Glide Performance

Introduction:

  • 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 an engine-out situation

Best Glide Vs. Minimum Sink:

  • The best glide speed provides the most distance for the least altitude lost
  • Minimum sink speed provides the most time
  • Optimial glide performance can only be attained when the aircraft is appropriately configured for best glide/minimum sink

Glide Ratio:

  • The glide ratio is the ratio between how far an aircraft travels against how much altitude lost
  • 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., not flying uncoordinated, increasing drag)

Glide Performance:

  • An aircraft with a glide ratio of 5 has a descent angle of 11+ degrees
    • This means if on a 3° glide slope and an engine is lost, the runway cannot be made, regardless of altitude

  • 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 lightens (i.e., fuel burn), the best glide speed reduces as well
  • Note that best glide is usually published for gross weight, and 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 the pitch up
    • Many single-engine twins can actually 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 is generally a little slower than the best glide speed

Avionics and Electronic Flight Bag Tools:

  • Some avionics and EFPs provide glide rings based on the (albiet, limited) conditions of flight
  • These range rings are estimates only
  • Setting avionics ranges to the anticipated glide distance also gives an idea of what options are available (i.e., if no airports are within that distance, any need to land will be off airport

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
  • If when gliding, the point of intended landing is moving lower in the windscreen, the aircraft has the glide performance necessary to make the intended point
  • Understanding glide performance is a regular part of flying a glider
    • Not so normal is thinking about how it applies in a powered aircraft when the power is no longer there
  • Math helps pilots determine expected performance, allowing pilots to make better decisions from the start
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