History of the Flight
On 17 October 2013 at 14:51 UTC, an Italian Air Force Dassault Falcon 900 (flight IAM3116) departed Tallinn Airport for Rome Ciampino Airport. The aircraft was cleared to climb to flight level (FL) 340, with this clearance limit set due to crossing traffic: British Airways flight BAW17, a Boeing 777-200, cruising at FL350 on route P862. The airways L729 and P862 intersect near Kihnu Island.
At 15:00, a scheduled change of the Area Control Centre (ACC) West executive controller occurred, and the relieving controller was briefed on the traffic situation. At 15:05, while IAM3116 was approaching FL339, both aircraft received a Traffic Advisory (TA). Two seconds later, IAM3116 reduced vertical speed and began leveling off at FL340. Just before reaching FL340, the crew of IAM3116 received a Resolution Advisory (RA) aural warning: “ADJUST VERTICAL SPEED, ADJUST!”. After descending approximately 80 feet, the aircraft resumed climb, reaching FL343, which decreased vertical separation. Following coordination with ATC, IAM3116 descended back to FL340.
Aircraft Information
IAM3116 was a Dassault Falcon 900 operated by the Italian Air Force, equipped with TCAS II version 7.0 (TTR920, Mode S 24-bit address 33FFFA). BAW17 was a Boeing 777-200 operated by British Airways, also equipped with TCAS II version 7.0 (Mode S 24-bit address 4007F7). Both aircraft were capable of generating Traffic Advisories and Resolution Advisories.
Injuries and Damage
No injuries to persons occurred. No damage to either aircraft or other property was reported.
Analysis
The radar data, reconstructed by Eurocontrol using InCAS simulation software, showed that the minimum separation between the two aircraft was 853 feet vertically and 0.1 to 0.2 nautical miles horizontally. The ACC radar display showed the conflict in red, indicating an unsafe condition. The IAM3116 crew reported receiving the RA and stated they applied the relevant procedure. The investigation determined that the crew's interpretation of the TCAS RA was erroneous. The aural warning “ADJUST VERTICAL SPEED, ADJUST!” was considered vague, which likely contributed to the crew's response of climbing instead of adjusting vertical speed as intended.
Conclusion
The cause of the serious incident was erroneous interpretation of the TCAS Resolution Advisory by the crew of IAM3116. The vague wording of the Resolution Advisory was a contributing factor.