Indicated Airspeed from True Airspeed Calculation

Provide a True Airspeed

True airspeed is the airspeed that we would read ideally. You can set the true airspeed in the True Airspeed block in the model.

Calculate the Calibrated Airspeed

To calculate the calibrated airspeed, you adjust the true airspeed for errors introduced through the pitot-static airspeed indicators used to determine airspeed. These measurement errors are density error, compressibility error, and calibration error. The Ideal Airspeed Correction block can apply the density error and compressibility error computing calibrated airspeed from true airspeed.

Density Error

An airspeed indicator reads lower than true airspeed at higher altitudes. This is due to lower air density at altitude. When the difference or error in air density at altitude from air density on a standard day at sea level is applied to true airspeed, the result is in equivalent airspeed (EAS). Equivalent airspeed is true airspeed modified with the changes in atmospheric density that affect the airspeed indicator.

Compressibility Error

Air has a limited ability to resist compression. This ability is reduced by an increase in altitude, an increase in speed, or a restricted volume. Within the airspeed indicator, there is a certain amount of trapped air. When flying at high altitudes and higher airspeeds, calibrated airspeed (CAS) is always higher than equivalent airspeed. Calibrated airspeed is equivalent airspeed modified with compressibility effects of air, which affect the airspeed indicator. True airspeed is the airspeed that we would read ideally (and the airspeed value easily calculated within a simulation). However there are errors introduced through the pitot-static airspeed indicators used to determine airspeed. These measurement errors are density error, compressibility error and calibration error. Applying these errors to true airspeed will result in indicated airspeed (the ideal airspeed correction block can handle the density error and compressibility error).

Adjust to Indicated Airspeed for Pitot-Static Airspeed Indicator

Calibration error of the pitot-static airspeed indicator is the last adjustment to the airspeed value. The adjustment results in an indicated airspeed displayed on the airspeed indicator in the cockpit.

Calibration Error

The airspeed indicator has static vent(s) to maintain a pressure equal to atmospheric pressure inside the instrument. Position and placement of the static vent, angle of attack, and velocity of the aircraft determines the pressure inside the airspeed indicator and the amount of calibration error of the airspeed indicator. The calibration error is specific to a given aircraft design. A calibration table is usually given in the pilot operating handbook (POH) or in other aircraft specifications.

Example Calibration Tables

As an example, here is the Cessna 150M airspeed calibration table from "Pilot's Operating Handbook, Cessna 1976 150 Commuter, Cessna Model 150M", Cessna Aircraft Company, Wichita, Kansas, USA, 1976.

Calibration table for 0, 10, and 0 degrees of flap:

flaps0IAS = 40:10:140;
flaps0CAS = [43 51 59 68 77 87 98 108 118 129 140];

flaps10IAS = [40:10:80 85];
flaps10CAS = [42 50 60 69 78 82];

flaps40IAS = [40:10:80 85];
flaps40CAS = [40 50 61 72 83 89];

plot(flaps0CAS,flaps0IAS,flaps10CAS,flaps10IAS,flaps40CAS,flaps40IAS)
xlabel('Calibrated Airspeed (CAS) (kts)');
ylabel('Indicated Airspeed (IAS) (kts)');
title('Cessna 150M Airspeed Calibration Table');
legend ('0 degrees','10 degrees','40 degrees','Location','southeast');

Simulate Model to Display Airspeeds

You can see this indicated airspeed adjustment based on these example calibration tables modeled in the Calculate IAS block. After simulating the model, indicated airspeed is displayed with calibrated airspeed in the scope and the display block.