Private Pilot Private Exam

Which condition results in the altimeter indicating a lower altitude than true altitude?

1. Low atmospheric pressure

2. Colder than standard air temperature

3. Warmer than standard air temperature

4. Normal atmospheric conditions

The correct answer is: Warmer than standard air temperature

The altimeter measures the atmospheric pressure to determine altitude. Under warmer than standard air temperature conditions, the actual pressure at a given altitude will be higher than what the altimeter is calibrated to recognize. This discrepancy occurs because altimeters are based on the standard atmosphere model, which assumes a normal lapse rate of temperature and pressure with altitude. When the air is warmer, it expands and leads to a higher pressure at a given altitude, causing the altimeter to read a lower altitude than the true altitude. Therefore, in warmer conditions, pilots may find themselves higher than indicated on the altimeter. While low atmospheric pressure can cause the altimeter to read higher than actual altitude, colder temperatures would also result in the opposite effect, causing the altimeter to read higher than true altitude because the air is denser at lower temperatures. Normal atmospheric conditions would not create any discrepancies between indicated and true altitude as they align with the standard measurement assumptions.