On 16 December 2002, a Saab 340 being operated by Swedish airline Skyways and arriving at Stockholm on a scheduled domestic passenger flight and an Embraer 145 being operated by Swiss on a scheduled passenger flight from Stockholm to Basel almost collided at the intersection between taxiways ‘Z’ and ‘A’ in normal night visibility. Upon seeing the Saab approaching on a conflicting track, the Embraer 145 was stopped very suddenly and the other aircraft passed within an estimated 3 metres. No persons were injured and neither aircraft was damaged. The diagram below taken from the official report shows the intersection involved.
ESSA Apron Diagram (Section)
An Investigation was carried out by the Swedish Accident Investigation Board. It was found that the Saab 340 had failed to stop at its acknowledged clearance limit - to Taxiway ‘A’ only and that only prompt action by the Embraer crew had avoided a collision. The Aircraft commander of the Saab had been aware that his aircraft was only cleared to taxiway ‘A’ and it was therefore apparent that the error was involuntary and without his awareness. The co pilot in the Saab who was acting as PM was reportedly “occupied with the shutdown of the left-hand engine” and first became aware of the situation when the (aircraft commander) applied the brakes after passing the Embraer.
The Investigation noted that aircraft ground movement control is shared between two controller positions, GND NORTH and GND WEST and that this means that crews often have to change ground frequency. According to the Investigation findings:
“It rather often happens, in connection with taxiing between the different portions of the airport, that pilots forget to hold at appointed positions. From the tape recording of the incident in question it is evident that such a mistake was made by another crew at intersection ZK less than one minute after the incident under investigation here. The number of such occurrences that remain unreported is unknown.”
It was also noted that although AIPs procedures refer to the normal departure taxiway when leaving Terminal 2 ‘U’ rather the ‘A’, which is envisaged as an inbound route, ATC clearance to use of taxiway ‘A’ outbound, whether this is requested or not, is common.
The formal conclusion of the Investigation was that:
“The incident was caused by the pilot in command mistakenly crossing a taxiway without permission from the air traffic controller.” Contributory factors were considered by the Investigation to have been:
- the airport taxiway system is complex
- there is a risk that operational mistakes can be made
- both pilots did not have full watch outwards
- air traffic controllers give aircraft clearance to taxi in deviation from AIP”
The Investigation led to two Safety Recommendations being made to the Swedish Civil Aviation Administration in respect of Stockholm Arlanda Airport to:
- perform an unbiased review of the airport’s taxi system with regard to measures which can be taken in order to diminish the risk of operational mistakes (RL 2003:32e R1)
- in connection with ground control, normally always clear aircraft to taxi in accordance with the AIP, unless a different taxi route is requested by pilots and the situation allows this (RL 2003:32e R2)
The Final Report of the Investigation was published on 29 August 2003 and may be seen in full at SKYbrary bookshelf Report RL 2003:32e