The Federal Aviation Administration on Tuesday mandated fan blade inspections before some Boeing 777 jetliners are allowed to fly again, responding to a harrowing flight Saturday from Denver that fit into a pattern of engine-related failures.

The emergency order covers 777s with Pratt & Whitney engines flown by United Airlines, which had already voluntarily grounded 24 of the jets after two broken fan blades sent hunks of metal into the body of one of its planes — as well as into yards, a home, a truck and a soccer field below.

The planes also were taken out of service in South Korea and Japan, where a similar engine failure with a Japan Airlines jet in December forced pilots to make a sudden return to Okinawa. Boeing recommended that airlines ground jets with the same kind of engine.

The FAA said Tuesday’s order was an interim step, with more action expected to follow. The agency noted that although United Flight 328 landed safely, the fan blade failure “resulted in damage to the engine, an in-flight fire, and damage to the airplane,” creating unsafe conditions.

“As these required inspections proceed, the FAA will review the results on a rolling basis,” the agency said in a statement, adding that it may also revise the requirements based on the investigation.

Boeing said Tuesday evening that it supports the move and “will work with our customers through the process.” The company estimates there are 128 777s with the affected engines globally, with 69 in service and the rest in storage.

United said it would comply with the order “to ensure all 52 of the impacted aircraft in our fleet meet our rigorous safety standards.”

In February 2018, pilots on another United 777 — also headed to Honolulu with the same model engine — had to make an emergency landing after a fan blade broke and large pieces of engine coverings known as fan cowls sheared off. The National Transportation Safety Board found that the fracture was probably the result of problems with Pratt & Whitney’s inspection process, which the company said it has since improved.

Following the 2018 incident, the FAA ordered new and recurring inspections on the engines, based on how many times they had been used. The FAA said “these thresholds provide an acceptable level of safety,” according to the 2019 order. On Sunday, FAA Administrator Stephen Dickson said the agency concluded that “the inspection interval should be stepped up.”

Tuesday’s order, known as an emergency airworthiness directive, requires an inspection process known as thermal acoustic imaging, or TAI, which is used to find cracks. It uses sound energy to create heat, and the resulting images are examined by inspectors, according to the NTSB.

“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,” the FAA said in its statement.

NTSB Chairman Robert L. Sumwalt said this week that a preliminary examination of one fan blade in Saturday’s incident “indicates damage consistent with metal fatigue,” though the cause remained unclear. A team of investigators is analyzing engine-related debris that had fallen on the Denver suburb of Broomfield and is pouring through records to determine the age of the fan blades and when they were last inspected, Sumwalt said. He said earlier incidents with the Pratt & Whitney engines will be part of its investigation.

Sumwalt also said investigators are eager to understand what caused a fan cowl to rip off the plane, a critical issue that outside aviation safety experts said is part of a pattern of similar incidents in recent years.

Those pieces are, by design, not supposed to shear off, even in the event of a broken fan blade, because of the danger to the aircraft and those below. In 2018, a metal latching mechanism on a Southwest plane’s fan cowl flew off after a fan blade cracked and blew out a window, killing Jennifer Riordan, an Albuquerque bank executive and mother of two.

Sumwalt said it was too early to know what caused the latest problem this past weekend near Denver. But the agency’s examination of the 2018 incident, which has some similarities, offers a glimpse into the difficulties that can be involved with inspecting critical fan blades for tiny but potentially catastrophic cracks.

The blade that failed on the 2018 flight from San Francisco International Airport to Hawaii had been inspected twice, and deemed safe, in 2010 and 2015, the NTSB said after its investigation into the incident. Pratt & Whitney had used the thermal acoustic imaging process for those inspections.

In 2010, that inspection tool detected “a small indication” at the same spot where the blade eventually cracked in the 2018 flight, investigators said. In 2015, the tool found a “larger indication” in the same area.

But both times, inspectors thought what they saw was “a defect in the paint” used in the inspection process itself, according to the NTSB, a failure the agency attributed to insufficient training and a broader lack of feedback from engineers on the inspectors’ findings. A longtime inspector who had examined the blade said “he would never find out if the blade was actually cracked or if it was a false positive,” according to the NTSB.

Another risk was found in the sunlight that would shine through large windows in Pratt & Whitney’s East Hartford, Conn., fan-blade overhaul facility. The inspection tool picks up temperature changes “and the afternoon sun would cause ghost images on the thermal scans,” according to the NTSB.

Underlying the inspection failures before the 2018 flight was Pratt & Whitney’s decision to classify the imaging process, which the company developed around 2005, as “new and emerging technology,” according to the NTSB’s June 2020 report. That meant the company “did not have to develop a formal program for initial and recurrent training,” inspector certification and added oversight, as would be done with some other established inspection methods, the NTSB said.

The inspectors who worked on the problem blade were given modest initial training, according to the NTSB. But for another training offered by Pratt & Whitney, the two inspectors “were not permitted to attend so that they could work on clearing the backlog of fan blades that were in the shop.”

Following the 2018 incident, the company reported that it developed a curriculum for “initial and recurrent” training for inspections using thermal acoustic imaging, the NTSB said.

In responding to Saturday’s incident in Colorado, Pratt & Whitney, a division of Raytheon Technologies, said it sent a team to work with investigators.

In a statement late Tuesday, Pratt & Whitney said the inspections ordered by the FAA require shipping the fan blades to the company, where they will be examined with its imaging process “to confirm airworthiness.”

“The directive applies only to the PW4000-112” model, and the fan blades that are unique to this engine type,” the company said, adding that it is coordinating with airlines, regulators and Boeing.

“The safe operation of the fleet is our top priority,” the statement said. “Pratt & Whitney commends the flight crew operating United Airlines flight 328 for their professionalism.”