Surveillance Display Freeze during Opposite-Direction Traffic at Tehran ACC
Tehran ACC sector at 0230 local. The controller is monitoring 12 aircraft on a 240-NM display. The main radar display freezes for 20 seconds; the backup display is available but has an 8-second latency and is not coupled to the flight-plan processor. IRB810 at FL350 and IRB811 at FL350 are 40 NM apart on the same airway, opposite direction, and both have been cleared for FL350. IRB812 is 15 NM behind IRB810 at FL350. The controller has no STCA alert on the backup and is due for a break after 2 hours 10 minutes on position. The voice switch is working, and both aircraft are in radio contact. The sector has no adjacent radar coverage below FL290. The controller must act before the two aircraft reach 20 NM separation.
1. What immediate action should the controller take to resolve the conflict?
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The closing speed is high: if each aircraft is doing about 450 kt, the closure is roughly 900 kt, so 40 NM is about 2.7 minutes. The controller must act on voice and not wait for the display to recover. Issue avoiding action to both aircraft immediately. A workable instruction is: 'IRB810, turn right 30 degrees, maintain FL350'; 'IRB811, turn right 30 degrees, descend FL330.' The turn creates lateral divergence and the descent creates vertical separation. If only one aircraft can be reached, turn that aircraft away from the other and then establish vertical separation as soon as possible. Do not rely on the backup display for the avoiding action because its 8-second latency can show a stale position; use pilot position reports and Mode C, but treat them as confirmation, not as the primary tool.
2. How should the backup display and procedural separation be used after the immediate conflict is resolved?
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The backup display can be used to monitor the general picture, but not to establish 5 NM radar separation while it has 8-second latency and no flight-plan coupling. After the avoiding action, establish procedural separation using pilot reports, mandatory position reporting, and confirmed levels. Increase spacing between IRB810 and IRB812, which are on the same level and same direction; if IRB810 turns, ensure IRB812 is not turned into the same airspace. If radar separation cannot be assured, apply a procedural interval appropriate to the route and speed, or hold one aircraft until the main display is restored. The controller should also ask the adjacent sector or military unit for any available surveillance information, but must not assume coverage below FL290 where none exists.
3. What human-factors and reporting issues arise from this event?
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The controller is at 2 hours 10 minutes on position and is managing 12 aircraft during a display failure. That combination increases workload, slows scanning, and makes confirmation bias more likely: the controller may keep looking at the frozen display because it is familiar. The correct human-factors response is to verbalize the failure, call the supervisor, reduce traffic if possible, and take the break as soon as relief arrives. The event must be reported as a system failure and as an occurrence because separation was compromised. Record the time of the freeze, the backup latency, the instructions issued, the positions at the time, and the traffic count. The report should distinguish the equipment failure from the controller's recovery actions so that the investigation can address both the display architecture and the fatigue risk.