To maintain a constant boost pressure with increasing altitude, what action does the A.B.C. take?

To maintain a constant boost pressure as altitude increases, it is essential to ensure that enough air enters the engine to compensate for the lower air density found at higher elevations. One effective action taken by the Automatic Boost Controller (A.B.C.) in this situation is to progressively open the throttle valve.

As altitude increases, the amount of oxygen available for combustion decreases, leading to a drop in manifold pressure if the throttle position remains constant. By progressively opening the throttle valve, more air can flow into the engine, which helps to maintain the required boost pressure. This adjustment allows the system to increase the airflow, compensating for the thinner atmosphere, thus ensuring that the engine operates efficiently and produces the desired power output.

In contrast, other options involve actions that would either restrict airflow or maintain it at a level that is inadequate for optimal engine performance at increased altitudes, which could lead to a decrease in boost pressure. Therefore, progressively opening the throttle is the correct approach to maintaining consistent boost pressure under changing altitude conditions.