6 June 2017 - Loss of separation AAS

On Tuesday the 6th of June 2017, an aircraft that had to execute a go-around on the Schiphol-Oostbaan and an aircraft that executed a departure from the Aalsmeerbaan, came closer to eachother 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 aircrafts in flight is referred to as their ‘separation’. Separation minima have been estabishled 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 aircrafts in its control zone. When two aircrafts come closer to eachother 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 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.

Description of the situation

An aircraft of the type Airbus A321 approaches from a north-easterly direction to land on the Schiphol-Oost runway and receives wind information from the air traffic controller at Schiphol. Above and around Schiphol there are rain showers and a strong south-westerly wind is blowing. An aircraft of the type Embraer E190 is ready for take-off from the Aalsmeer runway and is given permission to depart by the air traffic controller.

Airbus does a go-around

Just before landing on the Schiphol-Oost runway, the captain of the approaching Airbus reports that he is going to do a go-around as a result of a wind shear. The air traffic controller confirms that he has understood this. At that moment, the departing Embraer has commenced the start procedure.

Following the announcement of a possible go-around of an aircraft approaching to land on the Polder runway, a great deal of coordination goes on via the intercom between the air traffic controllers at Toren Centrum, Toren-West and the approach controllers. As a result, there is a slight delay in resolving the potential loss of separation between the aircraft and the air traffic controller eventually instructs the Airbus to maintain its current course. On this course, which is the runway direction, the Airbus remains sufficiently free from the departing Embraer and the Airbus passes behind the Embraer.

The instruction to the Airbus is not confirmed by the pilot and the aircraft banks to the left, as prescribed in the go-around procedures for the Schiphol-Oost runway. In response, the air traffic controller issues a corrective instruction to the Airbus to fly towards the east. This instruction is followed. 

Embraer takes off from Aalsmeer runway

The departing Embraer flies, according to procedure, on a route towards the east and after take-off banks to the left. The climbing Embraer passes the height of the Airbus at a height of around 2,000 feet – around 600 metres – which is also banking to the left, at a height of 2,000 feet.

At this height, both aircraft fly into the clouds, whereupon the air traffic controller is unable to see the aircraft and can no longer apply visual separation. Shortly afterwards, the departing Embraer climbs to a sufficient height and there is a vertical separation of a minimum 1,000 feet – around 300 metres.

Both flights continue without problem.

Minimum separation

Air traffic under the control of the air traffic controller may be visually separated. If this is not or no longer possible, then the separation criteria of the radar controller apply: 3 nautical miles - around 5.5 kilometres - lateral, or 1,000 feet - around 300 metres - vertical.

At the moment that the Airbus and the Embraer are both banking to the left, the minimum separation between them is reached: 1.3 nautical miles - around 2.5 kilometres - lateral and 200 feet - around 65 metres - vertical.

Conclusion

The incident resulted from the fact that the air traffic controller was rather late in instructing the Airbus to continue flying on its current course, i.e. towards the runway, so in accordance with procedure, the Airbus had alreadybanked to the left.

Immediately on detecting the loss of separation between the Airbus and the Embraer, corrective instructions were issued. There was no risk of collision.

Follow up in response to the investigation

From the safety investigation, it appears that in certain weather conditions an air traffic controller is not always able to apply visual separation, although this is assumed by current procedures for simultaneous use of the Schiphol-Oost runway and the Aalsmeer runway.

The possibility of improving the procedure when both runways are in use at the same time is being studied.

Classification: major incident