Originally Posted by FlexibleResponse
GPS routinely provides wind and true speed and direction and distance and thereby navigation information.
Any question in this area should, first of all, distinguish between avigation and navigation.
For navigation, GPS does not add very much to IRS. The pros and cons of GPS versus IRS have been discussed earlier on this thread. GPS does not provide wind. The airplane computers (ADIRU or FMS?) derive wind by comparing groundspeed to airspeed.
However, the first priority after loss of airspeed indication is to keep flying. Pitch and power should keep airspeed and altitude within safe limits. Groundspeed isn't needed immediately, but could perhaps help in the longer term if the pilot knows what groundspeed he had prior to the loss of airspeed indication.
Why is GPS or IRS groundspeed no substitute for airspeed? The liftforce that keeps the airplane flying is the result of aerodynamic
pressures around the wing. These
pressures are directly related to the
pressures measured by the pitot/static system. In fact, the needle of a pneumatic ASI indicates the dynamic
pressure directly, only the scale markings on the dial behind it are 'calibrated' in airspeed units.
EDIT:: Similarly
pressure altitude = static
pressure. To 'synthesize' pressure altitude from GPS height, the computer needs to know the pressure at sealevel, and the temperature profile between sealevel and flight altitude.
HN39