Back in 2016, an American Airlines (A1G) (AAL) aircraft caught fire during take-off from the O’Hare International Airport in Chicago leading to an emergency evacuation of the plane on the tarmac. An investigation from the National Transportation Safety Board (NTSB) has discovered that the incident was caused by a rare engineering failure.

On October 28, 2016, American Airlines Flight 383, carrying 161 people, was attempting take-off from Chicago O’Hare when the plane suddenly caught fire. Pilots of the aircraft aborted take-off when passengers and crew members saw flames outside the windows. All the people onboard the plane were forced to make an emergency evacuation on the tarmac. The plane came to a halt only about 3,775 feet from the end of the runway, and the cabin filled with smoke, federal investigators said.

Nobody was killed in the fiery incident at O’Hare aboard a flight headed to Miami, but 20 people suffered injuries, one of them serious. Dramatic photographs and videos of the aircraft going up in flames at the airport soon spread around the web. Investigators later found pieces of the engine thrown as far as a half-mile away from the aircraft.

The NTSB discovered that a disk broke apart in the engine of the Boeing 767-300ER aircraft on the right side of the plane sparking a fire in the General Electric engine. The company said the flaw was in the nickel-metal alloy which had been used to manufacture the disk – but that the failure is extremely rare. GE said it had been 30 years since the problem had been detected and this aircraft was the only plane still using a disk made from that bad batch of alloy, USA Today reported on January 30, 2018.

The NTSB ruled that the rare flaw could not have been detected during manufacturing or in inspections between flights required by the Federal Aviation Administration (FAA). Thus, the board recommended that the FAA develop new inspection guidelines for airliner engines to check for internal disk cracks.

Robert Sumwalt, the board chairman and a former airline pilot, noted that nearly every airliner engine uses a similar metal alloy and this was the only reported failure of the kind. “The fact is this is a very, very rare failure,” USA Today reported him saying.

In addition, the NTSB investigation pointed to the cabin crew, who they say were not knowledgeable on how to use the intercom system to talk to pilots during the evacuation. The crew members evacuated passengers behind an engine that was still running and took too long to get all the passengers out. Several passengers even attempted to pick up their carry-on luggage during the evacuation and refused to listen to instructions from the crew. The NTSB concluded that the emergency evacuation was unnecessarily chaotic, Travel and Leisure writes.