US aviation regulators on Tuesday ordered inspections of all Pratt & Whitney engines similar to the one that broke up on a Boeing 777 passenger plane over Denver at the weekend, directing the tests be carried out before any return to service.

The spectacular accident, in which an engine burst into flames and scattered debris over a Denver suburb shortly after takeoff for Honolulu, led to scores of Boeing 777s being grounded worldwide. No one was injured.

"US operators of airplanes equipped with certain Pratt & Whitney PW4000 engines (must) inspect these engines before further flight," the Federal Aviation Administration (FAA) said.

The engine maker said it would comply, examining 125 planes with blades similar to those that failed on the Boeing 777, using Thermal Acoustic Imaging (TAI) inspection "to confirm airworthiness."

"Pratt & Whitney is coordinating all actions with Boeing, airline operators and regulators. The safe operation of the fleet is our top priority," the company said in a statement.

The regulator said it was issuing the order "as a result of a fan-blade failure that occurred Saturday on a Boeing 777-200 that had just departed from Denver International Airport."

The FAA explained in a statement that "TAI technology can detect cracks on the interior surfaces of the hollow fan blades, or in areas that cannot be seen during a visual inspection."

Debris from the failed engine of a United Airlines airplane in the Denver suburb of Broomfield
Debris from the failed engine of a United Airlines airplane in the Denver suburb of Broomfield Broomfield Police Department / -

Metal fatigue has emerged as the chief suspect in the engine failure.

FAA chief Steve Dickson said Tuesday it was "fortunate there were no fatalities or injuries."

The near-miss over Denver was a fresh setback for Boeing, which only recently resumed deliveries of the long-grounded 737 MAX following two fatal crashes.

It also raises fresh questions about the FAA, which was roundly attacked for its oversight of Boeing in the certification of the MAX jets, and about whether maintenance was adequate on the plane, aviation experts said.

Even before the Denver incident, US air safety regulators had been weighing stricter inspections on the jets and their Pratt & Whitney engines, US officials said Tuesday.

The FAA reviewed inspection records and maintenance history after a Japan Airlines fan blade incident on December 4 last year "to determine the cause of the fracture and was evaluating whether to adjust blade inspections," an FAA spokesman said Tuesday. The flight landed in Japan without injuries.

Following a February 2018 incident on another United jet, the FAA reviewed 9,000 fan blade inspection reports and issued an airworthiness directive setting new rules on inspections.

Factfile on the Pratt & Whitney 4000-112 engine used by Boeing 777 aircraft.
Factfile on the Pratt & Whitney 4000-112 engine used by Boeing 777 aircraft. AFP / John SAEKI

In a briefing Monday, the National Transportation Safety Board (NTSB) said it was too soon to know if the issue in Denver was similar to those in the Japan Airlines flight, or the February 2018 incident, which involved another Boeing 777 and a Pratt & Whitney engine.

"A preliminary on-scene exam indicates damage consistent with metal fatigue," NTSB Chair Robert Sumwalt told the briefing.

He said two fan blades fractured on the number two engine on the Boeing 777-200 on Saturday. One of them was later found on a soccer field, while the other remained lodged in the engine.

The NTSB also plans to look at the inspection record on the United plane to see "who knew what when, what could have been done and what should have been done," Sumwalt said.

"Fatigue means that you can have a crack in the material and when you load it again and again, the crack slowly grows," said Robert Kielb, a professor at Duke University's school of engineering.

"This is an example of an event where we learn something about the design 20 years after it goes into service, and then we immediately ground the fleet, figure out what's going on and fix it."

In the wake of the Denver incident, Boeing said all 128 777s with Pratt & Whitney engines were grounded.

Besides United, which removed 24 planes from service, affected airlines included Japanese carriers Japan Airlines and All Nippon, and South Korea's Asiana and Korean Air.

On Monday night, a Delta Air Lines flight on a Boeing 757 en route to Seattle from Atlanta was diverted to Salt Lake City "out of an abundance of caution following an indicator warning of a possible problem with one of its engines," a Delta spokesman said.

"The flight landed safely without incident and taxied to the gate without assistance."