Definition
The airspeed at which an aircraft, in a power-off glide, loses altitude at the slowest possible rate measured in feet per minute. It produces the longest time aloft but not the greatest distance covered.
Plain English
The speed to fly if you want to stay in the air as long as possible while gliding without power. It keeps you up the longest, but it doesn't take you the farthest.
Context Anchor
Seen in glide performance and emergency procedures discussions, especially when considering what speed to fly after a loss of engine power.
Derivation
“Sink” comes from an old word meaning to go down. In aviation, “sink” means the aircraft is moving downward through the air; “rate-of-sink” means how fast that downward movement is happening.
Why Pilots Care
This speed maximizes time in the air during an engine failure, giving more opportunity to find a landing site or receive help.
Grounding Statement
At this speed, the airplane is trading altitude for time as efficiently as it can.
Intuition Check
Minimum rate-of-sink does not mean the airplane will glide the farthest. It means the airplane will descend the slowest and stay airborne the longest.
Example Sentence 1
After the engine quit, the pilot slowed to minimum rate-of-sink to buy extra time aloft while making the mayday call.
Example Sentence 2
The glider pilot held minimum rate-of-sink to conserve altitude while waiting for the next thermal.