FlarePath

What happened

NTSB summary, verbatim

The No. 1 (left) engine high pressure turbine (HPT) stage 1 aft blade retainer developed a fatigue crack that initiated and propagated through thickness and circumferentially until final overload failure that was not contained by the turbine case. Fractography of the HPT stage 1 aft blade retainer revealed the crack initiated in the upper collet radius at a single point of origin which corresponded to a surface connected material anomaly that measured 0.005 inches by 0.015 inches by 0.0005 inches. The fracture surface contained an area of intergranular fracture morphology with visible beach marks, consistent with hold-time fatigue growth and the remaining fracture surface was consistent with overload fracture. Further examination of the material anomaly was determined to be a cluster of particles enriched with Titanium, Aluminum, Zirconium, Magnesium, and Oxygen, which is consistent with Oxide/Carbonitride/Nitride/Carbide (OCNC) particles. Analysis of the HPT stage 1 aft blade retainer material found all dimensional and material characteristics to be conforming to GE Aerospace drawings and material specifications. The presence of OCNC particles and clusters of particles is inherent to Rene 65 material with quantity and size distributions accounted for within GE’s philosophy for material life limit calculations. The calculated stress conditions for the GEnx HPT stage 1 aft blade retainer collet feature were found to be uniquely high when compared to the stress conditions of other rotating hardware. This higher stress environment was determined to be the result of the assembly interference fits between the HPT stage 1 disk, the HPT stage 1 aft blade retainer, and the HPT mid seal in combination with the engine’s operational loads. Analysis of material test data suggests that surface connected OCNCs have a reduced crack incubation/nucleation life at high stress conditions, and that surface connected OCNCs do not behave according to traditional crack growth threshold behavior. Activation of surface connected OCNCs can in fact reduce material fatigue life capability but not every OCNC will initiate a crack, suggesting an unknown bimodal behavior.

Photographs

1the aircraft, and what the investigators found

The aircraft before the accident

Probable cause

the Board's determination

The uncontained left engine failure was caused by the failure and liberation of the HPT stage 1 aft blade retainer as a result of a fatigue crack in the upper collet radius that originated from a surface connected cluster of Oxide/Carbonitride/Nitride/Carbide (OCNC) particles. Contributing to the failure of the HPT stage 1 aft blade retainer was an unknown material behavior unique to OCNC clusters in Rene 65 material that are surface connected in highly stressed material that may result in a localized reduction of material fatigue life capability.

Occurrence sequence

1 stepsNTSB coding · CICTT taxonomy

1 · Initial climb
Uncontained engine failure
defining event

Findings

1 causal · 0 contributing

CAUSE
Aircraft › Aircraft power plant › Power plant — Failure

Sequence of events

8 timed eventsfrom the FDR factual report · claude-sonnet-5

1336 UTCdeparture/climbp.5
Aircraft departed Singapore and climbed uneventfully to 31,000 ft (FL310)
1408:48 UTCcruisep.5
With the aircraft stable at FL310 and 312 kts computed airspeed, engine 1 (the left engine) N1, N2, and fuel flow all abruptly decreased
1408:55 UTCcruisep.5
The left engine fire warning became active
1409:24 UTCcruisep.5
The engine 1 fuel cutoff switch was moved from the run position to the cutoff position
1409:28 UTCcruisep.5
The engine 1 fire switch was pulled
1409:40 UTCcruisep.5
The left engine fire bottle was discharged
1409:55 UTCcruisep.5
The left engine fire warning cleared
1504 UTClandingp.5
The aircraft returned to Singapore and made an uneventful single-engine landing

NTSB analysis

from the final report

The No. 1 (left) engine high pressure turbine (HPT) stage 1 aft blade retainer developed a fatigue crack that initiated and propagated through thickness and circumferentially until final overload failure that was not contained by the turbine case. Fractography of the HPT stage 1 aft blade retainer revealed the crack initiated in the upper collet radius at a single point of origin which corresponded to a surface connected material anomaly that measured 0.005 inches by 0.015 inches by 0.0005 inches. The fracture surface contained an area of intergranular fracture morphology with visible beach marks, consistent with hold-time fatigue growth and the remaining fracture surface was consistent with overload fracture. Further examination of the material anomaly was determined to be a cluster of particles enriched with Titanium, Aluminum, Zirconium, Magnesium, and Oxygen, which is consistent with Oxide/Carbonitride/Nitride/Carbide (OCNC) particles. Analysis of the HPT stage 1 aft blade retainer material found all dimensional and material characteristics to be conforming to GE Aerospace drawings and material specifications. The presence of OCNC particles and clusters of particles is inherent to Rene 65 material with quantity and size distributions accounted for within GE’s philosophy for material life limit calculations. The calculated stress conditions for the GEnx HPT stage 1 aft blade retainer collet feature were found to be uniquely high when compared to the stress conditions of other rotating hardware. This higher stress environment was determined to be the result of the assembly interference fits between the HPT stage 1 disk, the HPT stage 1 aft blade retainer, and the HPT mid seal in combination with the engine’s operational loads. Analysis of material test data suggests that surface connected OCNCs have a reduced crack incubation/nucleation life at high stress conditions, and that surface connected OCNCs do not behave according to traditional crack growth threshold behavior. Activation of surface connected OCNCs can in fact reduce material fatigue life capability but not every OCNC will initiate a crack, suggesting an unknown bimodal behavior.