Singapore B38M at Singapore on Dec 3rd 2021, both left main tyres damaged on landing
Last Update: November 30, 2022 / 18:38:57 GMT/Zulu time
Singapore's Transport Safety Investigation Board (TSIB) released their final report concluding the probable causes of the incident were:
- The damage to left MLG wheels was caused by skidding. The wheels skidded because the left MLG wheels were locked during the landing. The locking of the left MLG wheels was due to a brake application on the left MLG wheels by the SFO before the manual deployment of speedbrakes to ensure the aircraft weight was on the MLGs. The brake pressure acting on the left MLG wheels was sufficient to result in the locked-wheel situation.
- For the landing in PNH, the speedbrakes were deployed after reverse thrust selection. Subsequently, aircraft brakes were applied after the air/ground sensors were sensing "GROUND" (i.e. the aircraft had landed), and there were no locked-wheel situations and skidding of wheels.
- For the landing in SIN, although reverse thrust was selected, the speedbrakes did not automatically deploy as the air/ground sensors were still sensing “AIR” when the SFO was performing a gentle landing. Aircraft brakes were applied and this caused the left MLG wheels to lock. Subsequently, the locked wheels skidded on the runway and damaged the left MLG tyres.
- The flight crew’s action of selecting reverse thrust for speedbrakes deployment did not follow the MEL requirements of manually deploying the speedbrakes for the landings in PNH and SIN.
The TSIB reported: "The aircraft was delivered on 5 March 2019. It had accumulated 100 flight hours by the time of the incident7. The four tyres on the left and right MLGs were on the aircraft since delivery. The aircraft had operated for the following six flights prior to the flights on 3 December 2021. Three of these flights had reported issues of uncommanded disengagement of the autobrake upon touchdown and this was followed by the illumination of the autobrake disarm light during the landing roll. The maintenance crew performed ground tests on the Antiskid Autobrake Control Unit (AACU) following the aircraft maintenance manual."
The TSIB continued: "The test of the AACU after the HKT-SIN flight on 2 December 2021 revealed a fault message “Box AB/B” which indicated that there was an internal fault within the AACU or the antiskid valve. The aircraft manufacturer’s Fault Isolation Manual required testing of the antiskid valve and the autobrake shuttle valve, replacement of the AACU as well as wiring checks. However, as there would not be enough time to carry out all these maintenance actions before the next flight, the maintenance crew applied MEL items 32-42-01 and 32-42-03 and despatched the aircraft with the Antiskid8 and Autobrake systems inoperative."
For the sector from Phnom Penh (PNH) to Singapore (SIN) the captain (45, ATPL, 9090 hours total, 207 hours on type) was pilot monitoring and the senior first officer (36, ATPL, 4187 hours total, 106 hours on type) pilot flying.
The TSIB analysed:
Cause of the left MLG tyre damage
The damage to the left MLG’s tyres were caused by skidding. The wheels skidded because the left MLG wheels were locked (i.e. not spinning) during the landing, given that the Antiskid system was inoperative. The brake pressure (of 1500 psi) applied to the left MLG wheels by the SFO, before the speedbrakes were manually deployed to ensure the aircraft weight was on the MLGs21, was sufficient to result in the locked-wheel situation.
Non-deployment of speedbrakes after landing in SIN
MEL 32-42-01 requires, among others, that the flight crew should, during landing, deploy the speedbrakes manually, i.e. by moving the speedbrake lever to the “UP” position, before applying aircraft brakes. The PIC did not follow this requirement during the landing in PNH and the SFO did not follow this requirement during the landing in SIN.
Although the flight crew did not follow the procedure in MEL 32-42-01, the outcome of the two landings was different. There was no wheel locking and skidding during the landing in PNH but there was for the landing in SIN. The investigation team believes the reasons for the differing outcome are as follows:
Landing in SIN
The SFO elected to do a gentle landing and the MLG struts were not sufficiently compressed to cause the air/ground sensors to switch from the “AIR” mode to the “GROUND” mode. Thus, the condition needed for the automatic deployment of speedbrakes by means of reverse thrust selection was not satisfied. The SFO, believing that the aircraft had already touched down, applied brakes. Thus, the brakes were applied before the aircraft weight was on the MLGs. With the Antiskid system inoperative, the brake pressure applied on the MLG wheels was sufficient to cause a locked-wheel situation and resulted in the skidding of the MLG wheels when the aircraft eventually landed on the runway.
Landing in PNH
Although the Antiskid system was inoperative, there was no MLG locked-wheel situation. This is likely because, at the time of the reverse thrust selection, the aircraft had already landed positively on the runway and the MLG struts had been sufficiently compressed to make the air/ground sensors go into the “GROUND” mode, which enabled the speedbrake deployment following the reverse thrust selection (even though the PIC did not manually deploy the speedbrakes). By then, the lift on the wings had been dumped and the aircraft’s weight was fully on its wheels to allow the MLG wheels to spin up. The PIC applied brakes at about the time of the speedbrake deployment and thus there was no locked-wheel situation.
Flight crew deviating from MEL requirements
During the approach to PNH, the PIC initially armed the speedbrake system by moving the speedbrake lever to the “ARMED” position. The “SPEEDBRAKE DO NOT ARM” light on the control panel then illuminated. This reminded the PIC about the need to manually deploy the speedbrakes and he returned the speedbrake lever to the “DOWN” position. However, he did not follow the requirements of MEL 32-42-01 to manually deploy the speedbrakes. Despite his intention to move the speedbrake lever to the “UP” position, he selected the reverse thrust levers.
For the landing in SIN, the SFO anticipated that speedbrakes would deploy automatically upon the selection of reverse thrust. He might have been influenced by the following:
(a) The SFO had seen the deployment of the speedbrakes when the PIC selected the reverse thrust levers during the landing in PNH.
(b) During the approach briefing by SFO, the PIC advised the SFO that the reverse thrust selection would result in the deployment of the speedbrakes.
This incident highlights the importance of following the requirements in the MEL.
This article is published under license from Avherald.com. © of text by Avherald.com.
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