United Airlines' Boeing 777 accident, explained

A video taken from inside the aircraft, now viral on social media, shows the engine on fire, wobbling, as the pilots attempted a safe landing at the Denver International Airport. The Boeing 777 planes are equipped with Pratt & Whitney PW4000 engines. The one involved in the Sunday incident too had the same the engine

The United States Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB) have opened an investigation into the incident. The FAA was quick to suggest the airlines conduct a more frequent inspection for the type of hollow fan blades, “unique” to the Pratt & Whitney PW4000 engine, “used solely on Boeing 777 planes”.

According to a report in the Associated Press, the investigation revealed that one of the two fan blades of the engine which broke off showed "signs of metal fatigue or hairline cracks from the stress of wear and tear." Investigators say they believe the one with wear and tear broke off first, causing the second one to chip off too.

The chairman of the NTSB said the investigating officers will inspect the maintenance records for the engine and fan blades, adding the debris, too, shall be examined on Tuesday.

As a result of the ongoing investigation and following the FAA advisory, at least 69 planes were grounded in the United States, Japan and South Korea. These three are the only countries using Boeing 777 fitted with Pratt & Whitney PW4000 engines. The United Airlines said while the incident had compelled it to ground 24 of its Boeings, 28 others in its possession will remain parked.

Regulators in Japan ordered Japan Airlines and All Nippon Airways to ground 32 planes. Similarly, Korean Air and Asiana Airlines in South Korea, have said they will ground their Boeing 777s.