A fan blade on a jet engine broke off a Southwest Airlines plane last month in a violent failure that sent debris slamming into the plane, according to a preliminary investigative report released Monday.

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A fan blade on a jet engine snapped off a Southwest Airlines plane last month in a violent failure that sent debris slamming into the plane, according to a preliminary investigative report released Monday.

Investigators with the U.S. National Transportation Safety Board (NTSB) found evidence of a crack “consistent” with metal fatigue in the titanium-alloy blade, it said in a statement on the agency’s website.

The Boeing 737-700 was forced to make an emergency landing in Pensacola, Fla., on Aug. 27 after parts of the left engine broke apart, damaging the fuselage, wing and tail. The plane lost cabin pressure and passengers tweeted pictures of themselves with oxygen masks on.

While no one was hurt on the flight from New Orleans to Orlando, some of the 99 passengers reported on social media that the diversion was harrowing as they looked outside and saw the air intake known as a cowling had been ripped loose, exposing the front of the engine. The five crew members also weren’t hurt.

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The CFM56 engine was built by CFM International, a joint venture between General Electric and France’s Safran.

The NTSB has not determined the cause of the failure.

Lost pressure

An unidentified shard put a 5- by 16-inch (13- by 41-centimeter) gash in the side of the plane above the wing, according to investigators. The cabin leaked air and lost pressure, though the NTSB said no metal from the engine pierced the cabin and no debris was found within the plane.

NTSB investigators view so-called “uncontained” failures seriously because they can fling heavy metal parts into fuel lines, electronics and the passenger compartment. And under requirements in the U.S. and other nations, it’s never supposed to happen.

Regulations hold that jet engines must be built with a strong-enough exterior casing to prevent fan blades and other debris from breaking through in the event of a failure. Tests must be conducted simulating a fan-blade breakup to prove that the metal shards can’t escape.

Modern jet engines contain a series of spinning fans; if one breaks apart it can eject blades and other metal debris at high speeds.

Checking for cracks

Engine manufacturers and airlines conduct periodic inspections on planes to spot any evidence of cracks or weakening of the metal due to fatigue. Investigators did not say why they suspect the fan blade broke loose.

“GE and Safran continue to assist the NTSB in its investigation,” GE spokesman Rick Kennedy said.

The last maintenance check on the Southwest plane was Aug. 21, according to Brandy King, an airline spokeswoman.

Richard Aboulafia, an analyst at aviation-research firm Teal Group, said advances in materials and manufacturing have made uncontained engine failures like the one on the Southwest jet extremely rare, and both the Boeing 737 and the CFM engine have good records.

“Given the experience we have had with this aircraft and this engine, the odds of a systemic problem are basically nil,” Aboulafia said. He said it’s not necessary to inspect every CFM fan blade on all 737s unless the NTSB decides otherwise after completing its investigation.

John Cox, a former pilot and now an aviation-safety consultant, said the NTSB will probably focus on how long the fan blade that broke off had been cracked and why Southwest didn’t discover metal fatigue during maintenance checks. He said cracks usually start small and grow slowly.

Multi-engine passenger jets like the 737 are designed to fly on one engine, but so-called uncontained engine failures are dangerous because flying debris can damage systems needed to control the plane or can penetrate the cabin.

While increasingly rare, engine failures that propel shrapnel into the fuselage of a plane have proved fatal in the past. A mother and child seated in a Delta Air Lines plane were killed on July 6, 1996, in Pensacola when the left power plant on a Boeing MD-88 broke apart while accelerating for takeoff.

After the front fan blade disintegrated, it sent metal shards flying into the plane where people were sitting, according to the NTSB. A manufacturing defect in the engine, made by United Technologies’ Pratt & Whitney, should have been detected by airline maintenance workers performing routine inspections, the NTSB investigation concluded.

Highly reliable

Overall, engine reliability has improved dramatically since the technology was introduced, and airliners routinely fly across oceans with just two power plants because breakdowns are so rare.

While far below the leading causes of accidents and death, engine failures ranked fourth in the decade from 2006 through 2015 with 165 fatalities, according to Boeing statistics.

Southwest has begun repairs on the plane, the airline said in an emailed statement.

 The carrier deferred to the NTSB to release details of the investigation.