On 12 December 2014, two aircraft breached the airborne separation minima in the approach zone of Amsterdam Schiphol Airport. LVNL has reported this incident to the Dutch Safety Board and is conducting its own investigation.
Loss of separation
The horizontal or vertical distance between aircraft in flight is referred to as their “separation”. Separation minima have been established to maintain air traffic safety, whilst at the same time making optimum use of airspace. Air traffic control is responsible for maintaining these minimum distances between aircraft in its control zone. When two aircraft come closer to one another than the minima allow, the situation is known as a “loss of separation”.
The criteria for separation minima have been designed in such a way that they allow enough time to correct the situation before it presents a serious danger. An air traffic controller faced with a loss of separation must undertake a number of steps in a very short time.
- Detect the loss of separation;
- Identify an effective solution;
- Communicate that solution to the pilot(s) concerned, in the form of instructions regarding their altitude, bearing and speed;
- Ensure that the pilot(s) follow these instructions in order that safe separation is restored as quickly as possible.
LVNL’s primary safety task is to maintain the separation of aircraft from one another, and also from vehicles and other obstacles when on the ground. Air traffic controllers internally report any incidents falling within our area of responsibility, with the aim of learning lessons from them and so reducing the chance that similar occurrences will take place again in the future. All reported incidents are investigated by LVNL, as part of our ongoing commitment to improving safety.
On 12 December 2014, at approximately 08.00 CET, a loss of separation occurred in the approach zone of Amsterdam Schiphol Airport. The situation arose because a Piper P46T aircraft departing from Rotterdam climbed beyond its cleared flight level, causing a loss of separation with a Dassault Falcon 900 (F900) which was approaching Schiphol from the south. At their closest, the two aircraft were 1.6 nautical miles (horizontal) and 300 feet (vertical) apart. LVNL has reported this incident to the Dutch Safety Board and is conducting its own investigation.
Summary of investigation results
The primary responsibility of Air Traffic Control the Netherlands is ensuring the separation of aircraft from other aircraft and ground vehicles. All safety events that occur at Air Traffic Control the Netherlands are reported and investigated with the objective to learn and improve, and thus reduce the probability of similar incidents in the future. The results of this incident investigation
It is a busy morning at Amsterdam Airport Schiphol on December 12th 2014. Around 8 o’clock in the morning air traffic control is guiding all inbound traffic to the airport. Two runways (18R and 18C) are in use for inbound flights and one runway (18L) is used for outbound flights. At that exact moment runway 24 is used in mixed-mode at Rotterdam The Hague airport.
An Dassault Falcon 900 (F900) approaches Schiphol from the south to land on runway 22. The crew is in contact with approach control located at LVNL Headquarters (Schiphol east) and descends to FL 070. At Rotterdam The Hague airport a Piper Malibu (P46T) is cleared for take-off using runway 24. Air traffic control instructs P46T to use the LUNIX1B departure route and climb to 3000 ft. After this instruction the P46T is transferred to approach control at LVNL Headquarters. While following the LUNIX1B departure route the P46T is instructed to climb to FL060 and fly a direct route to point MISGO, which is a small deviation of the LUNIX1B departure route. ATC received an acknowledgement of these instructions by the pilot.
On the east side of Rotterdam the P46T pilot reports that he almost reached the instructed FL060. ATC instructs the P46T to maintain FL060. ATC receives acknowledgement of this instruction and confirms the height information by checking the radar plot, which indeed indicates FL060 at that moment. After a short while ATC notices that the P46T is still climbing instead of maintaining the instructed FL060. ATC repeatedly instructs the P46T to maintain FL060. ATC receives acknowledgement of this instruction, but meanwhile the height information of the radar plot shows FL064 and the P46T is still climbing. As a result, the P46T comes closer to the F900, which flies at FL070 approaching Schiphol, than the separation minimum. ATC instructs the F900 to turn left immediately heading 340 (NNW) and climb to FL080. The F900 executed these instructions immediately and separation was restored.
ATC again instructs the P46T to descend to FL060 and is executed as such. The maximum height of the P46T was FL068 according to the radar plot. The minimum separation occurred when the F900 passes behind the P46T and is determined at a horizontal distance of 1,6 NM and a vertical distance of 300ft. The mandatory separation minimum is 3NM or 1000ft. Both aircraft could not visually see each other while flying through a cloud layer. The crew of the F900 noted the P46T on the TCAS display in the cockpit, but they did not receive any kind of proximity warning. The P46T is a small aircraft and is not equipped with TCAS.
During the investigation the pilot of the P46T explained that one altimeter was switched to the local pressure (QNH 992Hpa) and the other one to standard pressure (QNH 1013Hpa). On top of that the aircraft experienced upward turbulence and the autopilot system could not handle this turbulence very well. When an aircraft is flying at FL060, which above transition altitude, the altimeter should be adjusted to standard pressure instead of local pressure. If the altimeter is still switched to local pressure, FL060 is actually FL066. The incorrect altimeter settings and the upward turbulence together explain the P46T flying at FL068 instead of the instructed FL060.
Classification: significant incident
The owners of the small aircraft P46T will pay more attention to the adjustment of the altimeter during annual ‘prof checks’.
A Flight Level (FL) indicates the altitude of an aircraft referenced to an imaginary isobaric base with the International Standard Atmosphere pressure at sea level of 1013.2 hectopascals. It is not necessarily the same as the aircraft's true altitude either above mean sea level or height above ground. Flight Levels are used to separate aircraft above the height where ground obstacles (like mountains) are no longer of concern. While use of a standardised pressure setting facilitates separation of aircraft from each other, it does not provide the aircraft's actual height above ground. Standard flightlevels are measured in feet (ft). For example, FL010 is 1000ft above the standard isobaric base, FL100 is 10.000ft and FL245 is 24.500 ft etc. At low altitudes the true height of an aircraft relative to an object on the ground needs to be known. The pressure setting to achieve this is called QNH or "altimeter setting" and is available from various sources, including air traffic control and the local METAR-issuing station.
When aircraft depart from an airport the pressure setting used is the the local pressure (QNH). The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of altitude to the use of flight levels. When operating at or below the TA, aircraft usually fly at the altitude referred to the QNH. When aircraft climb the aircraft altimeter pressure setting is normally adjusted, passing the transition altitude, to the standard pressure setting of 1013 hectopascals (millibars) or 29.92 inches of mercury and aircraft altitude will be expressed as a flight level. If en aircraft descends the crew switches the aircraft altimeter pressure setting from standard pressure to local pressure. In order to secure the vertical separation minimum of 1000ft between these transition levels a transition layer is defined.
The transition level is the lowest flight level above the transition altitude. When descending below the transition level, the pilot starts to refer to altitude of the aircraft by setting the altimeter to the QNH for the region or airfield. Note that the transition level is, by definition, less than 500 ft above the transition altitude. Aircraft are not normally assigned to fly at the transition level as this does not guarantee separation from other traffic flying (on QNH) at the transition altitude; the lowest usable flight level is the transition level plus 500 ft. The transition layer is the airspace between the transition altitude and the transition level.
Infographic loss of separation 12 December 2014