Definition
In a multi-engine airplane, the airspeed that produces the greatest gain in altitude per unit of time when one engine is inoperative and the other is producing takeoff power, with the airplane configured per the manufacturer's procedures. VYSE is marked on the airspeed indicator by a blue radial line.
Plain English
The speed that gets a twin-engine airplane climbing the fastest when one of its two engines has failed. It is shown on the airspeed indicator as a blue line, which is why pilots often call it 'blue line speed.'
Context Anchor
You see VYSE in multiengine training, emergency procedures, airplane performance charts, and on many multiengine airspeed indicators as the blue line.
Derivation
From aviation speed notation: V for velocity, Y for best rate of climb (the same Y used in VY), and SE for 'single engine.' So VYSE reads as 'best rate of climb speed, single engine.'
Why Pilots Care
Tells the pilot whether positive climb is possible after an engine failure and guides the decision to continue or return to the runway.
Intuition Check
VYSE is not the same as minimum control speed. It is the best climb speed on one engine, not the slowest speed at which the airplane can still be controlled.
Example Sentence 1
After the right engine failed on climbout, the pilot pitched for VYSE and held blue line until reaching a safe altitude.
Example Sentence 2
According to the POH, VYSE is 92 knots at 5,000 feet density altitude with the left engine feathered.