Abstract:

This Data Item describes four separate methods for determining the true airspeed of an aircraft for a single combination of angle of attack and Mach number by using a GPS unit carried within the aircraft. All the data required for this procedure are from instrumentation carried on the aircraft, namely the GPS unit and the aircraft's standard air data system. Although there is no reliance on equipment external to the test aircraft, the procedure requires the analysis of specially flown manoeuvres.

Each method requires that the aircraft perform three separate manoeuvres at the same airspeed, or similar airspeeds, the same altitude and, as far as is practical, in the same airspace.

The first two methods, which assume that turns of 90 degrees are made between each manoeuvre, are often described as horseshoe methods. For the third and fourth methods, flight directions that are approximately 120 degrees apart between manoeuvres are generally used. It is assumed that wind speed and direction are constant throughout the test manoeuvres. Hence, it is asserted that the manoeuvres should be flown on a calm day and within as small an airspace and time as possible.

Procedures to determine the wind speed and direction using each method are also outlined in the document, as is a technique for calculating the static pressure correction coefficient.