On Thursday 30 March, during the parallel approach along the Zwanenburgbaan (18C) and the Polderbaan (18R), 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);
- monitor that the pilot(s) follow these instructions so that safe horizontal or vertical distance is restored as quickly as possible.
LVNL’s primary safety task is to maintain the separation of aircraft in the air, and between vehicles and other obstacles when on the ground. Air traffic controllers internally report any safety related occurrence, with the objective to learn lessons from those occurrences, thereby reducing the chance that similar occurrences will take place again in the future. All reported occurrences are investigated by LVNL, as part of LVNL’s ongoing commitment to improve safety.
Description of the situation
An aircraft, type Boeing 737, was approaching from the east to land on the Zwanenburgbaan. An aircraft, type Embraer 190, was approaching from the northwest to land on the Polderbaan. Both aircraft descended.
The approach controller for the Polderbaan instructed the Embraer to descend to 2,000 feet. The pilot of the Embraer confirmed the instruction. Meanwhile, the Boeing descended to 3,000 feet, in accordance with instructions from the Zwanenburgbaan approach controller. However, the Embraer descended slower than expected. As a result, 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 next to each other, informed the pilots of the Embraer and the Boeing of the parallel operation and of the presence of another aircraft in their vicinity. The pilots of both aircraft confirmed that they understood the situation, and the Boeing pilot confirmed the Embraer was visible. Air traffic control monitored both aircraft precisely, and decided not to give any corrective instructions.
Both aircraft landed safely.
The moment at which the loss of separation occurred was the moment that the Embraer was still descending to the instructed height of 2,000 feet and the Boeing was already flying at the instructed height of 3,000 feet.
The minimum separation between the two aircraft was 1.4 nautical miles horizontally – approximately 2.6 kilometres – and 500 feet vertically – approximately 152 metres. The separation standard in the approach phase that the aircraft were in at that stage is 3 nautical miles horizontally 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). This occurred shortly after reaching the minimum separation.
The incident was the result of an incorrect estimation of the Embraer’s vertical flight profile. Air traffic control has acknowledged this. Based on the normal horizontal behaviour of the two aircraft and the confirmation from the Boeing pilot that the Embraer was visible, no corrective instructions were given.