Compressing Sequence on the ILS: A Slow Cessna in Front of a Jet Stream
Santo Domingo Approach is sequencing four arrivals for ILS RWY 17 in VMC, wind calm, no cloud below 5,000 ft. In order along the downwind: a B737 12 nm from touchdown at 180 kt, an ATR-72 at 20 nm at 170 kt, a Cessna 402 at 25 nm at 140 kt, and a Learjet 35 at 35 nm at 250 kt. The Cessna pilot has asked to fly 110 kt to the marker because of a nose baggage-door warning light, and his transponder is intermittent. Minimum radar separation in the TMA is 5 nm. The tower reports the previous landing vacated at the second highspeed turnoff, and the runway is dry. The ATR is on a company schedule and its crew has already asked for 'no delays'. The Learjet's crew wants to be on the ground before their duty limit, which is 40 minutes away. Radar is serviceable. Two VFR training aircraft are in the circuit on RWY 17.
1. Do the arithmetic, decide the landing order, and state your first transmission.
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Convert every aircraft to time to touchdown:
- B737: 12 nm at 180 kt ≈ 4.0 min
- ATR-72: 20 nm at 170 kt ≈ 7.1 min
- Cessna 402: 25 nm at 110 kt ≈ 13.6 min
- Learjet 35: 35 nm at 250 kt ≈ 8.4 min
The Cessna is first in the pattern behind the B737, but it will be caught by both the ATR and the Learjet well before 5 nm final. Slowing the ATR to its minimum clean speed (about 130 kt) only stretches it to roughly 9 min, which is still not enough and compresses everything behind. The Cessna therefore has to come out of the sequence.
First transmission, before anything else: 'Cessna 402, turn right heading 090, maintain 3,000 ft, expect re-sequencing, traffic an ATR-72 at your ten o'clock, five miles, eastbound.' Give the reason if time allows, give the new expectation, and keep it short. Provisional order: B737, ATR-72, Learjet, Cessna, unless the door warning escalates the Cessna to the front.
2. The Cessna's transponder is intermittent and the pilot reports a nose baggage-door warning. How does that change your handling and your separation?
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Two separate issues, and they must not be confused.
Surveillance: 5 nm radar separation rests on a verified radar position. If the transponder drops out, identify the aircraft by primary radar returns or by a position report over a known fix, and use the last verified position to protect it. If identification is lost, revert to a procedural minima or terminate radar service for that aircraft before you base separation on a track you cannot see. Do not let a frozen track symbol make a decision for you.
Urgency: a possible open baggage door is an urgency condition, not a routine delay. Ask 'say intentions', offer a tower fly-by or low approach so the crew can get a visual check, alert the RFFS to standby, and ask whether they want to land immediately. If they say yes, they become number one and everyone else extends downwind or goes around; a warning light outranks a company schedule. Do not diagnose the problem for the pilot and do not trade his safety for sequence convenience.
3. How do you integrate the Learjet, and what phraseology do you use to slow it?
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At 250 kt the Learjet needs about 8.4 min from 35 nm, so it will run over the ATR at 7.1 min. It must be slowed before it becomes a conflict, not after the two tracks touch.
Sequence it behind the ATR with at least 5 nm and issue speed control early, in stages:
- 'Learjet 35, reduce speed 210 knots.'
- Then, as the picture stabilises: 'Learjet 35, reduce speed 180 knots, maintain 180 knots to 5 DME.'
- Add the expectation: 'Expect ILS approach RWY 17, number three, landing interval about four minutes.'
Two rules govern this. Never assign a speed below the aircraft's minimum safe speed; if the crew says 'unable', accept it and re-vector rather than repeat the instruction. And speed control is only valid while you have identified traffic, so if the Learjet's separation depends on the Cessna's intermittent transponder, switch to distance-based vectors you can see on primary radar.
Finally, tell the Learjet crew the truth about the delay. Their duty limit is a crew-planning problem, not a separation problem, and they may need to plan a diversion or a standby crew.