Final answer:
A standby altimeter must be capable of operating continuously for at least 12 hours, or at half-power for 24 hours, to ensure aircraft safety in the event of primary altimeter failure.
Step-by-step explanation:
A standby altimeter is an essential safety device on aircraft, designed to function in the event that the primary altimeter fails. According to aviation regulations, a standby altimeter must have the ability to operate for a minimum amount of time to ensure pilots can safely land the aircraft. While specific requirements may vary by jurisdiction or aircraft type, a general rule of thumb is that the standby altimeter should be capable of operating continuously for a minimum of 12 hours, or at half-power for 24 hours. This ensures that the aircraft can maintain critical functionality during long-haul flights or in case of an extended emergency.