Final Glide into a Valley Field with a Wind Shift
You are 25 km from your home field at 1400 m AGL. The field is in a valley at 300 m elevation, with a 700 m grass strip aligned 09/27. The forecast wind was 270/10 kt, but the ATIS now reports 250/18 kt gusting 28 kt. Your glider's polar gives a best glide ratio of 40:1 at 90 km/h, and you have been flying at 100 km/h. The field elevation is 300 m, and your altimeter reads 1700 m with QNH set. There is rising terrain between you and the field, with a ridge at 1100 m elevation that you must cross. Your calculated final glide at 100 km/h gives a 200 m margin over the ridge. The valley is known for mechanical turbulence and rotor on the lee side when the wind is from 250° or more.
1. Calculate whether you can reach the field, and assess the margin.
Read the worked answer
Height available above the field: 1700 m - 300 m = 1400 m. Distance to run: 25 km. At a 40:1 glide ratio, still-air height required is 25,000 m / 40 = 625 m. So in still air you have a large margin - about 775 m.
But the margin over the ridge is the binding constraint. The ridge is at 1100 m, so your height above the ridge is 1700 m - 1100 m = 600 m. The calculated final glide at 100 km/h gives only a 200 m margin over the ridge, which is the number that matters, because the ridge is between you and the field and you must clear it before you can descend into the valley.
The wind is now 250/18 gusting 28 kt, which is a headwind component on a westerly track and increases the height required to reach the ridge. Gusts of 28 kt also mean the sink and the rotor on the lee side will be worse than the still-air calculation. The 200 m margin is not adequate for a lee-side crossing in these conditions.
The correct decision is not to press on to the ridge with a 200 m margin. Either gain height before the ridge, divert to an alternative field on the windward side, or turn back while you still have height.
2. Describe the approach and landing plan if you do reach the valley, including the wind and turbulence considerations.
Read the worked answer
If you reach the valley with height, the plan should account for the strong, gusty westerly wind and the lee-side rotor.
- Approach direction: with 250/18 gusting 28 kt, runway 27 gives a headwind component of about 17 kt gusting 26 kt, which is favourable for landing distance but demands active gust control. Runway 09 would give a tailwind and should be rejected.
- Turbulence: the lee side of the valley will have mechanical turbulence and possible rotor. Fly the approach with a higher approach speed (for example, add half the gust spread to the normal approach speed), and be prepared for a shear on short final.
- Height and profile: keep a stable approach, avoid steep turns low, and plan a go-around option. In a 700 m strip with a strong headwind, the landing roll will be short, but the flare and touchdown must be positive.
- Escape: identify an alternative field on the windward side before descending into the valley, and keep enough height to divert if the turbulence is worse than expected.
The judgement tested is that a strong gusty wind changes both the approach speed and the escape options, and that a lee-side valley is not a place to arrive with a marginal height margin.