Airspeed Indicator - Types of Airspeed Measurements

Types of Airspeed Measurements

Memory aid: "ICE-T" (iced tea), or Indicated->Calibrated->Equivalent->True.

Another memory aid: This is a Pretty Cool Drink, giving the errors compensated for between the speeds Position, Compression, and Density.

At increased density altitude, for the same given indicated airspeed the aircraft's true airspeed (TAS) will be higher, but the same indicated airspeed limits (IAS) apply. Likewise, most efficient cruise speed, total drag, available lift, stall speed, and other aerodynamic information depend on calibrated, not true airspeed. Most aircraft exhibit a small difference between the airspeed actually shown on the instrument (indicated airspeed, or IAS) and the speed the instrument should theoretically show (calibrated airspeed or CAS). This difference, called position error, is mainly due to inaccurate sensing of static pressure. It is usually not possible to find a position for the static ports which, at all angles of attack, accurately senses the atmospheric pressure at the altitude at which the aircraft is flying.

Bernoulli's principle states that total pressure is constant along a streamline. Pitot pressure is equal to total pressure so pitot pressure is constant all around the aircraft and does not suffer position error. (However, pitot pressure can suffer alignment error if the pitot tube is not aligned directly into the oncoming airflow.)

The position of static ports must be selected carefully by an aircraft designer because position error must be small at all speeds within the operating range of the aircraft. A calibration chart specific to the type of aircraft is usually provided.

At high speeds and altitudes, calibrated airspeed must be further corrected for compressibility error to give equivalent airspeed (EAS). Compressibility error arises because the impact pressure will cause the air to compress in the pitot tube. The calibration equation (see calibrated airspeed) accounts for compressibility, but only at standard sea level pressure. At other altitudes compressibility error correction may be obtained from a chart. In practice compressibility error is negligible below about 3,000 m / 10,000 feet and 100 m/s / 200 knots CAS.

The true airspeed can be calculated as a function of equivalent airspeed and local air density, (or temperature and pressure altitude which determine density). Some airspeed indicators incorporate a slide rule mechanism to perform this calculation. Otherwise, it can be performed with a calculator such as the E6B handheld circular slide rule. For a quick approximation of TAS add 2% per 300m / 1000 feet of altitude to IAS (or CAS). e.g. IAS = 52 m/s /100 Knots. At 3000 m / 10,000' Above Sea Level, TAS is 62 m/s / 120 Knots.