FAA Private Pilot Helicopter (PRH)Performance and LimitationsEasy
A helicopter is flying at a constant indicated airspeed of 80 knots while climbing from 2,000 feet to 10,000 feet pressure altitude. As altitude increases, the true airspeed (TAS) will:
- AIncrease because a given IAS represents more actual airspeed through thinner air
- BIncrease because less dense air causes less instrument error compensation
- CRemain exactly the same as IAS at all altitudes
- DDecrease because the air is thinner
Show answer & explanationAnswer & explanation
Correct answer: A. Increase because a given IAS represents more actual airspeed through thinner air
As air density decreases with altitude, the airspeed indicator (which measures dynamic pressure) reads lower than the actual speed of the aircraft through the air. Therefore, for a constant IAS, true airspeed increases as pressure altitude increases.
Why the other options are wrong
- B. The explanation of 'instrument error compensation' is not the correct aerodynamic reasoning.
- C. IAS and TAS are only approximately equal near sea level in standard conditions.
- D. TAS increases, not decreases, with altitude for constant IAS.
IAS vs TAS with Altitude
Because air density decreases with altitude, a constant indicated airspeed corresponds to an increasingly higher true airspeed as altitude increases.
- IAS measures dynamic pressure, not true speed
- TAS = IAS plus a correction for air density
- At high altitude, TAS can be significantly greater than IAS
Memory trick: Thinner air, same gauge reading, but you're really going faster (TAS grows)