K-Mile Asia B734 at Singapore on Sep 28th 2021, burst both left tyres on departure
Last Update: November 2, 2022 / 19:52:20 GMT/Zulu time
Singapore's TSIB released their final report concluding the probable causes of the incident were:
- An under-inflation condition probably existed in the previous Wheel #2 tyre (replaced prior to the flight due to air valve leakage) which led to the overloading and stressing of Wheel #1 tyre. This prolonged stressing of Wheel #1 tyre resulted in it failing first during the take-off roll at Changi Airport. The newly replaced Wheel #2 subsequently failed due to overloading from taking on the full load after Wheel #1 tyre had failed.
- The aircraft operator’s recording of tyre pressure was not in line with good maintenance documentation practice. Although the mechanic recalled that the tyre pressure of Wheel #2 tyre was 177 psi, there is no other evidence for the investigation team to corroborate with the mechanic’s recollection as this tyre pressure was not recorded. The lack of proper recording of tyre pressure had also resulted in the daily check records being amended without proper annotations and countersigning.
- The flight time was relatively short and the two AFTN messages sent by the Singapore ATC had not been accorded with an urgent priority indicator. Thus, the Jakarta ATC did not process the information on FOD immediately.
- The tyre debris and detached panel, being parts fallen from the aircraft, could have had an impact to safe aircraft operation. Hence the two AFTN messages sent to the Jakarta ATC, which were meant to alert the flight crew, should have been accorded with an urgent priority indicator so that the entity receiving the messages could take timely actions.
The TSIB analysed:
Possible cause of the tyre failure
Noting the result of the examination of tyre debris (paragraph 188.8.131.52), i.e. both Wheel #1 and Wheel #2 tyres had experienced casing rupture and tread separation, the investigation team opined that the sequence of tyre failures is probably as follows:
(a) Wheel #2 tyre had been operating in an under-inflated condition, as suggested by the air valve leakage problem of the previous Wheel #2 (which was replaced on 27 September 2021) and the purported tyre pressure of 177 psi before a mechanic replaced Wheel #2.
(b) The under-inflation would cause Wheel #1 tyre to be overloaded and to experience increased stress, resulting in tyre failure. After this failure, the load would be transferred to the newly replaced Wheel #2 tyre, which would in turn experience an overloading and a failure subsequently.
The investigation team has considered also two other possible causes but found them unlikely:
(a) Possibility that Wheel #1 tyre was under-inflated
The corrosion found on the over-pressure relief valve of Wheel #1 hub could have caused the wheel to lose pressure progressively over time and the tyre to operate in an under-inflated condition, thus stressing the Wheel #2 tyre. If Wheel #1 tyre had indeed been operating underinflated over some time, then Wheel #2 tyre should have incurred some damage. However, this was not the case as no damage was found on the previously replaced Wheel #2 tyre.
(b) Possibility of foreign object damage
Wheel #1 tyre might have been damaged by an FOD. However, no FOD damage was identified on the tyre debris and no FOD was recovered from the runway.
On the balance of evidence, the investigation team believes that under-inflation likely existed in the Wheel #2 tyre which had an air valve leakage problem, and was replaced in Bangkok. This under-inflation would then result in the overloading of Wheel #1 and its failure.
Maintenance practice by the aircraft operator
The aircraft operator’s mechanic who replaced the Wheel #2 on 27 September 2021 following the discovery of an air valve leakage problem claimed that he had noticed the tyre pressure to be 177 psi but did not record the tyre pressure before its replacement. This was not in line with good maintenance documentation practice, as the recording of tyre pressure prior to any tyre replacement would serve as evidence as to whether the tyre pressure was or was not beyond tyre pressure limit and would provide a clue as to the reason of any further tyre maintenance actions.
In the course of the investigation, the investigation team came across a number of daily check records where only the BEFORE row of the tyre pressure table were recorded with the tyre pressures of the main landing gear and nose landing gear, even though the aircraft operator’s mechanics had purportedly performed a top-up of the tyre pressures to the maximum allowed. This gives rise to doubts as to whether the mechanic really did perform the tyre pressure topping up and as to whether, even if they did perform the topping up, they did record conscientiously the measured tyre pressures. The records, as they were, did not allow the investigation team to estimate the condition of Wheel #1 and Wheel #2 tyres prior to the incident and to establish if Wheel #1 and Wheel #2 tyres had been able to maintain pressure over the period of service (i.e. if the tyres had been constantly losing pressure or had operated in an under-inflated condition).
The manager had amended the INFLATION and DIFF figures in the daily check records without proper annotations (e.g. the reasons for the amendment) and countersigning. Again this does not accord with good recording practice and may be misleading as the amended daily check records could be mistaken as part of the original document recording when they were not.
Transmission of debris information to the flight crew of the incident aircraft
When the Singapore ATC received the debris information from the aerodrome operator, the aircraft was already under the control of Jakarta ATC. The Singapore ATC therefore sent two AFTN messages to the Jakarta ATC so that the Jakarta ATC could inform the flight crew of the incident aircraft. The first AFTN message on the tyre debris was sent at 0814 hrs and the second AFTN message at 0834 hrs. Owing to the relatively short flight time and the AFTN messages not having been classified as urgent, the Jakarta ATC did not process the messages immediately and the debris information did not reach the flight crew prior to the landing in Jakarta.
It is essential that flight crew of aircraft be alerted in a timely manner on issues that may potentially affect the safety of their aircraft, so that they could take appropriate course of actions.
According to the Singapore ATC, it follows guidance provided in ICAO Document 44449 on the use of priority indicators for AFTN messages. The guidance mentioned that messages of an urgent nature should be classified as urgent. The two AFTN messages were sent using the GG priority indicator, instead of the DD priority indicator meant for urgent messages. Being from an aircraft, the tyre debris and the detached panel could have had an impact to the safe operation of the aircraft. The investigation team opined that the two AFTN messages should have been accorded an urgent priority indicator.
This article is published under license from Avherald.com. © of text by Avherald.com.
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