On 2 June 2010, an Airbus A318 (F-GUGJ) being operated by Air France on a scheduled passenger flight from Lyon to Toulouse was about 90nm northeast of Toulouse when it came into very close proximity in day Visual Meteorological Conditions (VMC) with a Pilatus PC12 (EC-ISH) being operated by Norestair SL on a non revenue positioning flight from Buochs, Switzerland to San Sebastian, Spain which was travelling on the same track and in the same direction. A last minute sighting enabled a slight descent and divergent turn to be made. There was no Airborne Collision Avoidance System (ACAS) activation on either aircraft and the minimum separation after the avoiding action was estimated at between 15 and 30 metres horizontally and about 100 feet vertically.
An Investigation into the accident was carried out by the French BEA. It was established that the PC12, with only two altimeters fitted, had earlier reported different readings on each of them, one showing FL270 and the other FL290, but had been reassured by an exchange with ATC that the former was correct. Despite the fact that the one stated by ATC to be correct was also on the same side of the cockpit as one of the two available airspeed indicators which had concurrently with the development of the altimeter discrepancy begun to register an impossibly low reading, the pilot had accepted the ATC assurance that his aircraft was at the correct cleated level of FL270. In fact, it was 2000 feet higher and the altimeter and airspeed on the other side of the cockpit were both correct. The rest of the flight was completed uneventfully by reference to those instruments.
The A318 crew reported that they had initially detected some “strange” slow roll oscillations of about 5 degrees maximum for about 5 seconds but seeing nothing abnormal on their PFD, had carried on with the preparation for their arrival. Then, the First Officer “intrigued by fresh oscillations that made him think of wake turbulence, looked outside. He was then in visual contact with an aircraft that was very close, slightly above and to the right. He disconnected the autopilot and made a pitch down input to the left, keeping in constant visual contact with the other aircraft while passing. He estimated that he had descended around 200 feet during this manoeuvre. During this time, he was watching his Navigation Display (ND) to be sure that no aircraft was located below. He saw the white diamond symbol on the TCAS indicating an aircraft 2,000 feet below, without realising at that time that it was in fact the same aircraft that he had just passed.”
The pitot static system on the PC12 was examined. It had a cabin differential pressure indicator which used the static pressure pneumatic line on the Pilot’s side to determine its indication. The Pilot’s side static line was found to have a leak on a connector that joined it to the cabin differential pressure indicator so that some of the pressurised air in the cabin was able to enter the static pressure line by (at FL290) an amount equivalent to 2,000 feet of altitude.
It was noted that the aircraft was on its first flight following disconnection and reassembly of the static system in order to carry out an EASA-mandated Airworthiness Directive. Although the static circuit was required to be subjected to an impermeability test after reassembly, with an unpressurised system, this test did not show any leak. It was considered that the vibrations in flight and the stresses created by the climb with pressurisation of the cabin “very likely led the tube to move thus causing the leak”.
The Investigation noted that the PC12 AFM Performance Tables showed that at the prevailing atmospheric conditions at cruise power, the IAS at FL290 would be 158 KIAS whereas at FL270, it would be 165 KIAS. Prior to the conflict, the flight instruments in front of the (single) pilot were reported to have been showing FL270 and 90 KIAS whereas the fight instruments in front of the front seat passenger were showing FL290 and 160 KIAS. However, it was noted that the Pilatus Quick Reference Handbook (QRH) drill for pitot / static system problems did not lead to the utilisation of such diagnostic evidence.
It was concluded that the near miss with the Airbus 318 had been the result of a leak in the static pressure line which supplied the left side barometric and speed unit which had led to erroneous altitude and speed information being displayed to the Pilot. It was found that this had caused the PC12 to fly into conflict with the A318 without the risk of collision between the two aircraft being detectable by the controller or by Short Term Conflict Alert (STCA) or TCAS. However, the last minute sighting of the PC12 by the A318 had fortuitously removed the risk of a collision.
The mistaken belief of ATC that it was possible to verify the actual altitude of the PC12 provided false reassurance to the PC12 Pilot and led him to continue his flight 2000ft higher (FL290) than the level he believed he was at (FL270).
Three Safety Recommendations were made as a result of the Investigation as follows:
On 26 August 2010:
- that the DSNA (French ANSP) implement, in the shortest possible time, an emergency procedure so that ATC ensures that there is a safety space around an aircraft as soon as the crew casts doubt on its vertical position, without waiting for the latter to declare a distress or emergency situation.
At the conclusion of the Investigation:
- that the European Aviation Safety Agency (EASA) should require that procedures in the Flight Manual relating to situations of doubtful or erroneous altitude be completed or developed by manufacturers
The Final Report of the Investigation was published on 10 February 2011. It included the statement that crews should be informed that:
- that they must strive to maintain external visual vigilance and to pay attention to “weak signals“. In the case under consideration, only the visual detection of the PC12 following a perception of “oscillations“ by the A318 crew made it possible to avoid a probable collision in flight.
- that the protection systems onboard (TCAS) and on the ground (STCA) are based on the altimeter values transmitted by the airplane via the transponder. Consequently, a false altimeter value makes it impossible for these systems to play their role as the final safety system.
- that ATC controllers do not have equipment that allows them to dispel any doubts expressed by a crew concerning its altitude. In fact, the only altitude information available on the ground comes from mode C, transmitted by transponders.