During autopilot ALT HOLD, loss of the bus bar requires altitude information to come from which source?

• A. Autopilot aneroid
• B. Static port
• C. Altimeter
• D. Inertial navigation system

Answer: (A)

When the autopilot is in ALT HOLD, it needs a stable altitude reference. If the electrical bus feeding the normal altitude sensing path fails, the system relies on an independent source built into the autopilot—the autopilot aneroid. This internal pressure-sensing capsule provides altitude information to the autopilot so it can continue to hold the commanded altitude even with the main bus down. The static port and the exposed altimeter depend on the standard air data system, which isn’t available to the autopilot in this fault scenario, and inertial navigation systems don’t provide a direct, reliable altitude input for hold mode.