Maintenance records from the Southwest Airlines Boeing 737-700 that experienced an inflight engine failure April 17 reveal the fan blades of its CFM56-7B turbofan engines, which are suspected of causing the failure, had been overhauled 10,712 engine cycles before the accident, the US National Transportation Safety Board (NTSB) reported.

In an investigative update released on May 3, the NTSB said the fan blades were last overhauled in November 2012, at which time they were inspected visually and by fluorescent penetrant. The fan blades of the accident engine had accumulated more than 32,000 engine cycles since new. Maintenance records indicated that they had been periodically lubricated as required by the Boeing 737 NextGen-series aircraft maintenance manual.

After Southwest 737-700 experienced an engine failure in which a fan blade fractured on Aug. 27, 2016, forcing pilots to make an emergency landing in Pensacola, Florida, eddy current inspections were incorporated into the overhaul process requirements, the NTSB said.

The pilots of Southwest Airlines flight 1380 executed an emergency landing at Philadelphia International Airport on April 17 after the 737’s left engine failed, blowing out a window and causing the cabin to depressurize. One of the 144 passengers on board was killed and eight passengers suffered minor injuries.

NTSB investigators responding to the scene immediately found that one of 24 titanium alloy fan blades was missing from the damaged engine. The number 13 fan blade had broken at the point where it attached to the disk hub and there was evidence of metal fatigue where the blade separated, the board said.

In the aftermath of the accident, FAA and European Aviation Safety Agency (EASA) issued emergency airworthiness directives (AD) requiring fan blade inspections within 20 days on high-time CFM56-7B engines. FAA published a separate AD on May 2, which becomes effective on May 12, requiring operators to conduct ultrasonic or electrical eddy current inspections on the fan blades before they reach 20,000 cycles, each consisting of an engine start, takeoff and landing, and full shutdown. Blades with 20,000-30,000 cycles, or blades of unknown age, must be inspected by Aug. 31. The FAA’s new AD aligns with what EASA earlier required in an April 20 directive.

Bill Carey, bill.carey@aviationweek.com