If your vacuum system fails, which instruments will you lose?

In the context of a vacuum system failure, the primary instruments that rely on this system for their operation are the heading indicator and the attitude indicator.

The attitude indicator is essential for determining the aircraft's orientation relative to the horizon, and it relies on a gyroscopic mechanism powered by the vacuum system to function accurately. If the vacuum system fails, the attitude indicator will be unable to maintain its accurate representations, leading to a critical loss of spatial awareness for the pilot during flight.

Similarly, the heading indicator also depends on the vacuum system to maintain its gyroscopic stabilization. This instrument provides essential directional information, which is crucial for navigation and maintaining the desired flight path. A failure in the vacuum system would cause the heading indicator to precess and become unreliable, significantly impairing navigational capabilities.

In contrast, other instruments listed in the options do not depend on the vacuum system. For instance, the vertical speed indicator and altimeter operate on static pressure rather than being mechanically powered by the vacuum. Therefore, a vacuum failure would not impact their functionality.

Understanding the roles of these instruments and their dependence on the vacuum system highlights why the correct answer is the combination of the heading indicator and attitude indicator, as these two are directly affected by such a system failure.