What scenario would lead to the replacement of an aircraft instrument?

The scenario that would lead to the replacement of an aircraft instrument is a case leaking. When an instrument case is leaking, it can compromise the accuracy and functionality of the instrument, potentially exposing internal components to moisture, contaminants, and pressure changes. This kind of damage can lead to catastrophic failures if the instrument is not operating correctly, as it may provide incorrect readings to the pilot, impacting flight safety.

In contrast, instrument calibration failure may allow for calibration adjustments rather than a full replacement, while loose mounting screws can be tightened without needing to replace the instrument. Fogged glass might impair visibility but typically does not necessitate replacement and could often be remedied through cleaning or replacing the glass element.