Swiss A320 and Swiss A320 at Zurich on Mar 15th 2011, close encounter between two simultaneously departing aircraft on crossing runways
Last Update: May 14, 2012 / 15:50:29 GMT/Zulu time
The serious incident is attributable to the fact that the air traffic control officer concerned gave take-off clearance to an aircraft on runway 28 although another aircraft on runway 16, to which he had given take-off clearance shortly before, was still on its take-off roll. The result was that an inadvertent convergence of these aircraft occurred, involving a high risk of collision.
The following factors significantly contributed to the genesis of the serious incident:
- At a time with a very high volume of traffic at Zurich airport, survey flights were being carried out, which increased the complexity of operation for air traffic control.
- The air traffic control officer concerned was engaged on tasks which did not have a high priority at this time.
- The aerodrome control centre work concept allowed only inadequate mutual support in the case of a high volume of traffic and in general did not feature any monitoring for early detection and correction of errors.
- The air traffic control's collision warning system was inappropriate for resolving the impending conflict.
The genesis of the serious incident was favoured by the complex operation on two intersecting runways which is subject to a small error tolerance in the event of a high volume of traffic.
The tower controller had instructed LX-2026 to line up runway 28 and wait.
The tower controller had intended to have LX-2026 depart from runway 28 before LX-1326 on runway 16, however, as an aircraft was on approach to runway 14 soon to pass 8nm to touch down which would have prohibited any departure from runway 16 until after the the aircraft had landed on runway 14, therefore the tower controller decided to change the departure sequence and coordinated that change with ground control. The flight strips were accordingly swapped.
When LX-1326 taxied about 700 meters before the holding point tower cleared the aircraft to taxi into position on runway 16.
Another aircraft taxiing to the apron was nearing runway 28 to cross the runway, ground control - in accordance with the changed departure sequence - therefore blocked runway 28, which now appeared red on all radar screens, and cleared the aircraft to cross.
Tower subsequently cleared LX-1326 for takeoff from runway 16. The crew of LX-2026 was still taxiing into position to runway 28, the crew was processing their takeoff checklist and did not notice the takeoff clearance although the cockpit voice recorder recorded that takeoff clearance on runway 16 clearly.
LX-1326 began their takeoff roll 41 seconds after their takeoff clearance.
The crossing Airbus vacated runway 28 and ground control de-activated the runway 28 block 42 seconds after the takeoff clearance had been issued on runway 16. Seeing the runway had turned black again and was available to him, tower gave takeoff clearance for LX-2026 on runway 28 4 seconds after the runway had turned black again.
35 seconds after the second takeoff clearance on runway 28 the RIMCAS (runway incursion monitoring and conflict alert subsystem) issued a stage 2 alert, at that point LX-1326 was rolling at a speed of 143 knots above ground and LX-2026 at 89 knots above ground.
The tower controller was surprised by the alert believing it was a false alert, he no longer had LX-1326 on his mind, and believed this was a runway incursion with a vehicle before he realised the conflict was between departures on runway 16 and 28. 9 seconds after the RIMCAS alert he ordered LX-2026 to "stop immediately".
The crew of LX-2026 did not acknowledge that instruction as they had seen the other departing traffic on runway 16 two seconds earlier and were already rejecting their takeoff. At the time the crew rejected takeoff the aircraft had already accelerated to 135 knots about 550 meters before the intersection of the two runways, LX-1326 was just rotating for takeoff at 162 knots.
LX-2026 came to a stop 16 seconds after rejecting takeoff short of edge runway 16 however already within the runway's protected zone. LX-1326 by then had also become airborne and had passed the intersection. LX-2026 taxied to the south apron via runway 16 and requested emergency service be ready due to their hot brakes.
The crew of LX-1326 had not noticed the incident and got to know about it through tower informing them of the serious incident.
The tower controller was relieved from duty a short while after the incident.
Flight LX-2026 departed on the same aircraft but a different crew three hours later.
The BFU analysed that the RIMCAS stage 2 alert indicative of a pending conflict of two aircraft on the runways was issued too late. The intention of RIMCAS however was not avoid conflicts between aircraft on conflicting takeoffs, but to alert of collisions between vehicles and aircraft on the ground. However, only 1 in 5 alerts was genuine, so that RIMCAS alerts were not given the importance they deserved. The BFU said: "The current situation with genuine, false and nuisance alarms makes it more difficult for air traffic control officers to assess the situation and therefore creates a hazard from the viewpoint of aviation safety."
The BFU analysed that the crew of LX-1326 was focussed on monitoring and steering their aircraft during their takeoff run and rotation, they therefore likely blanked out all transmissions without their callsign and because concentrated on the direction of their takeoff roll could not see the conflicting LX-2026.
The crew of LX-2026 was accelerating for takeoff on runway 28 when just prior to reaching V1 the captain, pilot flying, caught a glimpse of the conflicting LX-1326 and quickly decided to reject takeoff. Within 1-2 seconds he aborted the takeoff, the noise levels within the cockpit rose substantially due to the rejected takeoff, the crew therefore did not hear the instruction to stop.
According to the tower controller's testimony he no longer had LX-1326 on his mind when runway 28 turned black indicating the runway was back available to the tower. LX-1326 was clearly depicted on the ground radar screen at that point rolling down runway 16 still on the ground, nonetheless tower issued takeoff clearance to LX-2026 on runway 28. It can be concluded that the controller did not check the ground radar screen before issuing the takeoff clearance.
The ground controller, sitting next to the tower controller, did not notice the pending conflict as he was busy communicating with a business jet.
The same tower controller had been involved in another conflict on runways 28 and 16 on Jul 31st 2008, when he issued a takeoff clearance on runway 28 although a landing clearance on runway 16 had been issued earlier. The BFU analysed his behaviour and patterns in both incidents but "it was impossible to detect any uniformity in the form of a conjunction or reciprocal effect of quite specific a) personality-related and b) external circumstances which triggered these incidents."
The presentation of runways on the tower radar display was "ergonomically unfavourable" with the northern part of the runway system shown in a separate window making it more difficult to the controller to comprehend the entire runway system in one glance. The BFU continued: "Since at Zurich airport 73% of the departing traffic takes off from the intersecting runways which are in simultaneous operation, a comparable situation is common and the ergonomically poor display is therefore of particular importance."
There was only one tower controller with the ground controller sitting besides the tower controller meant to assist the tower controller, but not monitor the tower controller. However, during times of high volume of traffic the ground controller may be occupied with his tasks and therefore is not available to assist the tower controller, who then effectively is operating as a single controller. The BFU commented angrily: "Therefore, error-free working of the individual is assumed, which, as is well known, is not realistic." and went on to say: "The cited systemic interconnections are one reason why at European airports which are comparable to Zurich two or more ADC air traffic control officers are deployed to monitor the runway system and the airspace ... Experience shows that this extra ATCO can constitute a valuable safety net."
The BFU slams Skyguide: "It is striking that within the air navigation services company Skyguide there are no clear and unambiguous procedures according to which take-off clearances are given for precisely this concept with departures from intersecting runways, which is frequently applied and which is demanding. The argument put forward by Skyguide, i.e. that it is not possible or appropriate to define such procedures, is not convincing - other air navigation services companies working with comparable operating concepts have actually had such procedures established for a long time."
This article is published under license from Avherald.com. © of text by Avherald.com.
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