On Tuesday 5 February, during parallel approaches to the Zwanenburgbaan runway (18C) and the Polderbaan runway (18R) at Amsterdam Airport Schiphol, two aircraft flew closer together than is permitted by the separation minima.
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).
Loss of separation
The horizontal or vertical distance between aircraft during a flight is referred to as 'separation'. The purpose of the separation minima is to ensure air traffic flows safely and airspace is used optimally. Air traffic control is responsible for enforcing the minimum separation limits between aircraft flying in their air traffic area. When two aircraft come too close to each other despite the separation minima, this is known as a loss of separation.
The criteria for the minimum separation are designed to ensure there is sufficient time to resolve the loss of minimum distance or altitude. In such cases, air traffic controllers perform a number of measures within a short time:
- Detecting the loss of separation;
- Estimating an effective solution;
- Communicating this solution via instructions to the pilot(s) (altitude, direction, speed);
- Monitoring compliance with these instructions by the pilot(s) in order to restore the necessary distance or altitude as soon as possible.
Description of occurrence
A Boeing 747-800 aircraft was approaching to land on runway 18R/36L (Polderbaan) from the northwest. From the east, an Embraer 175 aircraft was coming in to land on runway 18C/36C (Zwanenburgbaan). Both aircraft were descending.
There were two approach controllers on duty at the time, one assigned to the Polderbaan and another to the Zwanenburgbaan.
The approach controller for the Polderbaan instructed the Boeing to descend to an altitude of 600 meters (2,000 feet). The pilot of the Boeing confirmed this instruction. The Embraer was instructed to descend to 900 meters (3,000 feet) by the Zwanenburgbaan approach controller. Both of these instructions followed the set procedure for this runway combination. The Boeing received instructions for a somewhat shorter route to the final approach. As a result, the aircraft was still descending to 600 meters (2,000 feet) while the aircraft was turning to pick up the Instrument Landing System (ILS) for the final approach. At that point, the Embraer had already engaged the Instrument Landing System (ILS) for the final approach and was at an altitude of 900 meters (3,000 feet). As a result, the vertical distance between the aircraft was not large enough and the required minimum vertical separation of 300 meters (1,000 feet) was not achieved.
The pilots of both aircraft did not notice anything unusual during their approaches.
Both aircraft landed safely.
The moment when the loss of distance occurred was the moment in which the Embraer picked up final approach at 900 meters (3,000 feet) while the Boeing was still descending to 600 meters (2,000 feet) and had not yet engaged the ILS.
During this occurrence, the minimum distance between the two aircraft was 3.4 kilometers (1.9 nautical miles) horizontally and more than 180 meters (600 feet) vertically. The separation standard for the phase of the final approach in which the flights were at that moment is: 5.5 kilometers (3 nautical miles) horizontally or 300 meters (1,000 feet) vertically. This standard remains in effect until both aircraft have a stable link to the part of the ILS that ensures an ideal flight path (localizer). Both aircraft were stabilized by the landing system shortly after the moment of minimal separation.
Conclusions of the investigation and follow-up actions
The occurrence was the result of a miscommunication between the two approach controllers. Because the Boeing was still descending when the signal was given that the vertical distance between the two aircraft was not sufficient, no corrective instructions were issued.
Based on this occurrence and a number of comparable occurrences, Air Traffic Control the Netherlands (LVNL) has conducted an internal investigation to thoroughly analyze all underlying causes of occurrences with parallel approaches at Schiphol Airport. The results of the investigation have led to the implementation of a number of improvements, such as paying extra attention to the specific procedures for parallel approaches among air traffic controllers as well as pilots, researching technological options for providing further support to air traffic controllers, and better monitoring of the parallel operation to learn from the resulting observations.
Classification: serious occurrence