Notice
On Thursday 2 May, two aircraft, on approach towards Groningen Airport Eelde, approached each other closer than the minimum separation requirements.
LVNL is investigating this incident and has reported it to the Dutch Safety Board and the Human Environment and Transport Inspectorate.
Situation and investigation
A Diamond DA40, a light single-engine training aircraft, is flying a holding pattern around OMFAR at an altitude of Flight Level 50 (approx. 1500 metres). The aircraft is waiting for permission to descend to a lower altitude to perform an approach to runway 05. The altimeter is set to the default standard atmospheric pressure (QNE) of 1013 hectopascals (hPa). At higher altitudes, where aircraft separation is the only factor of importance, the altimeter is set to this standard atmospheric pressure (QNE). This is standard procedure worldwide.
A Cessna 340, a light business jet, performs an approach to runway 05 at an altitude of 2000 feet (approx. 600 metres) via OMFAR. The C340's altimeter is set to local atmospheric pressure (QNH). Local atmospheric pressure is used at lower altitudes, where distance from the ground and obstacles has to be factored in alongside separation between aircraft. Air traffic control provides local atmospheric pressure.
The approach controller authorises the DA40 to descend to an altitude of 3000 feet (about 900 metres), relaying the local air pressure of 999 hectopascal. The DA40's pilot correctly reads back the altitude and local air pressure.
The pilot of the DA40 reports that the plane is approaching 3000 feet. The approach controller confirms this and instructs to maintain the altitude of 3000 feet in the holding pattern. This instruction is also read back correctly by the DA40's pilot.
However, the approach controller notes that the DA40 is descending below the cleared altitude of 3000 feet and asks the pilot to confirm that the altitude of 3000 feet will be maintained. This is confirmed by the pilot, whose instruments are still set to standard atmospheric pressure (QNE), making it seem on board that the aircraft is flying at 3000 feet. At this point, the flight instructor on board the DA40 notes that local atmospheric pressure (QNH) had not yet been set correctly and points this out to the student.
Since the altitude indicator on the radar screen still shows a lower altitude, the approach controller instructs the pilot of the DA40 to immediately climb back to an altitude of 3000 feet.
The moment the DA40 drops below the cleared altitude, the C340 flies out in front of the DA40 with the same heading and an altitude of 2000 feet.
Minimum distance
The minimum distance between the DA40 and the C340 is 0.8 nautical miles (about 1.5 kilometres) horizontally and 500 feet (about 135 metres) vertically. The standard separation at this stage is 3 nautical miles (about 5.5 kilometres) horizontally or 1000 feet (over 300 metres) vertically.
Conclusion
The loss of separation occurred because the air pressure setting on board the DA40 was not adjusted from standard air pressure (QNE) to local air pressure (QNH) for the descent to 3000 feet. The difference between QNE and QNH value led to a substantial difference between the cleared and actual altitude of the DA40.
Classification: serious incident