Description

On 2 September 2013, a B737-800 (VT-SPJ) being operated by Spicejet on a scheduled domestic passenger flight from Shamshabad to Dehli as SEJ234 initiated a late go around from an approach to runway 28 at destination when it became apparent that a landing clearance would not be issued because an Airbus AIRBUS A-320 (VT-IEB) being operated by Indigo on a scheduled domestic passenger flight from Dehli to Coimbatore was still not airborne from the same runway. The two aircraft subsequently came into close proximity twice as they followed similar flight paths in their respective climbs.

Investigation

An Investigation was carried out in accordance with ICAO Annex 13 procedures by a Committee of Inquiry appointed by the Ministry of Civil Aviation on 11 November 2013. It was noted that this significant event was not reported by the controllers involved using the applicable procedures.

Data from both aircraft FDRs were successfully downloaded as was that from the 737 Cockpit Voice Recorder (CVR) but relevant data from the CVR of the A320 were overwritten. ATC TWR voice recordings and the APP Radar and TWR A-SMGCS recordings were also available.

It was established that at the time the situation which led to the conflicts began to develop, a handover of the TWR position was in progress with the single controller in position (Junior Executive) being replaced by an OJT Instructor (Senior Manager ATC) who was supervising a licensed and experienced controller with previous experience at Cochin who had the status of a trainee undergoing validation training. It was also evident that Visual Meteorological Conditions (VMC) prevailed and although Advanced Surface Movement Guidance and Control System was in use, no mention was made of any real time access to an airborne radar display at the TWR controller position.

The total flying experience of the 55 year-old A320 Training Captain was 10,533 hours which included 5967 hours on type, almost all of it in command. The total flying experience of the 33 year-old A320 First Officer undergoing line training prior to anticipated promotion to Captain on completion was 3645 hours, including 1427 hours on type. The total flying experience of the 29 year-old 737 Captain was omitted but they were recorded as having 1620 hours as Captain on the 737 after being promoted to the rank of Captain 19 months previously. The total flying experience of the 26 year-old 737 First Officer was also omitted but they had flown 398 hours on type in the previous 180 days.

It was established that shortly before the conflict situation began to develop, an Instructor had arrived at the TWR position with a trainee who he then permitted to plug in and take over transmissions despite himself not also having plugged in (the area speaker was on) or recorded his arrival. Just over two minutes later, the controller previously in position signed off, still with no acceptance entry having been made by the Instructor. The trainee remained as controller for 5 minutes - until, shortly after the 737 go around had commenced, at which point the trainee ceased controlling with "the instructor who was still offline...also not in control". For a few seconds the position was "unattended" until the qualified controller who had earlier handed over the channel took over again on seeing the situation and began to pass instructions. He then remained in position until the conflict was over before "handing over to the instructor again".

When the 737 was at 8nm final, the A320 had been permitted by the student controller working the TWR position to line up and wait on runway 28 after a Boeing 787 had taken off. The student controller had been advised by the A320 crew that a wait on the runway until the desired wake vortex separation (3 minutes) had been achieved would be necessary. At the time that the A320 actually entered the runway, the 737 was on TWR frequency and was passing 1737 feet agl at a speed of 156 KCAS. One minute and 20 seconds later, the A320 was lined up and stationary by which time the 737 was passing 1482 feet agl and TWR advised the A320 that a further 1 minute 22 seconds wait would be required. The 737 CVR showed that by this time, its crew were of the opinion that "ATC is pushing them into a very tight situation" and had already reviewed the go around procedure.

With take off clearance, the A320 subsequently began its take off roll without delay once the required waiting time had elapsed. At this point, the 737 was 275 feet agl without a landing clearance but with advice from the trainee controller to expect this on "short short final". With the A320 22 seconds into its take off roll and at a groundspeed of 82 knots, the 737 commenced and announced on TWR frequency a go around - the minimum height as this was commenced was 29 feet agl. No go around or any other instruction was given by the trainee but before vacating the position soon afterwards without an immediate take over of the position by anyone else, he acknowledged the 737 go around transmission with "Expedite, report passing 2600 feet".

The qualified controller who had earlier signed off took over the position and the A320 became airborne with the 737 at 455 feet agl (1420 feet QNH) on a similar track to but behind the A320 and climbing more slowly. Separation was therefore being rapidly eroded. TWR instructed the 737 to expedite the climb and report passing 3600 feet QNH and the A320 to climb to and maintain 2600 feet QNH. As the 737 climbed though 1160 feet agl at 2070 fpm (and thereby exceeded the TCAS RA suppression threshold of 1100 feet agl for climbing aircraft) 17 seconds after the A320 had got airborne, a TCAS RA 'CLIMB' was received which immediately changed to 'DESCEND, DESCEND NOW' at 1500 fpm as the minimum height for descend advisories - 1200 feet agl was also exceeded. As the A320 was still below 1100 feet agl, no co-ordinated TCAS RA was annunciated - although a TA was - and it continued climbing as the 737 descended towards it from above following its RA.

Contrary to the requirement for maximum TCAS RA response times (for a reversal, 2.5 seconds), the 737 crew were still in the process of converting their 2070 fpm climb into the RA-required rate of descent as closure continued and the 737 was still faster than the A320 when, 12 seconds after its annuciation the 737 RA had strengthened to 'INCREASE DESCENT' whilst a TCAS 'CLIMB' RA was annunciated on the A320 flight deck. At this point, the two aircraft were estimated to be about 270 metres apart laterally and converging rapidly in the vertical plane. Their closest proximity (CPA) occurred 11 seconds later (40 seconds after the A320 had taken off) as both aircraft, respectively descending (737) and climbing (A320) crossed at 1600 feet agl (2400 feet QNH) with an estimated lateral separation of approximately 90 metres. Shortly before the CPA, the A320, approaching its clearance limit of 2600 feet QNH, was instructed by TWR to stop as cleared but responded TCAS CLIMB and continued. Just before the CPA, it was found that 737 crew had visually acquired the A320 and executed a 30º roll to the right to increase clearance.

EDITORS NOTE: The wingspans of both the aircraft involved are in the vicinity of 35 metres.

TCAS annunciations 'Clear of Conflict' occurred on both aircraft flight decks 11 seconds after the CPA.

Thereafter, with both aircraft still on approximately the same ground track, the 737 resumed its climb from below the A320 whilst the latter, aware of the previous TWR request to stop at 2600 feet QNH transmitted to TWR “Levelling out at 3000, Confirm that”. In absence of any TWR response to this call, the crew did so. A further conflict scenario then followed with new coordinated TCAS RAs generated 14 seconds after the original 'Clear of Conflict' annunciations. The 737 RA was a preventive 'DON'T CLIMB' and the A320 a 'CLIMB' and both were initially complied with. Then, TWR replied to the previous A320 call mentioning 3000 feet with "maintain 2600" which was followed in preference to continuing the CLIMB RA. Whilst the two aircraft were still in the TCAS RA-annunciated phase of conflict, TWR instructed both aircraft to change to Radar. Subsequently, after 35 seconds of RA, and 1 minute and 42 seconds after the A320 became airborne, 'Clear of Conflict' was again annunciated to both crews as the A320 passed 4440 feet QNH and the 737 passed 2800 feet QNH.

The diagram below shows the vertical profile of the flight paths of both aircraft.

The Investigation's overall findings included the following:

• It would have been expected that, in the knowledge that the departing A320 would need a 3 minute separation behind the 787, TWR would not have lined up the A320 given the range of the 737 on final.
• Once the A320 was on the runway, it should have been clear at an early stage that a landing clearance would not be possible unless the A320 was instructed to vacate the runway and when it was not, it was inappropriate to delay the go around instruction to the 737.
• The behaviour of the Instructor in respect of his failure to properly supervise the trainee was inappropriate.
• When the 737 announced it was going around, the A320 should have rejected its take off.
• Thereafter, neither flight crew made optimum use of their TCAS RA traffic displays, especially when below the various RA-activation thresholds which apply near the ground.
• Not all the TCAS RA responses were in accordance with the pilots' training.
• The necessary inhibition of TCAS II RAs when aircraft are close to terrain reduces the effectiveness of the system and without active ATC control requires pilots to make the most of the relative awareness information provided by TCAS Displays.
• There was a complete failure to follow the prescribed procedures for TWR position handover and the procedures which apply to trainee supervision by an OJT Instructor.

The Probable Cause of the Serious Incident was determined to have been "the initial error made by Air Traffic Control in placing two aircraft in conflicting positions (so that) conflict then followed" and that "no advantage (was taken) of any of the opportunities that were subsequently available to block the consequences of the error. (With) the possibility of a machine-related failure ruled out, human error, to a varying extent, is attributable to all three work areas, ATC Tower and the Pilots in both aircraft".

The following manifestations of Human Error were identified:

The ATC TWR personnel

• the decision to hold an aircraft on the runway until wake vortex separations are met whilst allowing an approaching aircraft to continue
• the inability to appreciate the potential implications of allowing the approaching aircraft to continue to a point where a very late go around would inevitably lead to a high risk of subsequent airborne conflict
• the inability to exercise judgement and order the rejection of the A320 takeoff and eliminate the airborne conflict risk
• the failure to exercise effective air traffic control using instructions to directly prevent airborne conflict or to facilitate awareness of the risk by transmitting relevant traffic information when TCAS Resolution Advisories were inhibited.
• the further failure in allowing both aircraft to continue on similar flight paths after the first conflict and thereby allowing a second conflict to develop and require resolution by coordinated TCAS RA responses.
• the failure to operate with a clear understanding of relative responsibility during normal /non normal / emergency situations with more than one controller on position.
• the complete failure to follow the established procedure for handover of the TWR control position which was in progress at the time
• the failure of the most experienced of the three controllers (the Instructor) who were assumed to be available at the TWR position in the absence of any formal handover to contribute to resolution of the emergency situation.

The A320 Flight Crew

• the failure of the A320 crew to reject their take off having become aware at a speed of just over 80 knots that the 737 was going around
• the failure to use the TCAS display or other means to monitor the progress of the 737 and thus prevent flight path conflict.

The B737 Flight Crew

• the failure to pre-empt a developing situation when at 6nm final, another aircraft on the landing runway is going to take approximately 2 more minutes before it begins is take off roll which will then last another 45 seconds before the runway is clear.
• the continuation without a landing clearance below DA as far as the runway threshold.
• the failure, having reached 1600 feet, to continue the expected go around flight path followed thus far and descending for a time before re-establishing a climb which was neither incompliance with the go around flight path nor attributable to response to TCAS RAs.
• the failure to use the TCAS display or other means to monitor the progress of the A320 and thus prevent flight path conflict.

However it is acknowledged that only the manoeuvre of right banking the aircraft after visually sighting the intruder aircraft (the A320) in close proximity saved this serious situation from a possible mid air collision accident.

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

1. that An Unspecified Recipient should ensure that the ILS Decision Height (DH) / Decision Altitude (DA) is the latest point at which a go around is initiated when a landing clearance has not been issued to an aircraft which has been cleared to make an approach.
2. that the Airports Authority India (AAI) as ANSP should ensure that when more than one controller is present at an active position, including situations where OJT is involved, a clearly defined command hierarchy is necessary to ensure appropriate transfer of controls and responsibilities in both Normal and Non-normal situations.
3. that the Airports Authority India (AAI) should ensure that the laid down Standard Procedures for ATC position hand over / takeover are always followed.
4. that the Airports Authority India (AAI) as ANSP should ensure that the thresholds for an OJT Instructor to take over from a trainee controller being given on-job practical operational experience prior to validation are clearly laid down so that this occurs before an error is manifested in consequences. These guiding threshold limits must be inculcated in the Instructor's training and standardisation checks.
5. that the Appropriate and Competent Authority should ensure that the professional and judgemental controller errors which created the conditions for the investigated traffic conflict to occur are addressed and corrective measures taken to ensure no future replication. Also, it should be ensured that Air Traffic Services review the non-reporting of an incident of such magnitude.
6. that An Unspecified Recipient should ensure that the limitations of the TCAS system in addressing conflicts of this nature are included in the crew training curriculum.

The Final Report of the Investigation was completed on 4 February 2014 but not published until 1 December 2015.

Further Reading

• Loss of Separation
• Airborne Collision Avoidance System (ACAS)
• ACAS Resolution and Traffic Advisories
• Hindsight 6 article "Airbus AP-FD TCAS MODE" - by Paule Botargues
• ACAS: Guidance for Controllers
• Loss of Separation - ATCO-induced Situations
• Loss of Separation - Pilot-induced Situations
• ACAS Guide - Airborne Collision Avoidance Systems (incorporating TCAS II versions 7.0 & 7.1 and introduction to ACAS X), May 2016