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Thread: Altitude error

  1. #1

    Altitude error

    I did a significant avionics upgrade on my plane that I completed in June 2020. Upgraded my PFD from an AFS4500 to AFS5600, installed SV-ADSB, SV-ADAHRS, SV autopilot servos and a Garmin G5. I have more than 150 hours on the plane since that upgrade and everything has been working fine. Last August (14 months ago) I had an altimeter certification done so I'd be legal for IFR training and flight. They certified the AFS5600 and the G5 for accuracy. The AFS5600 has always been pretty close to field altitude when I set it, within ~20 feet. The G5 typically reads ~30 feet high, but within the 75 foot variance required to be IFR legal.

    Today, after pulling the plane out of the hangar, starting it up and setting the baro setting on all devices, I noticed that the difference between the two units was about 100 feet, with the AFS5600 reading 100 feet higher than field altitude. I was in VFR conditions so I noted this difference and then flew to another nearby airport that has an ASOS on the field. After an hour on the ground, I restarted and again set the barometric pressure for both units and saw the exact same behavior, this time confirming that the AFS5600 was reading 100 feet higher than field altitude. I am scratching my head as to how this can happen. Both the AFS5600 and the G5 are connected to the exact same static system, which I have recently leak checked and found no problems. Regardless, if the static system were a problem then both the G5 and the AFS5600 would read incorrectly, but the problem I see is only on the AFS5600. This suggests that the SV-ADAHRS unit lost calibration somehow and is now producing an inaccurate altitude that it sends to the AFS5600.

    After confirming this problem at the second airport, I adjusted the altitude offset in the settings area to offset the error and went for a (VFR) training flight with my instructor. Somewhere during that ~2 hour flight, the AFS5600 began reading 100 feet lower than the G5, which suggests that the original problem went away and the offset I had entered was now causing the erroneous reading. After getting back on the ground, I removed the offset and the AFS5600 showed approximately the correct field altitude when I set the local barometric pressure. So in brief, this was a real problem (as confirmed at 2 airports), but it was intermittent because it went away somewhere during my flight today.

    Are there any known problems that would cause the ADAHRS unit to temporarily produce an inaccurate altitude reading? Any other ideas? I am in the midst of IFR training and will not be able to proceed unless I can prove that the altitude problem is resolved.

  2. #2
    Advanced Flight Systems Shawn McGinnis's Avatar
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    Sep 2012
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    The element that I'm concerned with is the "during that ~2 hour flight, the AFS5600 began reading 100 feet lower". This intermittence is the tricky part.


    We have seen, water, bugs, tape, thread compound, kinked hoses, over-tightened zip ties, et cetera. Normally the easiest diagnostics would be to just remove the static tube and see if the offset goes away. This would show a downstream blockage. If it remains then verify that there is no blockage in the fitting if any of the previous seal methods were used.

    These blockage offsets are normally most noticeable when the baro swings pre/post storms.
    Shawn McGinnis

    Advanced Flight Systems
    support@advanced-flight-systems.com

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