by dilbert » Sun Jan 29, 2017 11:19 pm
Thanks for the reply
I understand what you are saying, but that is not the issue. The issue is how the instrument is calibrated. It is a bourdon tube gauge. When the outside pressure drops the tube straightens causing the needle to turn to a higher setting. To calibrate the instrument an instrument maintenance person has to adjust the physical linkage between the needle and the bourdon tube. It is adjusted according to standard atmosphere. When properly calibrated, the gauge will always read 0 altitude when the atmospheric pressure is 29.92 inches HG.
I don't know how this has been programmed in flight gear. I just went back and rechecked the A6 at San Diego. This time, quite inexplicably, it read 780 ft as at KSFO with the instrument setting menu showing 29.92 HG, which should not be. It's understandable that the gauge will read higher at a lower atmospheric pressure-that's what it does. But if it reads higher, the instrument setting menu should reflect a commensurate lower pressure, which it does not. It consistently comes up showing 29.92 at start, even though the altitude is 900 feet or so. Yes, you can adjust the altimeter to the field elevation ( around 20 feet) using the instrument setting menu, but when you do so, the pressure drops to around 29.0 or so inches, which is incorrect, since the instrument should be calibrated to read 0 feet at 29.92 pressure, not 900. Nevertheless that's not how it's programmed. At start the instrument adjustment menu always shows the pressure to be 29.92 HG; and Generally, the needle is at 900; though sometimes it will slowly drop to a slightly lower
figure (I don't know what in the program causes this).
Lets say the program imports the actual atmospheric pressure at the location, and the altitude reads 900 feet instead
of 20. If that's the case, the altimeter adjustment menu should show the pressure to be less than 29.92, something like 29.0.
Why is this important? Because arriving pilots depend on controllers, ATIS, Towers, etc. to be informed of the correct existing pressure, so they can set their altimeter accordingly. Should a flightgear pilot do that, his altimeter will always read about 900 feet high, and he would be in danger of hitting the ground or going in the drink.
I first discovered the problem doing night carrier approaches, and quickly discovered I had to be very careful to reset the altimeter to field elevation at departure; and when flying over sea make sure it agreed with the radar altimeter, not to visit Davey Jones locker. If I had depended on a pressure supplied by ATIS or the LSO, I'd would have needed my Mae West. Cheers