FAA Private Pilot Airplane (PAR)Weather Theory and ServicesHard
An aircraft maintains a constant indicated airspeed while climbing from an airport with a density altitude of 2,000 feet to a higher airport with a density altitude of 8,000 feet. What happens to the aircraft's true airspeed?
- ABoth true airspeed and indicated airspeed increase equally
- BTrue airspeed increases while indicated airspeed remains essentially constant
- CBoth true airspeed and indicated airspeed remain unchanged
- DTrue airspeed decreases while indicated airspeed increases
Show answer & explanationAnswer & explanation
Correct answer: B. True airspeed increases while indicated airspeed remains essentially constant
As density altitude increases, air density decreases. For a given indicated airspeed, the aircraft must travel faster through the thinner air to generate the same dynamic pressure on the pitot-static system, so true airspeed increases even though indicated airspeed stays the same.
Why the other options are wrong
- A. IAS is designed to remain constant relative to dynamic pressure, not to increase with altitude.
- C. TAS changes with density altitude even when IAS is held constant.
- D. IAS does not increase from altitude alone if the pilot holds it constant.
IAS vs TAS with Density Altitude
As density altitude increases, true airspeed becomes progressively higher than indicated airspeed for the same indicated reading, because thinner air requires higher true speed to produce equivalent dynamic pressure.
- Rule of thumb: TAS increases about 2% per 1,000 ft of density altitude above IAS
- IAS reflects dynamic pressure, not actual speed through the air
- This is why aircraft need longer takeoff rolls and have faster true groundspeeds at altitude
Memory trick: Thin air means you're moving faster than your gauge admits