Definition
VYSE is the airspeed in a multi-engine airplane that produces the greatest gain in altitude per unit of time when one engine is inoperative and the remaining engine is producing maximum available power. It is published in the Aircraft Flight Manual and is marked on the airspeed indicator of most multi-engine airplanes by a blue radial line, commonly called the 'blue line.'
Plain English
If you lose one engine in a twin, VYSE is the speed that gets you the most height gain per minute on the engine you have left.
Context Anchor
Seen in multiengine training, engine-failure procedures, aircraft performance charts, and on the airspeed indicator as the blue line.
Derivation
V stands for velocity (speed), Y is the standard aviation code letter for best rate of climb, and SE means single engine. Put together: the velocity for best rate of climb on a single engine.
Why Pilots Care
Holding this speed after an engine failure gives the best chance of climbing to a safe altitude or returning to the airport rather than descending into terrain.
Intuition Check
“Best rate” does not mean the steepest climb path or the safest speed in every situation. Here it means the speed that gives the most altitude gained per minute with one engine inoperative, if the airplane can climb at all.
Example Sentence 1
After the right engine failed on departure, the pilot pitched for the blue line and held VYSE while securing the failed engine.
Example Sentence 2
The POH lists VYSE as 88 knots at maximum takeoff weight for this aircraft.