Certified Flight Instructor - Flight Instructor Airplane Practice Exam

When the manifold pressure gauge indicates approximately 29" Hg prior to engine start, what does this signify?

1. The pointer is stuck at full-power

2. The throttle is closed

3. Pressure is the same as atmospheric pressure

The correct answer is: Pressure is the same as atmospheric pressure

When the manifold pressure gauge reads approximately 29 inches of mercury (Hg) prior to engine start, it indicates that the ambient atmospheric pressure is approximately equal to 29 inches of mercury. This value is used as a standard barometric pressure at sea level under standard conditions (which is about 29.92 inches Hg). When the engine is off and the throttle is closed, the manifold pressure will stabilize at the ambient atmospheric pressure, hence why the gauge would read around 29" Hg. In contrast, if the throttle were fully open, the pressure in the manifold would be higher than atmospheric pressure while the engine is running, explaining why the gauge would not display a reading close to 29" Hg. Additionally, a stuck pointer situation would not lead to a meaningful reading reflecting atmospheric conditions, as it would not accurately represent the system pressure. Therefore, the reading of 29" Hg accurately reflects the standard atmospheric pressure when the engine is not running.