On Friday 15 April, two aircraft flew closer together than is permitted by the separation minima. One aircraft had departed from the Polderbaan (36L) and the other aircraft had departed from the Zwanenburgbaan (36C) from Amsterdam Airport Schiphol.
LVNL (Air Traffic Control the Netherlands) is conducting an investigation into this occurrence itself and reported the occurrence to the Onderzoeksraad voor Veiligheid (Dutch Safety Board).
Description of occurrence
Parallel departures occur from runway 36L (Polderbaan) and runway 36C (Zwanenburgbaan), departing from Amsterdam Airport Schiphol in an initially northerly direction. Standard practice during parallel departure operations is for aircraft departing from the Polderbaan to receive departure routes to the north and west. Aircraft departing from the Zwanenburgbaan are assigned routes heading east and south.
An Airbus A320-neo (A20N) aircraft and a Boeing 777-300ER (B77W) aircraft took off almost simultaneously from the Polderbaan and the Zwanenburgbaan respectively. The tower controller for the Polderbaan observed by looking out from the tower that the A20N was flying straight ahead. The tower controller for the Zwanenburgbaan also observed that the B77W was in a turn to the right. This is in line with the departure routes as instructed. After take-off, the flights were handed over to the radar controllers according to procedure.
Two radar controllers were handling the departing flights. One was responsible for the western part of the ATC terminal control area around Schiphol, which included the A20N. The other was responsible for the eastern part of the terminal control area, which included the B77W. Both air traffic controllers gave the flight under their supervision the instruction to continue on their intended departure route and then climb to Flight Level 130 (about 4 kilometres). At that time the instructed flight paths were free of each other and any other aircraft.
After flying straight for some time, the A20N departing from the Polderbaan turned right instead of following the expected departure route by a left turn. As soon as the radar controllers observed that the A20N was turning to the right, the radar controller controlling the A20N instructed the aircraft to accelerate its climb in order to increase the distance to the B77W. By that point the crew of the B77W had observed the A20N. In response the B77W reduced its climbing speed, also in order to increase the distance to the A20N. The A20N was then instructed to resume the correct flight path and both aircraft continued on their way.
The minimum distance between the A20N and the B77W was 1.61 nautical miles (approx. 3 kilometres) horizontally and 400 feet (122 metres) vertically. The separation standard for the applicable airspace is 3 nautical miles (more than 5.5 kilometres) horizontally or 1,000 feet (approx. 300 metres) vertically. When the minimum distance was reached the A20N began climbing faster than the B77W, thus increasing the distance.
Conclusions of the investigation
The occurrence was the result of the A20N flying on the wrong departure route, turning to the right and flying south instead of turning to the left and flying south-west. Due to various circumstances the aircraft was not prevented from flying this wrong route, nor was the fact that it was flying on the wrong route detected at an early stage.
The investigation revealed that when the aircraft was ready for departure at the gate, the crew of the A20N had requested an outbound runway and a flight path from the Outbound Planner: the official at Schiphol Tower who (among other things) gives route clearances. Due to a technical malfunction it was not possible to communicate with this aircraft via text messages. The Outbound Planner repeated the route to the (south)west several times over the radio frequency, because the crew of the A20N appeared to be having difficulty reading back the correct route. After the Outbound Planner was convinced that the crew had understood the instructions correctly, the A20N was handed over to the ground controller for pushback and taxi. However, the investigation revealed that the crew had not understood the instructions correctly, was uncertain about the departure route received, and had not been completely correct in reading the information back to the Outbound Planner. Subsequently, the crew selected the incorrect departure route in the aircraft's on-board computer.
After the A20N had taken off it was handed over to the radar controller responsible for the western part of the terminal control area around Schiphol. After initial contact on the radio frequency the radar controller did not notice that the crew of the A20N had reported the departure route that would involve flying south. The radar controller then checked whether the A20N's south-western route was clear of other flight traffic, and then instructed the A20N to follow the intended (south-western) route and climb to Flight Level 130. The radar controller then gave several instructions to other aircraft.
When the A20N started turning right instead of left, this was not immediately observed by the radar controllers. It takes several seconds before the radar shows that an aircraft is turning. The radar controller for the eastern part of the Schiphol terminal control area was the first to see the incorrect route and immediately informed the other controller who was covering the western part. The A20N was then instructed to accelerate its climb. Due to the positions of the departure routes and the positions of the aircraft when the reduced distance was detected, instructing the A20N to climb faster was the most effective solution. Because the pilot did not immediately understand the instruction, the air traffic controller had to repeat it once again. In the meantime the B77W had reduced its climbing speed and the distance between the two aircraft was already increasing.
Based on this occurrence and a number of other occurrences in which an outbound flight deviated from the instructed departure route, Air Traffic Control the Netherlands (LVNL) has conducted an internal investigation to analyse the underlying causes of such occurrences. The investigation included potential measures to mitigate the risk of such a deviation and to increase the likelihood that any deviation in the departure route is detected in good time.
For example, an article in the internal LVNL Safety Magazine focused particular attention on the Outbound Planner, highlighting the need to confirm the correct departure route with the pilots and the importance of reading back the route correctly. For the radar controllers the importance was emphasised of listening carefully during the initial contact on the radio frequency, focusing particularly on the departure route stated by the pilot, especially during parallel take-off.
LVNL is also exploring technical options to support the air traffic controllers in the earlier detection of an incorrectly selected departure route. LVNL is, for example, reviewing options for confirming the departure route selected by the pilot based on information from the aircraft. That would make it possible to identify the difference with the instructed route at an earlier stage.
The airline companies involved were also contacted. The crew of the A20N indicated that they would in future always check with the air traffic controller to establish whether they had understood the instruction correctly, if they had any doubts about the instruction they received. The airline in question also distributed this information among the other pilots at that airline, to prevent such situations in the future.
Classification: serious occurrence