US Federal Aviation Administration (FAA) on Tuesday issued an Emergency Airworthiness Directive (AD) ordering US operators to inspect Boeing 777-200 equipped with Pratt & Whitney PW4000 engines before further flights.

Under the directive, operators must conduct a thermal acoustic image (TAI) inspection of the large titanium fan blades located at the front of each engine.

“As these required inspections proceed, the FAA will review the results on a rolling basis. Based on the initial results as we receive them, as well as other data gained from the ongoing investigation, the FAA may revise this directive to set a new interval for this inspection or subsequent ones”, FAA said in the statement.

Previously, the inspection interval for this type of engines was 6,500 flight cycles, with one cycle defined as one takeoff and landing.

The damaged starboard engine of United Airlines flight 328, a Boeing 777-200, is seen following a February 20 engine failure incident

The directive was issued after the fan-blade failure that occured as the Boeing 777 plane was conducting a United Airlines flight from Denver to Honolulu on Saturday. Although the aircraft managed to turn back and safely land, the incident resulted in damage to the plane and in-flight engine fire.

Debris from the damaged jet engine landed on Denver’s suburb of Broomfield. No injuries were reported.

The US National Transportation Safety Board (NTSB) chairman said on Monday that it is believed that metal fatigue could be one of the reasons behind the incident.