On Thursday 16 July 2020, an aircraft and a helicopter flew closer together than is permitted by the separation minima. The aircraft were both in the control area of Groningen Airport Eelde. LVNL (Air Traffic Control the Netherlands) conducted an investigation into this occurrence itself and reported the occurrence to the Onderzoeksraad voor Veiligheid (Dutch Safety Board).
Description of occurrence
A Cirrus SR20 aircraft and a B0-105 helicopter, both in the airspace on the north side of Groningen Airport Eelde, were handled by the approach controller via the radar screen. Both aircraft were flying according to Instrument Flight Rules (IFR). The aircraft was flying at an altitude of approximately 4,000 feet (1,200 metres) precisely behind the helicopter, which was flying at an altitude of approximately 3,000 feet (900 metres). Both aircraft intended to land at the airport and therefore needed to descend. Due to the faster speed of the aircraft compared to the helicopter, the air traffic controller intended to have the aircraft land earlier. This means that the aircraft would have to overtake the helicopter.
The air traffic controller instructed the helicopter pilot to descend from 3,000 feet to 2,000 feet (600 metres), an instruction that the helicopter pilot read back correctly and then carried out. The air traffic controller then informed the pilot of the aircraft about the new position of the helicopter (traffic info) before instructing the pilot of the aircraft to descend from 4,000 feet to 3,000 feet. This left 1,000 feet (300 metres) of separation between the aircraft and the helicopter, which is mandatory in this situation. The pilot of the aircraft did not hear the instruction properly and asked: 'down to where?'. The air traffic controller then made a mistake and responded by saying 'down to 2,000 feet', whereas it was supposed to be 3,000 feet, as in the original clearance. On the flight strip, where notes are made per flight, the air traffic controller then noted 3,000 feet. The pilot of the aircraft responded with 'down to 2,000 feet', which the air traffic controller did not notice. The air traffic controller’s radar screen showed that the aircraft is descending from 4,000 feet, but does not show the altitude to which the aircraft would descend; the air traffic controller keeps track of the cleared altitude on the flight strip.
Just before the aircraft descended below 3,000 feet, the air traffic controller instructed the pilot not to reduce speed, because at that moment it was overtaking the helicopter, which was flying at 2,000 feet. The air traffic controller then focused on other air traffic in their air traffic control area, and there was a period of 40 seconds during which the aircraft descended below 3,000 feet, which was visible on the radar screen. As soon as the air traffic controller focused on the aircraft and the helicopter again, the air traffic controller observed that the aircraft had descended to 2,200 feet (670 metres), and the air traffic controller realised that the aircraft was descending to the same altitude at which the helicopter was located, i.e. 2,000 feet. At that time, the helicopter was flying approximately 2 nautical miles (3,700 metres) behind the aircraft at a slower speed.
The air traffic controller instructed the pilot of the aircraft to immediately start climbing and initiate a turn to increase the distance to the applicable separation standard, i.e. 1,000 feet (300 metres) vertically or 3 nautical miles (5,500 metres) horizontally.
Both the helicopter and the aircraft subsequently landed safely at Groningen Airport Eelde and both pilots indicated that they had not noticed that they had been closer to each other than prescribed in the separation standard.
The analysis after the occurrence showed that the minimum distance between the two aircraft had been 1.6 nautical miles (3,000 metres) horizontally and 500 feet (150 metres) vertically. The separation standard for the phase of the approach in which the flights were at that moment is 3 nautical miles horizontally or 1,000 feet vertically.
The occurrence was the result of the incorrect altitude instruction that was issued by the approach controller and not noticing that the pilot read back an different altitude than the approach controller had intended. The situation was recognised after the minimum separation standard had been exceeded, after which the air traffic controller intervened.
Classification: serious incident