Notice
On Sunday, September 22, 2024, two aircraft came closer to each other than the separation minima prescribe. The occurence occurred between two aircraft at higher level under the control of the Amsterdam Area Control Centre.
LVNL conducted its own investigation into the occurence and has also reported it, in accordance with the investigation process, to the Dutch Safety Board, the Human Environment and Transport Inspectorate, and the Public Prosecution Service.
Description of occurrence
Two single-engine aircraft were both en route at cruising altitude, under the control of the Amsterdam Area Control Centre (ACC). Amsterdam ACC is responsible for guiding air traffic up to an altitude of about 7.5 kilometres. One aircraft ('aircraft A’) was flying at Flight Level 120 (at approximately 3.7 kilometres altitude), the other aircraft ('aircraft B’) was flying at Flight Level 100 (just over 3 kilometres altitude). The pilots had requested this altitude when they submitted their flight plans.
The pilot of aircraft A flying at Flight Level 120 asked the ACC controller to be allowed to descend to Flight Level 100 because of so-called ‘icing’ conditions. These conditions can cause icing to form on the outside of the aircraft and that could potentially lead to a risky situation. The ACC controller replied that initially descending to Flight Level 110 (about 3.3 kilometres altitude) was permitted and that a further descent to Flight Level 100 was possible, but that the ACC controller must then give additional heading instructions because the other aircraft (aircraft B) was flying 3-4 nautical miles (about 6.5 kilometres) behind aircraft A. The ACC controller asked for confirmation of the Flight Level 110 instruction and to be kept informed by the pilot. Both instructions were confirmed by the pilot.
The ACC controller then issued aircraft A with a heading instruction to a waypoint (a point along the route which is designated using a name consisting of five letters). The pilot did not know this waypoint and asked how it was spelt and the ACC controller replied accordingly. Shortly afterwards, the plane made a left turn and dropped to below Flight Level 110. This caused the aircraft to fly closer to the other aircraft than prescribed by the separation minima. The ACC controller noticed this and asked the pilot to check whether he was flying towards the specified waypoint, as the route to the waypoint was north of where the pilot was currently flying. The pilot responded by saying “standby”, meaning “please wait, I will get back in touch with you later”. In the meantime the ACC controller issued a heading instruction to aircraft B, which was still flying at Flight Level 100, to turn away from aircraft A, thereby restoring the necessary separation.
Minimum separation
The minimum distance between the two aircraft was 3.0 nautical miles of horizontal separation (slightly more than 5.5 kilometres) and 400 feet of vertical separation (slightly more than 120 metres). ACC traffic controllers use separation criteria of 5 nautical miles (about 9.3 kilometres) of horizontal separation or 1,000 feet (about 300 metres) of vertical separation in the area in question.
Conclusion and follow-up
The investigation revealed that the ACC traffic controller was trying to prepare for the possible further descent of aircraft A in order to create space between it and aircraft B. That was why the heading instruction to the waypoint was given. The pilot stated in the investigation that he did not know the waypoint and it therefore took a lot more effort and time to look it up.
In addition, the pilot was concerned about icing conditions and was busy putting the aircraft into a descent. As a result the pilot descended further than indicated in the instruction. When the ACC controller discovered this and checked with the pilot whether he was flying in the right direction, the pilot replied by saying “standby”. The ACC controller then felt that the pilot's priority was to pilot the aircraft to get clear of the conditions causing the icing and therefore decided to give aircraft B a heading instruction away from aircraft A.
Both pilots were able to see each other's location throughout the flight on the Automatic Dependent Surveillance–Broadcast (ADS–B) system in their aircraft (a system that allows aircraft to share their position, altitude, speed and other information in real time with other aircraft) and they were therefore constantly aware of each other's position.
Feedback on the investigation result and lessons learned from this event will be provided to all ACC controllers at LVNL
Classification: significant occurrence
