Final answer:
The flight mode annunciator displaying ALT indicates that the aircraft is in altitude hold mode, meaning it maintains a constant altitude. This autopilot feature reduces pilot workload and enhances flight efficiency and comfort.
Step-by-step explanation:
When the flight mode annunciator on an aircraft's autopilot system displays ALT, this typically indicates that the autopilot is in altitude hold mode. It means that the aircraft is maintaining a constant altitude. The system will automatically adjust the plane's control surfaces and engine power to keep the aircraft at the selected altitude until the pilot inputs a command to change the mode or disengage the autopilot. This feature is essential for reducing pilot workload during cruising and allows for a more comfortable and efficient flight.