Definition
The airspeed at which an airplane in a power-off glide loses altitude at the slowest possible rate, providing the maximum time aloft for a given altitude. It is slower than best glide speed, which provides the greatest distance covered per foot of altitude lost.
Plain English
The speed that keeps the airplane in the air for the longest time when the engine is not producing power. Flying at this speed makes the airplane drop toward the ground as slowly as possible, but it does not cover the most ground.
Context Anchor
Encountered when learning glides, power-off descents, and the difference between staying airborne longer versus gliding farther.
Derivation
"Sink" describes the airplane's downward movement through the air when it is not producing enough lift to stay level. "Minimum sink" means the smallest downward rate possible, so "minimum sink speed" is the airspeed that produces that smallest rate of descent.
Why Pilots Care
Using this speed during an engine failure gives the greatest time aloft to troubleshoot, select a landing site, or attempt a restart.
Grounding Statement
At minimum sink speed, the airplane is still descending, but it is descending at its slowest rate.
Intuition Check
Minimum sink speed does not mean the slowest speed the airplane can fly. It means the speed where the airplane loses altitude the slowest.
Example Sentence 1
After the engine quit at altitude over open water, the pilot slowed to minimum sink speed to maximize time aloft while preparing the emergency radio call.
Example Sentence 2
The glider pilot held minimum sink speed while searching for lift, conserving altitude until a thermal was found.