What should a manifold pressure gauge read when the engine is not operating?

What should a manifold pressure gauge read when the engine is not operating?

• A. The current sea level barometric pressure
• B. The existing barometric pressure as indicated on the altimeter barometric scale when pointers are set to the surveyed field elevation
• C. Zero psi
• D. The pressure of engine oil

Answer: (B)

When the engine isn’t operating, there’s no suction in the intake, so the pressure inside the intake manifold equals the surrounding ambient air pressure. The manifold pressure gauge therefore reads the current barometric pressure at your location. To know that value in the cockpit, you compare it to the barometric pressure the altimeter would show if you set it to the surveyed field elevation—i.e., use the current local barometric pressure as indicated on the altimeter when its pointers are set to the field elevation. This is why the correct reading is the existing barometric pressure as shown by the altimeter at the field. The other options don’t fit: sea level pressure ignores altitude, zero psi would imply vacuum, and engine oil pressure is unrelated.

When the engine isn’t operating, there’s no suction in the intake, so the pressure inside the intake manifold equals the surrounding ambient air pressure. The manifold pressure gauge therefore reads the current barometric pressure at your location. To know that value in the cockpit, you compare it to the barometric pressure the altimeter would show if you set it to the surveyed field elevation—i.e., use the current local barometric pressure as indicated on the altimeter when its pointers are set to the field elevation. This is why the correct reading is the existing barometric pressure as shown by the altimeter at the field. The other options don’t fit: sea level pressure ignores altitude, zero psi would imply vacuum, and engine oil pressure is unrelated.