On the morning of Wednesday 22 June, during the approach along the Zwanenburgbaan (18C), two aircraft came closer to each other than is allowed by the separation minima. LVNL is investigating the occurrence and has reported the occurrence to the Dutch Safety Board.
Loss of separation
The horizontal or vertical distance between aircraft in flight is referred to as their ‘separation’. Separation minima have been established to maintain air traffic safety whilst at the same time making optimum use of air space. Air traffic control is responsible for maintaining this minimum separation between aircraft in its control zone. When two aircraft come closer to each other than the separation minima allow, the situation is known as a loss of separation.
The criteria for separation minima have been designed in such a way that they allow enough time to restore the minimum horizontal or vertical distance. An air traffic controller must undertake a number of steps in a very short time:
- detect the loss of separation;
- identify an effective solution;
- communicate that solution to the pilot(s) concerned, in the form of instructions (regarding their altitude, bearing and speed);
- ensure that the pilot(s) follow these instructions so that safe horizontal or vertical distance is restored as quickly as possible.
LVNL’s primary task with regard to safety is to keep aircraft separate from one another (including aircraft combined with vehicles on the ground). Air traffic control reports all occurrences that take place in practice within LVNL, in order to learn from them and to reduce the risk of such occurrences in the future. Within LVNL, all reported occurrences are investigated in order to be able to continuously improve safety.
Situation and investigation results
An aircraft type Embraer E190 was approaching from the west for a landing on the Zwanenburgbaan. At the same time, an aircraft type Boeing B737 was approaching from the east for a landing on the same runway.
The approach controller instructed the Embraer to descend to 2,000 feet (approximately 600 metres) and then instructed the Boeing to descend to 3,000 feet (approximately 900 metres). It was anticipated that both flights would maintain a minimum distance of 1,000 feet (approximately 300 metres) to create a height buffer between the two flights.
The approach controller was too late in instructing the Embraer (flying on an easterly course) to turn off to the right and to intercept the signal from the Zwanenburgbaan instrument landing system. As a result, the Embraer flew past the extended track through the middle of the Zwanenburgbaan, bringing it into the vicinity of the Boeing. Because at the time of turning in the Embraer was still flying higher than the 2,000 feet instructed, the intended height buffer of 1,000 feet was not attained.
The approach controller then gave both flights corrective instructions. The Boeing received a brief heading instruction to turn away to the right in a north-westerly direction and the crew of the Boeing reported that they had the Embraer in their sights. The Embraer was instructed to descend more quickly. The Embraer continued its final approach, and shortly afterwards the Boeing was able to continue its final approach.
Both aircraft landed safely.
At the time that the Embraer flew past the centre line of the Zwanenburgbaan, the minimum reciprocal distance with the Boeing was attained: 400 feet vertical (approximately 120 metres) and 1.6 nautical miles (approximately 2.9 kilometres) horizontal. The separation standard in the approach phase that the aircraft were in at that stage is 3 nautical miles horizontal - approximately 5.5 kilometres - or 1,000 feet vertical.
The incident was the consequence of the controller giving the instruction too late, in combination with the lack of a good height buffer. Immediately after the Embraer was instructed to commence its final approach, corrective instructions were given and so there was no danger of collision.
Classification: significant incident