Night Approach with a Microburst and Low-Level Wind Shear
At 2240 UTC your aircraft is 25 nm west of destination on a night visual approach. The 2230 METAR reads: wind 250°/12 kt gusting 28 kt, visibility 1,800 m in heavy thunderstorm rain, few cumulonimbus at 1,500 ft, broken cloud at 3,000 ft, temperature 24 °C, dew point 22 °C, QNH 1004 hPa. The TAF contains TEMPO 2230/2330 250°/25 kt gusting 45 kt, visibility 800 m, heavy thunderstorm rain. A SIGMET warns of embedded cumulonimbus tops FL420 with severe turbulence. A departing aircraft reports a 25 kt airspeed loss at 600 ft AGL on the runway centreline, and the low-level wind shear warning is active. Fuel remaining is 75 minutes; the alternate is 25 minutes away and final reserve is 30 minutes. The weather radar shows a bow-shaped core 8 nm southwest of the field.
1. What evidence indicates a microburst or gust front, and why is the 600 ft PIREP significant?
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The evidence points strongly to a microburst or strong gust front. The temperature/dew-point spread is only 2 °C, so the boundary layer is nearly saturated and evaporative cooling can produce a dense downdraft. Heavy thunderstorm rain, cumulonimbus, a bow-shaped radar echo and gusts to 28 kt with a forecast of 45 kt all indicate a strong outflow. The bow echo is a classic signature of a descending downdraft and outflow surge. The PIREP is the most operationally important item: a 25 kt airspeed loss at 600 ft AGL on the centreline means the shear is active in the final-approach segment, not merely near the ground. At night, with 1,800 m visibility in heavy rain, visual references will be poor and the first indication may be a sudden sink or airspeed loss. The visual approach should not be continued on the assumption that the preceding aircraft's smooth report still applies.
2. Using the fuel figures, decide whether to continue, hold, or divert. Show the arithmetic.
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Fuel remaining is 75 minutes. Final reserve is 30 minutes and the alternate is 25 minutes away, so 55 minutes are committed to reserve plus diversion. That leaves 20 minutes for approach, go-around and any holding. A single approach and go-around in this weather could consume 15–20 minutes, and a second approach would leave no fuel to reach the alternate. The TEMPO condition runs until 2330, the core is only 8 nm southwest of the field, and the active wind-shear warning plus the 600 ft PIREP make a missed approach likely. Holding is not a realistic option because there is no hold fuel and the weather is forecast to persist. Divert now to the alternate, declare the fuel situation if necessary, and do not attempt the approach on the strength of the earlier smooth report. The safe decision is diversion while the fuel margin still exists.
3. If the aircraft inadvertently enters the shear on short final, describe the recovery and the reports you would make.
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Recognise the shear from airspeed loss or gain, unusually high sink, glidepath deviation or a sudden change in pitch attitude. Apply go-around thrust immediately, set the go-around attitude, and maintain wings level. Do not chase the airspeed indicator with pitch; use the established pitch attitude and let thrust and energy recovery follow. Do not retract gear or flaps until a positive climb is established. If the autopilot is engaged and is not coping, disconnect and hand-fly. Report the wind shear to ATC immediately, including position, altitude, airspeed loss or gain and the phase of flight, and file a PIREP after landing. Also report the bow echo and the 600 ft loss so that following aircraft receive the warning. If a second approach is attempted, use the wind-shear recovery procedure briefed before take-off and consider a higher approach speed only if approved and landing distance permits.