Description

On 24 July 2018, a Boeing 777-200 (HS-TJD) being operated by Thai Airways on a scheduled international passenger flight from Bangkok to Dhaka as TG 321 encountered adverse weather at its destination and after an unsuccessful first approach, the landing off the second one was followed by the right hand main landing gear being off the paved surface for part of the landing roll during which obstructions were encountered that severely damaged it. After finally leaving the runway onto an RET and stopping on it, aware that significant tyre damage must have occurred, the flight crew had not called TWR, who were unaware of the excursion due to restricted visibility until the next landing aircraft reported debris on the runway. There were no injuries to the 170 occupants but substantial damage was caused to the aircraft, the runway and installations in the vicinity of the right hand runway edge.

Investigation

An Investigation was carried out by the Bangladesh Aircraft Accident Investigation Group (AAIG). Relevant recorded flight data was available from the FDR and the CVR which were both removed from the aircraft. A Preliminary Report detailing initial progress with the Investigation was published on 23 August 2018.

It was noted that the 40 year-old Captain had a total of 12,000 hours flying experience of which all but 220 hours were on type. The 36 year-old First Officer had a total of 2,437 hours flying experience of which 1,762 hours were on type. The 34 year-old TWR Controller had a total of 5 years’ experience as a controller and the 41 year old APP controller had a total of 10 years’ experience as a controller.

It was established that an initial ILS Cat 1 approach to runway 14 at Dhaka had been flown with the First Officer as PF. On 6½ nm final, the aircraft had been transferred to TWR and on checking in had been cleared to land and advised that the surface wind was 230°/11 knots and that the runway surface was wet. The actual weather conditions at the time included a visibility of 2000 metres in “moderate to heavy rain” with a 900 foot cloud base and FEW CB at 2,500 feet. Neither pilot obtained visual contact with the runway and they subsequently stated that they had commenced a go around “from the MAP”. FDR data showed that the aircraft had descended to 79 feet agl before beginning the go around.

The crew requested and received radar vectors towards a second ILS approach and three minutes after reporting a go around from the first approach, the aircraft was reporting established on the 14 ILS LLZ at 10 nm, this time with the Captain having taken over as PF. After reporting fully established on the ILS, the aircraft was transferred to TWR and cleared to land with the surface wind at 250°/10 knots onto the wet runway. At this time, unknown to the crew, a SPECI was issued which materially differed from one issued ten minutes earlier only by the change of present weather from RA (rain) to TSRA (thunderstorm rain).

The crew subsequently reported having sighted the approach lights with “the Captain then sighting the runway followed by the First Officer just prior to minima at 135-145 feet agl” during what was stated by the Investigation to have been “moderate to heavy rain”. The AP was disconnected at 135 feet agl and the aircraft was then de-crabbed and the right wing banked into wind to compensate for what was estimated as having been an 8 knot crosswind component with left rudder applied to counteract the resultant drift. Touchdown with the right main gear first then occurred ahead of the TDZ (touchdown zone) just 175 metres past the threshold with an estimated 15° right bank and to the right of the runway centreline. A drift right on the 45 metre wide runway continued for approximately 230 metres until the right main gear wheels departed the paved surface onto the adjacent “grassy-mud” surface. The aircraft then turned left to track “almost parallel” to the runway with the right main gear around 2 metres from its edge for a further 470 metres during which time the right main gear assembly ran over a series of nine concrete cable-laying pits, each about 1.5 metres by 0.6 metre in size, covered with iron sheets and protruding above the ground by 10/15 cm which resulted in significant impact damage to all six tyres and their wheels.

The aircraft then fully regained the runway surface and continued on it for about 1500 metres whilst gradually tracking back to the centreline before turning onto the RET which began 830 metres from the end of the 3,200 metre long runway and subsequently stopping near its junction with the parallel (southern) taxiway. Runway and taxiway surface damage caused by right main gear wheel rims was found from the point where that gear assembly returned to the runway until the final stopping position, a total distance of 1980 metres.

The excursion was not visible to the TWR controller because of the poor visibility and they therefore gave the usual frequency change to GND after seeing the aircraft clear the runway. In the absence of any report from the 777 crew, it was only after another aircraft, a Jet Airways Boeing 737-800, had landed two minutes later and reported “scattered debris on the right edge of the runway” that the controller became aware of anything untoward having occurred. They then immediately suspended further use of the runway and ordered a runway inspection. This then confirmed that the 777 had been involved in a runway excursion and so the controller then despatched the emergency services to the stationary 777.

Damage to the right main landing gear was found to include all six tyres “partially and/or completely torn and damaged with wheel hubs broken and exposed with the entire wheel-well having grass-mud deposition”. The brake assemblies of some wheels were also damaged with a number of cables detached from the wheel assembly and minor damage was found to the right inner flap and the right engine exhaust area had ingested “grassy mud”.

It was noted that the runway had centreline lighting which, like all the other runway and approach lighting, had been working normally. The flight crew stated they had been aware that the aircraft had touched down near the right hand edge of the runway but that they had not been aware of the excursion which followed despite the right main gear impact with significant obstructions. They stated that only when the aircraft began “juddering mildly” as they slowly regained the runway centreline had they recognised that right main gear tyre failure must have occurred. The Investigation noted that retread tyres were fitted to the aircraft.

The Cause of the excursion was formally documented as ‘Flight Crew Human Factors’ as follows:

• The flight crew’s inadequate coordination during manual handling of the aircraft while on the approach to land resulted in the aircraft neither touching down within the ‘touchdown zone’ nor at or close to the centreline of the runway because it was inappropriately aligned.
• According to the DFDR read out, the PIC applied left rudder immediately after touchdown to bring the aircraft back to centreline. The PIC provided left rudder input for approximately 19.6 seconds reaching a peak recorded value of 25.7679 inches (left rudder pedal forward). However, either the left rudder pedal input was insufficient or it was necessary to also use the left wheel brakes (which were not applied initially) to prevent the aircraft from veering off the right hand edge of the runway due to inertia.

Seven Contributory Factors were also identified as:

1. Flight Crew Human Factors:
   • The flight crew decision not to hold in the vicinity of the airport for the weather to clear before commencing the second approach for landing.
   • The flight crew decision not to go around from the second approach.
2. Weather Factors:
   • Reduced visibility during the approach due to the prevailing rain and thundershower.
   • Slant visibility restricted pilot vision and resulted in a late sighting of the runway.
3. Maintenance Factor:
   • The right main landing gear tyres being retreads may have been less able to sustain the shock and resulting major damage after hitting, one after another, the nine manholes/handholds adjacent to the paved runway surface.
4. Airport Factors:
   • It was observed that there was increased risk of damage to the aircraft from striking hard vertical surfaces (the isolation transformer concrete pits for runway lighting system) which were close to the runway shoulder.
   • The many manholes/handholds (in excess of nine) which were located too close to the runway edge, having the approximate dimensions 5 feet (length) x 4 feet (width) x 2 feet (depth) with sharp vertical edges protruding above the ground surface by about 4-6 inches, adversely contributed to the severe damage to the aircraft tyres as they rolled over them at high speed.

Four Safety Recommendations were made as a result of the Investigation as follows:

• that Thai Airways’ Flight Crew Simulator Training should include and emphasise:
  • Landing with a cross wind component, in rain/thundershower with poor visibility conditions and a wet runway surface.
  • Piloting technique while flying manually with cross wind component, rain / thundershower and poor visibility conditions yet making a landing in the vicinity of the ‘touchdown zone’ and on the runway centreline.
  • Pilot decision making on whether or not to commence an approach in marginal weather conditions.
  • To be prepared to initiate a go around at any time during an intended approach and landing should it be considered appropriate in the prevailing conditions.
• that the Bangladesh Airport Operator should upgrade the radar equipment in use so that it can show the weather phenomena in the vicinity of the airport to assist aircraft vectoring by approach controllers during difficult weather conditions.
• that Thai Airways should ensure that its maintenance reliability programme monitors the in-service reliability of retread aircraft tyres.
• that the Bangladesh Airport Operator should consider locating drains or any other underground electrical systems installation, particularly for those to be installed with concrete encasement, manholes and handholds as far as practicable from the edge of runway and/or designing them with chambered or tapering edges.

The Final Report was published on 27 March 2019.

Related Articles

• Runway Excursion
• Missed Approach
• Go-around Decision Making
• Decision Making
• Foreign Object Debris (FOD)
• Visual References
• Weather