On the morning of Thursday 2 June, during the parallel approach along the Zwanenburgbaan (36C) and the Aalsmeerbaan (36R), 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 Boeing B737, was approaching from the west to land on the Zwanenburgbaan. An aircraft, type Beechcraft Beechjet BE40, was approaching from the east to land on the Aalsmeerbaan.
The approach controller for the Zwanenburgbaan instructed the Boeing to descend to 2,000 feet - or 600 metres. The approach controller for the Aalsmeerbaan instructed the Beechjet to descend to flight level 45 - or 1.3 kilometres. This anticipated the continuation of both flights in order to realise a vertical separation distance of at least 1,000 feet - or 300 metres - between the Boeing and the Beechjet, before both aircraft turned for a parallel approach to their landing.
Upon reaching the cleared altitude of flight level 45, the approach controller for the Zwanenburgbaan instructed the Beechjet to descend further, accidentally giving an altitude of 2,000 feet instead of 3,000 feet - or 900 metres. The pilot of the Beechjet confirmed the instruction. The Boeing was already flying at 2,000 feet and was instructed by the approach controller for the Zwanenburgbaan to intercept the radial of the instrument landing system of the Zwanenburgbaan.
When the approach controller for the Aalsmeerbaan instructed the Beechjet to intercept the radial of the instrument landing system of the Aalsmeerbaan, the Beechjet descended to below the altitude of 3,000 feet. The approach controller saw this immediately and instructed the Beechjet urgently to remain at an altitude of 3,000 feet. The planned vertical separation of at least 1,000 feet was not realised. Both aircraft followed the planned horizontal path as instructed.
Both approach controllers, who were seated directly next to each other, informed the pilots of the Boeing and the Beechjet of the parallel operation and of the presence of another aircraft in their vicinity. Both approach controllers gave instructions rectifying the situation. The approach of the Beechjet was interrupted and the aircraft was instructed to fly in an easterly direction. The Boeing’s heading was briefly changed to north-west. Soon thereafter, the Boeing could continue its final approach. The Beechjet made a new approach.
Both flights landed safely.
When the Boeing and the Beechjet turned in to intercept the radial of the instrument landing systems on their respective runways, the minimum separation distance was reached: 400 feet - or 120 metres - vertically and 1.9 nautical miles - approximately 3.1 kilometres horizontally. In the approach phase in which the flights found themselves at that point, the separation norm is 3 nautical miles horizontally - 5.5 kilometres - or 1,000 feet vertically. This norm applies until both aircraft are stabilised at that part of the instrument landing system that provides the ideal course line (localizer).
The incident was a result of an instruction by air traffic control for an incorrect altitude for the Beechjet. As soon as it was detected that the Beechjet’s altitude was not what it was excepted to be, instructions rectifying the situation were given, which meant that there was no danger of collision.
Classification: significant incident