If the vacuum pump fails in a small aircraft, which instruments will no longer function?

The correct answer identifies the instruments that rely on vacuum pressure for their operation. In a small aircraft, the altitude indicator and heading indicator typically depend on the vacuum system. If the vacuum pump fails, it affects the instruments that rely on that system to function properly.

The attitude indicator is a primary instrument that provides information about the aircraft's orientation relative to the horizon, and it relies on a gyroscopic system powered by the vacuum pump. Similarly, the heading indicator, which shows the aircraft's directional flight path, also uses a gyroscope that requires vacuum pressure to maintain its accuracy.

The altimeter, while critical for altitude information, operates on barometric pressure and is not reliant on the vacuum system. The airspeed indicator functions based on dynamic pressure and is likewise unaffected by vacuum system failure. Additionally, neither GPS nor the magnetic compass utilizes vacuum pressure, making them independent of the pump's operation.

Therefore, in the event of a vacuum pump failure, the instruments that cease to operate are specifically the attitude indicator and the heading indicator, confirming the selection of the heading indicator and attitude indicator as the correct response.