TCAS RA During an Opposite-Direction Crossing
At 03:10 UTC, a radar controller is working two aircraft on the same airway in opposite directions. Flight ABX401, a Boeing 777, is level at FL290. Flight NGR502, an Airbus A320, is climbing from FL270 to FL310 and has just passed FL284; the controller had expected vertical separation to exist by the crossing point. The aircraft are 22 NM apart with a closure rate of 950 kt. The controller passes traffic information. Seconds later, ABX401 reports 'TCAS RA, climb, climb.' NGR502 reports 'TCAS RA, descend.' Both aircraft are in controlled airspace with no other known traffic within 15 NM. The controller must decide what instructions may be issued, how separation is restored, and what reports are required after the event.
1. What instructions may the controller issue while the TCAS RAs are active?
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While an RA is active, the controller must not issue any instruction contrary to the RA. ABX401 is climbing, so the controller must not say maintain FL290 or descend. NGR502 is descending, so the controller must not instruct it to continue climbing or maintain FL310. The correct response is acknowledgement and traffic information: ABX401, roger TCAS RA, traffic an Airbus A320 opposite direction descending. NGR502, roger TCAS RA, traffic a Boeing 777 opposite direction climbing.
The pilots have priority and will follow the RA. The controller should keep the frequency clear, stop any other clearances to these aircraft, and wait for clear of conflict. Do not attempt to vector either aircraft back to its cleared level until the RA is over; doing so could worsen the conflict.
2. How should standard separation be restored after both aircraft report clear of conflict?
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After both report clear of conflict, the controller must first establish the actual levels because the RA may have taken the aircraft well away from their cleared levels. Ask: ABX401, confirm level. NGR502, confirm level. Then assign non-conflicting levels. For example, keep ABX401 at FL310 and NGR502 at FL270, giving 4,000 ft vertical separation, or assign FL300 and FL280 if RVSM vertical separation of 1,000 ft is assured. Do not assume the original clearance still applies.
Restore lateral or vertical standard separation before the aircraft converge again. If the geometry is tight, vector one aircraft away and re-clear it on route once separation is positive. Record the RA, the resulting levels, and the new clearance in the flight progress strip.
3. What reports and follow-up are required after the event?
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The event is a TCAS RA and a potential loss of separation, so the controller must file an ATS occurrence report and a TCAS RA report, notify the duty supervisor, and preserve radar and voice recordings. The report should include: the initial clearances, the traffic information passed, both RA messages, the resulting levels, and the restoration of separation.
Investigate why the climb was cleared such that vertical separation was not assured by the crossing point. Because two aircraft received opposing RAs, the occurrence may meet the criteria for a serious incident. Inform the operators, record the involved aircraft and crews, and forward the report to the safety office within the State's prescribed time frame. Do not alter or discard the flight data.