Says it all really and i'm thinking emergency here by the way.



I've had two alternator failures an one suction failure already this year.

The point of the thread is how can SD show us a relevant vertical deviation pointer and I highlighted why I felt that was not possible or could be confusing. It would be great if it could but I can’t see how without the added complication of an air pressure input. I think you’re letting your fascination with flight levels get in the way and hopefully my examples below will show why……I won’t make it more complicated by bringing temperature into it.



You’re flying along below transition altitude at 5000’ with a Qnh of 993 and SD agrees you’re at about 5000’. Without climbing or descending you pass into airspace that now puts you above transition level so you set 1013.2. What does your aircraft’s altimeter and SD now say? My guess is SD still says 5000’ but your altimeter now indicates about 5550’.



Alternatively, fly at FL80 from a high pressure area (1033) to a low pressure area (993). Is your physical distance AMSL always 8000’ and does SD show you at 8000’ at both ends of the flight? My guess is you’ve changed altitude by about 1100’ and SD will reflect this.



Everyone else feel free to ignore the following bit if you’re bored with our conversation as it’s a little off thread!



Work out the difference between two pressures and you can determine a height/altitude. An old fashioned altimeter does it mechanically and an ADC does it electronically. Both are in abundance, as can be confirmed by a quick look at many websites (Airbus, Boeing, Bombardier, Cesna, Embraer, Garmin, Piper, etc, etc, etc…). ADC’s are used regardless of the pressure setting, they don’t turn on/off dependent on transition level or standard being set. If you want any more help feel free to make it a private message so we’re not cluttering things up here.



Cheers BJ

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