Originally Posted by
Jet Jockey A4
Sorry for the slight detour off thread but since they brought up the subject of Baro Altitude versus GPS Altitude I'm curious...
Besides the fact the Earth is not perfectly round thus rendering wrong GPS altitudes information (now taken care of by software adjustments), isn't one of the major problems for using GPS altitude the fact that we fly a constant pressure wave/altitude as we set 29.92/1013 on our altimeters and this pressure wave is not at a constant altitude above the Earth's surface?
When I look at our GPS altitude in our FMS, sometimes it is 1000 feet if not more off the Baro altitude and sometimes it is really close but I have never seen them match each other.
Could/can they even have a solution to solve this problem?
Also since aircraft performance is based on a "standard pressure" and if we now decide to switch to GPS altitude and now fly a "true" altitude above the Earth's surface how would that affect the aircraft's performance because now instead of being at a constant FL410 pressure altitude you might be at a 42,300 feet altitude?
The answer is that everyone has to fly using the same datum. (Posted earlier) So either everyone flies on GPS altitude or on a pressure datum altitude. For engines pressure is important as is temperature with temperature sometimes having more impact. As I posted earlier as you follow the pressure level you will descend and climb along that pressure level. This is an issue that is in the 'too difficult' pile at the moment but will eventually have to be approached as satellite based systems become more ubiquitous and reliable. The question will be which of the systems is more efficient from a fuel burn point of view and which is the safest. A brief read of the NASA ASRS (confidential reporting system) is frightening for the number of altimeter setting errors. If the decision was to be made without all existing grandfathered in altimetry equipment and procedures, nobody would ever suggest using barometers (altimeters) with multiple datum changes for measuring aircraft altitude.