Boeing 737NG operators must conduct more frequent inspections of engine fan blades as regulators move to mandate a CFM International-recommended reduction in the repetitive-check interval needed to ensure cracks are not forming at the blade roots.
The European Aviation Safety Agency (EASA) on Sept. 28 published an airworthiness directive requiring operators to inspect CFM56-7B-series fan blade dovetails every 1,600 cycles, or nearly twice as frequently as the previous 3,000-cycle interval. FAA will issue a similar directive Oct 1, and other regulators are expected to follow. Blades that currently have 1,150 cycles or more since the last inspection must be re-inspected within 450 cycles.
The move is based on a CFM recommendation communicated to operators in a July 27 service bulletin that revised the original interval, issued in an April 20 bulletin. Regulators mandated the April bulletin's recommendations, which included both initial and repetitive inspection thresholds for the entire CFM56-7B fleet.
The April bulletin was triggered by the April 17 in-flight engine failure on a Southwest Boeing 737-700 that investigators quickly linked to a fractured blade caused by low-cycle fatigue cracking at the part's root. The blade separation caused part of the engine inlet to break away and strike the aircraft, causing what NTSB called "significant impact damage." One passenger died as a result.
"CFM gained a better understanding of the fan blade failures based on the inspections and further analysis of the detected cracks and the April 2018 event," FAA said in the new directive's preamble. "As a result, CFM reduced the repetitive inspection interval to prevent a fan blade failure."
Cracked fan blades have been identified as the root causes of two CFM56-7B engine failures on Southwest 737-700s. The first, in August 2016 near Pensacola, Fla., triggered an initial set of inspection recommendations from CFM. FAA was in the process of mandating checks based on CFM's bulletins when the April accident occurred.
Post-accident, both EASA and FAA used emergency directives to fast-track fleet-wide checks. Regulators set initial-inspection deadlines based on engine's service life, with the highest-cycle engines requiring checks within 20 days. Subsequent orders modified some of the initial deadlines and inspection methods, but the latest directive is the first change to the fleet-wide repetitive inspection interval.
Inspecting all 24 fan blades on each CFM56-7B using ultrasonic or eddy current inspection methods, as the directive requires, takes about four hours. While each operator's utilization varies, CFM has said that the average CFM56-7B accumulates about 1,500 cycles in a calendar year. There are about 14,000 CFM56-7Bs in service.