Definition
The green-colored band on an aircraft instrument's dial that marks the normal operating range for that instrument. On a tachometer with a constant-speed propeller, the green arc shows the range of engine RPM approved for continuous operation.
Plain English
The green section of a gauge that shows the safe, everyday operating range. As long as the needle stays inside the green, you're in normal limits.
Context Anchor
Seen on cockpit gauges during engine operation, especially when setting or checking propeller speed with a constant-speed propeller.
Derivation
“Green” is used because green commonly signals normal or safe operation. “Arc” means a curved part of a circle, which matches the curved colored band on a round aircraft gauge.
Why Pilots Care
Operating inside the green arc keeps the engine and propeller at efficient, safe speeds; going outside it for extended periods can reduce performance or cause damage.
Intuition Check
Do not read “green arc” as just a decorative green mark. In an aircraft, it is an operating range marker: green means normal range on that gauge.
Example Sentence 1
After setting cruise power, the pilot confirmed the tachometer needle was settled comfortably inside the green arc.
Example Sentence 2
After leveling off, the pilot fine-tuned power to place the RPM needle in the middle of the green arc.