Definition
VYSE is the airspeed that produces the greatest gain in altitude per unit of time when a multi-engine airplane is operating with one engine inoperative, at maximum gross weight and sea level conditions. It is published in the Pilot's Operating Handbook and is marked on the airspeed indicator of most twin-engine airplanes by a blue radial line, which is why pilots commonly call it 'blue line' speed.
Plain English
If one engine quits in a twin, VYSE is the speed that lets the airplane climb the most feet in the shortest amount of time on the remaining engine. It is the speed you fly to gain altitude fastest after losing an engine.
Context Anchor
Seen in multiengine airplane limitations, performance charts, emergency procedures, and on the airspeed indicator as the blue-line speed.
Derivation
From the standard FAA airspeed notation: V for velocity, Y for best rate of climb (the long-established symbol for that climb speed), and SE for single engine. Knowing the parts makes any V-speed easier to decode the next time you meet one.
Why Pilots Care
Using this speed after an engine failure maximizes climb performance to clear obstacles and reach a safe altitude before further action is taken.
Grounding Statement
Picture one engine stopped and the airplane heavy after takeoff: VYSE is the speed that gives the airplane its best chance to keep climbing.
Intuition Check
Do not read “best” as “always safe” or “always able to climb.” VYSE is the best climb speed for specific conditions, but weight, altitude, temperature, and airplane condition can still prevent a climb.
Example Sentence 1
After the right engine failed, she pitched for blue line and held VYSE while securing the failed engine.
Example Sentence 2
At sea level and maximum weight the published VYSE was 88 knots; the pilot adjusted slightly lower once weight decreased.