Southwest’s deadly engine accident turns attention to fan blades
On November 14, 2018, The National Transportation Safety Board (United States) held an investigative hearing, as part of an ongoing investigation of the fatal CFM International engine failure on Southwest Airlines flight 1380 on April 17, 2018. The hearing examined CFM56-7B engine fan blade design, development history and maintenance. A similar failure of this model engine also happened two years earlier, in 2016.
The deadly Southwest flight 1380
Southwest Boeing 737-700 (N772SW) was en route from New York LaGuardia (LGA) to Dallas Love field (DAL) on April 17, 2018, when it experienced a left engine failure and cowling during climb, forcing the flight to divert into Philadelphia International Airport (KPHL).
At first it was thought to be engine fire, the cockpit voice recorder reveals, and only later it became known that the emergency situation was caused by a loss of engine fan blade inlet. Fragments of shattered engine struck the plane’s wing, fuselage, and broke one cabin window, causing a depressurization. The airplane sustained substantial damage.
There were 149 people on board: five crew and 144 passengers, eight of whom sustained minor injuries, while one was partially sucked out of the cabin, receiving fatal injuries.
The left engine installed on the incident was a CFM56-7B24 turbofan engine. The flight 1380 was not the first failure of this model engine. In fact, as Powerplant group chairman’s factual report states, as of November 12, 2018, the have been 15 confirmed cracked CFM56-7B fan blades. In two of these cases, cracked fan blades resulted in a fan-blade-out (FBO) separation. One of these cases is the flight 1380. The second, which also happened to Southwest, occurred two years earlier − on August 27, 2016.
Back then, a Southwest Boeing 737-700 (operating flight 3472) was en route from New Orleans, Louisiana, to Orlando, Florida. After experiencing an uncontained engine failure (and cabin depressurization), the plane successfully diverted to Pensacola International Airport (PNS), Florida. Although the plane sustained substantial damage, none of 99 passengers and five crewmembers onboard were injured.
So what is happening with CFM56-7B?
As it turns out, the fact that CFM56-7B fan blades can brake was a known fact since the design stages of the engine, and there have been a series of engineering changes (including shim installation and adding lubrication) implemented to prevent such failures.
The attitude towards the problem, as regulation changes show, has also been shifting. Post August 2016 incident, CFM issued several AOWs with changes to the recommended fan blade maintenance practices. In 2017, the FAA started mandating an UI of fan blades installed on CFM56-7B engines in certain conditions. Just before flight 1380 accident, EASA issued a demand for a one-time inspection of several CFM56-7B fan blade versions within 9 months of the effective date of April 2.
After that, a series other inspection recommendations and mandates were issued, essentially mandating more fan blade inspections at shorter intervals (as per latest revisions, to 1,600 flight cycles instead of 3,000).