FAA Ground School Practice Test

Under what condition will true altitude be lower than indicated altitude?

1. In warmer than standard air temperature

2. In colder than standard air temperature

3. Under standard pressure

4. At sea level

The correct answer is: In colder than standard air temperature

True altitude is defined as the actual height of an aircraft above sea level, while indicated altitude is what the altimeter displays based on the pressure setting. The relationship between true altitude and indicated altitude is influenced by atmospheric conditions, particularly temperature and pressure. In colder than standard air temperature, the air is denser than standard, which causes the altimeter to read higher than the actual altitude above sea level. The altimeter assumes that the altitude is being measured in standard atmosphere conditions (which is based on a specific pressure and temperature). Since the altimeter does not compensate for this colder temperature, it results in the indicated altitude being higher than the true altitude. Consequently, when flying in a colder environment, the true altitude can indeed be lower than what is indicated on the altimeter. In contrast, warmer than standard air temperature decreases the density of the air, leading to a lower indicated altitude compared to true altitude. Under standard pressure, both altitudes generally align, and at sea level, the altimeter and true altitude are also roughly equal as long as the altimeter is correctly set.