Korean B773 at Tokyo Narita on Jun 29th 2018, main gear axle fractured on landing

Last Update: September 30, 2019 / 18:35:34 GMT/Zulu time

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Photo of Korean Airlines HL7573, Boeing 777-300 — Korean Airlines HL7573, Boeing 777-300 (Photo credit: lkarasawa / Flickr / License: CC by)

Incident Facts

Date of incident
Jun 29, 2018

Classification
Accident

Airline
Korean Airlines

Flight number
KE-703

Departure
Seoul, South Korea

Destination
Tokyo Narita, Japan

Aircraft Registration
HL7573

Aircraft Type
Boeing 777-300

ICAO Type Designator
B773

A Korean Air Boeing 777-300, registration HL7573 performing flight KE-703 from Seoul (South Korea) to Tokyo Narita (Japan) with 319 passengers and 16 crew, landed on Narita's runway 16L at 12:36L (03:36Z), slowed and vacated the runway. While taxiing towards the terminal the aircraft became disabled on the taxiway due to the aft right main gear axle having fractured.

Japan's TSB reported the aircraft stopped on a taxiway due to damage to the right main gear received while landing on Narita's runway. The JTSB opened an investigation into the occurrence.

The airline reported the aircraft made a normal landing and was taxiing towards the terminal when the aircraft was forced to stop due to the fracture of a landing gear. The passengers disembarked about 90 minutes later.

The runway was closed for about 20 minutes to clear the debris.

On Jul 24th 2018 the JTSB reported the aft right main axle had fractured after landing. Part of the fracture surface was discoloured black. The axle had been installed on the aircraft in July 2009 when the right main leg had been replaced. The investigation is going to detail the causes of the fracture.

On Sep 30th 2019 the JTSB released their final report concluding the probable causes of the accident were:

It is certain that the aircraft had the right main landing gear aft axle fractured when landing in the serious incident, and subsequently, it was forced to halt on taxiway and could not continue taxiing.

It is highly probable that the fractured axle was attributed to the stress corrosion cracking (SCC) originated from the corrosion generating on the pivot bore and ongoing operations of the aircraft thereafter with cracking occurred.

It is highly probable that the corrosion generated on the pivot bore was contributed by water penetration caused by the torn fillet seal due to rotation of the bushings and corrosion inhibitor that was not applied.

The JTSB analysed:

It is highly probable from the statements of the captain and the first officer and FDR records that there did not occur hard landing when the aircraft landed at Narita international airport.

From the fact that all parts scattered on runway were identified as ones of the right main landing gear of the aircraft, it is certain that the aircraft was forced to halt on taxiway due to the axle fractured at landing and could not continue taxiing further.

It is highly probable that leakage of hydraulic fluid was attributed to the hydraulic hoses of the brake and the hydraulic hoses of the steering system, which were cut by the fractured axle.

It is highly probable that the temperature display of brake No. 12 of EICAS, which was not displayed, and status messages of the “MAIN GEAR STEERING” and the “BRAKE TEMP SYS”, which appeared, were caused by the damaged right main landing gear steering system and the damaged temperature sensor associated with the fractured axle.

It is highly probable that sighted smoke or something, which seemed like it, was a hydraulic fluid that squirted in misty condition from the cut hydraulic fluid hoses, and then became smoky after being sprayed on the brake, the temperature of which had become high due to braking action.

It is highly probable from the maintenance records of the aircraft that the Scheduled maintenance task was performed in accordance with the maintenance requirements of the Operator, which had been established originating from the manual of the manufacturer of the aircraft.

It is highly probable that the forward fractured surface had an initiating region of corrosion on the lower side of the pivot bore, then was generating SCC and finally was fractured due to repetitively imposed loads. Besides, it is highly probable that a long period of time had passed since the fracture occurred from the entire fractured surface, which was discolored black by dusts and corrosion.

However, it could not be determined when the fracture had occurred due to the nature of corrosive condition of the fractured surface, which varies depending on the operating environment of aircraft.

It is probable that the aft fractured surface was caused by SCC initiated by the corrosion generated on the lubrication passage, which then led to progressing cracking due to repetitively imposed loads. Besides, part of the aft fractured surface was discolored black by dusts and corrosion. In view of what is stated above, it is highly probable that the forward side of the pivot bore had fractured first; and, even thereafter, the aircraft kept operations with the aft side of the pivot bore partially cracked. Consequently, it is highly probable that the axle was finally fractured at the time of landing in the serious incident.

It is probable that the measured diameter of the pivot pin noted in table 2 exceeded the one specified in CMM and could possibly have contributed to interference between the pivot pin and bushing, which impeded the free rotation of the pin and contributed to the rotation of bushings.

It is also probable that “dried and hardened grease” sampled from the pivot could possibly contributed to the pivot pin and the bushings, which impeded the free rotation of the pin. It is possible, from the fact that the Operator did not have records of using Clay-thickened grease in the grease up work of the pivot joint, that the grease in question was Clay-thickened grease (ASG 7), which had been applied during the overhaul in 2009 and had remained in the lubrication passage until the serious incident. Dispense grease into the grease fitting until the used grease is visually removed and only new grease comes out in one or more locations at this joint interface; however, it is probable that AGS 7 grease applied during the overhaul had not completely been replaced with new grease ASG 33, and then, it dried up and became solid due to the absence of circumferential grease groove on bushing ID surface and the limited grease to no more than 10 - 15 pumps from a grease gun according to AMM.

Majority of the ingredients of the grease sample from the G3 passage were overall consistent with ASG33 grease. The brown portion contained corrosion products and cadmium plating. It is probable that this was the grease pushed back from the axle bore into grease G3 passage by rotation of the bushing.

Investigation of the condition of the axle pivots of the same type of the aircraft conducted by the Operator revealed that some of axles had corrosion.

It is highly probable, from the fact that those axles had been overhauled before CMM was revised, that the corrosion inhibitor, which had not been coated on bushings assembly portions, contributed to the corrosion. Besides, it is observed that even the axle pivot, which kept the pivot pins and grease in good condition, had evidence of bushings rotation. This implies that the bushings could possibly rotate even if the pivot pin did not impede the free rotation of the pin to the bushings.

From the aforementioned, it is highly probable that the fillet seal of bushing wore out by rotation of the bushing during the operation of the aircraft, which then led to water penetration and corrosion generated in the pivot bore.

It is highly probable that generation of corrosion was attributed to the landing gear, which had been overhauled before CMM was revised, and because of that, corrosion inhibitor had not been applied to it. It is highly probable that corrosion led to SCC generated on the pivot bore, and ongoing operations of the aircraft thereafter with the axel cracked finally resulted in the fracture of the axle.

In view of possible detection of the cracked axle from the feeling and situation of pumping during grease up work, it is required that the Operator confirm that there exists no clog up or leakage of the grease during the grease up work and stipulate in the job card the ways to cope with if such has occurred.

Metars:
RJAA 290500Z 22020G32KT 180V260 9999 FEW030 30/23 Q1011 WS R16L NOSIG=
RJAA 290430Z 23017G30KT 200V270 9999 FEW030 31/23 Q1011 WS R16R WS R16L NOSIG=
RJAA 290400Z 22017G28KT 190V250 9999 FEW030 32/22 Q1011 WS R16R WS R16L NOSIG=
RJAA 290330Z 21014G24KT 180V250 9999 FEW030 31/22 Q1011 WS R16L NOSIG=
RJAA 290300Z 22017G28KT 180V240 9999 FEW030 32/23 Q1011 WS R16R WS R16L NOSIG=
RJAA 290230Z 22017G31KT 190V250 9999 FEW020 31/22 Q1011 WS R16L NOSIG=
RJAA 290200Z 21021G34KT 9999 FEW020 31/22 Q1011 WS R16R WS R16L NOSIG=
RJAA 290130Z 22019G33KT 9999 FEW020 31/22 Q1011 WS R16R WS R16L NOSIG=
RJAA 290100Z 22017G30KT 190V260 9999 FEW020 30/22 Q1011 WS R16R WS R16L NOSIG=
RJAA 290030Z 22017G27KT 9999 FEW020 30/22 Q1011 WS R16R WS R16L NOSIG=
RJAA 290000Z 22018G28KT 9999 FEW015 30/22 Q1011 WS R16L NOSIG=