On Sunday 25 February 2018 two aircraft came closer to each other than is allowed by the separation minima. Both aircraft are approaching Rotterdam The Hague Airport and are under control by Rotterdam air traffic control. The situation arises when one of the two aircraft descents below the level which is instructed by the air traffic controller. During the occurrence the pilots of both flights have each other’s aircraft in sight, so there is no collision risk.
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.
Description of situation
The traffic controller in the tower provides approach air traffic control within the area of responsibility of Rotterdam Approach. Visibility is good and there are no clouds.
The two flights
The pilot of a Mooney aircraft contacts Rotterdam tower while at an altitude of 2,000 feet (around 600 metres), flying in the direction of the STD beacon. The traffic controller instructs the pilot to continue the flight for an instrument approach to runway 06.
A Citation aircraft approaches from the west and the pilot informs the traffic controller in the tower that he is descending to flight level 60 (around 1,800 metres). The traffic controller instructs the pilot of the Citation that he can expect to proceed directly for an instrument approach to runway 06. The traffic controller also instructs the Citation to fly to the PS beacon and to descend to 3,000 feet (around 900 metres).
The traffic controller intended to allow the Citation to land first, followed by the Mooney. That is the reason why the traffic controller instructs the pilot of the Mooney to follow a north-westerly course after reaching the STD beacon. The traffic controller informs the pilot of the landing sequence, stating that the route to be followed is necessary in order to maintain the necessary separation behind the Citation.
The Citation is given permission to make a final approach
When the pilot of the Citation reports having reached an altitude of 3,000 feet, the traffic controller instructs him to continue flying at that altitude until commencing the instrument approach by means of the ILS glide-path. The pilot is given permission to continue the final approach to runway 06.
A short while later, the Mooney is instructed to fly to the PS beacon.
The Citation descends below 3,000 feet
The traffic controller sees on the radar screen that the Citation has descended below 3,000 feet to 2,200 feet, earlier than intended. The minimum separation with the Mooney is reached at that point: 0.4 nautical miles horizontal separation (around 740 metres) and 200 feet (around 60 metres) vertical separation. After that, their flight-paths diverge.
The pilot of the Citation reports having received a “false glideslope indication” (a false ILS glide-path signal) and that he would return to an altitude of 3,000 feet. Both flights then went on to be completed without any irregularities.
Outcomes of investigation
The most significant cause of the occurrence was the fact that the Citation descended to below 3,000 feet earlier than intended.
During the investigation, the pilot of the Citation repeatedly stated that the aircraft was following a “false glideslope indication” and that that signal could not have been correct due to the aircraft’s position in relation to the airport. The operator of the Citation turned out not to be in possession of any data recordings from the aircraft’s systems.
During the investigation carried out by LVNL, no evidence was found that a false glide-path was the cause leading to the aircraft descending to below 3,000 feet. Both the glide-path equipment and the recorded signal transmissions were examined and no irregularities were found. When descending below 3,000 feet, the Citation was, furthermore, travelling at a position that lay outside of the coverage area of the glide-path signal.
The pilots establish visual contact and receive a notification
The pilot of the Citation saw the Mooney on his right-hand side and had increased altitude, partly as a result of receiving a TCAS notification. The pilot of the Mooney saw the Citation approaching from a distance and received a PCAS indication. PCAS is a portable version of TCAS that provides simplified functionality that issues a warning if aircrafts fly too close together. These are used on a regular basis in small-sized aircraft. The Citation passed by in front and the pilot of the Mooney did not see any need to alter his course.
Instruction to descend
According to current traffic control procedures, the instruction to leave an altitude of 3,000 feet on the ILS glide-path is a correct one. However, previous occurrences have shown that pilots sometimes interpret that instruction in different ways.
Though this is not relevant in the case of this particular occurrence, the investigation revealed that the rules and working method for the purpose of controlling approaching air traffic is not always applied clearly by the traffic controllers in the Rotterdam tower.
Follow-up as a result of the investigation
A description of this occurence has been published in the LVNL Safety Magazine. In the case of critical altitude separations with other aircraft during the final approach, air traffic control has been recommended to exercise caution when instructing aircraft to leave an altitude of 3,000 feet on the ILS glide-path.
A recommendation was also made to tighten up the working method employed when controlling approaching air traffic from the tower and to ensure that staff are aware of that working method.
Classification: serious incident