FAA Powerplant Written Practice Test

At any given RPM, what engine monitoring instrument can indicate a change in power output?

1. Fuel Flow Meter

2. Oil Pressure Gauge

3. Manifold Pressure

4. Temperature Gauge

The correct answer is: Manifold Pressure

The manifold pressure instrument is designed to measure the pressure within the intake manifold of a reciprocating engine. This measurement directly correlates to the amount of air and fuel mixture entering the engine, which is essential for generating power. As the power output of the engine changes (due to throttle adjustments or changes in load), the manifold pressure will also fluctuate. At a constant RPM, if the throttle is opened further or closed, the manifold pressure will increase or decrease accordingly. This makes the manifold pressure gauge a reliable indicator of the engine's power output under various operating conditions. It effectively reflects changes in fuel/air mixture delivery, which is fundamental for assessing the engine's performance. In contrast, while a fuel flow meter can also relate to power output because it measures the amount of fuel consumed, it does not provide a direct and immediate response related to changes in power like manifold pressure does. Oil pressure and temperature gauges are primarily used to monitor the engine's condition and do not directly indicate changes in power output.