On Monday 11 December two aircraft came closer to each other than is allowed by the separation minima. The incident takes place between a plane departing from Schiphol and a flight with destination Schiphol. The situation arises because in one of the two aircraft the altimeter is set differently from what is customary in the phase the flight is in.
LVNL is investigating the occurrence and has reported the occurrence to the Dutch Safety Board.
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 air space. Air traffic control is responsible for maintaining this minimum separation between aircraft in its control zone. When two aircraft come closer to each other than the separation 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 restore the minimum horizontal or vertical distance. An air traffic controller 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);
- monitor that the pilot(s) follow these instructions so that safe horizontal or vertical distance is restored as quickly as possible.
LVNL’s primary safety task is to maintain the separation of aircraft in the air, and between vehicles and other obstacles when on the ground. Air traffic controllers internally report any safety related occurrence, with the objective to learn lessons from those occurrences, thereby reducing the chance that similar occurrences will take place again in the future. All reported occurrences are investigated by LVNL, as part of LVNL’s ongoing commitment to improve safety.
Altimeter and settings
The indication from an altimeter varies with differences in air pressure, and air pressure differs according to location. During various phases of a flight, a pilot must adjust the altimeter to the agreed air pressure. Global agreements have been made about this.
In addition to the separation between airplanes, the distance of planes from the ground and obstacles is important near an airport. That is why the altimeter is set to the local air pressure (QNH) for departing and incoming flights. The local air pressure is reported by the air traffic control.
At greater heights, only the separation between airplanes is significant. There, the altimeter is set to the standard air pressure (QNE), which is 1013 hectopascal, or hPa. This is the standard global procedure. This ensures that all airplanes use the same altimeter setting, so that the mutual separation is maintained.
The standard procedure for an airplane that departs Schiphol dictates that a plane climbs to a height of FL060 (more than 1,800 meters) and that the altimeter is set to the standard air pressure at 3,000 feet (more than 900 meters). From that moment, the height is expressed in flight levels (FL), where 10 flight levels is equal to 1,000 feet (more than 300 meters).
Situation and investigation
The departing airplane, an Embraer 170, has started from the Buitenveldert runway in an easterly direction. The incoming airplane, a Boeing 737, is coming from the northeast for an approach to the Aalsmeer runway in a northerly direction. At the moment of the event, the local air pressure at Schiphol was low, namely 983 hPa.
The Embraer has departed with the correct setting of the altimeter (local air pressure). The crew then switches to the standard air pressure later than the prescribed height of 3,000 feet. This is partly a consequence of turbulence, which distracted the pilots somewhat.
As a result, the Embraer ends up higher than the prescribed height of FL060 (FL060 = more than 1,800 meters). This occurs at the moment that the Boeing is flying at FL070 (more than 2,100 meters). The air traffic controller immediately notices this and gives a heading instruction to the Boeing.
The minimum distance is 1.2 nautical miles (more than 2.2 kilometers) horizontally and 200 feet (approximately 60 meters) vertically. The separation standard in this phase of the flight is 3 nautical miles (approximately 5.5 kilometers) horizontally or 1,000 feet (more than 300 meters) vertically.
At the moment that air traffic control gives a heading instruction, a TCAS alert sounds in the cockpit of the Boeing. In this case, it is a warning to climb. A TCAS warning takes priority over an instruction from air traffic control and therefore the Boeing performes the climbing instruction. As a result, the Boeing keeps flying higher than the Embraer. The Boeing flies along the back of the Embraer.
At the same moment, a TCAS alert also sounds in the cockpit of the Embraer, in this case to descend. The Embraer does not perform the TCAS instruction in a timely manner due to an incorrect action in the cockpit.
The relevant airline has also investigated this event and has taken measures to prevent incorrect air pressure settings and incorrect actions regarding TCAS in the future.
Classification: serious incident