Primary-Radar VFR Traffic and a Mode C Discrepancy in the TMA
At 0910 UTC, Nairobi Approach is providing radar service in the TMA. 5N-BBB, a Beechcraft King Air, is IFR at 9,000 ft inbound to JKIA and squawking 4521. A VFR Cessna 172, 5Y-CSA, is at 8,500 ft, transponder failure, and has been identified by primary radar only 4 NM ahead of the King Air on a converging track. The King Air's Mode C indicates 9,300 ft while the pilot reports 9,000 ft. A third aircraft, a Q400, is at 7,000 ft, 8 NM behind. The secondary radar has intermittent failure and the display shows a 'data age' warning. You have 5 NM radar separation and 1,000 ft vertical.
1. What immediate separation action would you take between the King Air and the Cessna, and why?
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The Cessna is primary-only, 4 NM ahead on a converging track, and the vertical difference is only 500 ft based on the King Air's reported level. That is not sufficient: 1,000 ft vertical or 5 NM radar separation is required. Issue traffic to both aircraft and instruct the Cessna to descend to 8,000 ft: '5Y-CSA, traffic, King Air one o'clock, 4 NM, 9,000 ft, descend and maintain 8,000 ft.' Tell the King Air: '5N-BBB, traffic, Cessna twelve o'clock, 4 NM, 8,500 ft, primary only, maintain 9,000 ft.' The descent to 8,000 ft gives 1,000 ft against the King Air and 1,000 ft against the Q400 at 7,000 ft. Do not rely on the Cessna's Mode C because the transponder has failed; use primary position and pilot-reported level.
2. How do you handle the King Air's Mode C discrepancy?
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The displayed 9,300 ft differs from the pilot-reported 9,000 ft by 300 ft, which is significant for separation. First ask the pilot to verify level and confirm the altimeter setting: '5N-BBB, verify level, confirm altimeter setting.' If the pilot confirms 9,000 ft and the discrepancy persists, stop using that Mode C for separation and use the pilot-reported level, while informing the next controller and requesting a transponder/altitude reporting check. Do not average the two figures or assume the higher value is correct. If the aircraft is in RVSM airspace later, a persistent 300 ft error must be treated as a height-keeping issue and reported. In the TMA, maintain 1,000 ft vertical separation based on the verified pilot level until the Mode C is reliable.
3. The display shows a 'data age' warning and secondary radar is intermittent. What do you do?
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Treat the 'data age' warning as a loss of valid surveillance data for separation. Do not continue to apply 5 NM radar separation on stale SSR data. Revert to primary radar if it is serviceable and you can identify the aircraft, or apply procedural/non-radar separation. Reduce traffic flow, issue delaying vectors or holds, and coordinate with the adjacent sector and tower. Inform affected aircraft if radar service is terminated: '5N-BBB, radar service terminated, maintain 9,000 ft, report over [fix].' If primary radar is also unreliable, separate the King Air, Cessna and Q400 by 1,000 ft vertical and known positions until surveillance is restored. Record the time of the failure and the separation method used.