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AXP340 long delay for Press Alt at startup

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CardinalRG View Drop Down
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    Posted: 12 Nov 2016 at 4:44pm
Recent problem with my AXP340.   Was fine for past year or so....

Most days, on first flight of the day, I don't get Pressure altitude output for 10 minutes or so.  It starts in ALT mode and transponder squawk works fine, but The AXP screen shows Pressure Altitude shows FL---.  After 10 minutes or so, the AXP340 starts reporting the correct Press Alt.

This started a few weeks ago.  No recent panel work done.

Any troubleshooting suggestions?

Joe Brunski
1977 C-177RG with two IFD540s + AXP340 + MLB100


Edited by CardinalRG - 12 Nov 2016 at 5:01pm
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DavidBunin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DavidBunin Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 2016 at 8:08am
Sounds like the heater is going out in your altitude encoder.  Altitude encoders normally contain a heater to bring the pressure sensor to an above-ambient operating temperature (usually around 140°F) which means they can eliminate temperature as a variable in the pressure sensor logic.

Most encoders have a warm-up time of no more than 30 seconds before they go valid.

Have your altitude encoder removed and bench tested at an avionics shop.  My guess is you'll be buying a new one.  They are not terribly expensive.

David Bunin
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