Aircraft and Operators
BRT 695, a BAe146-200 (registration G-GNTZ) operated by British Airways City Express, was scheduled to fly from Zurich to Manchester. SWR 1499, an Airbus A320-214 (registration HB-IJL) operated by Swiss International Air Lines, had arrived from Prague.
Sequence of Events
On the evening of 2 December 2004 at 19:37 UTC, SWR 1499 landed on runway 14 and vacated via taxiway H3. The apron controller instructed the crew to taxi via taxiways KILO, BRAVO, and JULIET to the holding point of runway 28. The crew was informed of a short delay due to traffic on the south side. The commander later stated he saw an aircraft in take-off position on runway 28.
At 19:36:13 UTC, BRT 695 contacted the aerodrome controller (ADC) and was given a departure time of five minutes. At 19:37:38 UTC, the ADC cleared BRT 695 to line up on runway 28. BRT 695 arrived at the line-up position around 19:39:26 UTC.
At 19:39:38 UTC, SWR 1499 reported to the ADC, stating it was holding short of runway 28 at taxiway JULIET. The ADC responded at 19:39:42 UTC with clearance to cross runway 28 and a frequency change to apron control. The SWR 1499 crew read back the clearance at 19:39:48 UTC.
Immediately after this readback, at 19:39:53 UTC, the ADC issued take-off clearance to BRT 695 on runway 28. The ADC later stated that during the readback from SWR 1499, he heard a synthetic voice saying "wait cancelled" from the speaker between the ADC and ground control. This distracted him, as he had expected a time release for BRT 695. He instructed ground control to investigate but received no further information. Meanwhile, the lights on taxiway JULIET had changed: the red stop bar lights were off and the green taxiway centerline lights were on.
The commander of SWR 1499 stated that as his first officer completed the readback, he rolled forward, passing the stop bar with the cockpit approximately over it. He then heard the take-off clearance issued to BRT 695. The commander observed BRT 695's landing lights come on as it began its take-off roll.
At 19:40:07 UTC, SWR 1499 called the ADC, asking to confirm that they had stopped off the runway. The ADC instructed them to hold position. After BRT 695 departed, the ADC again cleared SWR 1499 to cross runway 28 and contact apron control at 19:40:42 UTC.
The crew of BRT 695, which was conducting a line training flight, reported that they were unaware of the incident.
Coordination and Communication
The incident involved the use of a coordination procedure known as "wait/wait cancelled" between the departure controller (DEP) and the aerodrome controller (ADC) via the TACO (Tower Approach Communication) system. The TACO system calculates time spacing for departures and can suggest a "wait" if the performance of the preceding aircraft is uncertain. The departure controller can manually override the time release with a wait release, which can then be cancelled with a "wait cancelled" signal, producing an audible alert in the tower.
The departure controller stated he had used this procedure to expedite traffic without needing to telephone the ADC. He stated he was unaware of any previous difficulties with the procedure and has since ceased using it. The ADC stated it was the first time he had experienced this type of coordination.
Analysis
The investigation determined that the rapid sequence of clearances — the ADC issuing a crossing clearance to SWR 1499 and a take-off clearance to BRT 695 on the same runway — was due to the ADC being distracted by the unexpected "wait cancelled" signal. The ADC did not consider aborting the take-off of BRT 695 because he assessed that SWR 1499 was not on the runway and posed no danger. However, at that time, the red stop bars were off, the green taxiway lights were on, and SWR 1499 had already started to cross the runway, with its nose past the stop bar. The exact position of the aircraft was difficult to ascertain due to night conditions and rain. The radar image showed that at least part of the A320 had passed the stop bar. The take-off of BRT 695 had just begun, so an aborted take-off would have been possible and advisable.
The instruction to SWR 1499 to cross runway 28 and simultaneously change frequency is common practice at Zurich. The wording of the frequency change instruction allowed the crew discretion on when to switch. Had the crew immediately changed frequency, they would have been unable to hear the take-off clearance. The fact that the crew remained on the ADC frequency was beneficial.