Notification
On 22 July, during the approach along the Buitenveldertbaan (27) of Amsterdam Airport Schiphol, 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.
Occurrence investigation
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 and results of the investigation
There are a lot of birds in the air over Schiphol that day - particularly at the start of the Buitenveldertbaan runway. That is why it is decided after consultation with bird control to handle incoming aircraft via Aalsmeerbaan.
Two airplanes, a Cessna C25B and a Fokker F70, are approaching Aalsmeerbaan from the south and are instructed by the approach controller to intercept the signal of the runway’s instrument landing system.
After both aircraft have intercepted the signal of the instrument landing system (with the Fokker flying behind the Cessna) and have switched to the tower controller’s frequency, it turns out the aforementioned birds are starting to move towards Aalsmeerbaan. At the advice of bird control, it is decided to inform the Cessna and the Fokker that they need to abort their approach to Aalsmeerbaan and change course so they can land at Buitenveldertbaan.
From Aalsmeerbaan to Buitenveldertbaan
The approach controller and the tower controller agree to instruct the Cessna and the Fokker to keep an eastward heading at an altitude of 2,000 feet - approximately 600 metres - and switch to the approach controller’s communications frequency.
The tower controller instructs the Cessna to adopt an eastward course at an altitude of 2,000 feet and orders the pilot to switch to the approach controller. Immediately after this, the tower controller orders the Fokker to fly east-north-east at an altitude of 2,000 feet. The tower controller refrains from communicating this east-north-eastward course, which is intended to put the Fokker on the same approach route as the Cessna, to the approach controller.
The Fokker is instructed to switch to the approach controller’s frequency. The flight crew do not respond to this instruction. After receiving the instruction a second time, the Fokker confirms that it will be switching frequencies.
Course and airspeed
The Fokker’s east-north-eastern course, and the fact that Fokker’s airspeed is higher than that of the Cessna, creates a risk of the Fokker moving within the Cessna’s minimum separation distance. The approach controller therefore orders the Fokker to change its heading to east-south-east. However, it becomes clear at this point that the Fokker has not yet established radio contact with the approach controller.
The approach controller proceeds to coordinate actions with the tower controller. It turns out that the Fokker is still tuned to the tower controller’s frequency. The Fokker is immediately instructed to reduce airspeed and adopt a course heading east-south-east. The Fokker complies with these instructions without delay.
The Fokker flight crew indicate afterwards that due to the large number of activities that had to be handled in the cockpit, they did not immediately switch to the approach controller’s frequency.
Both aircraft subsequently landed safely on Buitenveldertbaan.
Minimum distance
The minimum distance between the two aircraft occurred when the Fokker took a flight path heading east-north-east in the direction of the Cessna. This separation was 1.8 nautical miles - some 3 kilometres - horizontally and 0 feet vertically. The Fokker flight crew had a continuous visual of the Cessna.
During the approach phase that the two airplanes were in at that stage, the applicable separation minima are 3 nautical miles horizontally - over 5.5 kilometres - or 1,000 feet vertically.
Conclusion
On the one hand, this loss of separation was the consequence of an instruction issued to the Fokker by air traffic control, which put the airplane on a course in the direction of the Cessna. On the other hand, the Fokker did not immediately comply with the instruction to switch communication frequencies. Immediately after it was detected that the distance between the Fokker and the Cessna was less than expected, the Fokker was issued with corrective instructions which averted the danger of collision.
Classification: major incident.