On Tuesday 5 February two aircraft flew closer together than is permitted by the separation minima. The aircraft were flying in the local air traffic control area of Rotterdam The Hague Airport.
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 flight trainer, type S22T aircraft, performs a practice approach on runway 24 at Rotterdam airport. During such a maneuver, a pilot practices the approach procedures and prepares for landing, without actually touching down. After the practice approach, the flight trainer is instructed by the tower controller Rotterdam to climb to an altitude of 2,000 feet (more than 600 meters), following the runway centreline.
After that, a type M20P private aircraft is instructed by the tower controller to taxi onto runway 24 and hold there for take-off. The private plane plans to depart towards the southwest. In the meantime, the tower controller and the Rotterdam approach controller have made intercom contact and discussed both aircraft, agreeing that the private plane can take off once the flight trainer has reached an acceptable distance. The tower controller instructs the flight trainer to alter course to the left, heading towards point ROT. The pilot repeats this instruction and heads left. Once the tower controller has established that the flight trainer is turning to the left, the private aircraft receives permission to take off, and the flight trainer is handed off to the approach controller.
However, the flight trainer does not fly on towards point ROT; instead, it continues veering left. By now, the private aircraft is wheels up. The approach controller currently guiding the flight trainer sees on the radar that the aircraft is not flying towards point ROT, but curving 360 degrees and heading back towards the airport. The air traffic controller asks the flight trainer to confirm the route, at which point the pilot indicates that he is not heading towards point ROT. The approach controller then instructs the pilot to immediately adjust course to fly northeast, taking avoiding action. After the heading instruction has been issued a second time, the aircraft changes course.
In the meantime, the approach controller contacts the tower controller, requesting that the departing private aircraft not be allowed to climb higher than 1,000 feet (over 300 meters). Since the flight trainer is at 2,000 feet, this should have maintained the minimal distance required between the two aircraft. However, when the tower controller issues the instruction, the private plane is already at 1,200 feet and continues climbing to 2,000 feet. When the tower controller detects this movement, the flight trainer has already turned onto the new course instructed by the approach controller, putting more distance between the two aircraft. Both planes are allowed to fly on from there.
The minimum distance between the flight trainer and the private aircraft was 1.53 nautical miles (over 2,800 meters) horizontally and 500 feet (approx. 150 meters) vertically.
Results of the investigation
Because the flight trainer flew a different route than instructed, it returned to the airport instead of flying southwards. The pilot flying the trainer has stated that the incorrect route was due to incorrect programming of the flight trainer’s navigation system. Once the aircraft approaches a point (in this case ROT), the navigation system is set to continue on towards the next point along the route (in this case Rotterdam Airport). Because ROT is situated close to the airport, the autopilot automatically sent the aircraft on towards Rotterdam Airport after completing the practice approach.
During the occurrence, there were low-hanging clouds at an altitude of 600 feet (about 200 meters). As a result, the tower controller did not have a visual confirmation of where the two aircraft were once they had climbed into the clouds. In such a case, aircraft are required to maintain more distance than in situations involving good visibility which would allow the tower controller to perform a visual check of the air traffic. The additional distance had not yet been ensured when the private aircraft took off. That issue was resolved by the heading instruction that the approach controller issued to the flight trainer.
Because the flight trainer was flying in the clouds, the pilot also had no reference points for his position and did not realize that the route he was flying was incorrect. The pilot of the flight trainer initially entered the heading instruction provided by approach control into the autopilot. The approach controller issued the instruction a second time, at which point the pilot took manual control to execute the instruction more quickly.
The tower controller issued the instruction not to climb higher than 1,000 feet when the aircraft was already flying at 1,200 feet. The pilot of the private plane later stated that a new engine had been installed in the aircraft, causing it to climb so strongly that it was not possible to interrupt this climb at 1,000 feet. This resulted in the aircraft climbing to 2,000 feet.
The occurrence was the consequence of deviation from the instructed route by the flight trainer. The pilot of the flight trainer has indicated that the navigation system had been programmed incorrectly and that the aircraft had therefore flown the wrong course.
In the meantime, the tower controller had instructed the private aircraft to take off but, due to low-hanging clouds, was unable to achieve visual confirmation of either aircraft once they flew into the clouds. As a result, it was not possible to visually check air traffic, making it necessary to maintain more distance between the aircraft than had been ensured at that point.
The radar controller identified the occurrence, issued a heading instruction to the flight trainer, and requested the tower controller not to allow the departing aircraft to continue climbing. Because the heading instruction to the flight trainer was not followed until after it was repeated, and the altitude instruction to the private aircraft could not be followed, the distance between the two aircraft initially decreased before the minimum required distance was achieved.
Classification: significant occurrence