Gust front and wind shear on final: go-around and runway change
At 16:10 local, a line of thunderstorms lies 8 NM west of the airport, moving east at 25 kt. The ATIS gives wind 210/08 kt, but the threshold anemometer now shows 290/25G40 kt. A Boeing 737 is at 2 NM final to Runway 24 and reports 'moderate to severe wind shear, plus 15 then minus 20 knots'. The tower has a low-level wind shear alerting system. The approach controller's weather radar shows a magenta core over the final approach path. A second aircraft is at 8 NM final; a third is waiting to depart. The MET office has not issued a new SIGMET. The airport has one runway; the cross runway 15/33 is available but has no ILS and would give a 12 kt tailwind on Runway 15.
1. What do you do with the two arrivals in the next two minutes?
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Send the 2 NM aircraft around immediately. The reported shear of plus 15 then minus 20 kt is severe and the magenta core is over the final path; a landing is not worth the risk. Instruct: 'Go around, climb to 3,000 ft, wind 290/25G40, wind shear warning.' Do not clutter the frequency with an explanation.
For the 8 NM aircraft, stop the approach before it reaches 4 NM: instruct it to hold at 3,000 ft or vector it downwind, and pass the wind shear report and the go-around. Issue a general wind shear warning on the frequency and update the ATIS. Warn the departing aircraft not to take off into the gust front; hold it.
The LLWAS alarm confirms the shear, but the pilot report is the strongest evidence. The aim is to keep aircraft out of the shear zone until the gust front passes.
2. Do you change to Runway 15/33? Give the reasoning with the numbers.
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No, not now. First, the wind is 290/25G40; on Runway 15 that is a 12 kt tailwind, and on Runway 33 the crosswind component is high. Many operators limit tailwind to 10 kt, with 15 kt only by approval, so 12 kt may exceed the aircraft limit. Second, Runway 15/33 has no ILS, and the ceiling and visibility may deteriorate as the line arrives; a visual approach is not a safe alternative in a gust front. Third, changing runway during an active gust front means re-sequencing aircraft in shear and with a tailwind, which adds risk.
Wait until the squall line passes, then reassess. If the wind settles at 290/20, Runway 24 is still into wind. If it settles at 330/15, consider Runway 33 after checking tailwind and crosswind limits and pilot acceptance. A runway change is a whole-system decision, not a reflex.
3. What reports and coordination are required after the event?
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Record the pilot reports and the LLWAS alarms in the log with times and values. Pass the pilot report to the MET office as a PIREP and ask whether a SIGMET is warranted; ATC does not issue SIGMETs, but the MET office can. Update the ATIS and, if the shear persists, keep the warning on the ATIS.
Inform the supervisor and the next controller at handover. If an aircraft was damaged or a serious incident occurred, preserve the recordings and file an occurrence report. For a routine weather go-around no mandatory occurrence report is needed, but a wind shear report is valuable safety data and should not be lost. The lesson is that pilot reports and LLWAS must reach the MET office and the next shift, not just the voice tape.