If you depart a base with an altimeter setting of 29.82 and an hour later receive a new setting of 30.22, prior to adjusting to the new setting, how does the indicated altitude compare to the true altitude?

• A. 400 feet lower
• B. Not determinable
• C. The same
• D. 400 feet higher

Answer: (D)

The key idea is that an altimeter translates ambient pressure into a reading of altitude using a reference pressure setting. If the pressure setting increases, the indicated altitude decreases for the same actual altitude.

Here, the setting rises by 0.40 inHg (from 29.82 to 30.22). If you stay at the same true altitude, the ambient pressure at that level would be higher because the sea-level pressure has increased. Since the altimeter is still set to the old, lower value, it will interpret the higher ambient pressure as being lower in altitude. Using a rough lapse rate of about 1 inHg per 1,000 feet, a 0.40 inHg difference corresponds to about 400 feet. Therefore, the indicated altitude will read about 400 feet lower than the true altitude at that moment. Once you update the setting to 30.22, the indicated altitude would adjust accordingly, but the question asks about the indication before making that change.