Commercial Pilot License (CPL) Practice Exam

The magnetic compass is the correct instrument for providing bank information in the event of a vacuum failure. This is because the magnetic compass is an autonomous instrument that does not rely on the aircraft's electrical or vacuum systems to operate. It utilizes the Earth's magnetic field to show heading, which can help a pilot maintain orientation during flight.

When a vacuum failure occurs, instruments such as the artificial horizon, which depend on the vacuum system for accurate readings of the aircraft's attitude, will become unreliable. In such situations, the magnetic compass can be used to infer the aircraft’s bank angle indirectly by maintaining a reference to the heading. Although the compass does not measure bank angle directly, a change in heading relative to the ground during a turn can indicate that the aircraft is banking.

In contrast, the altimeter, vertical speed indicator, and fuel gauge do not provide information relevant to bank angle or orientation and are therefore not useful for this purpose in the event of a vacuum system failure. The altimeter indicates altitude, the vertical speed indicator shows the rate of climb or descent, and the fuel gauge provides information about fuel levels, none of which can assist in assessing a bank angle or maintaining proper aircraft control when other instruments are compromised.