Definition
The optimum airspeed for a glider or sailplane to fly between thermals or areas of lift, calculated to maximize cross-country performance by balancing the loss of altitude against the distance covered in the prevailing air mass conditions. Speed to fly increases when flying through sinking air or with a headwind, and decreases when flying through rising air or with a tailwind.
Plain English
The best airspeed for a glider to use when flying from one patch of rising air to the next. The pilot flies faster through sinking air to get out of it quickly, and slower through air that is sinking less, to cover the most ground for the altitude lost.
Context Anchor
Seen in glider and sailplane flying, especially when choosing how fast to fly between areas of rising air.
Why Pilots Care
In soaring, a poor speed-to-fly choice can mean the difference between reaching the next thermal and landing in a field. Flying too slow through sink wastes altitude; flying too fast through good air covers less ground per foot lost.
Grounding Statement
In still or helpful air, the best speed may be slower; in sinking air or a headwind, the best speed is often faster.
Intuition Check
Speed To Fly does not mean any speed that will keep the aircraft flying. It means the selected best speed for the conditions and the pilot’s goal.
Example Sentence 1
As the variometer showed strong sink, the pilot pushed the nose down to the speed to fly indicated on the MacCready ring.
Example Sentence 2
With a strong headwind the speed to fly was increased to maintain the best glide angle over the ground.