Description

On 13 November 2018, a Boeing 737-800 (B-1918) operated by China Southern Airlines on a scheduled domestic passenger flight from Yiwu to Hong Kong as CSN6045 was on final approach at destination and following a Bombardier Global 5000 (P4-AVA) operated by Austrian company MS Aviation for a private client and on a non-revenue international positioning flight from Clark, Philippines to Hong Kong in night VMC. The 737 received a very late landing clearance for runway 07L, and it touched down before the business jet had cleared the runway. This resulted in reducing separation, contrary to ATC procedures.

Note: A third runway to the north of and parallel to the two runways which existed at the time of this event was being constructed at Hong Kong when this event occurred. It was completed whilst the Investigation was in progress and the original runway 07L/25R was then re-designated as 07C/25C in preparation for the commissioning of the new runway in early 2022.

Investigation

A Serious Incident Investigation was opened by the Hong Kong Air Accident Investigation Authority (AAIA) and conducted in accordance with ICAO Annex 13 principles. Relevant data were successfully downloaded from both the CVR and the FDR of both aircraft and recorded ATC data were also available.

Both pilots of the Global 5000 were qualified as captain on type, but the aircraft commander was not identified. The PF had a total of “more than 8,900 hours” flying experience, of which more “than 1,500 hours” were on type, and the PM had a total of “more than 8,006” hours flying experience, of which “more than 753 hours” were on type. The 737 commander was a training captain supervising line training for a first officer new to type and was acting as PM. He had a total of 14,277 hours flying experience, of which all but 200 hours was on type. The first officer under training had a total of 1,286 hours flying experience, of which all but 250 hours was on type. 

What Happened

The runway controller in position for TWR North and responsible for runway 07L at the time of this event had recently returned from a 90-minute break. Fourteen minutes after taking over, he cleared the Global 5000, which was at about 2nm and moving at a ground speed of around 120 knots, to land. The controller considered that this speed was about 20 knots slower than usual for most aircraft types at that range. He anticipated the possibility of a catch-up by the following 737 and therefore instructed the aircraft in front to expedite exiting the runway after landing.

Under a minute after landing clearance had been given to the Global 5000, the 737 checked in on the TWR frequency and 45 seconds later, the controller saw the first aircraft touch down at a recorded speed of 99 KIAS with the 737 still 3.2 nm away. Thrust reversers on the landed aircraft were promptly deployed and braking commenced, but after 20 seconds, the landed aircraft had prematurely slowed down to less than 10 knots by the time it reached the reverse direction RET exit A6, when the first possible RET exit for 07L landing traffic was A7. (See the illustration below.)

The central section of runway 07L/25R showing the first RET for 07L - taxiway A7. [Reproduced from the Official Report]

 At this point, the controller checked his Situation Display (which provided controllers with way to easily see the spacing between aircraft on approach and on the runway) and saw the aircraft moving slowly near A6. He then asked the pilots if they were taking (reverse) RET A5 (which the aircraft had already passed by more than 400 metres) and received no reply and followed this by “inadvertently instructing the aircraft to vacate the runway via RET A5, whereas his intention was, in fact, to instruct the aircraft to vacate via RET A7." The crew responded with “approaching A7 now” and the controller instructed them to “keep the speed up and vacate the runway via RET A7”.
Just over 40 seconds after the Global 5000 had slowed down to around 10 knots, the controller cleared the 737 to land “as it was about to cross the 07L threshold." However, at that moment, the Global 5000 still had than 200 metres to go before reaching A7 as shown on the controller’s A-SMGCS display and was not clear of the runway until the on-ground distance between the two aircraft had reduced from 1,470 metres to just over 1000 metres.

Why It Happened

It was noted that the purpose of multiple RETs available for all runways was to minimise the runway occupancy time for arriving aircraft, and that the optimum 30° angle enables arrivals to safely vacate the runway at speeds up to 50 knots even in wet conditions. Studies carried out jointly by the Civil Aviation Department and the Airport Authority showed an average post-touchdown runway occupancy of approximately 50 seconds, whereas after unnecessarily decelerating too quickly, the Global 5000 had been on the runway for 1 minute and 25 seconds. It had speeded up slightly from the initially recorded 9 knots but still took 40 seconds to taxi the 400 metres from abeam A6 to A7.

Part 3 of the applicable Manual of Air Traffic Control stated that “landing aircraft will not normally be permitted to cross the beginning of the runway on final approach until the preceding landing is clear of the runway”. Both Global 5000 pilots had previously landed at Hong Kong - in the case of the PF “6 or 7 times."

The 737 flight crew, aware of the traffic ahead of them which they had seen on final approach, stated that they had conducted a quick scan of the TDZ before touching down, but had not observed any abnormal lights and therefore assumed that the aircraft ahead had cleared the runway. The Investigation accepted that the failure of the B738 pilots to visually detect the aircraft ahead when they were about to land would have been hindered by its small size and by the nose-up attitude of their own aircraft in the landing flare. However, it was observed that “an ATC clearance to land does not relieve pilots of their responsibility to go around if required” but whilst the 737 pilots stated that they had been “ready and prepared to go around if necessary," they did not detect the aircraft ahead as they decelerated.

It was found that the controller had only obtained his Aerodrome Control Rating two months before the event occurred. At interview he “recalled” that when he issued the landing clearance to the 737, “his visual observation showed that the aircraft ahead had reached the entry of ‘A7’ with its nose turning onto ‘A7’ and its speed increasing” whereas at that time it was still on the runway. When he eventually realised that in fact it had still not vacated the runway, he “considered it too late to instruct the 737 to go around as it had already landed on the runway”.   

The Investigation recognised the greater difficulty of determining the position of aircraft on the runway during darkness, but noted that controllers are encouraged during training to also make use of both their Situation Display and their A-SMGCS display, especially at night.

On the basis of the ICAO Runway Incursion outcome severity scale, on completion of the Investigation the event was rated as a C which is described as “an incident characterised by ample time and/or distance to avoid a collision." The description of the event in the Preliminary Report as "Serious Incident" was therefore downgraded to an "Incident" at its conclusion.

The Cause of the Loss of Separation was formally documented as “the controller cleared an arriving aircraft to land when he inadvertently misperceived that the runway was clear whereas the preceding landing had not yet vacated the runway”.

A Contributory Factor was identified as “the preceding aircraft took an unusually long time to vacate the runway after landing by as much as 70% more than the average runway occupancy time of arrivals at the airport and thus increased the likelihood that a tight catch-up situation might develop”.

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

• that MS Aviation should remind its pilots that after landing at Hong Kong they should, as stipulated in AIP Hong Kong, vacate the runway as quickly as practicable to enable ATC to apply minimum spacing on final approach to maximise runway utilisation and minimise missed approaches.

The Final Report of the Investigation was completed in February 2023 and the updated version of it was released online in July 2023.

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• Controller Detection of Manoeuvring Area Conflicts - Guidance for Controllers
• Controller Detection of Manoeuvring Area Conflicts - Safety Barriers
• Visual Scanning Technique
• Rapid Exit Taxiway (Nano)