How is manifold pressure maintained when the pilot selects MAP and the aneroid capsule responds?

• A. The pilot selects MAP with throttle control and the aneroid capsule adjusts the butterfly to maintain it.
• B. MAP is fixed and cannot be changed.
• C. MAP is determined by engine RPM only.
• D. MAP is set by an electronic control unit.

Answer: (A)

Manifold pressure is the pressure inside the intake manifold, and this system uses a feedback loop to hold a user-set value. You set the target MAP with the throttle, and the aneroid capsule—sensing ambient pressure changes with altitude—drives the throttle butterfly via a linkage to keep the actual manifold pressure matching that target. As you climb and ambient pressure drops, the capsule expands and nudges the throttle to open enough to maintain the chosen MAP; as you descend, it contracts and reduces throttle to prevent MAP from increasing beyond the set value. This mechanical feedback lets the engine maintain a constant MAP despite changes in air density and engine load. The other ideas—MAP being fixed, determined by RPM alone, or set by an electronic control—don’t fit this mechanical, altitude-compensated arrangement.