Boeing 737 N30401 incident near Orlando, Florida

Probable cause

A No. 1 (left) engine high pressure compressor stage 2 blade separation. The blade separated due to a mixed-mode fatigue crack that originated on a blade disk post and was initiated by a misaligned (out-of-schedule) high pressure compressor stage 1 variable stator vane that was missing a washer and retaining nut on the vane trunnion stem.

NTSB analysis

The No. 2 engine high pressure compressor (HPC) failure was caused by a stage 2 blade separation. A HPC stage 1 variable stator vane (VSV) trunnion stem, identified as VSV #33, was missing a washer and retaining nut that allowed VSV #33 to rotate independent of the other vanes in the stage. The out-of-schedule VSV created a 1 per revolution pulse on the HPC stage 2 rotor that eventually created fatigue cracks on the HPC stage 2 blade posts that secure the rotor blades in the disk spool. When one of the fatigue cracks transitioned to overload, a section of a blade post separated and allowed a blade to enter the gas path. Titanium fragments created by secondary impact damage as a result of the separated blade were caught between the rotating blade tips of the stage 1 blades and the steel HPC cases. The contact created a brief but intense titanium fire that melted the HPC case material around the HPC stage 1 rotor plane of rotation. The damage observed both immediately upstream of the separated HPC stage 2 blade and through all remaining stages aft was secondary impact damage.

The GE Aviation Materials Laboratory performed a binocular analysis on the HPC VSV #33 trunnion stem and D-head and found wear patterns and witness marks that confirmed that a washer and retaining nut were present at some point. According to engine maintenance records, the last time the HPC VSVs were removed was during an engine overhaul at the GE Aviation- Celma maintenance, repair, and overhaul (MRO) facility, Petrópolis, Brazil in July 2014. When the HPC was assembled during the engine overhaul, the VSV #33 washer and retaining nut were likely either omitted or improperly torqued, which may have allowed the retention nut to back off during engine operation.

Text from the NTSB. Open the NTSB docket for ENG19IA013 for the full record.