If a pilot changes the altimeter setting from 30.11 to 29.96, what is the approximate change in indication?

• A. Altimeter will indicate 150 feet lower
• B. Altimeter will indicate 150 feet higher
• C. Altimeter will indicate 300 feet lower
• D. No change in indicated altitude

Answer: (A)

Lowering the altimeter setting changes the reference sea level pressure the instrument uses. When you decrease the setting, you’re telling the altimeter that sea level pressure is lower than it actually is, so the instrument interprets the current pressure as coming from a higher position above the surface. In this case, the setting drops from 30.11 to 29.96, a 0.15 inHg change. Roughly 1 inHg corresponds to about 1,000 feet near the surface, so a 0.15 inHg change translates to about 150 feet. Therefore, the indicated altitude would be about 150 feet higher. A quick practical rule: decreasing the setting raises the indicated altitude, while increasing the setting lowers it.