Definition
On a multi-engine airplane airspeed indicator, the blue radial line marking VYSE — the best rate-of-climb airspeed with one engine inoperative. Flying at or above this speed gives the airplane its best chance of climbing, or at least minimizing altitude loss, after losing an engine.
Plain English
A blue mark on the airspeed indicator of a twin-engine airplane that shows the speed to fly if one engine quits. At that speed the airplane climbs best on the remaining engine.
Context Anchor
Seen on the airspeed indicator and in multiengine training, especially during engine-failure practice and performance discussions.
Derivation
The term comes from the actual blue line or blue mark printed on the airspeed indicator. Pilots use the color as a quick cockpit reference for the one-engine best-climb speed.
Why Pilots Care
Flying at blue line speed after an engine failure gives the best chance of climbing to a safe altitude or returning to the airport.
Intuition Check
Blue Line does not mean any blue line on a chart or a general “safe speed.” In this context, it means the blue airspeed-indicator mark for the best climb speed after one engine fails in a multiengine airplane.
Example Sentence 1
After the simulated engine failure, the instructor reminded him to pitch for blue line before troubleshooting.
Example Sentence 2
During the multi-engine checkride the examiner asked the applicant to point out the blue line and explain its purpose.