Description

On 20 October 2023, a Boeing 737-800 (G-TAWD) being operated by TUI Airways on an international passenger flight from Corfu to Leeds Bradford veered off the side of the wet runway after a normal touchdown in day IMC in a limiting crosswind was mismanaged. Juddering of the nose landing gear occurred and main and nose landing gear tyre damage was subsequently found but damage to the aircraft was otherwise minimal and there were no injuries to any of the 201 occupants who were eventually able to disembark the aircraft normally and board ground transport to the terminal.

The aircraft where it finally stopped. [Reproduced from the Official Report]

Investigation

A Field Investigation was carried by the UK Air Accident Investigation Branch (AAIB). Relevant recorded data were recovered from the aircraft CVR and FDR. Supporting information was also provided by both ATC and the airport operator.

It was noted that the 41 year-old Captain, who was acting as PF, had a total of 14,250 hours flying experience which included 2,800 hours on type. Similar experience and age information for the First Officer was not recorded.

What Happened 

During the descent, landing performance calculations for the 2,250 metre-long runway 14 at Leeds Bradford (the displaced landing threshold means that the LDA is only 1,801 metres) using the surface wind provided of 060°/19 knots were made. Both pilots subsequently recalled - without explanation - that they had concluded that this wind velocity would place an approach outside the allowable limits which was why they had initially asked to take up the hold at the overhead NDB. However, as the aircraft approached the hold, ATC advised that the surface wind was 070°/21-33 knots and that another Boeing 737-800 had just landed. Having confirmed that this wind velocity was in limits, an ILS approach to runway 14 was accepted.

Conditions on the approach were “smoother than expected” and it was flown with minimal airspeed fluctuations. The aircraft was configured for landing earlier than usual and autobrake max was selected and a landing clearance was given when descending through approximately 2,000 feet with a wind check of 070° 23/37 knots which meant a maximum crosswind component of 35 knots. With just over 1½ nm to go, the AP and A/T were disconnected.

The aircraft was de-crabbed in the flare and a smooth landing within the TDZ followed. The autobrake engaged one second later and reverse thrust was selected with the target deceleration rate achieved. FDR data showed a left control column input and a rudder pedal movement from right to neutral soon after landing and the aircraft responded by yawing to the left. In response, the yaw was reduced by restoring right rudder. However, decelerating through 107 KCAS, the autobrake was disconnected and for the following eight seconds, there was no manual braking.

Reverse thrust was cancelled at around 86 KCAS and the thrust levers were returned to the ‘forward idle’ position but as right rudder pedal input was increased, juddering of the nose landing gear tyres became evident and the Captain responded by returning the rudder pedals to neutral. Braking was briefly asymmetric but was then equalised and as a result the aircraft continued to yaw to the left. Some asymmetric braking and then the steering tiller were used as the left runway edge was approached and three seconds prior to reaching the runway edge, maximum manual symmetric braking was applied. The aircraft left the runway at a groundspeed of approximately 55 knots and then travelled around 220 metres before stopping six seconds later. Normal shutdown procedures were followed and all occupants remained on board to await transport to the terminal.

The aircraft track showing the uncorrected deviation left from the runway centreline beginning soon after passing taxiway ‘L’. [Reproduced from the Official Report]

Boeing was asked to assess the FDR landing data, the actual recorded weather conditions and a wet runway in their simulation model. They confirmed that the aircraft deceleration had not been “friction-limited” at any point and that the aircraft had “likely achieved a braking action commensurate with a dry runway”. They noted that in the prevailing circumstances it would be expected that to maintain the runway centreline, “approximately 2-3 degrees of right rudder pedal, increasing to 4 degrees by the time the aircraft comes to a stop” would be required. They stated that they had previously seen similar control wheel inputs used in an attempt to maintain directional control during deceleration in a crosswind in runway excursion events as in this case and “stated that the contribution of this control wheel input to the loss of lateral control was negligible....and may have been an instinctive response”.

A subsequent examination of the runway by the Investigators concluded that the nosewheel track had changed from two clear tyre marks to a single tyre mark approximately 840 metres after touchdown by which time the aircraft was about half way between taxiways ‘L’ and ‘D’ and this single mark then continued for until the aircraft left the runway. This change in marking confirmed the point where the nosewheels “were turned to one side instead of in the direction of the aircraft’s travel”. No runway markings indicative of reverted rubber aquaplaning were found nor was any corresponding evidence found when the tyres were inspected. Both nosewheel tyres had abrasion surface damage around their entire circumference confirming the condition which would be expected when they were turned almost 90° to the direction of travel whilst still rotating.

Following the removal of the left nose wheel, the origin of the juddering reported to have occurred was found - the inner axle bearing had failed (the outer axle bearing was undamaged). These bearings are replaced on condition and do not have a fixed service life - the failed bearing was found to have be been fitted during a routine wheel overhaul in September 2022.

The runway 14/32 surface was grooved concrete and a check of the Minimum Friction Level (MFL) after the accident found that the average values for each section of the runway exceeded the MFL. A runway inspection made shortly before the aircraft landed found that although the surface was wet along its entire length, it had not been contaminated and “rainwater could be seen draining away”. The Investigation “found no evidence to suggest the reported runway condition was not reflective of the actual runway condition”.

Why It Happened

After eliminating other possibilities, the relevant operating procedures and guidance in respect of aircraft directional control after touchdown on a wet runway in a strong crosswind were reviewed.

It was noted that company procedures stated that any landing roll out not involving use of the autobrake must involve smooth manual application of the wheel brakes “with steadily increasing pedal pressure as required for runway condition and runway length available (whilst maintaining) deceleration rate with constant or increasing brake pressure as required until stopped or desired taxi speed is reached”.

The Boeing FCTM also stated that “rudder control (is) effective until approximately 60 knots (and that) rudder pedal steering is sufficient to maintain directional control during the rollout”. It further stated that “a combination of rudder, differential braking, and control wheel input should be used to maintain (the) runway centreline during strong crosswinds, gusty wind conditions or other situations” with these control inputs “maintained until reaching taxi speeds”. The FCTM also “recommended that autobrake is used when the runway is limited or when landing in a crosswind and that the speed at which manual braking is made will depend on the deceleration rate of the aircraft, runway conditions and stopping requirements”. Associated guidance on manual braking stated that immediately after main gear touchdown, smooth application of a constant brake pedal pressure should be used to achieve the desired braking with “full brake pressure used for short or slippery runways” (whilst no evidence was found that the runway had been slippery, it was relatively short).

In respect of reverse thrust use, the operator’s procedures recommended that “when stopping is assured and the airspeed approaches 60 KIAS start reducing the reverse thrust so that the reverse thrust levers are moving down at a rate commensurate with the deceleration rate of the airplane” to ensure that reverse idle is reached by taxi speed and reverse thrust stowed once the engines have decelerated to idle.

It was also noted that (although there was no evidence that the runway was slippery), guidance in the Boeing FCTM was found to state that maintaining directional control when landing in crosswinds on a slippery runway if deviation from the centreline occurs during strong crosswinds, pilots should release the brakes and reduce reverse thrust to idle to prevent the reverse thrust side force component adding to the crosswind (see the illustration below). The benefits of brake release were described as increasing tyre-cornering and contributing to retaining or regaining directional control and the benefit of setting reverse idle as “a reduction in the reverse thrust side force component without the requirement to go through a full reverser actuation cycle”. 

The Boeing FCTM illustration on the use of reverse thrust in crosswinds on a slippery runway. [Reproduced from the Official Report]

Finally, in respect of landing performance using flap 40 and reverse thrust, it was noted that whilst the distance available was sufficient to allow a MAX AUTO BRK landing with 100 metres to spare or maximum manual braking with 200 metres to spare, cancelling the auto brake selection early and not then not maintaining continuous manual braking and cancelling reverse thrust had “invalidated the landing performance calculated by the crew” although no evidence was found that “the aircraft did not have sufficient stopping distance available or that the crew assessed this to be the case”. It was considered likely that the inner axle bearing of the left nosewheel, which was found to have failed, had probably failed during the investigated landing creating the vibration felt through the rudder pedals by the Captain. However, these bearings are routinely replaced ‘on condition’ and it was noted that a single bearing failure “did not prevent the nosewheel from rotating nor did it affect the ability to steer the nosewheels” and no evidence which indicated that the bearing failure “physically reduced or restricted the pilot’s ability to control the aircraft in yaw”.

Overall, it was considered that although “deviation from the runway centreline may have been considered a challenging situation by the crew (and) there was limited time for the crew to assess the cause of the judder and the practical impact it had on the directional control of the aircraft” as it approached the edge of the runway, “it is not clear why the PF Captain did not attempt to use all right rudder available, in spite of the judder, to prevent the runway excursion”. It was accepted that “although a nosewheel judder would not necessarily be considered an ‘intense stimulus’”, it was possible that it had created a surprise which, as the aircraft approached the side of the runway, resulted in the Captain’s failure “to attempt to use all right rudder available.....to prevent the runway excursion".

The formal narrative Conclusion of the Investigation was as follows:

The initial phase of the landing roll was normal, with the aircraft touching down in the touchdown zone and meeting the target deceleration rate. The PF disconnected the autobrake and then stowed the reverse thrust early in the landing rollout and a constant deceleration was not maintained.

The deviation from the centreline, resulting from the strong crosswind from the left, required more right rudder input than was applied, in order to correct it. Additional use of differential braking to assist was also available. There was an unusual juddering from the nosewheel reported by the crew likely resulting from the failure of a nosewheel bearing. There was no mechanical defect identified by the investigation which would have prevented the crew from applying the additional right rudder that was available to keep the aircraft on the runway. However, the crew’s actions may have been influenced by the nosewheel juddering.

The Final Report was published on 24 October 2024. No Safety Recommendations were made.

Related Articles

• Runway Excursion
• Cross Wind Landings
• Surprise
• Brakes
• Rudder
• Aircraft Performance
• Landing Distances
• Standard Operating Procedures (SOPs)