A precaution for operation of an engine with a constant-speed propeller is to avoid which operating condition?

• A. High manifold pressure settings with low RPM
• B. Low manifold pressure with high RPM
• C. High RPM with high manifold pressure
• D. Low RPM with low manifold pressure

Answer: (A)

When a constant‑speed propeller is used, the throttle sets the engine’s manifold pressure (how much air/fuel goes into the cylinders) while the prop governor maintains a chosen RPM. A key precaution is to avoid having high manifold pressure while the RPM is low. This means you’d be asking the engine to produce a lot of power, but with insufficient cooling because the engine isn’t turning fast enough to push air over the cylinders and around the fins.

The result can be excessive cylinder-head temperatures and higher overall thermal stress, which raises the risk of overheating and detonation. So, keep the RPM up (or at least not too low) when you have high manifold pressure, and avoid staying in a high‑MAP, low‑RPM condition.