On 18 March 2016 an Airbus A319 (B6039) being operated by China Southern Airlines on a scheduled international passenger flight from Cheongju to Dalian as CZ8444 continued beyond its departure clearance limit at the mid-runway holding point for 24R at night with thick fog present and began to enter the runway for backtrack just as a Boeing 737-800 (HL7786) touched down on the same runway. On sighting the A319 which had just stopped, the 737 crew made an instant turn to the left whilst still at high speed and just avoided a collision before completing their landing roll.
A Serious Incident Investigation was carried out by the Korean Aviation and Railway Accident Investigation Board (ARAIB) based on relevant data from the FDR in both aircraft and recorded ATC and meteorological data which were all aligned to a common time base.
The 37 year-old Training Captain in command of the Airbus A319 was occupying the right hand seat and had a total of 11,032 hours flying experience which included 9,473 hours on type and 5,224 hours in command. A 31 year-old First Officer undergoing command upgrade line training and occupying the left seat had a total of 6,870 hours flying experience of which all but 250 hours were on type. Another First Officer with a total of 2,190 hours flying experience all but 290 hours on type was occupying the supernumerary crew seat and acting as Safety Pilot.
The 48 year-old Training Captain in command of the Boeing 737 had a total of 9,225 hours flying experience which included 3,856 hours in command. The 39 year-old First Officer was undergoing line training on type and had a total of 3,277 hours flying experience and the investigated flight was the third sector of his second day of line training. A 31 year old Captain was occupying the supernumerary crew seat and acting as Safety Pilot. He had a total of 6,415 hours flying experience which included 2,491 hours in command.
The 24 year-old Air Force GND controller had just over three years experience and was also qualified to act as TWR (Runway) controller. The 26 year-old Air Force TWR controller had almost seven years experience and was also qualified to act as GND controller and, for the nine months preceding the investigated event, as a shift leader. Both controllers had approval to control civil traffic issued by the Korean Transportation Safety Authority and were working the night shift.
Cheongju is a joint military/civil use airport with ATC provided by Korean Air Force (KAF) controllers approved to handle civil air traffic. There are two parallel concrete runways, 24R/06L and 24L/06R, with the northern runway which is 2,744 metres long and 60 metres wide allocated for civil use and the narrower (45 metres wide) southern runway used for military flights. The northern runway does not have a parallel taxiway or direct access from the terminal apron to either end of the runway which means that departing civil aircraft have to enter via taxiway B3 at the mid-point and then backtrack to begin their takeoffs.
The Cheongju Aerodrome Diagram with the tracks of the two aircraft superimposed. [Reproduced from the Official Report]
The ground tracks of the A319 (red) as it entered the runway to back track for takeoff and the 737 (yellow) which had just touched down. [Reproduced from the Official Report]
The runway in use was 24R and a Boeing 737 was on an ILS final approach and working the approach radar frequency. Ninety seconds after calling established on the LOC, it had 4 nm to touchdown and the radar controller contacted TWR to obtain permission to issue a landing clearance which was received and issued with caution that the RVR had just dropped marginally below the 550 metre Cat 1 minimum. The 737 crew replied that they had the runway in sight.
Ten seconds later, the A319 called GND ready for taxi and received the immediate response “runway in use 24R, taxi via B3 is approved, altimeter 29.87”. The A319 replied with “runway 24R via taxiway B3” and began to taxi. The METAR current at the time gave the wind calm, visibility 3,200 metres in mist but with some cloud/fog patches present and identical Air and Dew Point Temperatures. Within the previous five minutes, the 24R touchdown RVR had dropped from 1500 metres to 450 metres.
After 90 seconds taxiing, and without any further communication with ATC, the A319, now with much reduced forward visibility, crossed the runway 24R holding point unseen by either controller, entered the runway and after a further 16 seconds started a left turn to backtrack along the runway. Three seconds earlier, the 737 had touched down within the TDZ at a position approximately 600 metres past the runway threshold with the (as yet unseen) A319 less than 600 metres ahead.
Nine seconds after touching down and with groundspeed reducing though 100 knots, the 737 crew saw the other aircraft at the same time as it saw the 737 and stopped. The 737 crew immediately used a left rudder input and left differential braking to deviate left of the runway centreline by what was subsequently calculated to have been six metres, enough to pass 3 metres clear of the A319 (see the illustration below).
The position of the two aircraft as they passed annotated with relevant distances/ dimensions. [Reproduced from the Official Report]
The 737 completed its landing roll and then backtracked to exit the runway via the B3 taxiway. In the meantime, the A319 had been cleared to backtrack and line up at the threshold. However by the time it reached this position, the RVR had dropped below the minimum required for takeoff so once the 737 was clear, it was cleared to taxi to the 06L threshold where with the visibility 2,500 metres it was cleared for takeoff and departed without further event 36 minutes after the near miss.
It was found that just one second before minimum separation between the two aircraft occurred, the GND controller, without either controller realising that the A319 was on the runway, had instructed it to change to TWR.
Why It Happened
Neither aircraft was aware of the presence of the other until the very late acquisition of visual contact in the prevailing fog affecting the north eastern half of the runway because the departing A319 was still on the GND frequency and the landing 737 was on the TWR frequency.
FDR data confirmed that when the 737 crew saw the other aircraft they responded by making an immediate rudder and brake input to achieve a deviation to the left of the centreline which was sufficient to prevent a collision.
The presence of fog patches on the aerodrome meant that the controllers would have needed to supplement their visual monitoring of clearance compliance by reference to surface radar. The fact that the conflict occurred in such conditions at night would have complicated the controllers’ judgement of the limits of visual surveillance and the GND controller subsequently stated that he had not seen the A319 enter the runway. He had handed the aircraft to the TWR controller without confirming its position as procedurally required and by this time it had already entered the runway.
The responsibility of China Southern Airlines to ensure that its pilots were provided with training compatible with their operational exposure to risk was reviewed and it was concluded that it had been “insufficient” and too reliant on “self learning”. The importance of immediately clarifying any lack of clarity in respect of any clearance limit was considered to have been inadequately emphasised based on the “arbitrary interpretation” by all three pilots of the taxi clearance limit and the universal distinction between GND controller authority and the exclusive runway control authority of a TWR controller.
The performance of the two KAF controllers involved was reviewed. It was observed that the critical requirement for verbal communication of clear and unambiguous clearance limits and the corresponding requirement to obtain a correct read back from pilots had been ignored by the GND controller. The absence of adequate monitoring of the progress of the A319 by the same controller was also noted.
The prevailing ATC procedures at Cheongju were reviewed and found to state that it was “recommended” that a taxi clearance should use either of the following alternative formats:
- Runway (number), Taxi via (route as necessary) or
- Runway (number), Taxi via (route and/or hold short instructions as necessary)
This overall guidance was observed to allow the issue of a departing aircraft taxi clearance without necessarily including a clearance limit at or prior to entering the runway. It was noted that this was contrary to ICAO Doc 4444 section 126.96.36.199 which requires that a taxi clearance must, where relevant, include specification of a “holding point” or “hold short of runway number”. It was considered that “ground controllers need to use clear control terms when issuing taxi clearances" by supplementing the procedures which had been in force at Cheongju Airport at the time. Since all departing aircraft must enter and backtrack the runway before lining up for takeoff, this was considered to be of particular local importance.
In respect of approval to enter active runways, it was observed that the China Southern Airlines flight crew did not properly understand the taxi instruction issued by the GND controller but did not seek to confirm its contents which it was considered should be of particular concern at airports that have few aircraft movements. It was noted that R/T communications for pilots at the airline were conducted as part of “communication training” and it was found that a pilot holding an ICAO English Level 4 qualification did not receive any additional training related to air traffic control communications. It was noted that at other similar large airlines, much more pilot training on ATC control procedures and terminology was normal and that in comparison, China Southern Airlines “lacked control procedure training courses and training time for flight crew”. It was concluded that “as a result, flight crew lacked an adequate understanding of control procedures and control terminology”. This was considered in particular to extend to a lack of pilot appreciation of the fact that ground movement control on any active runway is reserved to TWR (runway) controllers. It was judged that in respect of ATC procedures and terminology, China Southern Airlines’ pilots needed to be provided with improved training runway usage procedures.
It was noted that Cheongju was categorised in the China Southern OM as a Class ‘B’ airport in line with their policy for military airports or others with slight operational anomalies. The necessary route qualification required Captains to obtain “1-2 hours of self-study” and complete 2 flights to the airport concerned, with route qualification received after the second one. In the case of the investigated flight, the visit to Cheongju was the route qualification flight to operate there in command. The Investigation considered that compared to other airlines' air traffic control procedures training courses and training hours for similar airports, this was below the normal and such training needed to be enhanced.
A consequence of the absence of both a parallel taxiway and direct access to the ends of both runways was noted as an unusually short taxi out to reach the runway holding point. This routine use of the active runway for taxiing was considered to introduce vulnerability to mistakes by controllers or pilots in respect of taxi clearances and would have justified the use by the GND controller of a supplementary instruction such as “Hold short of Runway 24R on B3”. It was also noted that although flashing Runway Guard Lights were in place at the B3 holding point, these would have no relevance to a pilot who mistakenly believed that they had been cleared to enter the runway. The absence of a controllable illuminated stop bar at the holding point was noted and it was considered that had one been installed and illuminated, it would have reliably identified the runway holding point and indicated even in low visibility conditions that the clearance limit had been reached.
The fact that the A319 flight crew had mistaken a poorly-given clearance "Runway 24R, taxi via B3" to mean "Taxi to Runway 24R" was clearly critical to the incursion. However, it was considered that despite the non-standard phraseology used, such a clearance could not have authorised entry to the runway and should not have been “interpreted arbitrarily”. Had it raised any doubt, this should have been queried. It was concluded that CRM between the flight crew in respect of ATC communications did not function properly because “there was a lack of understanding” of procedure so that more pilot training was necessary if repetition of such errors was to be avoided.
The Probable Cause of the Serious Incident was formally documented as “the China Southern Airlines Airbus A319 was permitted to taxi up to the runway 24R stop line on taxiway B3 by the Ground Controller at the Cheongju Control Tower but misunderstood this as a taxi clearance to enter the runway and backtrack (no parallel taxiway) to line up for takeoff on 24R and crossed the hold short line onto runway 24R on which the Korean Airlines Boeing 737 was landing”.
Two Contributory Factors were also identified as:
- The Ground Controller failed to stop the A319 before it entered the runway due to negligent visual monitoring.
- The Ground Controller did not clearly specify the clearance limit as the holding point for runway 24R (training on terminology was insufficient).
Four Safety Recommendations were made as a result of the Findings of the Investigation as follows:
- that China Southern Airlines instructs its flight crew in respect of the runway entry stop line and runway entry procedures, air traffic control procedure training and reinforcement of local airport training. [AIR1603-1]
- that the Korean Air Force (Cheongju TWR) prepare an improvement plan for visual surveillance procedure of aircraft entry to a runway for use by Ground and Runway controllers throughout Korea. [AIR1603-2]
- that the Korean Air Force (Cheongju TWR) prepare measures to prevent taxiing aircraft from entering the runway in use by improving Ground Control procedures and Ground Control instruction terminology. [AIR1603-3]
- that the Korea Airports Corporation, considering the reality of Cheongju Airport where fog often occurs, should prepare a plan to install buried runway warning lights on the runway entry stop lines on taxiways A3 and B3. [AIR1603-4]
The Final Report was released in the Korean language only on 27 March 2018.