What occurs when switching the altimeter setting from 29.15 to 29.85?

When switching the altimeter setting from 29.15 inches of mercury (inHg) to 29.85 inHg, the indicated altitude will increase because the altimeter is designed to reflect changes in atmospheric pressure. The altimeter shows the altitude based on the standard pressure setting of 29.92 inHg at sea level.

When the altimeter is set to a lower pressure of 29.15 inHg, the aircraft's altitude indicated is lower than its actual altitude. By changing the setting to a higher pressure of 29.85 inHg, the altimeter reflects a higher altitude. The difference in these settings (29.85 - 29.15 = 0.70 inHg) translates to an increase in indicated altitude of about 700 feet when using the rule of thumb that each 0.01 inHg change corresponds to approximately 100 feet of altitude change.

This is important for pilots to understand as it ensures they maintain accurate altitude readings for safety and for navigation purposes. This adjustment is crucial during altitude transitions, especially when flying near terrain or within controlled airspace.