K-Mile Asia B734 at Singapore on Jun 5th 2019, main gear torsion link damage on landing
Last Update: November 7, 2020 / 14:07:57 GMT/Zulu time
Incident Facts
Date of incident
Jun 5, 2019
Classification
Incident
Airline
K-Mile Asia
Departure
Bangkok, Thailand
Destination
Singapore, Singapore
Aircraft Registration
HS-KMC
Aircraft Type
Boeing 737-400
ICAO Type Designator
B734
A post flight examination revealed the right main gear upper torsion link was fractured. The main gear damper manifold was recovered from the taxiway along the taxi route to the parking bay.
Singapore's Transportation Safety Investigation Board (TSIB) rated the occurrence a serious incident and announced, via an interim statement of Jun 5th 2020, that an investigation was opened. A draft final report is currently being prepared.
On Nov 7th 2020 the TSIB released their final report concluding the probable causes of the serious incident were:
- There were no anomalies during the five flights after the landing gear replacement by MRO-I, except for the vibrations that occurred after the touchdown on the incident flight. The investigation team believed that the RH MLG torsion link assembly broke shortly after the vibrations had come about.
- A probable cause of the aircraft MLG vibrations following touchdown was the presence of air in the hydraulic line of the RH MLG damper. It could not be established whether the bleeding had been satisfactorily performed when the RH MLG of the aircraft was replaced at MRO-I.
- The missing damper manifold was reported to the aerodrome operator only four hours after the occurrence. During these four hours, the missing parts were lying on a taxiway and constituted an FOD hazard.
The TSIB described the sequence of events:
The B737-400 cargo aircraft involved in this incident had all the three landing gears recently replaced by a maintenance, repair and overhaul organisation in Jakarta, Indonesia (hereinafter referred to as MRO-I). The set of replacement landing gears was sourced by the aircraft operator from a maintenance, repair and overhaul organisation in the United States (hereinafter referred to as MRO-US) and was shipped by MRO-US direct to MRO-I. The aircraft returned to service on 3 June 2019.
On 5 June 2019, the aircraft flew from Bangkok, Thailand, to Singapore. This was the aircraft’s fifth flight after the landing gears replacement at MRO-I. The flight crew, consisting of a Captain and a First Officer (FO), were accompanied by a travelling Licensed Aircraft Maintenance Engineer (TLAME) who would perform maintenance checks during the transit in Singapore. For the approach to and landing at Singapore, the FO was the Pilot Flying and the Captain the Pilot Monitoring.
The aircraft landed on Runway 20R at Singapore Changi Airport at about 0407LT. During the landing roll, the flight crew experienced unusual vibrations when the thrust reversers were deployed. At the same time, the aircraft deviated to the left of the runway centreline. According to the FO, he had to apply greater than normal right rudder pedal input to keep the aircraft on the runway centreline. The Captain then took over control of the aircraft and applied brakes to slow down the aircraft. The vibrations subsided by the time the aircraft was slowed to about 60 knots on the runway. The aircraft left the runway via Taxiway W7 and taxied on Taxiways WP and NC2 to reach the parking bay 308.
The aircraft arrived at parking bay 308 at 0415LT. It was received by a team from a ground handling agent at Singapore Changi Airport which provided ramp services (e.g. aircraft marshalling, cargo handling). After the aircraft was parked, the T-LAME exited the aircraft and carried out a post-flight inspection. He found that the upper torsion link of the right-hand main landing gear (RH MLG) was broken and the RH MLG damper manifold (which was a part of the RH MLG damper) was missing (see Figure 2). There was also a hydraulic leak from the RH MLG because the hydraulic return line connecting the aircraft hydraulic system to the damper manifold was disconnected.
The TSIB analysed the cause of the vibrations:
According to the FDR data, there were no anomalies during the five flights after the landing gears replacement by MRO-I, except for the unusual vibrations that occurred after the touchdown on the incident flight. The investigation team believed that the the RH MLG torsion link assembly broke shortly after the unusual vibrations had come about. Paragraph 1.9.5 lists the possible factors of aircraft MLG vibrations following touchdown. The factor of “presence of air in the MLG damper” (i.e. item (d) in paragraph 1.9.5 and item (d) in paragraph A.2 in Appendix A) appeared to match the circumstances of this occurrence. As explained in item (d) in paragraph A.2 of Appendix A, MLG vibrations could occur within a few flights after a new or overhauled damper was installed. In these cases, a reason for the MLG vibrations could be that a thorough bleeding of air from the damper was not performed. This prevented proper damper operation. An ineffective damper would be unable to reduce the MLG torsional vibrations during landing.
The investigation team considered the other possible factors listed in paragraph 1.9.5 but concluded that these possibilities could be eliminated having taken into account the following:
(a) It was unlikely that wear or free play was a factor given that the RH MLG had been recently overhauled, and the aircraft had only made five flights after the installation of the overhauled RH MLG.
(b) FDR data showed no instance of extremely low sink rate.
(c) The damper piston was bent as a result of the vibration event. There is no evidence of pre-existing damper piston fracture.
(d) Shock strut pressure and extension readings were within the operating band.
(e) There is no evidence of wrong components or wrong part number having been used in the damper installation.
(f) There was no evidence of hydraulic fluid leak or loss prior to the occurrence.
In view of paragraphs above, the investigation team concluded that a probable cause of the aircraft MLG vibrations following touchdown was the presence of air in the RH MLG damper which likely affected the damper’s effectiveness and resulted in the breaking of the torsion link assembly. However the investigation team was not able to ascertain if the bleeding was done satisfactory.
The TSIB analysed the need to notify the aerodrome operator of foreign object hazard amongst others:
The T-LAME and the aircraft operator would be expected to appreciate that any aircraft missing parts constituted potentially an FOD situation and such cases should be reported to the aerodrome operator. However, there was a delay in reporting the potential FOD situation to the aerodrome operator. The T-LAME was preoccupied with reporting the situation to the aircraft operator’s headquarters and the follow-up repair actions and was not aware of the need nor the avenues to report the situation to the aerodrome operator. The aircraft operator did not seem to have a system that could ensure its flight personnel know how to contact the aerodrome operator at a destination or transit airport immediately and directly.
Incident Facts
Date of incident
Jun 5, 2019
Classification
Incident
Airline
K-Mile Asia
Departure
Bangkok, Thailand
Destination
Singapore, Singapore
Aircraft Registration
HS-KMC
Aircraft Type
Boeing 737-400
ICAO Type Designator
B734
This article is published under license from Avherald.com. © of text by Avherald.com.
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