Glide Performance
Expected Glide Performance drives airfield suitability decisions and impacts the conduct of the approach and landing.
Introduction to Glide Performance
- Expected Glide Performance drives airfield suitability decisions and impacts the conduct of the approach and landing.
- Ultimately, glide performance translates into options for gliders or aircraft in the event of an engine-out situation.
Best Glide Versus Minimum Sink
- The best glide speed provides the greatest distance for the least altitude lost.
- Minimum sink speed provides the most time.
- Pilots can achieve optimal glide performance only when they correctly configure the aircraft for the best glide/minimum sink.
Glide Ratio
- The glide ratio is the ratio of the distance an aircraft travels to the altitude it loses.
- An aircraft that travels 50' horizontally for every 10' of vertical altitude is said to have a glide ratio of 5 (50/10).
- Factors that impact glide distance include wind, configuration, and technique (i.e., pilots must avoid uncoordinated flight, which increases drag and reduces glide distance).
Glide Performance
- An aircraft with a glide ratio of 5 has a descent angle of 11+ degrees, meaning if you lose an engine on a 3° glide slope, you cannot reach the runway, regardless of altitude.
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Effects of Winds:
- Headwinds decrease glide distance, as the aircraft has a reduced ground speed (the wind is pushing against the aircraft).
- Tailwinds increase glide distance, as the aircraft will maintain a higher ground speed (the wind is pushing the aircraft forward.
Calculating Best Glide Speed
- Refer to POH for the best glide tables.
- Note the best glide is at the highest lift-to-drag ratio, roughly halfway between Vx and Vy.
- As an aircraft becomes lighter (i.e., as an aircraft burns fuel), the best glide speed reduces as well.
- Manufacturers typically publish the best glide for a given gross weight, and the best glide will decrease as the aircraft's weight decreases.
- Pitch for best glide by controlling the nose to achieve the published speed.
- Without the published speed, point the nose in such a way as to keep the wings level; you may even need to pitch up.
- Several single-engine twins can achieve the best glide speed by applying full nose-up trim.
- No matter the technique, be mindful of stall conditions.
Calculating Minimum Sink Speed
- The minimum sink may not be in most POHs, but it is generally slower than the best glide speed.
Avionics and Electronic Flight Bag Tools
- Some avionics and EFPs provide glide rings based on the (albeit limited) flight conditions.
- These range rings are estimates only.
- Setting avionics ranges to the anticipated glide distance also provides an idea of the available options (i.e., if no airports are within that distance, any need to land will be off-airport).
Glide Performance Conclusion
- As a rule of thumb, any aircraft should be able to glide to a distance of approximately 45° down.
- If, when gliding, the point of intended landing is moving higher in the windscreen, the aircraft will land short of the intended point.
- Understanding glide performance is a regular part of flying a glider.
- What's not so normal is considering how it applies in a powered aircraft when the power is no longer there.
- Mathematics helps pilots determine expected performance, enabling them to make better decisions from the outset.
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