There are new details emerging about the deadly Southwest Airlines engine failure yesterday. The NTSB reports that a fan blade on the engine was missing and there are striking parallels between yesterday's accident and a 2016 incident on another Southwest 737.
The 737-700 uses CFM 56-7B engines manufactured by CFM International, a joint venture between GE and Safran Aircraft Engines. The original CFM 56 first ran in 1974 and has been continuously updated since then. The -7 version of the engine dates back to 1995 and is used on a large number of 737 variants. The shutdown rate of the CFM 56 is reported to be only only one for every 333,333 hours and the engines log an average of one million flight hours every eight days.
The CFM 56 is classified as high bypass turbofan, meaning that most of the air that passes through the engine flows around the sides through bypass ducts rather than through the engine's combustion section. The engine's fan is the large rotating set of blades that can be seen at the engine inlet. Behind the fan is a series of compressors, turbines and igniters that first compress the intake air, mix it with jet fuel and then it burn it under high pressure to create thrust.
The CFM 56 has had a history of fan blade problems, although most of the incidents occurred with older versions of the engine. In 1989, a 737-400 fitted with CFM 56-3C engines suffered a fan blade failure. The pilots of the British Midlands jet shut down the wrong engine and the damaged engine subsequently quit completely. The resulting crash killed 47 people and injured 74 in what became known as the Kegworth air disaster. Two other 737-400s fitted with the same engines experienced fan blade problems shortly after, resulting in a fleet wide grounding of all 737-400s while fans were replaced and engine controls were modified to reduce the engine's maximum thrust.
The -7 version of the engine experienced a comparable problem in 2016 when Southwest Flight 3472 experienced a fan blade separation very similar to yesterday's incident. On the flight from New Orleans to Orlando, the 737-700 suffered a fan blade failure that also caused engine parts to puncture the cabin pressure vessel and led to a depressurization. There were no serious injuries and the incident led to a requirement to inspect fan blades for corrosion and cracks caused by metal fatigue.
CNN reports today that NTSB Chairman Robert Sumwalt said that one of the accident aircraft engine's 24 fan blades was missing and that there was evidence of metal fatigue where the blade attached to the spinner hub.
Sumwalt cautioned against drawing conclusions based on the earlier Southwest incident. “We want to look at this particular event and see what the factors are surrounding this and maybe they’re related and maybe not,” he told Flight Global. “We need to understand what’s going on here.”
Southwest CEO Gary Kelly said after the incident that the engine had been inspected on April 15 and returned to service. It is not clear if the recent inspection included checks for metal fatigue or focused on the fan blades.
Although it is still early in the investigation, it seems that fan blade separation due to metal fatigue is the likely cause of the accident. If this is confirmed, the NTSB and FAA may issue an inspection directive similar to 2016. With the CFM 56-7B powering some 13,400 airliners worldwide, inspections could be an expensive proposition in terms of both compliance and scheduling. In addition to Southwest, the 737-700 is operated in the US by several other carriers including Delta, United and Alaska Airlines.
When asked about the parallels between Southwest 3472 and whether the NTSB would recommend a fix or inspection for fatigue in the fan section, Sumwalt answered, “I don’t have that information right now. Our focus is getting out the door at headquarters and getting on this airplane so we can get up to Philadelphia.”