PLT257: Speed To Fly, Glide Ratio, And Ballast
What code PLT257 covers, why the FAA tests it, and how to study Speed To Fly, Glide Ratio, And Ballast before your checkride.
Recall glider performance - speed / distance / ballast / lift / drag
What this means for your checkride
Speed to fly is read from the polar by drawing a tangent that accounts for wind and for the sink of the air between thermals, so the pilot flies faster when the air is sinking. The speed to fly in a tailwind lies between minimum sink and best L/D, but never below minimum sink speed. Glide ratio equals the best L/D speed divided by the sink rate at that speed, computed in calm air. Water ballast increases the optimal speed to fly because the extra weight lets the glider hold the same lift to drag ratio at a higher airspeed, and flight computers adjust their speed-to-fly output for the ballast being carried.
Cross-country efficiency depends on flying the correct speed for the conditions and understanding how ballast changes it.
Carrying ballast does not improve the still-air glide ratio. It shifts best glide to a higher speed, which pays off mainly when cruising fast through sink.
Where to study it
A code on your test report is an area of deficiency. An authorized instructor must review it with you and endorse that review before you take the practical test, and the examiner may revisit it during the oral. Study it until you can explain it out loud, not just recognize the answer.