B738, Darwin Australia, 2016
B738, Darwin Australia, 2016
On 6 December 2016, a Boeing 737-800 approaching Darwin at night in the vicinity of thunderstorm activity suddenly encountered very heavy rain just before landing which degraded previously good visual reference. After drifting right of centreline just before and after touchdown, the right main gear left the runway for 400 metres before regaining. The landing and taxi-in was subsequently completed. The Investigation attributed the excursion to difficulty in discerning lateral drift during the landing flare to an abnormally wide runway with no centreline lighting in poor night visibility and noted similar previous outcomes had been consistently associated with this context.
On 6 December 2016, a Virgin Australia Airlines Boeing 737-800 (VH-VUI) being operated on a scheduled domestic passenger flight from Melbourne to Darwin departed the side of the 60 metre-wide 3354 metres long destination runway 29 at night after a ILS Category 1 approach in night VMC when shortly before landing, the previously good visibility had been suddenly reduced in very heavy rain. A drift away from the unlit runway centreline thereafter went undetected and the crew did not realise that the aircraft had left the paved surface soon after touchdown. Inspection of the aircraft on its parking gate after engine shut down found all right main gear tyres had sustained significant damage. After TC had been advised of this, six runway edge lights were found to have been destroyed leaving associated debris on and near the runway edge.
An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). Data from the aircraft FDR was downloaded and provided useful information on the precise trajectory of the aircraft.
The B737 flight crew were noted to have been a Captain with a total of 22,559 flying hours of which 9,206 hours were on type and a First Officer with a total of 7,000 flying hours of which 3,500 hours were on type.
It was established that after discovering en route that the destination weather was forecast to be significantly worse than that forecast prior to departure, the flight crew made appropriate advance preparations. By the time they arrived in the area, there was thunderstorm activity in the vicinity of the airport but the crew decided that they would continue their ILS Category 1 approach to the 60 metre-wide, 3354 metre-long runway 29 since they had good visual reference with the runway and its surrounds after emerging from the lowest cloud at about 2500 feet. At this time, a “rain shower” was reported to have been visible “towards the far end of the runway”. It was agreed that in the event that a go around became necessary, it would be flown to the right rather than straight ahead and the Captain, who was PF, asked the First Officer to be “ready to use the windscreen wipers”.
The approach was stable and the runway lights remained clearly visible below DA with the wipers on ‘fast’ from 500 feet agl until, at a recorded 179 feet agl, the aircraft suddenly entered heavier rain which rapidly intensified over the next few seconds. A small but slightly variable crosswind component from the left began to increase and analysis of FDR data indicated that from about 140 feet agl, which occurred about 5 seconds before crossing the runway threshold at 50 feet agl, the aircraft had been banked slightly right and had begun to drift away from the extended centreline without correction until touchdown. It was concluded that this continuing drift to the right could be attributed to “a combination of momentum and continuing crosswind”. It led to the runway threshold being crossed at 50 feet agl with the aircraft about 3 metres right of centreline and continuing to drift to the right at about 3°. By the time the landing flare was initiated at 35 feet agl, the aircraft was about 11 metres to the right of the runway centreline.
A main gear touchdown wings level and within the TDZ followed with the aircraft about 21 metres right of centreline and still drifting about 3° to the right. As the Captain lowered the aircraft nose, he subsequently stated that he had “realised the aircraft had landed ‘well right’ of the centreline but with the obscured visibility he could not readily ascertain how far”. He also stated that although he recalled having tried to steer the aircraft back towards the centre of the runway, he had been “very wary of over-steering and losing control on the wet runway”.
Four seconds after touchdown after travelling about 235 metres and at a recorded groundspeed of 126 knots, the right main landing gear left the sealed surface of the runway. It then remained no more than 1.5 m from the edge of the runway and impacted 6 runway edge lights mounted on concrete pads until it regained the runway after an excursion lasting just under 400 metres at a recorded groundspeed of 92 knots and thereafter remained on it. The Captain then deployed the thrust reversers, subsequently commenting that he had delayed this action until he was sure the aircraft was away from the edge of the runway. An illustration below shows the aircraft ground track prior to, during and immediately after the excursion.
Unaware that an excursion had occurred, the crew then taxied the aircraft to its allocated gate but stopped short for around 10 minutes until the weather conditions had eased sufficiently for ground crew to attend the aircraft and facilitate passenger disembarkation. Only after a subsequent external inspection did the crew report realising that the aircraft had left the runway edge and immediately reported this to ATC. After an initial brief runway inspection during which at least four runway lights were found inoperative but no debris other than grass was seen on the runway, the next approaching aircraft was advised of the situation and elected to continue. After its uneventful landing a few minutes later, a more thorough inspection then found debris from the runway lights was present on the runway surface.
During the excursion, the Captain recalled having heard “a repeated light thudding noise” which prompted him to ask the First Officer if they had hit the runway lights and was advised that they were just missing them. Since the aircraft nose was oriented to the left of the direction of travel, the front of the aircraft was within the painted runway edge markings making this assessment difficult. The First Officer later reported having felt “a shudder, as though the wheels were skidding” and both pilots later commented that “the physical sensation and noise were not as strong as they would have expected from the wheels departing the runway”. Both Melbourne-based pilots also stated that they were familiar with operations into Darwin with the most recent landings there in the case of both pilots having been at night. The Captain also stated that he had only seen rain conditions there like those which had affected the final stages of the investigated approach once, about 30 years previously and the First Officer said that he had never seen such conditions there before.
Why it happened
It was considered that in the absence of any evidence of aircraft unserviceability or crew fatigue, the excursion under investigation followed a normal approach and landing in all aspects except the undetected extent of lateral drift prior to touchdown in a context of reduced visibility at night. In particular, the crosswind component “was not sudden or strong enough to be significant under most circumstances” and was essentially a landing which suddenly became additionally challenging due to gusty, turbulent conditions in reduced night visibility which reduced the extent of information available to assess the aircraft behaviour and respond appropriately.
It was noted that the existence of at least some visual reference had continued throughout the final part of the approach through to touchdown and completion of the landing, but the problem for the crew was the degraded nature of this reference. It was also reported that the TWR controllers, located just under 1 nm northwest of the runway 29 threshold “had been able to see the lights of the aircraft including the illuminated logo light on the vertical stabiliser throughout the final approach and landing”.
A number of factors of potential relevance to the conduct of the approach were examined:
- The impaired visibility through the windscreen in heavy rain.
The aircraft type was certified for operation with windscreen wipers alone and was not fitted with a rain repellent fluid dispensing system of the sort which had been fitted to earlier versions of the type. However, windscreens could be treated with a hydrophobic coating if specified by the operator but it was noted that the service life of such coatings would depend on the operating environment in which the aircraft was used. Virgin Australia reported that 43 of its 79 Boeing 737 aircraft, including the aircraft involved in the event under investigation which had been delivered new in 2013, had arrived with a hydrophobic coating applied but that their maintenance schedule did not include any regular maintenance of these coatings and they “had not been routinely replaced in service”.
- The effect of the wide runway.
Darwin was identified as the only 60 metre-wide Australian runway which did not have centreline lighting. The only other Australian runway that width was runway 16/34 at Melbourne, which was equipped with centreline lighting. The other four Australian runways which had centreline lighting were all 45 metres wide - Brisbane 01/19, Melbourne 27 and Sydney 16L/34R and 16R/34L. These runways had Cat 2/3 approaches whereas Darwin runway 29 was Cat 1 only. It was found that the only other Australian occurrence in which a transport aircraft had been involved in a similar runway excursion in a similar context (specifically involving reduced forward visibility rather than a total loss of it) during the period 1997-2017 had also been at Darwin. As a result, the operator in that case had made considerable relevant additions to their Darwin airport briefing material whereas it was noted that the corresponding guidance to Virgin Australia pilots had not been similarly improved.
It was observed once the aircraft had begun to drift to the right soon after entering the heavy rain below 200 feet agl, maintaining the required flight path would have required corrective action. It was concluded that the available visual cues to achieve this - the runway edge lights and the marked but unlit runway centreline - were likely to have provided insufficient reference to allow the deviation to be detected and thus appropriate corrections made. It was also noted that in the prevailing conditions, and given that the runway threshold had been crossed at 50 feet agl, the forward visibility available to crew below about 100 feet agl would no longer have included either the approach lighting or the runway threshold lighting and the only visual clues to the relative lateral position of the aircraft would have been the (widely spaced) edge lights with the centreline markings not visible to a useful - if any - extent.
This prompted the Investigation to conduct a simple simulation of the edge lights as they would have been seen by the Captain by using a simple model representative of the prevailing night visual conditions. As illustrated below, this demonstrated that the 3.4 metre deviation from the centreline at 60 feet agl would have been “almost undetectable” and that the 11 metre deviation at 24 feet agl would have been “inconspicuous”. The model was also used to show that the addition of centreline lighting would have materially improved the ability to detect the deviation at both points and that this improvement had centreline lighting been available would have been equally true with a 6° bank angle.
It was noted that the 7th edition of ICAO Annex 14 Volume 1 published earlier in 2016 contained a Standard requiring runway centreline lighting for Category 2 and 3 runways but only a Recommendation that these should be provided on Category 1 precision approach runways - although with a particular emphasis on such runways if used by aircraft with high landing speeds or if the distance between the edge lights on each side of the runway was greater than 50 metres. It was noted that this same Recommendation had been included in the Annex and not subsequently upgraded to a Standard since 1966. It was also noted that there was no corresponding Recommendation for TDZ lighting on such runways, although this was a Standard for Cat 2 and 3 runways.
- Runway surface water drainage.
Since it was likely that there was already considerable surface water on the runway when the aircraft touched down, it was considered of note that although the runway was 60 metres wide, only the central 45 metres was grooved to assist surface water lateral drainage and as such “did not meet the relevant texture or surface friction requirements”. However, it was not considered that this had contributed the excursion.
- AIP Information for Darwin.
Aerodrome operators were obliged under the prevailing regulations to ensure that “adequate particulars” were included in the AIP En Route Supplement Australia (ERSA). Guidance on what such particulars should be were noted as including “the width and lighting information for each runway and important cautionary […] information relating to the use of the aerodrome”. It was noted that although ERSA information for many aerodromes included specific hazards such as birds, weather balloon launches or the likelihood of turbulence on particular runway approaches, “specific guidance with regard to visual issues on approach was uncommon”. The ERSA information for Darwin did not include any information on “the potential for visual illusions or potential problems with visual guidance during landing associated with the runway width, lack of centreline lighting or touchdown zone lighting”.
- The decision to continue the approach to a landing.
The crew were fully aware that the approach was being commenced and continued in deteriorating weather conditions. Frequent updates were provided by ATC and the aircraft weather radar was being fully utilised. However, in the five minutes before landing, there had been several recorded lightning strikes within 5 nm of the airport and two within 5 nm of the aircraft and an instrumented rain gauge situated near to the threshold of the landing runway had begun recording heavy rainfall 15 minutes prior to landing and this had peaked at a rate of 24 mm/hour for “a period of several minutes around the time the aircraft landed”.
It was noted that the operator’s FCOM recommended that an approaching thunderstorm should be avoided by at least 5 nm at any time during descent, approach and landing unless, in the landing case, they have cleared the immediate area since “the rear of storms is considered to be comparatively inactive”. However, it was also accepted that this was not a strict requirement and that the decision to continue was a matter of judgement. Overall, it was concluded that “the flight crew’s interpretation of the information available to them gave no compelling reason to initiate a missed approach and they did not detect the deviation from centreline until after touchdown”. In particular, it was considered that whilst a decision to continue an approach after entering heavy rain “would usually not result in an adverse outcome […] the combination of a wider runway and relatively limited visual cues” at night had increased the potential for adverse consequences. In this respect, it was observed that “a flight crew that is informed about the potential risk associated with a sudden reduction in visibility close to landing, particularly at airports with 60 metre-wide runways and no centreline lighting” is likely to have greater readiness to recognise and respond appropriately to the risk or onset of such circumstances.
The Findings of the Investigation were as follows:
Three Contributing Factors were identified as:
- After passing the decision height for the approach, the aircraft drifted under the influence of a slight crosswind and landed 21 m to the right of the runway centreline.
- Due to heavy rain, darkness and limited visual cues, the flight crew did not detect the aircraft’s deviation from the runway centreline prior to landing.
- The absence of centreline lighting and the 60 m width of runway 11/29 at Darwin result in very limited visual cues for maintaining runway alignment during night landings in reduced visibility. [Safety Issue]
Six Additional Factors that increased risk were also identified as follows:
- Category 1 runways that are wider than 50 m and without centreline lighting are overrepresented in veer-off occurrences involving transport category aircraft landing in low visibility conditions. The installation of centreline lighting on wider Category 1 runways is recommended but not mandated by the International Civil Aviation Organisation Annex 14. [Safety Issue]
- Virgin Australia did not have formal guidance for flight crews regarding the limited visual cues for maintaining alignment to runway 11/29 at Darwin during night landings in reduced visibility. [Safety Issue]
- The En Route Supplement Australia (ERSA) did not have formal guidance for flight crews regarding the limited visual cues for maintaining alignment to runway 11/29 at Darwin during night landings in reduced visibility. [Safety Issue]
- While the aircraft was in descent, the airport’s primary anemometer failed due to a lightning strike. Although there were windsocks and a second test anemometer at the airport, the reliability and timeliness of the wind information that the controllers could provide to flight crews was temporarily degraded.
- Although air traffic services broadcast details of an amended aerodrome forecast (TAF) for Darwin issued at 2207, the flight crew were not made aware of a subsequent amended TAF issued at 2237. However, the flight crew were provided with multiple updates of current weather conditions prior to landing at 2302.
- Because they were unsure what had occurred, the flight crew did not report the possibility of a runway excursion to air traffic control until 29 minutes after it occurred. This delayed the response by the airport operator and air traffic control to ensure that the runway was safe for subsequent operations.
Two Safety Recommendations were made as a result of the Investigation as follows:
- that the International Civil Aviation Organisation review the effectiveness of Annex 14, recommendation 184.108.40.206 (for the installation of runway centreline lighting on Category 1 runways that are wider than 50 m), given that Category 1 runways that are wider than 50 m and without centreline lighting are over-represented in veer-off occurrences involving transport category aircraft landing in low visibility conditions. [AO-2016-166-SR-013]
- that Darwin International Airport address the risk of very limited visual cues for maintaining runway alignment during night landings in reduced visibility that arise from the combination of the absence of centreline lighting and the 60 metre width of runway 11/29 at Darwin. [AO-2016-166-SR-014]
Safety Action known to have been taken as a result of the investigated event included, but was not been limited to, the following:
- Virgin Australia Airlines introduced additional guidance to flight crews for approach to Darwin airport, including notes about runway surface, slope, width, lighting and ambient light and initiated specific training to pilots for loss of visibility in heavy rain. It also commenced a program to investigate the potential to install hydrophobic windshield coatings on its 737 fleet.
- Darwin International Airport initiated relevant additions to the En Route Supplement Australia (ERSA) entry for the airport under the headings ‘Additional Information’, ‘Aerodrome and approach lighting’ and (runway) ‘Physical Characteristics’.
On the basis of the findings of the Investigation, the ATSB formally documented a Safety Message as follows:
Approaches in darkness and poor weather can be challenging. Centreline lighting greatly assists flight crews align the aircraft with the runway but many runways, including most in Australia, are not equipped with it. A wide runway without centreline lighting, such as that in Darwin, poses a particular challenge. Pilots and operators who are aware of any circumstances that are different to what is usually encountered and account for it in their planning are more likely to avoid being ‘caught out’ at a critical time.
The Final Report was released on 15 May 2019.
- Runway Excursion
- Missed Approach
- Cumulonimbus (Cb)
- Aeronautical Information Publications (AIPs)
- Visual References
- Low Visibility Procedures (LVP)
- Aircraft Rain Protection
- Werfelman, Linda. Lighting the Path. AeroSafety World, Flight Safety Foundation. June 2019