Serious incident Boeing 737-924 (WL) N30401, Thursday 21 February 2019
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Date:Thursday 21 February 2019
Type:Silhouette image of generic B739 model; specific model in this crash may look slightly different    
Boeing 737-924 (WL)
Owner/operator:United Airlines
Registration: N30401
MSN: 30118/820
Year of manufacture:2001
Engine model:Cfm Intl. CFM56 SERIES
Fatalities:Fatalities: 0 / Occupants: 181
Other fatalities:0
Aircraft damage: None
Category:Serious incident
Location:Orlando, Florida -   United States of America
Phase: Initial climb
Nature:Passenger - Scheduled
Departure airport:Orlando International Airport, FL (MCO/KMCO)
Destination airport:Houston-George Bush Intercontinental Airport, TX (IAH/KIAH)
Investigating agency: NTSB
Confidence Rating: Accident investigation report completed and information captured
United Airlines Flight UA1768, a Boeing 737-924, N30401, powered by two CFM International CFM56-7B26 turbofan engines, experienced a No. 2 (right) engine failure during initial climb from Orlando International Airport (MCO), Florida. At about 7,000 feet altitude, the flight crew reported an "abrupt loud grinding noise and instantaneous boom," followed by a loss of No. 2 engine power and subsequent uncommanded engine shutdown. The crew initiated quick reference handbook procedures, closed the No. 2 engine fuel shutoff valve, declared an emergency, and returned to MCO, where they made an uneventful overweight single engine landing. Airport rescue and firefighting (ARFF) crews met the airplane on the adjacent high speed taxiway, but no fire or smoke was visible, and the airplane was cleared to taxi to the gate under its own power. No passenger or crew injuries were reported.

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, Petropolis, 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.

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.

Accident investigation:
Investigating agency: NTSB
Report number: ENG19IA013
Status: Investigation completed
Duration: 1 year and 2 months
Download report: Final report



History of this aircraft

Other occurrences involving this aircraft

23 February 2018 N30401 United Air Lines 0 Green Bay–Austin Straubel International Airport, WI (GRB/KGRB) non
Runway excursion
29 December 2019 N30401 United Airlines 0 near Houston, TX unk
Engine failure


Revision history:

04-May-2023 05:26 ASN Update Bot Added
04-May-2023 05:36 harro Updated

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