Final answer:
The propeller control should be set to 'full forward' or 'high rpm' during engine operation when checking the manifold pressure to reflect true engine conditions. This setting is essential for accurate readings on the manifold pressure gauge and is part of the pre-flight run-up checklist for piston-engine aircraft with constant-speed propellers.
Step-by-step explanation:
During engine operation, to accurately check the manifold pressure, the propeller control should be set to ‘high rpm’ or the ‘full forward’ position. This setup allows the manifold pressure gauge to reflect the true pressure within the intake manifold, which is not being artificially limited by the propeller governor.
In piston-engine aircraft with constant-speed propellers, pilots adjust the propeller pitch to manage the engine load and performance, which in turn affects manifold pressure. For example, in a typical pre-flight run-up scenario, an aircraft pilot would first set the mixture control as required (usually full rich for normally aspirated engines at low altitude), ensure the propeller is at the full forward setting for high RPM, and then use the throttle to increase engine power to the recommended run-up value. It is at this point that the pilot would check the manifold pressure gauge to ensure it is within acceptable limits for that engine power setting.