Definition
In a multi-engine airplane, VYSE is the airspeed that produces the greatest gain in altitude per unit of time when one engine is inoperative and the remaining engine is at maximum available power. It is marked on the airspeed indicator with a blue radial line, commonly called the 'blue line,' and is published in the Pilot's Operating Handbook for the specific aircraft.
Plain English
If you lose an engine in a twin, VYSE is the speed that gets you climbing fastest on the one engine you have left. Pitch for blue line and you are giving the airplane its best chance to climb away from the ground.
Context Anchor
Seen in multiengine airplane training, engine-failure procedures, the airplane flight manual, and on the airspeed indicator as the blue line.
Derivation
The 'V' comes from the French 'vitesse,' meaning speed, which is why most aircraft speed limits start with V. 'Y' is the long-standing aviation symbol for best rate of climb, and 'SE' stands for single-engine. So VYSE literally reads as 'speed, best rate of climb, single-engine.'
Why Pilots Care
Maintaining VYSE after an engine failure provides the quickest altitude gain, improving chances of obstacle clearance and a safe return.
Intuition Check
VYSE does not mean the airplane is guaranteed to climb on one engine. It means this is the speed that gives the best climb result available under the conditions.
Example Sentence 1
When the right engine quit shortly after takeoff, he pitched for blue line and held VYSE while he identified and secured the failed engine.
Example Sentence 2
The performance section lists VYSE as 88 knots for this airplane at sea level with the propeller feathered.