How old can the altimeter be before the aircraft must be informed?

The correct answer is that the altimeter must be updated if it is older than one hour. In aviation operations, it is critical for pilots and air traffic controllers to have accurate altimeter settings to ensure safety and correct altitude awareness. Changes in atmospheric pressure can significantly affect altimeter readings, so timely updates are necessary.

An altimeter setting that is not current can lead to altitude discrepancies, potentially causing unsafe flight situations, especially during approach and landing phases where precise altitude is crucial. As a best practice, significant changes in air pressure can occur over a period, and maintaining an altimeter setting that is no more than one hour old helps in mitigating these risks.

Regarding the other options, older altimeter settings would not be compliant with safety regulations and could compromise the integrity of altitude reporting. Therefore, the one-hour limit is established to keep aircraft operations safe and precise.