What does the red arc on the tachometer indicate?

What does the red arc on the tachometer indicate?

• A. A normal operating range.
• B. A safe idle range.
• C. A resonant frequency RPM range requiring quick passage.
• D. An indication to shut down.

Answer: (C)

The red arc marks a resonant-frequency RPM range where the engine/propeller system can vibrate intensely. It’s not a normal operating zone, and staying in this range can cause damaging vibrations and fatigue in components. The idea is to pass through this RPM range quickly when changing power, rather than holding the engine at those speeds. Once you’ve moved through it, you return to a safe operating range. In practice, you’d avoid lingering in that band and focus on keeping RPMs in the normal operating range for regular use. The other options describe normal operating conditions, a safe idle, or an instruction to shut down, which aren’t what the red arc is signaling.

The red arc marks a resonant-frequency RPM range where the engine/propeller system can vibrate intensely. It’s not a normal operating zone, and staying in this range can cause damaging vibrations and fatigue in components. The idea is to pass through this RPM range quickly when changing power, rather than holding the engine at those speeds. Once you’ve moved through it, you return to a safe operating range.

In practice, you’d avoid lingering in that band and focus on keeping RPMs in the normal operating range for regular use. The other options describe normal operating conditions, a safe idle, or an instruction to shut down, which aren’t what the red arc is signaling.