On Tuesday, December 28, 2021, two aircraft came closer to each other than prescribed by the separation minima. One aircraft had taken off from Rotterdam The Hague Airport. The other aircraft was on approach to Amsterdam Airport Schiphol.
Air Traffic Control the Netherlands (LVNL) conducted its own investigation into this occurrence and has reported the occurrence to the Dutch Safety Board (Onderzoeksraad voor Veiligheid).
Description of occurrence
A Piper Malibu (PA46) aircraft took off from runway 24 at Rotterdam The Hague Airport. The Piper flew on the SOMEL2B departure route, heading south-west.
A Bombardier Regional Jet (CRJX) headed for Schiphol was flying in from the south.
The air traffic controller covering the South sector at that time had both aircraft under control. The Piper was cleared to climb to flight level 80; this instruction was confirmed by the pilot. Due to delays at Schiphol, the Bombardier was cleared to proceed to the RIVER holding area and was instructed to descend to flight level 90; this instruction was confirmed by the pilot. After a few minutes, the Bombardier received permission to leave the holding area and fly towards Schiphol Airport, still at flight level 90. Following these instructions, the Bombardier and Piper should have passed each other at 1,000 feet of vertical separation. However, the Piper continued climbing past flight level 80. Identifying this, the air traffic controller instructed the Piper to descend to flight level 80 and instructed the Bombardier to climb to flight level 100. Separation was restored but fell below the minimum separation standard.
During this occurrence, the minimum distance between the two aircraft was approximately 4 kilometres (2.2 nautical miles) horizontally and 122 metres (400 feet) vertically. The separation standard is approximately 9 kilometres (5 nautical miles) horizontally or 300 meters (1,000 feet) vertically. When the minimum distance was reached, the Piper and the Bombardier flew away from each other, thus increasing the distance.
Conclusions of the investigation
The Piper and the Bombardier were flying regular routes. However, the Piper did not proceed to flight level 80 when granted clearance, although the pilot did confirm. This is because, when passing the transition altitude (3000ft), the pilot did not switch the altimeter to the standard air pressure (QNE, 1013 hPa) that should be used above the transition altitude. The pilot continued to use the very low, local Rotterdam air pressure (QNH), which was 988 hPa. The 25 hPA difference resulted in an altitude difference of approximately 206 metres (675 feet, 7 flight levels). As a result, the Piper climbed higher than the cleared flight level 80 and continued to climb to within the separation minimum. The pilot of the Piper stated that a malfunction report occurred on board the Piper during the climb, and that the pilot forgot to switch to standard air pressure due to the increased workload.
Training sessions for air traffic controllers will cover the need to be alert to altitude deviations with very high or very low air pressure as a result of this occurrence.
LVNL is also investigating how information about the air pressure selected by the pilot can be presented to the air traffic controller.
Classification: major incident