Definition
On a multiengine airplane's airspeed indicator, a blue radial line marks VYSE — the best rate-of-climb airspeed with one engine inoperative (single-engine best rate of climb). Flying at this speed gives the airplane its greatest altitude gain per unit of time when one engine has failed and the other is producing maximum available power.
Plain English
It's the blue line painted on the airspeed dial of a twin-engine airplane. It shows the speed to fly to climb the most feet per minute when one engine has quit and the other is doing all the work.
Context Anchor
Seen on the airspeed indicator in many multiengine airplanes, especially during engine-failure training and single-engine climb discussions.
Derivation
Radial comes from radius, meaning a line drawn outward from the center of a circle. On a round airspeed indicator, the blue mark is called a radial line because it points outward from the center of the instrument face.
Why Pilots Care
Maintaining this speed after an engine failure gives the best climb performance, which is essential for clearing obstacles and completing safe single-engine procedures.
Intuition Check
Do not read this as just any blue line in the cockpit. In this context, the blue radial line is a specific airspeed indicator marking for best climb performance with one engine not producing power.
Example Sentence 1
After the right engine failed on departure, the pilot pitched for the blue radial line and held it until reaching a safe altitude.
Example Sentence 2
During the multi-engine checkride the examiner asked the applicant to demonstrate a climb at the blue radial line with the left engine secured.