Description

On 28 June 2023, a Boeing 777-300ER (HZ-AK28) operated by Saudi Arabian Airlines on a scheduled international passenger flight from Riyadh to Dhaka as SVA 806 touched down to the left of the landing runway centreline before drifting farther to the left. The left main landing gear departed the edge of the paved surface and paralleled the runway edge for over 400 metres before the aircraft was returned to the runway. It was brought to stop and then cleared to backtrack and exit the runway to taxi to its parking position without either pilot reportedly being aware of the excursion. Once the aircraft reached its parking position, ground crew saw that there was significant damage to the left main gear and informed the captain.

Investigation

An investigation was opened by the Aircraft Accident Investigation Committee Bangladesh (AAIG-BD) of the Bangladesh Ministry of Civil Aviation and Tourism in accordance with the prevailing Annex 13 guidance. The cockpit voice recorder (CVR) and the flight data recorder (FDR) were removed from the aircraft and their data were downloaded for the investigation at the Saudi Arabian National Transport Safety Centre in Jeddah.

The 60-year-old captain was a Malaysian national who had a total of 23,313 hours flying experience, of which 13,154 hours were on type. The 50-year-old first officer, who was acting as pilot flying (PF) for the approach, was a Saudi National who had a total of 5,300 hours flying experience, of which 1,750 hours were on type.    

What Happened

An instrument landing system (ILS approach) was flown to runway 14 with the tower reporting the visibility as 1,500 metres in heavy rain with the wind 240° at 15 knots two minutes before touchdown. The autopilot (AP) was disconnected at 98 feet agl, by which time the wind velocity recorded on the FDR was 250° at 8 knots. The aircraft began to drift left from 50 feet agl and continued to do so as it crossed the threshold to the left of the centreline, as the first officer made a left aileron input which worsened the deviation. From 10 feet agl over the runway, the captain began calling “centreline." Touchdown occurred 150 metres past the threshold with the left main gear already approximately 11 metres to the left of the 45 metre-wide runway’s centreline, i.e. about the same distance from its edge as from the centreline. The technique used was the one normally used for a dry runway touchdown - autobrake set, reversers symmetrically deployed without delay and the nose gear initially held well clear of the surface. FDR data showed that “almost instantly," the aircraft weathercocked slightly into wind as would be expected as the first officer made a “significant left control column input” on what was a wet (and at least potentially slippery) runway. The captain continued to call “centreline” but after five further such calls, with the first officer making no attempt to correct the drift and return to the centreline, the captain took control. However, by then the left main gear had already departed the paved surface and was digging a trench in the wet grass parallel to and approximately 5 metres from the edge of the paved surface (see the first illustration below).

Eight seconds after touchdown as the left main gear continued on the grass, the CVR recorded impact sounds as it hit one of six concrete manhole access ducts to underground cables which projected above the surface and passed over its approximately 2 metre x 2 metre metal cover (see the second illustration below). After 440 metres with the left main gear on the grass, the captain was able to return it to the runway and then to its centreline. The landing roll was continued to the end of the 3,200 metre-long runway before a 180° turn followed by a backtrack to exit the runway. During the backtrack, two EICAS messages appeared - ‘TIRE PRESS’ and ‘BRAKE TEMP’. The crew stated that as they were unaware of the excursion, the taxi in to stand 5 had been completed without any action in response.

The channel made by the left main gear as it continued parallel to the runway. [Reproduced from the Official Report]

One of the inspection manholes the landing gear crossed over with its metal cover broken. [Reproduced from the Official Report]

Once the engines had been shut down on stand, passenger disembarkation “proceeded normally” without event awareness. The ground mechanics meeting the flight then spoke to the flight crew and told them that the left main gear was “covered with grass and mud” and that all six tyres were damaged/deflated with the hubs of two of the wheels exposed. The captain reported this to the tower and a runway inspection was conducted. Minor impact damage to the inner left flap was also subsequently found.

The left main gear looking aft. [Reproduced from the Official Report]

Why It Happened

It was established that the aircraft had no relevant pre-event airworthiness deficiencies and that aircraft handling was the origin of the excursion. The crosswind component was minimal (equivalent to a less than 3° deviation if no compensation made) and it was consistent. Although the forward visibility was reduced, it was assessed that this had not in itself interferred with the pilots’ ability to accurately assess the aircraft trajectory relative to the runway before or after touchdown.

The Investigation reviewed the guidance in the Boeing Flight Crew Training Manual (FCTM) on the wet runway landing centreline correction manoeuvre and it was found to say as follows:

“As the airplane starts to weathervane into the wind, the reverse thrust side force component adds to the crosswind component and drifts the airplane to the downwind side of the runway. Also, high braking forces reduced the capability of the tires to corner. Guidance to correct back to the centreline: “release the brakes and reduce reverse thrust to reverse idle”. Releasing the brakes increases the tire-cornering capability and contributes to maintaining or regaining directional control. Setting reverse idle reduces the reverse thrust side force component without the requirement to go through a full reverser actuation cycle. Use rudder pedal steering and differential braking as required, to prevent over correcting past the runway centreline. When directional control is regained and the airplane is correcting toward the runway centreline, apply maximum braking and symmetrical reverse thrust to stop the airplane. Essentially, Boeing recommends disconnecting the autobrakes and releasing brake pressure to regain directional control.”

The Investigation noted that this guidance on restoring directional control during a landing rollout on a slippery runway “was not efficiently applied [...] or perhaps applied too late." It was observed that the one manoeuvre not considered was a rejected/balked landing whilst still on the runway. The investigation also took the view that “improper, inadequate in-flight centreline compensation and no flight crew excursion awareness or corrective technique implementation” were primarily causal and shared by both pilots.

The captain’s training records during his initial line training on type included the entry “slow response with situation awareness” although that observation had been addressed and cleared before his successful final line check at the conclusion of that training. It was considered that both pilots’ “lack of awareness of when the aircraft veered off the left edge of the runway” was of “more concern” given the captain’s failure to take over until this had already occurred.

In respect of crew performance, it was also noted that:

• Although the aircraft was properly and functionally authorised for autoland, the First Officer’s decision to disconnect the autopilot at 98 feet agl was a deviation from both Boeing and Saudi Arabian Airlines’ accepted practice
• Lack of response to the Captain’s repeated “centreline” deviation verbal warnings called for an immediate takeover of control and the delay contributed to the excursion

The Probable Cause of the Serious Incident was formally documented as “the captain’s delay in (the) mandated action of taking over flight controls from the first officer’s inappropriate/aggravating response to an off-centreline wet/slippery runway landing”.

Two Contributory Factors were identified:

1. The deteriorating changing weather of medium to heavy rain and crosswind conditions during landing. 
2. Damage to the aircraft and escalated safety risk was attributed to the abrupt encounter with projecting concrete around the underground cable access manholes.

Two Safety Recommendations were made based on the Findings of the Investigation as follows:

• that Saudi Arabian Airlines should:
  • conduct Scenario Based Training (SBT) for this critical phase of flight for the purpose of managing the risk of:
    • inaccurate performance of a crew and 
    • provision of inaccurate information to crew
  • introduce and/or design ‘Line Oriented Flight Training’ Simulator Modules to cover wider scenarios exceeding the basic required flying skills (such as runway centreline alignment and effective crosswind handling with associated cross-wind) emphasising:
    • flight crew communication, management and leadership; and,
    • effective decision making and technique for rejecting a landing soon after initial touchdown
  • utilise Flight Data Monitoring (FDM) to monitor and collect data of real-time 
• that the Dhaka Aerodrome Authority should ensure:
  • that ATC provide accurate and timely reporting of ambient weather conditions, especially wind strength, direction and variation, runway surface state and braking action.
  • act to resolve the risk presented by the proximity of the cable-laying manholes to the runway edge (ICAO Annex 14 and ICAO Doc 9981 PANS-ADR or ICAO Doc 9859-SMM).
  • to resolve, in a similar manner to the AAIG-BD’s Safety recommendation ‘d’ of the report into the similar runway excursion off the western side of the same runway by a Thai Airways’ B-777 in 2018, this time by mitigating the risk presented by the proximity of the cable laying manholes to the eastern side of the runway edge (ICAO Annex 14 and ICAO Doc 9981 PANS-ADR or ICAO Doc 9859 Safety Management Manual).

The Final Report of the Investigation was published on 23 June 2024. 

Related Articles

• Runway Excursion
• Landing on Contaminated Runways
• Runway Surface Friction
• Situational Awareness
• Crew Resource Management
• Decision-Making
• Rejected Landing