On Thursday 14 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 Groningen Airport Eelde.
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
Two flight trainers are performing practice approaches
The aircraft involved are type TB-10 and type C172. Both pilots are performing practice approaches flying under instrument flight rules in order to obtain their pilot license, The approach controller at Eelde instructs them to fly to a position lined up with the approach runway, from which the pilots can start their instrument approach. The approach controller is responsible for achieving enough distance between the two aircraft: at least 3 nautical miles (approx. 5,600 meters) horizontally or 1,000 feet (approx. 300 meters) vertically.
The air traffic controller’s planning
The TB-10 flies northeast, parallel to the landing runway, and is guided by the approach controller to a point about 8 nautical miles from runway 23 to commence the instrument approach. This means that the TB-10 continues flying until the air traffic controller instructs the aircraft to turn to the southwest for the approach. The C172 also flies northeast, and the approach controller is planning to have the C172 line up behind the TB-10 for the approach to runway 23.
Short line-up by C172
By this point, the TB-10 has been guided by the approach controller on a southwest heading towards runway 23 and is handed off to the tower controller according to the procedure. The C172 is still flying northeast, parallel to the landing runway, when the pilot of the C172 requests permission to turn towards the runway in order to start the approach earlier: a ‘short line-up’. The approach controller approves the request and instructs the C172 to turn to the southeast, moving to a position from which it can start the approach to runway 23, behind the TB-10.
Then the approach controller sees on the radar display that the C172 is flying faster than the TB-10 and that the distance between the aircraft is dropping below the required 3 nautical miles. The approach controller corrects the situation by having the C172 continue flying on the southeast heading provided, until the aircraft has achieved sufficient distance from the TB-10 again. The approach controller then guides the C172 to the west to start the approach to runway 23.
The minimum distance between the TB-10 and the C172 was 1.8 nautical miles (approx. 3,300 meters) horizontally and 100 feet (30 meters) vertically.
Results of the investigation
The occurrence is the result of a combination of factors. During the event, another aircraft was performing a survey flight in the same area. That flight increased the complexity of the overall traffic situation. The air traffic controller estimated that it would be possible for the C172 to turn towards the runway earlier, with the traffic pattern at that time. However, the C172 was flying faster than expected, which ultimately led to a loss of distance with the TB-10. The air traffic controller then corrected the situation.
Classificatie: significant incident