FAA Private Pilot Helicopter (PRH)Performance and LimitationsHard
A pilot is flying a helicopter at 90 knots indicated airspeed (IAS) at 5,000 feet pressure altitude on a standard day. What effect will climbing to 10,000 feet pressure altitude at the same IAS have on the true airspeed (TAS)?
- ATAS will decrease.
- BTAS will increase.
- CTAS will fluctuate due to changing air density.
- DTAS will remain the same.
Show answer & explanationAnswer & explanation
Correct answer: B. TAS will increase.
True airspeed (TAS) increases with altitude for a constant indicated airspeed (IAS). This is because IAS measures dynamic pressure, which decreases with lower air density at higher altitudes. To maintain the same dynamic pressure (IAS) in less dense air, the helicopter must fly faster through that air, resulting in a higher TAS.
Why the other options are wrong
- A. Incorrect. TAS increases with altitude for a given IAS.
- C. Incorrect. While air density changes, TAS will steadily increase, not fluctuate, assuming stable flight conditions.
- D. Incorrect. TAS changes with altitude for a given IAS.
IAS vs. TAS with Altitude
Indicated Airspeed (IAS) is what the airspeed indicator shows. True Airspeed (TAS) is the actual speed through the air. For a constant IAS, TAS increases with altitude due to decreasing air density.
- IAS is based on dynamic pressure.
- Air density decreases with altitude.
- To maintain constant dynamic pressure in less dense air, true speed must increase.
Memory trick: Thin air makes the true speed 'fly' faster than indicated.