To reduce the power output of an engine equipped with a constant-speed propeller and operating near maximum BMEP, what should be done first?

To effectively reduce the power output of an engine equipped with a constant-speed propeller operating near maximum brake mean effective pressure (BMEP), it is crucial to first adjust the manifold pressure using the throttle control before changing the RPM with the propeller control.

When an engine operates at maximum BMEP, it is delivering its peak performance based on the amount of air and fuel being introduced into the cylinders, which corresponds to the manifold pressure. By reducing the manifold pressure first, you are effectively decreasing the amount of fuel-air mixture entering the engine, which reduces the power output right away. This initial step ensures that the engine is not operating at high power levels that could lead to excessive stress or potential damage.

Once the manifold pressure is decreased, the constant-speed propeller system can then be used to adjust the RPM as necessary. This sequence ensures that the engine maintains a healthy operating environment, preserving engine longevity while effectively managing power output.

This approach also aligns with the principles of engine management, where adjusting the fuel and air supply (manifold pressure) is typically prioritized over changing RPM, particularly in conditions where maximum efficiency and engine health are paramount.