On Monday 28 January, during parallel approaches to the Zwanenburgbaan runway (36C) and the Aalsmeerbaan runway (36R) 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.
Air traffic control reports occurrences that take place during LVNL's everyday practice in order to learn lessons and minimize the risk of such occurrences from presenting themselves in the future. Within LVNL, all reported occurrences are investigated in order to continually improve safety.
Description of occurrence
A Boeing 737-800 aircraft was approaching from the northwest to land on runway 18C/36C (Zwanenburgbaan). The aircraft was flying at an altitude of 1.2 kilometers (4,000 feet). Coming in from the east, an Embraer 175 aircraft was on approach to land on runway 18L/36R (Aalsmeerbaan). This aircraft was descending.
There were two approach controllers on duty at the time, one assigned to the Aalsmeerbaan and another to the Zwanenburgbaan.
The approach controller for the Aalsmeerbaan instructed the Embraer to descend to 900 meters (3,000 feet) according to the established procedure for this runway combination. The pilot of the Embraer confirmed this instruction. The Boeing was already flying at an altitude of 1.2 kilometers (4,000 feet), as instructed by the approach controller for the Zwanenburgbaan, according to the procedure for this runway combination. However, the Embraer descended slower than expected. As a result, during the turn to pick up the Instrument Landing System (ILS) for the final approach for the different landing runways, the vertical distance between the two aircraft was less than the standard distance of 300 meters (1,000 feet).
Neither of the two approach controllers, who were seated next to each other, specifically informed the pilots of the Embraer and the Boeing about the parallel operation and about the presence of the other aircraft in their proximity. The pilots of both aircraft did not notice anything unusual during the approaches. Both aircraft landed safely.
The moment when the loss of distance occurred was the moment in which the Embraer was still on descent to the instructed altitude of 900 meters (3,000 feet) and the Boeing was already flying at the instructed altitude of 1.2 kilometers (4,000 feet).
During this occurrence, the minimum distance between the two aircraft was approximately 3 kilometers (1.7 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 the estimation of the vertical flight profile of the Embraer by the approach controller. The aircraft was not descending as fast as expected. Because the Embraer was still descending when it was detected that the aircraft was not descending as fast as expected, 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