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Use of GPS to assist an emergency descent through cloud

A glider GPS is not a primary navigation aid. Extreme care should be taken to avoid being caught above cloud cover during a wave flight. If you have to make a descent in cloud below 4500’ QFE in the vicinity of the airfield you are risking your life and breaking the Instrument Flight Rules.

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Descending through cloud when the temperature is below zero is particularly dangerous due to icing of airframe, canopy and controls. Always know the freezing level before you take off, and if that is within a few 1000 feet of the surface, it is very important to avoid a cloud descent because there won’t be time to shed the ice before the ground is reached.

 

Should you have the misfortune to find yourself above complete cover, or in poor visibility, then the following advice is offered for a GPS-assisted emergency descent to Feshie Airfield.

 

The descent is based on two waypoints in the Spey Valley 5.6nm apart. Tracking from one waypoint to the other will keep you clear of the mountains and over good outlanding fields. The Insh waypoint is 1.2 nm from Feshie, Ruthven is 6nm from Feshie.  See co-ordinates below. These are standard BGA turn points.

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  1. Make sure the waypoints are in your navigation system before take-off, and are visible on the moving map. Ensure that your projected track line is showing on the map. It may be advisable to create a route that takes you between the two waypoints so that there is a line showing the track.
     

  2. To start the descent, track towards the nearest waypoint. This is done by matching the current track with the bearing to the waypoint, or matching the projected track line with the route line. You should arrive at or above 5000ft on Feshie QFE. This will keep you clear of the surrounding mountains.
     

  3. Wind can create extremely large drift angles which is confusing and can be dangerous. Drift angle can be reduced by flying faster eg best glide speed +15 to 20 kts. Fly even faster if the wind is very strong – never fly at a speed only a little above the wind speed and certainly never below wind speed – you will go backwards!
     

  4. On reaching the first waypoint turn via into wind to track to the second waypoint. Use airbrake to maintain a good rate of descent. During the leg be sure to apply enough drift towards the into-wind direction to track direct to the waypoint or you may drift into the mountains. Remember, if there is a large crosswind component fly faster than usual to reduce the drift angle.
     

  5. At the second waypoint, turn through the into-wind direction to track back to the first waypoint. Remember any drift required must now be applied in the opposite direction. Repeat until clear of cloud.

  6. Bear in mind that the going one way is likely to have significant headwind, and the other way significant tailwind so be prepared for grossly different leg times.
     

  7. Keep in contact with Feshie Base for weather information.
     

  8. When clear of cloud, close your airbrakes and if possible return to the airfield, or land out. Call Feshie once you break cloud.
     

  9. All turns in cloud should made at a moderate angle of bank, via the into wind direction

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Ruthven Barracks (RVN) N57 04.32 W004 02.36
Feshie Descent Point (FE3) N57 06.884 W003 56.020 (near Insh Watersports)

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