WASHINGTON (REUTERS, NYTIMES) - The US Federal Aviation Administration (FAA) said on Wednesday (April 18) it would order an inspection of some 220 jet engines after investigators said a broken fan blade touched off an engine explosion on a Southwest Airlines flight, shattering a window and killing a passenger.

The order, called an air-worthiness directive, would require an ultrasonic inspection within the next six months of the fan blades on all CFM56-7B engines that have accrued a certain number of takeoffs.

The CFM56 engine on Southwest flight 1380 blew apart over Pennsylvania on Tuesday, about 20 minutes after the Dallas-bound flight left New York's LaGuardia Airport with 149 people on board.

The explosion sent shrapnel ripping into the fuselage of the Boeing 737-700 plane and shattered a window.

Bank executive Jennifer Riordan, 43, was killed when she was partially pulled through a gaping hole next to her seat in row 14 as the cabin suffered rapid decompression.

Fellow passengers were able to pull her back inside but she died of her injuries later on Tuesday.

Philadelphia's medical examiner ruled that the cause of death was blunt trauma to the head, neck and torso, and ruled the death an accident, spokesman Jim Garrow said.

"As captain and first officer of the crew of five who worked to serve our customers aboard Flight 1380 yesterday, we all feel we were simply doing our jobs. Our hearts are heavy," Captain Tammie Jo Shults and First Officer Darren Ellisor said in a written statement released by the airline.

Capt Shults and Mr Ellisor said they were focused on working with investigators and would not be speaking to the media.

Earlier on Wednesday, National Transportation Safety Board Chairman Robert Sumwalt told a news conference that the incident began when one of the engine's 24 fan blades snapped off from its hub.

Mr Sumwalt said investigators found that the blade had suffered metal fatigue at the site of the break.

Mr Sumwalt said he could not yet say if the incident, the first deadly airline accident in the United States since 2009, pointed to a fleet-wide issue in the Boeing 737-700.

"We want to very carefully understand what was the result of this problem, and as I mentioned a few minutes ago, I'm very concerned about this particular event," Mr Sumwalt said at the news conference at the Philadelphia airport. "To be able to extrapolate that to the entire fleet, I'm not willing to do that right now."

Southwest crews were inspecting similar engines the airline had in service, focusing on the 400 to 600 oldest of the CFM56 engines, made by a partnership of France's Safran and General Electric, according to a person with knowledge of the situation.

It was the second time that style of engine had failed on a Southwest jet in the past two years, prompting airlines around the world to step up inspections.

A National Transportation Safety Board inspection crew was also combing over the Boeing 737-700 for signs of what caused the engine to explode.

Mr Sumwalt said the fan blade, after suffering metal fatigue where it attached to the engine hub, suffered a second fracture about halfway along its length.

Pieces of the plane were found in rural Pennsylvania by investigators who tracked them on radar.

The metal fatigue would not have been observable by looking at the engine from the outside, Mr Sumwalt said.

Although the FAA said the directive would apply to about 220 engines, airlines said that because fan blades may have been repaired and relocated, it would affect a far greater number.

The jet was travelling at 305km per hour when it made an emergency landing at Philadelphia International Airport, according to Mr Sumwalt, much faster than the typical 249kmh touchdown.

Passengers described scenes of panic as a piece of shrapnel from the engine shattered a plane window, almost sucking Ms Riordan out.

"The window had broken and the negative pressure had pulled her outside the plane partially," Ms Peggy Phillips, a registered nurse who was on the plane, told WFAA-TV in Dallas. "Two wonderful men... they managed to get her back inside the plane, and we laid her down and we started CPR."

Ms Riordan was a Wells Fargo banking executive and well-known community volunteer from Albuquerque, New Mexico, the company said.

Videos posted on social media showed passengers grabbing for oxygen masks and screaming as the plane, piloted by Capt Shults, a former US navy fighter pilot, prepared for the descent into Philadelphia.

"All I could think of in that moment was, I need to communicate with my loved ones," passenger Marty Martinez told ABC's Good Morning America on Wednesday.

During the incident, he logged on to the in-flight Wi-Fi to send messages to his family.

"I thought, these are my last few moments on Earth and I want people to know what happened," said Mr Martinez, who live-streamed on Facebook images of passengers in oxygen masks as the plane made a bumpy descent into Philadelphia.

Southwest Airlines experienced an unrelated safety incident early on Wednesday when a Phoenix-bound flight was forced to land at the Nashville airport shortly after takeoff because of a bird strike.

The airline expected to wrap up its inspection of the engines it was targeting in about 30 days.

The GE-Safran partnership that built the engine said it was sending about 40 technicians to help with Southwest's inspections.

Pieces of the engine including its cowling - the smooth metal exterior that covers its inner workings - were found about 97km from Philadelphia airport, Mr Sumwalt said.

The investigation could take 12 to 15 months to complete.

In August 2016, a Southwest flight made a safe emergency landing in Pensacola, Florida, after a fan blade separated from the same type of engine and debris ripped a hole above the left wing.

That incident prompted the US Federal Aviation Administration to propose last year that similar fan blades undergo ultrasonic inspections and be replaced if they failed.

When the Southwest Airlines plane was inspected on Sunday, no problems with the aircraft or its engines were uncovered.

The accident is focusing renewed scrutiny on the engine inspection process.

Investigators say the routine visual inspection might not have been sufficient to uncover problems in the engine.

The plane, a Boeing 737-700, and its engine are mainstays in the sky. Southwest's fleet, more than 700 planes, is made up entirely of 737 models. They have a stellar safety record, as do the engines.

After the accident, several airlines, including Southwest, Korean Air and WestJet Airlines, say they plan to inspect the fan blades in their engines.

Whether regulators will push for more action will depend on the early findings of their investigation.

The Federal Aviation Administration said a directive that it proposed last year to compel airlines to perform such inspections would take effect within the next two weeks.

The European Aviation Safety Agency established such regulations for European carriers in late March.

Finding flaws isn't always easy.

Metal fatigue, which investigators suspect is a factor in this week's engine failure, can be a visible or an invisible weakness that is the result of bending, vibration or other stress.

While it is often associated with older airplanes and engines, it can sometimes be the result of manufacturing flaws that cannot be seen.

"The forces in the engine are extraordinary," said Captain John Gadzinski, a 737 pilot and the founder of Four Winds Aerospace Safety, an aviation consultant. "Those small cracks that you may only see with an electron microscope will go from small to catastrophic failure in an instant."

What occurred in mid-air on Tuesday - a failure in which the engine parts are not contained in the housing and threaten the integrity of the airplane - is exceedingly rare, aircraft analysts said.

That it has happened twice with the same airline in such a short time makes it even more worrisome.

"It's unusual that a fan blade would fail twice on the same engine model, with the same carrier, over two years," said Mr Kevin Michaels, the president of AeroDynamic Advisory in Ann Arbor, Michigan, who has worked as a gas turbine engineer.

In the 2016 incident, a fan blade and another component separated from the engine. The debris did not enter the cabin. It did produce a gash in the aircraft's side, causing depressurisation, according to a preliminary report from the safety board.

If it turns out that this week's engine failure had the same root cause, said Mr Robert Mann, an airline analyst based in Port Washington, New York, the need to inspect 737 engines could be much more urgent.

"It could drive a significant event industry-wide, but it's hard to tell at this point," he said. "It could be a defect of design, which would mean it's subject to fatigue failures."

In February, a United Airlines Boeing 777, which uses a different kind of engine, also experienced a blade separation and loss of other engine parts.

That flight landed in Honolulu virtually on schedule, with no injuries reported and only minor damage, according to the safety board.

Aircraft experts say there's no reason to worry about Southwest in particular, or about what this might say about the safety of flying in general.

Southwest got into the airline business to "compete with the family car, which means they have saved tens of thousands of lives", said Mr Richard Aboulafia, an aviation analyst at the Teal Group in Fairfax, Virginia, alluding to the statistical fact that flying is safer than driving.

"The extreme worst-case scenario from these two incidents is that it might be prudent to inspect slightly more often," he said.

Mr Gary Kelly, Southwest's chief executive, said in a news conference on Tuesday that the accident had not caused him to doubt the 737.

"The airplane, in my opinion, is proven," Mr Kelly said. "It's very reliable. It has the greatest success of any other aircraft type."