Definition
The airspeed at which an unpowered airplane loses altitude at the slowest possible rate, allowing it to remain airborne for the maximum amount of time. It is slower than the best glide airspeed, which gives the greatest distance covered per unit of altitude lost.
Plain English
The speed that keeps the airplane in the air the longest while gliding down, even though it won't travel as far as it could at a faster glide speed.
Context Anchor
Encountered when learning glide performance, especially when comparing staying airborne longer versus covering the greatest distance during an engine-out glide.
Derivation
"Sink" refers to the rate at which the airplane descends through the air. "Minimum sink" means the smallest possible descent rate -- so this is the speed that produces the slowest downward movement.
Why Pilots Care
It gives the pilot the maximum time aloft to locate a landing site or correct a problem before touchdown.
Grounding Statement
At minimum sink airspeed, the airplane is descending, but it is descending at its slowest rate.
Intuition Check
Minimum sink airspeed does not mean the slowest speed the airplane can fly. It means the speed where the airplane loses altitude most slowly during a glide.
Example Sentence 1
After the engine quit, the pilot had a field made and slowed to minimum sink airspeed to give themselves more time to complete the engine-out checklist.
Example Sentence 2
The student compared descent times at minimum sink airspeed versus best glide speed during a practice power-off maneuver.