FAA Ground School Practice Test

If it is necessary to set the altimeter from 29.15 to 29.85, what change occurs?

1. A 500-foot increase in indicated altitude

2. A 700-foot increase in indicated altitude

3. A 400-foot decrease in indicated altitude

4. A 300-foot increase in indicated altitude

The correct answer is: A 700-foot increase in indicated altitude

When adjusting the altimeter from a setting of 29.15 inches of mercury to 29.85 inches, it is important to understand how the altimeter works. The altimeter measures the atmospheric pressure and translates that into altitude. When you increase the altimeter setting, as in this case, you are effectively indicating a higher pressure, which correspondingly means that the altitude indicated by the altimeter will decrease. The difference in these altimeter settings is 0.70 inches of mercury (29.85 - 29.15 = 0.70). In standard atmospheric conditions, a change of one inch of mercury corresponds to an altitude change of approximately 1,000 feet. Therefore, a 0.70 inches increase in pressure would translate to approximately a 700-foot change in indicated altitude. In this specific scenario, you would be seeing a decrease in indicated altitude by 700 feet because you are making an adjustment to a higher pressure setting. Thus, the correct understanding here focuses on the relationship between pressure and altitude, confirming that changing the altimeter from 29.15 to 29.85 results in a 700-foot increase in indicated altitude. This aligns with the principle that higher pressure readings indicate lower altitudes.