What aircraft instrument can be used to check the manifold pressure gauges for proper indication?

What aircraft instrument can be used to check the manifold pressure gauges for proper indication?

• A. Altimeter
• B. Airspeed indicator
• C. Attitude indicator
• D. Magnetic compass

Answer: (A)

The main idea is using a pressure reference to verify a pressure-reading instrument. The manifold pressure gauge shows the pressure inside the engine’s intake, while the altimeter shows the ambient atmospheric pressure. Because both respond to the same atmospheric pressure, the altimeter can serve as a quick cross-check for the manifold pressure indication. A practical check is: set the altimeter to the local pressure (QNH) so it reflects the true ambient pressure at your location, then run the engine to a known manifold pressure condition (for example, at a given RPM and throttle setting). If the manifold pressure gauge is indicating properly, its reading should be in the expected relationship to the ambient pressure shown by the altimeter (typically approaching ambient pressure as throttle is opened toward wide-open throttle, within the engine’s normal intake losses). If the manifold pressure reading deviates significantly from what the ambient pressure implies, the gauge may be inaccurate or miscalibrated. Other instruments mentioned—airdynamics gauge measures ram air pressure, while attitude indicator and magnetic compass rely on gyros and magnetic fields—do not provide a valid cross-check for manifold pressure.

The main idea is using a pressure reference to verify a pressure-reading instrument. The manifold pressure gauge shows the pressure inside the engine’s intake, while the altimeter shows the ambient atmospheric pressure. Because both respond to the same atmospheric pressure, the altimeter can serve as a quick cross-check for the manifold pressure indication.

A practical check is: set the altimeter to the local pressure (QNH) so it reflects the true ambient pressure at your location, then run the engine to a known manifold pressure condition (for example, at a given RPM and throttle setting). If the manifold pressure gauge is indicating properly, its reading should be in the expected relationship to the ambient pressure shown by the altimeter (typically approaching ambient pressure as throttle is opened toward wide-open throttle, within the engine’s normal intake losses). If the manifold pressure reading deviates significantly from what the ambient pressure implies, the gauge may be inaccurate or miscalibrated.

Other instruments mentioned—airdynamics gauge measures ram air pressure, while attitude indicator and magnetic compass rely on gyros and magnetic fields—do not provide a valid cross-check for manifold pressure.