The effectiveness of required, routine inspection techniques are emerging as a key focus in the investigation of the April 17 CFM56-7B engine failure onboard a Southwest Airlines Boeing 737-700, the National Transportation Safety Board said in a May 3 update on the probe.
The fan blades on the failed engine, including the fractured one, underwent detailed fluorescent penetrant and visual inspections as well as full overhauls in November 2012–10,700 cycles before the accident, investigators discovered. The engine had accumulated more than 30,000 cycles since new.
Links between the April accident, an emergency landing in Philadelphia that included the death of one passenger, and a non-fatal August 2016 accident that also involved a Southwest 737-700 are becoming more apparent. In each instance, a No 1. engine failure was linked to a cracked fan blade, with fatigue cracking within the dovetailed-shaped blade root, where the blade attached to the hub, identified as the likely cause.
Following the 2016 accident, which led to an emergency landing in Pensacola, Florida, CFM revised its overhaul procedures, adding eddy current inspections, NTSB said. CFM also issued a service bulletin (SB) in 2017 recommending inspections of all blades at 15,000 cycles since their last overhaul.
Days after the most recent accident, CFM revised its inspection recommendations, urging ultrasonic inspections for blades with at least 30,000 cycles since new. Both the European Aviation Safety Agency (EASA) and FAA mandated the checks, giving operators until May 10 to perform the inspections on the 680 affected engines.
Each CFM56-7B has 24 fan blades.
The blades are made of a titanium alloy, and the root's face is coated with a copper-nickel-indium alloy. NTSB said the fan blade that fractured during the April accident flight, mounted in position no. 13, separated at the root, with the dovetail remaining installed in the fan disk. Two other pieces, a 12-in. fragment and a 2-in. fragment, were recovered from within the engine, between the blades and outlet guide vanes.
Inspection of the blades revealed signs of fatigue cracking within the dovetail. "The origin area was located outboard of the dovetail contact face coating, and the visual condition of the coating appeared uniform with no evidence of spalls or disbonding," NTSB said. An electron microscope revealed "striations consistent with low-cycle fatigue crack growth were observed," the board added.
The engine's blades had been lubricated six times since their 2012 overhaul. "At the time each of these fan blade lubrications occurred, the fan blade dovetail was visually inspected as required for the fan blades installed in the accident engine," NTSB explained. "The NTSB materials group is working to estimate the number of cycles associated with fatigue crack initiation and propagation in the No. 13 fan blade and to evaluate the effectiveness of inspection methods used to detect these cracks."
NTSB said detailed inspections of the other 23 blades were done. No cracking was found.
While attention has focused on the fractured fan blade, the damage triggered by the part's failure—a similar pattern to the August 2016 non-fatal accident—is emerging as an equally important focus.
In both cases, engine failures caused significant parts of engine inlets to break away. Debris caused wing and fuselage damage, and each cabin depressurized. The 2016 accident aircraft suffered a 5-in.-by-16-in. hole in the fuselage above the left wing, though the passenger cabin was not penetrated.
In the most recent accident, a chunk of the fan cowl from the left-aft area of the engine is believed to have struck the left-side fuselage and broke the window.
"A large gouge impact mark, consistent in shape to a recovered portion of fan cowl and latching mechanism, was adjacent to the row 14 window," NTSB said. "The window was entirely missing."
Flight attendants moving through the cabin after the engine failure found the passenger next to the window "partially out of the window and attempted to pull her into the cabin," NTSB said. They succeeded with the help of two other passengers. The passenger seated next to the broken window was killed.
NTSB also suggested that the April 2018 accident was not an uncontained engine failure. "The inlet cowl containment ring was intact but exhibited numerous impact witness marks," NTSB said. "Examination of the fan case revealed no through-hole fragment exit penetrations; however, it did exhibit a breach hole that corresponded to one of the fan blade impact marks and fan case tearing."
The board's most recent update on the 2016 accident, issued in September 2016, characterized that accident as an uncontained engine failure. NTSB did not provide details on the evidence that supports its conclusion.
The 18-month gap without an update, while not unprecedented, is rare for an agency that emphasizes the importance of accepted accident-investigation protocol, and rarer still for a non-fatal aviation accident. The International Civil Aviation Organization's Annex 13 accident investigation practices call for a minimum of one update per year.
NTSB says the 2016 probe "is an ongoing investigation" and that a final report is expected "soon."