Following the April 17 fan blade separation in a CFM56-7B powering a Southwest Airlines Boeing 737-700, the FAA issued an emergency Airworthiness Directive (AD) calling for ultrasound inspections of the affected engines’ fan blades. An AD had been expected as it was proposed in response to a fractured fan blade that caused an uncontained engine failure of a CFM56-7B on a Southwest Airlines 737-700 on Aug. 27, 2016.
The April 20 emergency AD coincided with issuance by engine manufacturer CFM International of Service Bulletin CFM56-7B S/B 72-1033. The AD requires within 20 days “a one-time ultrasonic inspection (USI) of all 24 fan blade dovetail concave and convex sides to detect cracking,” per the instructions in the bulletin. The AD applies to various models of the CFM56-7B that have accumulated 30,000 or more total flight cycles.
Preliminary examination of a separated fan blade on the CFM56 that failed in the most recent case, in which pieces of the engine shattered a cabin window above the left wing and killed one of the 144 passengers, also showed metal fatigue in the interior of the blade, isolated from visual detection, said NTSB chairman Robert Sumwalt. The incident forced the pilots of the 737-700 to divert to Philadelphia International Airport, where, during an April 17 press conference, Sumwalt confirmed the death of the passenger. At the time, the NTSB official said he did not know whether the preliminary AD would have covered the particular engine in question.
Notwithstanding the issuance of the proposed AD, and before the issuance of the emergency AD, Southwest Airlines on Wednesday said it would accelerate inspections of the CFM56-7Bs in its fleet of 737s. Several other airlines, including Delta, United, and American, have followed suit. A service bulletin issued by CFM in March 2017 describes the procedure for the one-time ultrasound inspection. The SB recommended that operators conduct their inspections on engines with more than 15,000 flight cycles since the time of their last shop visit “as soon as possible and not later than September 24, 2017.”
On Friday, however, the new service bulletin recommended ultrasound inspections within the next 20 days of CFM56-7B fan blades that have run more than 30,000 cycles since new, regardless of the timing of the last shop visit. It also recommends inspection of fan blades with 20,000 cycles by the end of August. The SB also calls for operators to repeat the inspection every 3,000 cycles, which amounts to roughly two years of service. The emergency AD doesn’t include the requirement for inspection of blades with 20,000 cycles or the repetitive inspection every 3,000 cycles, although the FAA said, “We are considering further rulemaking to address these differences.”
CFM, which is a 50/50 joint venture between GE and Safran Aircraft Engines, said the service bulletin's 30,000 cycle/20-day specification will cover 680 engines, while the recommendation for inspection of engines with 20,000 cycles by the end of August covers 2,500 units. So far operators have inspected more than 150 of the 30,000-cycle engines.
According to a statement by Southwest Airlines chairman and CEO Gary Kelly, “Southwest conducts inspections at a higher frequency that what is called for by GE or CFM. And in addition to our accelerated inspections, we’re meeting with GE and Boeing on a daily basis regarding the progress of the inspections and will continue to work with them throughout the investigation.”