FAA Ground School Practice Test

What happens to the altitude indication if air pressure decreases?

1. Nav displays will increase

2. Indicated altitude decreases

3. Indicated altitude increases

4. Indicated altitude stays the same

The correct answer is: Indicated altitude increases

When air pressure decreases, the indicated altitude on an altimeter increases. This occurs because the altimeter is a barometric device that measures the weight of the air above it. An altimeter is calibrated to read the atmospheric pressure at a specific altitude under standard conditions. As the air pressure decreases—possibly due to gaining altitude or weather changes—the altimeter interprets this reduced pressure as a higher altitude, causing the indicated altitude to increase. In practical terms, when a pilot is climbing, the actual altitude is increasing but the pressure is decreasing due to the less dense air at higher elevations. If a pilot were to maintain the same setting on the altimeter and ascend into an area of lower pressure, the altimeter would continue to indicate a higher altitude than the aircraft is actually at. Thus, a decrease in air pressure directly correlates with an increase in the indicated altitude on the altimeter.