimmering and missing most of its nacelle and all of its cowling.

MORE: Four important things to know about the United Airlines flight that rained debris on the Denver area

Debris rained over the Denver area, causing damage but no injuries. The plane landed safely back at Denver International Airport after 25 minutes.

The National Transportation Safety Board is investigating. It said Sunday that its preliminary investigation revealed:

  • One fan blade was fractured near the root
  • An adjacent fan blade was fractured about mid-span
  • A portion of one blade was imbedded in the containment ring

The Federal Aviation Administration on Sunday said it would require immediate, stepped-up inspections of Boeing 777 airplanes equipped with Pratt & Whitney 4000 series engines. It’s likely, the agency said, that planes will be removed from service as a result.

“We received all available safety data following yesterday’s incident,” Administrator Steve Dickson said in a written statement. “Based on the initial information, we concluded that the inspection interval should be stepped up for the hollow fan blades that are unique to this model of engine, used solely on Boeing 777 airplanes.”

Dickson said the FAA is meeting with Boeing and Pratt & Whitney to discuss the situation.

A Boeing 777 taxis at Denver International Airport on July 19, 2019, which was forecast to be the hub’s busiest day ever. (Jesse Paul, The Colorado Sun)

Boeing is recommending that all airlines flying Boeing 777s with Pratt & Whitney 4000-112 engines should be grounded. The company said it supported decisions by the Japan Civil Aviation Bureau and FAA to suspend operations of all 777 aircraft powered by Pratt & Whitney 4000-112 engines. It said there were 69 of the engines in service and another 59 in storage.

“We are working with these regulators as they take actions while these planes are on the ground and further inspections are conducted by Pratt & Whitney,” it said in a statement issued Sunday.

United is the only U.S. airline with the Pratt & Whitney PW4000 in its fleet, the FAA said. United says it currently has 24 of the 777s in service.

Pratt & Whitney has not publicly commented on the situation.

The Associated Press contributed to this report.

Jesse Paul is a Denver-based political reporter and editor at The Colorado Sun, covering the state legislature, Congress and local politics. He is the author of The Unaffiliated newsletter and also occasionally fills in on breaking news coverage....