Thread: QNH or QFE ?
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Old 9th Aug 2018, 10:54
  #46 (permalink)  
Join Date: Feb 2007
Location: Amsterdam
Posts: 4,592
Originally Posted by Discorde View Post
Not too long from now all primary altimetry will be GPS derived and barometric altimeter setting will be retained purely as back-up. On commercial aircraft local QNH will be auto-uploaded by data link and inputted to back-up barometric altimeters, both to confirm GPS altitude and to give redundancy. For GA pilots the option will be available to adjust indicated altitude to height for local flying.
I don't think so. Not because it's not possible, but because there needs to be a transition period to get from one system to the other. And during that transition period, some aircraft will base their altitude measurement on a GPS reading, and others on a barometric reading. However, there is no way to convert an accurate barometric-based altitude reading into the equivalent GPS-based altitude reading and vice versa unless you know not only the local QNH, but also the exact environmental lapse rate at your given position. Heck, you even need to know the actual humidity across the whole column of air below you. Using the standard ISA lapse rate for this instead may lead to hundreds of feet of error if you're flying at FL300. That's a whole order of magnitude worse than the normal accuracy of analog, mechanical barometric altimeters, and even more orders of magnitude worse than GPS-derived altitude. So during the transition period we would need to use significantly more separation than what's applied now. While the current trend is to reduce separation (RVSM) to improve airspace capacity. I don't think that would work.

Even transitioning from 25 kHz-spaced VHF frequencies to 8.33 kHz spacing will eventually take something like 10 years. And that's a transition that's backwards compatible (8.33 kHz radios can be used in 25 kHz airspace).
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