On 17 October 2013, a Falcon 900 being operated by the Italian Air Force on a flight from Tallinn to Rome Ciampino as IAM 3116 lost prescribed vertical separation in Class 'C' airspace in day VMC against a Boeing 777-200 being operated by British Airways on an international passenger flight from London Heathrow to Seoul Incheon as BAW 17. Both aircraft were working the same ATC frequency and on crossing tracks.
An Investigation was carried out by the Estonian Safety Investigation Bureau. It was noted that the State Aircraft involved "did not fall under the scope of Civil Accident Investigation Authorities neither in either Estonia or in Italy [but that] the Bureau had received full and valuable support from Italian Air Force Flight Safety Inspectorate" during the Investigation. Recorded ATC voice and radar data from the Estonian and Latvian ANSPs provided the primary evidence for the Investigation, the latter including Mode S downlink data which was analysed using the EUROCONTROL InCAS program, v 2.10 (see the output reproduced below).
The InCAS output of the relative vertical and lateral tracks of the two conflicting aircraft. [Reproduced from the Official Report]
It was established that the F900 had been cleared to climb only to FL 340 as its track would cross that of the eastbound 777 at FL 350. Its crew had been advised of the crossing traffic. Five minutes after a scheduled change of executive controller for the sector, during which the relieving controller had been briefed about the two crossing track aircraft, the F900 was approaching FL 339 and showing a radar indication of an 800 fpm rate of climb. At this point, both aircraft received a TCAS TA generated by their proximity to each other. The F900 crew reported having visually identified the 777 in approximately their 2 o`clock relative position and the 777 crew reported having visually identified the F900 in their 11 o`clock relative position. Then, 2/3 seconds later as the F900 reached FL 340, its crew received TCAS RA 'ADJUST VERTICAL SPEED ADJUST'. Mode S downlink data showed that the aircraft had initially descended approximately 80 feet before beginning to climb back to and beyond FL 340 in response, reaching FL 343 and triggering an STCA Alert at the radar controllers position. The F900 crew did not advise ATC of their TCAS RA until after it had ceased and they were returning to FL 340. When reporting it, the F900 crew stated that "we had a TCAS climb, now TCAS completely is ended, we continue descent FL340" followed by "we had a TCAS conflict that commanded us to climb […] it was maybe a problem of the wrong indication of the TCAS, but my rules say that I have to follow". The 777 did not receive a TCAS RA in respect of this proximity. The recorded radar picture of the two aircraft about to cross is shown below. Actual minimum separation was 853 feet vertically when between 185 and 370 metres apart laterally.
The controller’s radar picture shortly before the two tracks crossed. [Reproduced from the Official Report]
It was evident from the reconstructed relative flight paths that the initial reaction of the F900 crew to the RA - a reduction in vertical speed whilst about to level as cleared - had been correct, but this had then been followed by an incorrect reversal into a climb which had continued through FL340 until and beyond the Closest Point of Approach (CPA) and 'Clear of Conflict' annunciation until the exchange with ATC shortly afterwards in which the crew were reminded of their cleared level. It was stated that the commander of the F900 had been an "experienced pilot rated as instructor in the Italian Air Force who was experienced with TCAS II". It was noted that "in this instance the STCA [alert had] provided little help to the controller to avoid this conflict".
The primary concerns of the Investigation were the incorrect pilot response to a TCAS RA and the lack of any international requirements for carriage of TCAS II equipment by State aircraft operating in airspace shared by civil traffic and controlled by a civil ANSP.
The Cause of the occurrence was found to be "that the crew of the Dassault Falcon 900 misinterpreted the TCAS RA 'Adjust Vertical Speed, Adjust' and increased their rate of climb instead of levelling off".
One Contributory Factor was also identified as follows:
- The wording of the TCAS II version 7.0 RA involved in this occurrence might have contributed to the incorrect pilot response.
Two Safety Recommendations were made as a result of the Investigation as follows:
- that all ICAO and EASA Member States should, for safety reasons, upgrade their State aircraft to TCAS ll version 7.1 as soon as possible, including when forward-fit opportunities arise.
- that ICAO, EASA and the European Commission should consider encouraging their Member States to take further action to ensure the adoption of TCAS ll version 7.1 on State aircraft when operating in common airspace to mitigate the risks of loss of separation between civil and State aircraft.
The Final Report was completed during 2015 and subsequently published in English.