A squall line, a 15 kt crosswind limit and a 20:00 return
On 28 June you plan a VFR flight from Groningen (EHGG) to a private strip 25 NM east, departing 14:00 UTC and returning by 19:00 UTC. The aeroplane's demonstrated crosswind limit is 15 kt and the strip's only runway is 300/12. The 11:00 UTC TAF reads: EHGG 281100Z 2812/2912 22012KT 9999 SCT035 BECMG 2815/2817 25018G30KT 6000 SHRA BKN014 PROB30 TEMPO 2816/2820 25025G45KT 3000 +TSRA SCT008 BKN010CB BECMG 2822/2824 28008KT SCT040. A squall line is analysed 120 NM west of the field at 11:00 UTC and is moving east at 35 kt. There is no weather radar on board, no alternate strip within 15 NM of the destination, and the destination is flat farmland.
1. Decode the TAF, identify the most hazardous window, and relate the squall line to it.
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Reading the forecast in sequence:
- 2812/2815Z: 220/12 kt, 10 km or more, scattered at 3,500 ft - good VFR.
- BECMG 2815/2817Z: becoming 250/18 gusting 30 kt, 6 km in rain showers, broken at 1,400 ft. A BECMG is a permanent change within the window, so from about 17:00 the ceiling and gusts are the standing forecast, not a temporary fluctuation.
- PROB30 TEMPO 2816/2820Z: a 30 % probability of temporary conditions between 16:00 and 20:00 - 250/25 gusting 45 kt, 3 km in heavy thunderstorm rain, scattered at 800 ft and broken cumulonimbus at 1,000 ft.
- BECMG 2822/2824Z: 280/08 kt, scattered at 4,000 ft - recovery, but after your planned return.
The most hazardous window is 16:00-20:00Z, and it covers your entire intended return. The 30 % probability is not a small number to gamble on: it carries the only CB, the only thunderstorm rain, the strongest gusts and the lowest ceiling in the whole forecast.
Squall line timing: 120 NM at 35 kt = 3.4 hours, so it reaches the field at about 14:25 UTC - 25 minutes after your planned departure, far earlier than the worst TAF window.
2. Decide whether and when to go, using crosswind calculations for the strip runway 300.
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Crosswind component = wind speed x sin(angle between wind and runway). Runway 300, wind 250, angle = 50 degrees, sin 50 = 0.77.
- After 15-17Z (250/18G30): steady 18 x 0.77 = 13.8 kt - just inside the 15 kt limit. Gusting 30 x 0.77 = 23 kt - outside it.
- In the PROB30 window (250/25G45): 25 x 0.77 = 19.2 kt, gusting 45 x 0.77 = 34.7 kt - nearly twice the demonstrated limit, with a thunderstorm and a 1,000 ft CB base.
- Before 15Z (220/12): 60 degree angle, sin 60 = 0.87, 12 x 0.87 = 10.4 kt - comfortably inside.
Decision:
- Do not depart at 14:00. The squall line arrives around 14:25, so you would be landing at a 12 m wide strip with no alternate as the gust front hits.
- Either go early - depart around 11:30 and be back before 15:00, accepting that the wind is already veering - or postpone until after 20:00, when the PROB30 window closes.
- The absence of an alternate within 15 NM is decisive: if the strip becomes unusable there is nowhere to go, so the crosswind margin must be planned, not discovered overhead.
3. Against your better judgement you are airborne and the squall line arrives early. You see a low roll cloud ahead and the wind shifts 30 degrees with heavy rain. What do you do?
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A roll cloud marks the gust front - the leading edge of the cold downdraught outflow. What follows is a sudden wind shift and increase, severe low-level turbulence, a downdraught that can exceed any light aircraft's climb rate, heavy rain, a real risk of hail and a sharp drop in visibility.
- Turn away, now, and open the distance. Do not try to out-climb it or cross underneath; the hazard is at and below cloud base, exactly where you fly. Ten nautical miles of separation is a reasonable target.
- Do not attempt to land at the strip as the front passes. A 30 degree wind shift on runway 300 means a sudden tailwind or crosswind on short final, plus wind shear on the approach path.
- Slow to manoeuvring speed or below if turbulence is encountered, keep the wings level, accept the drift and do not fight the aircraft with large control inputs.
- Divert to a hard runway with better orientation and an instrument approach if available, or hold clear at altitude until the line passes; the TAF indicates improvement after 22Z.
- Note the decision that should have prevented this: the PROB30, the squall line's ETA and the 15 kt crosswind limit were all available on the ground, and all three pointed the same way.