United B739 at Orlando on Feb 21st 2019, uncontained engine failure

Last Update: April 28, 2020 / 22:02:03 GMT/Zulu time

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Photo of United N30401, Boeing 737-900 — United N30401, Boeing 737-900 (Photo credit: TDelCoro / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Feb 21, 2019

Classification
Incident

Airline
United

Flight number
UA-1768

Departure
Orlando, United States

Destination
Houston Intercontinental, United States

Aircraft Registration
N30401

Aircraft Type
Boeing 737-900

ICAO Type Designator
B739

A United Boeing 737-900, registration N30401 performing flight UA-1768 from Orlando,FL to Houston Intercontinental,TX (USA) with 181 people on board, was in the initial climb out of Orlando's runway 18L when the crew stopped the climb at 7000 feet after hearing a sudden grinding sound, a loud boom followed by the loss of power from the right hand engine (CFM56) and uncommanded shutdown of the engine. The crew worked the related checklist, closed the fuel shutoff valve, declared emergency and returned to Orlando for a safe landing on runway 18R about 20 minutes after departure.

The NTSB reported on Apr 10th 2020, that a preliminary visual inspection of the engine revealed a burn through in the high pressure compressor's case. The engine was removed from the wing and sent to the manufacturer for further analysis. The NTSB wrote:

The engine had no visible indications of undercowl fire or high energy uncontainment. The fan blades were all intact and in good condition, and the fan spun smoothly with concurrent rotation of the low pressure turbine (LPT). HPC case burn through was observed around the case, but was not continuous, and was estimated to have affected approximately 60% of the circumference. HPC case material remained intact around the rub button pads and at the HPC stator case split flanges. The case surfaces and accessories/lines in proximity to the burn through holes were discolored, sooted and coated in metal spray. The forward sump magnetic chip detector (MCD) plug had metal debris ("fuzz") accumulation on the magnet and in the filter screen. A sample of the debris was collected for analysis and identification by the GE Materials Laboratory.

A visual examination of the HPC stator cases was performed after removal of the external engine components, and in addition to the case burn through, one HPC stage 1 variable stator vane (VSV) trunnion stem was missing a washer and retaining nut at the 1:30 position. The VSV alignment mark, located on the top of the trunnion stem was positioned (clocked) at a different angle relative to the rest of the vanes in the stage.

During engine disassembly, eight HPC stage 2 rotor blades, including the blade dovetails, were found separated and missing. The HPC stage 2 disk blade slots for each of the eight missing blades had one or more separated disk post corners (Photo 1). There was secondary impact damage observed throughout the gas path aft of HPC inlet guide vanes. The HPT stage 1 blades and the LPT stage 1 nozzle vanes exhibited thermal damage. Metal flakes and debris were collected in the aft sump, adjacent to the No. 4 bearing.

Material analysis revealed: "The HPC rotor assembly, HPC cases, and the metal debris samples collected from the aft sump and the forward sump MCD were moved to the GE Aviation Materials Laboratory in Evendale, Ohio for analysis. A visual and binocular examination of the HPC stage 1 VSV (identified as VSV #33) trunnion stem confirmed that the missing washer and retaining nut identified during the engine teardown resulted in disengagement of the lever arm from the trunnion D-head allowing the vane to go off-schedule approximately 31 degrees relative to the other HPC stage 1 VSVs. The HPC VSV #33 trunnion stem had a uniform coating of dirt/debris along the full length of the stem consistent with engine operation over an extended period of time without the washer and retaining nut. The laboratory analysis also identified witness marks on HPC VSV #33 indicating that a washer and retaining nut were present at some point, but it could not be determined when the parts were removed or separated."

The NTSB further reported: "The last incident engine shop visit was an engine overhaul at the GE Aviation-Celma maintenance, repair, and overhaul (MRO) facility in Petrópolis, Brazil in July 2014. GE Aviation located photographs taken during the overhaul, but the HPC VSV actuation hardware condition could not be evaluated from the photos, because many of the VSV trunnions and lever arms were obstructed by external engine components. According to the shop records, 22 of the 82 HPC S1 VSVs were replaced and the remaining VSVs were overhauled. Installation position of the replaced and overhauled S1 VSVs were not recorded."

On Apr 28th 2020 the NTSB released their final report concluding the probable cause of the incident was:

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.