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On Thursday, October 23, 2025, two aircraft came closer to each other than the separation minima prescribe. This involved a departing flight from Schiphol Oostbaan (runway 22) and an approaching flight scheduled for the Buitenveldertbaan (runway 27).
LVNL is investigating this occurance and has reported it to the Dutch Safety Board, the Human Environment and Transport Inspectorate, and the Public Prosecution Service in accordance with the investigation process.
Due to stormy weather, the Buitenveldertbaan (27) was being used for both landing and departing traffic (mixed-mode operation). At that time, the Schiphol Oostbaan (22) was occasionally being used for General Aviation (GA), such as business jets. During the storm, the air pressure above Schiphol was exceptionally low: 983 hectopascals (hPa). Such low air pressure occurs only a few times a year around Schiphol.
A Cessna 525B (C25B) business jet is ready to depart from Schiphol Oostbaan (22) bound for Budapest. Before the aircraft departs, the pilot receives route instructions, including a climb to flight level 60 (approximately 1,830 meters). This climb is standard after departure from Schiphol to ensure a minimum altitude difference of 1,000 feet (approximately 300 meters) with air traffic approaching Schiphol at flight level 70. While repeating the route instructions, the pilot of the C25B mentions a different altitude: 6,000 feet instead of flight level 60. Air traffic control immediately corrects this to flight level 60, and the pilot then confirms the correct altitude.
After taxiing, the C25B receives clearance to enter the runway. The tower controller provides additional instructions: after departure, fly a heading of 120 and remain on the tower controller’s frequency instead of switching to the radar controller upon passing 2,000 feet. After the pilot correctly acknowledges the instructions, the air traffic controller grants the pilot clearance to depart from Schiphol Oostbaan (22).
After takeoff, the aircraft climbs very rapidly, at a rate of 4,200 feet per minute. However, the pilots do not switch to the standard air pressure setting above the transition altitude. As a result, with this exceptionally low air pressure, they are able to reach an altitude 810 feet (approximately 250 meters) higher than what the air traffic controller authorized.
In the Netherlands, the transition altitude—the altitude at which pilots must change the air pressure setting—is 3,000 feet. Above this altitude, pilots switch from local air pressure to an international standard setting of 1013 hPa. If an aircraft does not switch to the standard setting above this altitude, its actual flight altitude will differ from its intended flight altitude relative to other air traffic. In this case, this means that an aircraft climbing to 6,000 feet may actually end up higher than an aircraft climbing to flight level 60. Due to the exceptionally low air pressure of 983 hPa, this difference was approximately 810 feet (about 250 meters).
At the same time, a go-around is taking place on the Buitenveldertbaan (27), causing the tower controller to briefly attend to another flight. Afterward, the tower controller turns his attention back to C25B and instructs the pilot to switch to the radar controller. The tower controller has to repeat this instruction once more before the pilots actually switch to the radar controller. When C25B finally makes contact with the radar controller, the aircraft is already at around 5,200 feet due to its high rate of climb and is still climbing.
At that moment, an Airbus A321neo (A21N) is en route to Schiphol, flying south of Schiphol in an easterly direction at flight level 70 (2,130 meters) and is scheduled to land on the Buitenveldertbaan (27). Normally, in such situations, a minimum altitude difference of 1,000 feet is maintained. The C25B should therefore have stopped climbing at flight level 60, while the other aircraft was flying at flight level 70. Because the C25B continued to fly with an incorrect barometric pressure setting, the aircraft climbed higher than permitted and approached the crossing A21N.
The radar air traffic controller receives a Barometric Pressure Setting (BPS) warning indicating that the aircraft is flying with an incorrect barometric pressure setting. The controller responds immediately with corrective instructions: the C25B must level off at flight level 60 because there is traffic flying above them, and specifies the standard barometric pressure setting of 1013 hPa. The pilot misreads the barometric pressure as 983, which the controller must correct again. The air traffic controller also instructs the C25B to descend immediately to flight level 50, as it has already passed flight level 60 by a wide margin due to its high rate of climb. They are now approaching an altitude of flight level 68 (approximately 2.07 km) and will begin descending again from this point at a rate of over 3,000 feet per minute.
In the meantime, the approaching A21N receives a TCAS Resolution Advisory (TCAS RA) warning. The TCAS initially instructs the A21N to descend. Because the C25B has also begun to descend by this point, the TCAS shortly thereafter instructs the A21N to climb instead in order to resolve the conflict. This resulted in the A21N briefly descending to flight level 68 and then climbing back above flight level 70. For unknown reasons, the TCAS on board the C25B issued a TCAS RA warning later than the TCAS on board the A21N did. By that time, the C25B had already begun its descent.
*Flight levels are set in the cockpit above the transition altitude to a single fixed value for air pressure, 1013 hPa, or QNE. If the local air pressure is 1 hPa lower than QNE, 1012 hPa, and the flight were to continue climbing at local air pressure, it would end up approximately 27 feet higher than if it were flying at QNE. For example, climbing to 6,000 ft with an air pressure of 1012 hPa means that, relatively speaking, it would not reach FL60, but FL60.27. In this case, the air pressure was 30 hPa lower than QNE, namely 983 hPa as opposed to 1013 hPa. The 30 hPa difference means that a flight climbing to 6,000 feet at local air pressure ends up about 30 times 27 feet too high, or about 810 feet higher, and thus reaches FL68.1.
At the moment the C25B and the A21N came closest to each other, both aircraft were flying at flight level 68. The horizontal distance between the two aircraft at this point was 0.4 nautical miles (approximately 740 meters). The situation quickly returned to normal thereafter. The A21N continued its approach to the Buitenveldertbaan (27), and the C25B resumed its flight as scheduled following the brief intervention. Less than two minutes elapsed from the moment the C25B took off ( ) until the conflict was resolved.
The investigation shows that the incident was the result of a combination of circumstances. Key factors included the failure to switch to the standard barometric setting in a timely manner, the very high rate of climb, and the exceptionally low barometric pressure during the storm, which caused the aircraft to climb to a much higher flight level than cleared by the air traffic controller and, as a result, brought it closer to other air traffic than permitted.
The investigation also shows that the air traffic controllers corrected incorrect readbacks and intervened as soon as it became clear that the aircraft was climbing too far.
Contact with the operator indicates that the C25B crew experienced high workload in the cockpit during the storm. Additionally, they were less familiar with Schiphol and not accustomed to the low transition altitude at which the transition from local pressure to standard pressure occurs.
Although not all causal factors fall within LVNL’s sphere of influence, the incident has been discussed within operational management, and potential improvements are being explored to prevent recurrence. The risk and recommendation to implement mitigation measures have been documented in a safety recommendation and are being followed up within LVNL. These recommendations focus primarily on better informing operators about the low transition altitude within the Netherlands and switching to standard air pressure settings in a timely manner. The findings have also been shared with operational staff so that lessons can be learned from them.
Classification: major incident
