JAL B772 at Okinawa on Dec 4th 2020, engine shut down in flight after uncontained failure, parts of engine cowl dropped
Last Update: September 29, 2022 / 19:12:17 GMT/Zulu time
Japan's Ministry of Transport rated the occurrence a serious incident, Japans TSB have dispatched investigators on site and opened an investigation. The ministry reported one of the fan blades of the left hand engine was damaged at the root.
Passengers reported there was a bang and sudden noise. The captain subsequently announced the left hand engine had suffered damage, they were returning to Okinawa on the right hand engine only.
On Dec 8th 2020 Japan's Transport Safety Board (JTSB) reported they opened an investigation into an occurrence of a "situation similiar to damage to an engine (when debris penetrates the case of the engine)". The aircraft was climbing through about 5000 meters above sea level (FL164) when abnormal noise and vibrations were generated from the left hand engine, the engine was shut down and an emergency was declared. The aircraft returned to Naha Airport, stopped on the runway and was subsequently towed to the apron about 40 minutes after landing.
On Dec 9th 2020 The Aviation Herald received another photo showing one fan blade is fractured at the root and the next to it fractured about mid span (see below).
On Sep 29th 2022 the JTSB released their final report concluding the probable causes of the serious incident were:
The JTSB concludes that this serious incident was certainly caused by the fan blades of the left engine were fractured during take-off climb, resulting in parts and cowlings of the engine were departed, and the airframe was damaged by scattered parts.
The JTSB concludes that it is highly probable that the fracture of the fan blade had initiated from the nodule, which bonded to the internal surface of a hollow structure during the polishing process of manufacturing of the fan blades, and the crack was generated, in addition to this, the Aircraft continued flights without detecting the crack at the subsequent regular inspections led to fatigue fracture.
The JTSB concludes thatit is probable that the cracks were not detected in the subsequent regular inspections were contributed by method and intervals of the used inspection were insufficient to detect the defect in the fillet region.
The JTSB analysed:
The JTSB concludes that from the detailed inspection results of the Fan Blade No. 16 as described in 2.11.5; certainly, the fatigue fracture had the origin in the fillet in the cavity FA.
The nodule was observed at the origin. The constituent analysis result of the nodule indicated that it is highly probable that the nodule is resulted from the Fan Blade material (Ti-6Al-4V), which was caused by polishing performed in the condition of an insufficient cooling in the machining process of manufacturing fan blades, and was the molten metal scattered as sparks with high temperature as it stands, collided and landed on the base material, as having been notified through SB PW4G-112-72-139 and ASB PW4G-112-A72-268. The nodules observed only on the convex side were most likely due to individual differences in machining and the polishing conditions for each on the convex and concave sides. Alpha-case was observed in the base material to which the nodule was deposited. Probably, this was an area of alpha + beta phase of the base material regionally changing to alpha-phase when the molten metal with high temperature landed on the base material. In view of the characteristics of an alpha-case that it is relatively brittle, and its strength deteriorates, the JTSB concludes that the alpha-case was most likely the origin of the cracking.
It is highly probable that up to 0.020 inches was an early faceted region suggestive of fatigue fracture. The region in 0.027 through 0.124 inches from the origin was most likely the growth region of stable LCF mixed with some area of faceted growth consistent with MTRs. It is highly probable that the stable LCF region was caused by repetitive large tensile stress in the fan blades at the time of the rotation of the fan during engine operation. Beyond the stable LCF region, the fracture mode exhibited a mixture of coarse striations, faceted growth, and continued unstable progression of cracking, leading ultimately to a final fracture.
To identify the time when fatigue fracture occurred, based on a striation count, crack growth assessment was conducted. From the number of the striation, the fatigue crack was estimated to initiate over 6,000FC ago, and approximately 0.055-inch-deep when inspected in the last TAI (3,633 FC prior) conducted in June 2018. However, most likely this was not detected at that time (Figure 19). The weight of the departed portion was estimated to be approximately 23.4 lbs.
The JTSB analysed that blade #15 fractured due to overload as result of collision with the fragments of blade #16.
With respect to departed engine parts and damage to fuselage and horizontal stabilizer the JTSB analysed:
As described in 2.9.2, the cause of departed parts in the serious incident and their estimated weight are as follows (excluding the Fan Blade No. 15 and No.16):
(1) The departed parts of the inlet were a part of acoustic panels of the inlet inner damaged by the scattered fragments of the Fan Blades in the Impact Phase after the Fan Blades were fractured. The weight of the departed parts could not be determined.
(2) Portions of the fan cowls likely departed in the Rundown Phase attributable to either
(a) instability failure caused by the unrestrained forward edges of the cowl or
(b) overload failure caused by the imbalanced rotation. It is highly probable that the weight of the departed fan cowls on the left was approximately 83 kg and on the right was approximately 26 kg.
(3) The departed part of the reverse cowls was most likely attributable to the reverse cowls that were damaged by fragments of the fractured Fan Blades or scattered liberated FEGV. It is highly probable that the departed parts were light and limited in the area of the CFRP panel.
(4) The departed parts of FEGV were most likely attributable to the pins engaged to inner of FEGV, which were detached from the engagement by impact accompanying approximately 10-inch deflection of the fan case generated in the Impact Phase and imbalanced rotation and vibration generated in the Rundown Phase, and the locked part bolted to OD of FEGV were torn a part due to single supported FEGV and backward bending force by ram pressure. It is highly probable that among the 82 FEGVs in total, 79 FEGVs were departed as one FEGV was found in the left horizontal stabilizer and two in the reverse cowls, respectively, and the total weight of the departed FEGV was approximately 93 kg.
Damaged Horizontal Stabilizer and Fuselage
The JTSB concludes that the damage to the leading edge of the left horizontal stabilizer with the approximately 28 cm-hole and approximately 20 cm-dent observed as described in 2.9 was most likely caused by the collision of some of the FEGV, components of the engine, which were departed and scattered as described in 3.8, Because one of FEGV was found inside the damaged position (hole).
The JTSB concludes that the damage to the left aft fuselage (approximately 8 cm hole) observed was more likely caused by some of the FEGV that scattered and collided with the leading edge of the left horizontal stabilizer as explained above.
The JTSB concludes that as the damage to the left forward fuselage (approximately 2 cmdent) observed was minor and seen almost right beside the engine, it is probable that a small fragment of fan blade of the left engine has collided.
The JTSB concludes that it is probable that the damage to the horizontal stabilizer and fuselage in the serious incident did not seriously affect the subsequent flight performance of the Aircraft.
This article is published under license from Avherald.com. © of text by Avherald.com.
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