Certified Flight Instructor - Flight Instructor Airplane Practice Exam

If the outside air temperature decreases while flying at a constant altitude, what happens to true airspeed?

1. Decreases, and true altitude increases.

2. Increases, and true altitude increases.

3. Decreases, and true altitude decreases.

4. Stays the same, but true altitude changes.

The correct answer is: Decreases, and true altitude decreases.

To understand what happens to true airspeed when the outside air temperature decreases while flying at a constant altitude, it's important to consider how true airspeed (TAS) relates to air density and temperature. As the outside air temperature decreases, the density of the air increases. True airspeed is calculated based on the indicated airspeed (IAS) and the air density at a given altitude. When flying at a constant indicated altitude, a decrease in temperature results in a higher air density, which changes the relationship between indicated airspeed and true airspeed. In this scenario, as the air becomes denser due to the lower temperature, the true airspeed will decrease. This is because the aircraft is moving through denser air, and therefore, for a given indicated airspeed, the actual speed of the aircraft relative to the surrounding air (true airspeed) will be lower in such conditions. Furthermore, true altitude is determined based on pressure. Since you are maintaining a constant indicated altitude, the true altitude will actually decrease with cooler temperatures if you remain at the same indicated altitude. The aircraft is still at the same pressure altitude, but due to the increase in air density, the true altitude is less than what would be expected at a warmer temperature.