Quote:
Originally Posted by Ryan Coke
sa226 explains it well, Ken. Most modern aircraft use GPS (WJ's 737's have 2 on board, as well as other types of nav systems), however groundspeed from the GPS is really very different, though related, information. Airspeed is what is critical from an aerodynamic standpoint, hence the importance of the pitot/static system, which measure ram (dynamic) air pressure vs. static air pressure.
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Yeah, I guess I was thinking this might be like an episode of Mayday that I watched where the pilots had a controlled flight into terrain; the ocean. Both pitot tubes were blocked by tape and gave conflicting readings. One showed a speed of 300 kts, and the other 400; when they were really only doing 170-180. (I forget the exact numbers; but both were false readings of "fast enough to gain altitude"; but in reality they were on a "final approach" speed range.)
I guess my thinking was that if they had GPS telling them they were flying at 140-210 ground speed; they would have known they were going too slow. Also the altitude could have told them they were not at 10,000 feet; but 200 feet.
And please, don't take my post as me thinking I am right and you are wrong; I just want to get a better understanding of this all.
I guess my other question; how do wind speeds at altitude compare to ground level? For example if the wind on the ground is 15 kts; would it likely be no more than 30 at altitude? Or can it vary so much, and maybe even be as high as 75 or 100? I know from a United flight they showed the outside temperature once, and I was surprised to see the temperature 45 C below the ground temperature.