On Saturday, United Airlines Flight 328 , returning to the airport safely with no injuries to anyone onboard. What we didn’t realize over the weekend was that ejected engine parts actually punched a hole in the fuselage. This incident could have had a much worse ending.

The chairman of the National Transportation Safety Board says that based on a preliminary assessment, the damage is consistent with metal fatigue in the jet engine’s fan blades.

On Monday, NTSB Chairman Robert Sumwalt and Investigator-in-Charge Dan Bower about Flight 328. In a report from , Sumwalt said that the Pratt & Whitney PW4000 engine failed with a loud bang only four minutes after takeoff from Denver. The failure resulted in a hole cut into the fuselage under the wing.

Thankfully, the fuselage damage is purely cosmetic, the NTSB says. Most of the damage is on the body fairing under the wing. This fairing is not a structural member.

That Pratt & Whitney PW4000 lost an entire fan blade and a portion of another blade. Damage to the engine is extensive, with its inlet and cowling completely separated from the unit. Parts of the engine rained down on yards, parks and even through the roof of a home in the Denver area.

This engine model is used on 128 planes, fewer than 10 percent of the 1,600 Boeing 777 planes the company has delivered.

Based on its preliminary findings, Sumwalt says that the damage is consistent with metal fatigue in the fan blades. What is metal fatigue? It’s the weakening of a metal part through stress. Over time, this causes damage and can cause the growth of cracks. Inspections are supposed to detect metal fatigue so that damaged blades can be replaced.

While an engine failure isn’t an ideal situation, it’s possible for twin and quad-engine commercial aircraft to be able to takeoff with failed engines. Pilots train for these exact scenarios and know how to get the plane back on the ground safely.

Sumwalt says quick actions are being taken, via Reuters:

“United Airlines has grounded all of the affected airplanes with these engines, and I understand the FAA is also working very quickly as well as Pratt & Whitney has reiterated or revised a service bulletin,” Sumwalt said. “It looks like action is being taken.”

Japan Airlines and ANA Holdings as well.

This latest incident comes a few months after a Japan Airlines Boeing 777 with a PW4000 engine suffered two damaged fan blades, one of which had a metal fatigue crack.

Because of that incident, the U.S. Federal Aviation Administration was evaluating whether to make adjustments to fan blade inspections. FAA says that it plans to issue an emergency airworthiness directive that will require more frequent fan blade inspections.

The investigation at NTSB is ongoing, with the cockpit voice recorder and flight data recorder transported to the NTSB laboratory in Washington. These devices will give the team insight into what happened in the cockpit along with any readings the plane recorded.

NTSB says that an analysis along with conclusions and a determination of probable cause will come on a later date in a final report.