On May 3, 2018, the NTSB provided an investigative update on the cause of the tragic incident of April 17 aboard Southwest Airlines Flight 1380. As previously postulated, it appears that the root cause involved metal fatigue in one of the left engine’s fan blades.
A full copy of the update can be accessed through this link. In sum, however, the number 13 fan blade separated at the root, which is the part just above where the blade attaches to the disk that holds all of the fan blades. The report references this area as the “dovetail.” The dovetail sort of looks like an upside down Christmas tree and the disk and the blades mate together with the blade fitting into the notched area of the disk.
An examination of the number 13 dovetail area under a scanning electron microscope showed characteristics of low cycle metal fatigue. Low cycle metal fatigue involves a failure after a relatively low number of cycles (in this case engine starts and stops) but high loads (the fan blades spinning at a high rate of speed under the pressure of the entering air).
A simple illustration of low cycle metal fatigue would be taking a paper clip and straightening it out, then bending it over in one direction until the tips touch, and then doing the same thing in the other direction. It doesn’t take many times of doing that to break the paper clip in half.
The problem is that commercial aircraft fan blades are, at least in theory, designed to handle the loads that they experience. The flip side of that is that all metal parts that are under a load like the engine fan blades will experience fatigue forces. Nothing lasts forever. As a result, it seems logical either that there is a batch of fan blades that are not strong enough to withstand the design loads of the engine or that the current inspection and/or replacement periods are too long. There could be other causes too, but these seem, based on the information known so far, to be the most likely.
In this case, the blades were overhauled in November 2012, and according to the NTSB, the incident occurred 10,712 engine cycles after that overhaul. Perhaps allowing that time interval and number of cycles between overhauls is imprudent.
The NTSB update states that the fan blades had been lubricated and visually inspected six times between the overhaul and the accident, but there were not any more extensive types of inspections done to check for cracking in the blades. Perhaps this policy too is inadequate.
Another Southwest Airlines plane experienced a fan blade break in Pensacola on August 27, 2016. Following that incident, Southwest Airlines and the FAA began looking at more thorough inspection protocols for fan blades, but unfortunately, no such procedures were put in place in time to avoid this tragedy.