Testimony Given During NTSB Hearing On The Incident

The cracks that developed in engine fan blades that led to an uncontained failure on a Southwest Airlines 737 may have gone undetected during a 2012 engine overhaul and inspection, according to testimony given during an NTSB hearing held November 13th.

The Philadelphia Inquirer reports that Mark Habedank, a lead engineer at engine maker CFM International, testified at the hearing that by examining the wear patterns from the fan blades on the engine from Flight 1380, the initial cracks were present during the 2012 inspection, but because the blades were inspected visually using fluorescent dyes to highlight flaws, they were missed by the inspectors. At that time, inspectors were not required to use ultrasound or other advance technology to detect such issues.

Habedank said that engineers were able to trace the history of the crack by looking at "striations" in the metal, much like determining the age of a tree by counting the rings. If we look at the striation count and go backward, it appears that during that inspection, the size of the defect was about 1/16 of an inch," he said.

CFM had determined that the fan blade roots were subjected to excessive friction in flight before the April 17 engine failure. They had recommended that the blades be removed and lubricated every 3,000 flights. That interval has since been reduced to every 1,600 flights. Crack detection must also now be conducted using ultrasound or eddy-current devices.

Habedank testified that eight more blades have been removed from service since the more stringent requirements have been put in place.

The CFM-56 series engines of the type that failed are in use by more than 300 airlines worldwide.

(NTSB image)

FMI: Source report