FAA Powerplant Written Practice Test

Which engine monitoring instrument indicates a change in power output at any given RPM?

1. RPM gauge.

2. Manifold Pressure.

3. Oil pressure gauge.

4. Fuel flow meter.

The correct answer is: Manifold Pressure.

The manifold pressure gauge is the correct answer because it provides critical information about the engine's power output, particularly in piston engines. Manifold pressure reflects the air pressure within the intake manifold, which is directly related to the engine's power production. When the engine is operating, any change in manifold pressure indicates a variation in power output at a specific RPM, as it shows how effectively the engine is drawing in air to mix with fuel for combustion. In a typical reciprocating engine, an increase in power output usually corresponds to an increase in manifold pressure, assuming that the RPM remains constant. This makes the manifold pressure gauge an essential tool for pilots and maintenance personnel to assess engine performance and efficiency. Other instruments, while important for overall monitoring and safety, do not directly measure changes in power output in the same way. The RPM gauge indicates how fast the engine is running but does not provide information about power levels. The oil pressure gauge tracks the health of the engine's lubrication system but does not reflect power output changes. The fuel flow meter indicates how much fuel is being consumed but does not directly correlate to power output without considering manifold pressure and other factors.