When checking the manifold pressure during engine operation, where should the propeller control be set?

When checking manifold pressure during engine operation, the propeller control should be set to low pitch. This is essential because low pitch settings allow the propeller blades to have a greater angle of attack, which enhances the thrust produced by the engine. This, in turn, results in a more accurate representation of the engine’s manifold pressure under operational conditions, as the engine is effectively loaded.

When the propeller is set to low pitch, it allows the engine to reach higher RPMs, which is crucial for achieving the optimal performance level and providing a clearer reading of manifold pressure. Moreover, operating in low pitch is particularly important for specific checks, such as during takeoff or climb when maximum power is required and it helps ensure that the manifold pressure reflects the engine's capability under high load.

In contrast, if the propeller were set to high pitch or variable pitch with a higher angle of attack, it would reduce the engine's loading, thereby affecting the manifold pressure readings. Intermediate settings may not provide the consistent and maximum performance necessary for accurate measurements during critical checks.