Passengers on a United flight from San Francisco to Honolulu, Hawaii, were shaken on Tuesday when one of the plane’s engines began to fall apart mid-flight.

Photos and videos posted on social media show the parts peeling off the Boeing 777-222 aircraft’s engine in mid-air, with its inner workings becoming steadily more exposed. “Scariest flight of my life,” one passenger tweeted.

Flight UA1175 sent out a distress alert on its approach to Hawaii, and ultimately landed safely at 12.38 p.m. local time.

In a statement to Business Insider, the airline said:

United flight 1175 traveling to Honolulu from San Francisco landed safely after the pilots called for an emergency landing because of a loss of the engine cowling (the covering of the engine). Our pilots followed all necessary protocols to safely land the aircraft. The aircraft taxied to the gate and passengers deplaned normally.”It is not immediately clear what caused the problem and United Airlines was unable to confirm whether the engine beyond the cowling had been damaged during the incident.

However, the airline did confirm to Business Insider that the aircraft involved in the incident is tail number N773UA. It’s on a Boeing 777-222 delivered to the airline in 1994. In fact, it is the fourth Boeing 777 ever produced. According to Airfleets.net, the plane was powered by a pair of Pratt & Whitney PW4000 turbofans.

Pratt & Whitney was not immediately available for comment.

It should be noted that the Boeing 777 is designed to fly on a single engine.

It’s not the first time in recent memory an aircraft engine broke apart mid-flight.

In October 2017, an Air France Airbus A380 suffered an uncontained engine failure which caused the front of one of its four engines to come off mid-flight while on its way from Paris to Los Angeles. Fortunately, no injuries were reported and the plane was able to divert to Goose Bay in Canada.

In 2010, a Qantas A380 had a near-disaster when huge chunks of one of its engine casings blew off four minutes after take-off. The pilot managed to shut down the engine and safely land the aircraft after the incident, which was caused by a faulty oil pipe.

The damage in that incident, however, was to the rear of the engine casing on the Airbus super-jumbo rather than the front.