Definition
On a multiengine airplane's airspeed indicator, the blue radial line marks VYSE — the best single-engine rate-of-climb airspeed. It is the indicated airspeed that produces the greatest gain in altitude per unit of time when one engine is inoperative and the airplane is configured per the manufacturer's procedures.
Plain English
It is the blue line on the airspeed indicator. If you lose an engine in a twin, this is the speed that gives you the best climb — the most altitude gained for each minute of flight.
Context Anchor
Seen in multiengine training, engine-failure procedures, and discussions of engine-inoperative climb performance.
Derivation
Color-coded markings on airspeed indicators use standard meanings: white arc for flap range, green for normal operating range, yellow for caution, red for never-exceed, and a blue radial line specifically for VYSE on twins. The blue line was chosen as a distinct, easy-to-find color so a pilot under stress can locate the engine-out climb speed instantly.
Why Pilots Care
Maintaining this speed after an engine failure maximizes climb performance, helping the aircraft clear obstacles and reach a safe altitude or return to the airport.
Intuition Check
Do not read “blue” as meaning automatically safe. It is a target speed marker, not a guarantee that the airplane will climb. Do not read “indicated” as the airplane’s exact true speed through the air. It means the speed displayed on the airspeed indicator.
Example Sentence 1
After the simulated engine failure, the instructor told her to pitch for blue line and identify the dead engine.
Example Sentence 2
The airspeed indicator shows the blue radial line at 88 knots, the published speed for single-engine climb in this airplane.