Delta Flight 1418, which was traveling from Atlanta to Orlando, Florida, last Wednesday, was forced to shut down the plane’s turbines and return to the airport after one of the engines failed at 18,000 feet, Bloomberg reported.

The 27-year-old Boeing 57-200 was able to safely return to Hartsfield–Jackson Atlanta International Airport. The National Transportation Safety Board will now conduct an investigation into why the Pratt & Whitney engine failed. The aerospace manufacturer has agreed to cooperate with the NTSB. Luckily, there were no injuries reported among the 121 passengers and 6 crew aboard the Boeing 757-200 flight.

Although planes are able to fly with one engine, the incident is still considered an “uncontained” engine failure since debris from the engine could have damaged the fuselage of a plane. Most engines, however, are designed to contain debris, like fan blades, if they fail.

An uncontained engine failure occurs when fragments of rotating engine parts puncture and escape the engine case, which can directly damage an airplane and injure its passengers since high-energy disk fragments are capable of penetrating the cabin or fuel tanks, damaging flight control surfaces, or severing flammable fluid or hydraulic lines.

In April, Southwest Airlines Flight 1380 encountered a similar problem, which resulted in the death of one passenger. And in October 2016, American Airlines Flight 383, which was flying from Chicago, also experienced an uncontained engine failure that led to a fire breaking out.

According to Bloomberg, the Flight 1418 is the fourth uncontained engine failure involving Delta Airlines since August 2016. Anthony Black, a Delta spokesperson, said the airline is cooperating with the NTSB on the investigation. “Once they have completed their investigation we will change the engine and the aircraft will be ready to be placed back into service,” he added.

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In 1996, Delta Air Lines Flight 1288, a regularly scheduled flight from Pensacola, Florida to Atlanta, Georgia, experienced an uncontained, catastrophic turbine engine failure. The McDonnell Douglas MD-88, which was equipped with Pratt & Whitney JT8D-219 turbofan engines, was on takeoff roll from Runway 17 when engine failure resulted in debris from the front compressor hub of the number one left engine penetrating the left aft fuselage. The impact left two passengers, a mother and son, dead and two others severely injured. The pilot managed to abort the takeoff, stopping the plane on the runway.