Description

On 3 December 2021, a Boeing 737MAX-8 (9V-MBF) being operated by Singapore Airlines on a scheduled international passenger flight from Phnom Penh to Singapore Changi and subject to an MMEL operating restriction due to malfunctioning of either the Antiskid Autobrake Control Unit (ABACUS) or the antiskid valve sustained left main landing gear tyre and related damage during landing in day VMC on a dry runway. This was only realised following an external inspection after arrival at the assigned terminal parking gate. 

Investigation

A Serious Incident Investigation was carried out by the Singapore Transport Safety Investigation Bureau (TSIB). The FDR and the CVR were removed from the aircraft and relevant data were successfully downloaded from both recorders. 

The Flight Crew

The 45 year-old Captain had a total of 9,090 hours flying experience of which 207 hours were on type and a further 3,673 hours were on the 737-800. He had flown 36 hours in the preceding 90 days. The 36 year-old Senior First Officer, who had been acting as PF for the sector, had a total of 4,187 hours flying experience of which 106 hours were on type and a further 1,623 hours were on the 737-800. He had flown 27 hours in the preceding 90 days. 

What Happened

The aircraft involved had been delivered new on 5 March 2019 and had accumulated 100 flight hours by the time of the investigated event. The MLG tyres fitted at the time of the event were the ones it had arrived with. During the second of two flights by the aircraft on 2 December, uncommanded disengagement of the autobrake on touchdown was reported and ground testing of the Antiskid Autobrake Control Unit (AACU) in accordance with the AMM was performed. This found a fault message which indicated an internal fault within either the AACU or the antiskid valve. 

Time pressure prevented completion of the further investigation required and so the aircraft was released to service for the flights on 3 December in accordance with MEL provisions with the Antiskid and Autobrake systems inoperative. These provisions required the Autobrake system to be OFF and the speedbrakes to be operated manually. Also, the ‘antiskid inoperative braking procedure’ must be used in the event of a rejected takeoff or during a landing:

• minimum aircraft braking consistent with runway length with initial braking using light steady pedal pressure and increasing the pressure only as ground speed decreases. Do NOT pump the brakes.
• no application of aircraft brakes until the nose wheel is on the ground and the speedbrakes have been manually deployed.  

During the approach to Phnom Penh, the Captain was PF and when he armed the speedbrake system, the ‘SPEEDBRAKE DO NOT ARM’ annunciation appeared which reminded the pilots that manual deployment of the speedbrakes would be required in accordance with the MEL and the Captain then returned the speedbrake Lever to the off position. For the landing, the Captain had intended to manually deploy the speedbrakes as instructed but when he selected reverse thrust, “he saw that the speedbrake lever was moving to the deployed position by itself and realised that the reverse thrust selection [had] resulted in speedbrake deployment".

Prior to the subsequent approach at Changi, the Senior First Officer (PF) asked the Captain whether the armed speedbrakes would move automatically when reverse thrust was selected and received the response that “the speedbrakes could be deployed manually or could move by itself to the deployed position when reverse thrust was selected”. On landing, he selected reverse thrust expecting that, as on the previous landing, the speedbrakes would deploy automatically, although he was aware of the MEL operational requirement whereby he should manually deploy the speedbrakes. He intentionally made a gentle touchdown and then initially applied minimal brake pressure because he was “concerned that a positive touchdown would entail excessive braking and induce tyre damage, given that the antiskid system was inoperative”. 

However, when reverse thrust was selected, the Captain saw that the speedbrake had not moved to the “UP” position by itself and alerted the PF who manually selected the speedbrakes. As the aircraft decelerated, the PF “noticed a dark object flying forward on the right-hand side of the aircraft” and suspected that it could be FOD so he advised the Captain who informed ATC accordingly. At around the same time, a Boeing B747F crew taxiing in the opposite direction on the parallel taxiway called ATC to say that they had seen smoke coming from the left main gear of the 737.

This led to the 737 stopping as soon as it had cleared the runway so that ATC could visually check for any fire or smoke coming from its left main gear but nothing abnormal could be seen and as there was no vibration or directional control abnormality and no excessive thrust was needed to taxi the aircraft, it was decided to continue taxiing to the assigned terminal parking gate. As this was approached, it was seen that the left main gear outboard wheel was indicating a lower brake temperature than the other three wheels. A requested inspection once parked then found that the left main gear outboard tyre had burst and its inboard tyre had deflated and that there was associated damage to the left main gear bay door frame. 

The outboard (left) and inboard (right) left main landing gear tyres. [Reproduced from the Official Report]

The damaged frame on the fixed door of left MLG. [Reproduced from the Official Report]

Why It Happened 

The sequence of relevant events during the landing was reconstructed from available sources and depicted on the illustration below:

A composite depiction of the landing at Changi. [Reproduced from the Official Report]

Examination of the damaged tyres showed that the flat spots on both tyres had been caused by the wheels skidding and the outboard tyre had burst as a result of the flat spot grinding through the tyre material. There was no evidence of FOD that could have caused the damage to both tyres and no structural abnormalities were found in the carcases of either tyre.

It was noted that if the speedbrakes are armed, they will be deployed when:

• the ‘speedbrake armed’ annunciation is illuminated
• the sensed height above runway is less than 6 feet
• both thrust levers are retarded to “IDLE” position. 
• the main landing gear wheels spin up (more than 60 knots)

Any individual wheel spin-up/compression will enable flight spoiler deployment whereas compression of both the left and right MLG struts is required for the ground spoilers to deploy.

It was further noted that if the speedbrakes are not armed for landing or in the case of a rejected takeoff, they will still automatically deploy if:

• the MLG wheels spin up (more than 60 knots) and
• both (forward) thrust levers are retarded to idle and 
• reverse thrust is selected

It was noted that the FCTM provided the following guidance for operations with the antiskid system inoperative:

• ensure that the nose wheels are on the ground, and the speedbrakes are extended before applying the brakes. 
• initiate wheel braking using very light pedal pressure and increase pressure as ground speed decreases. 
• apply steady pressure. 
• use minimum braking consistent with runway length and conditions to reduce the possibility of tyre blowout. 
• do not pump the brakes - each time the brakes are released, the required stopping distance is increased. Also, each time the brakes are reapplied, the probability of a skid is increased.   

The crew followed the same (incorrect) procedure for the landing at Singapore and the previous one at Phnom Penh but whilst wheel locking occurred at Singapore, it did not at occur at Phnom Penh. The Investigation found that the reasons for this difference were as follows:

• The Singapore landing did not result in the MLG struts being sufficiently compressed to cause the air/ground sensing system to change from air to ground so that the condition for automatic speedbrake deployment by means of reverse thrust was not met. On the basis that ground sensing had occurred, braking was commenced before the aircraft weight was on the MLG but with the antiskid system inoperative, the applied pressure was sufficient to lock the wheels and skidding followed as ground sensing criteria were met.
• At Phnom Penh, the wheels did not lock because by the time reverse thrust was selected, the more positive touchdown had already compressed the MLG struts sufficiently to cause the air/ground sensing system to change from air to ground so that automatic speedbrake deployment followed. Brake application followed “at about the time of the speedbrake deployment” so no wheel locking was triggered.   

The Conclusions of the Investigation were formally documented as follows:

1. The damage to [the] 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 before the manual deployment of speedbrakes which would have ensured that the aircraft weight was on the MLG. The brake pressure acting on the left MLG wheels was sufficient to result in the locked-wheel situation.  
2. For the previous landing in Phnom Penh, the speedbrakes were deployed after reverse thrust selection. Subsequently, the aircraft brakes were applied after the air/ground sensors were sensing "GROUND" and there were no locked-wheel situations or skidding of wheels.  
3. For the landing in Singapore, although reverse thrust was selected, the speedbrakes did not automatically deploy as the air/ground sensors were still sensing “AIR” when the Senior First Officer was performing a gentle landing. When the aircraft brakes were applied, this caused the left MLG wheels to lock. Subsequently, the locked wheels skidded on the runway and damaged the left MLG tyres.
4. The flight crew’s action in selecting reverse thrust for speedbrake deployment did not follow the MEL requirements for manual deployment of the speedbrakes for both landings.    

One Safety Recommendation was made as a result of the Investigation findings as follows: 

• that Singapore Airlines emphasises to all its pilots the importance of following the operational requirements of the Minimum Equipment List. [RA2022-006]

The Final Report was published on 17 November 2022.

Related Articles

• Minimum Equipment List (MEL)
• Tyres
• Spoilers and Speedbrakes