teknosmurf
Well-Known Member
Hi all,
After I purchased my Davis back in August, I immediately got the transponder check done. As part of that, we went above and beyond the normal check to also check the static system to make sure things were behaving correctly. At the time, the altitude read from the encoder on the guys test box was "close enough" to the altimeter. This was back in the Aug./Sept. timeframe of last year.
A few weeks ago I started getting complaints from ATC that I was 300' off on my altitude. This was consistent throughout my 3 hours of flying...so I am sure I had enough warmup time. Additionally, the ACK A-30 does not transmit the code until it is warmed up (according to the manual).
To debug this, I built an encoder test box so I could read what was being reported. I then hooked up a suction pump to the altimeter/encoder part of my static system (disconnecting the rest of the static system). Sure enough, all the way up to 20k feet read 300' low.
If the reading were 500' low, I can see possibly one of the bits of the gray-coded output having issues, but at 300', this is not possible as all bits are being toggled. I confirmed this with my test box.
Before I simply recalibrate it and have it checked again, I would really like to figure out what the real issue is. I find it odd that it would jump a consistent 300' with no apparent changes to the system. The best guess I have is that we checked the system in the heat of the AZ summer, and when winter hit, things started acting differently. Unfortunately this does not seem likely either because 1) the error does not increase/decrease as the encoder warms up, and 2) it is consistent throughout the altitude range. The only other thing that I can see as a possibility is maybe the adjustment pots were somehow vibrated loose. Apparently it does not take much to tweak the calibration, and maybe the dust/dirt affected it by that minisucle 300'. I just find it odd that it would "jump" like that and not be a gradual progression.
BTW, yes, the altimeter does read correct at ground level both with the correct pressure set as well as what I would expect with 29.92.
Can anyone think of something that I may have missed here?
Thanks
After I purchased my Davis back in August, I immediately got the transponder check done. As part of that, we went above and beyond the normal check to also check the static system to make sure things were behaving correctly. At the time, the altitude read from the encoder on the guys test box was "close enough" to the altimeter. This was back in the Aug./Sept. timeframe of last year.
A few weeks ago I started getting complaints from ATC that I was 300' off on my altitude. This was consistent throughout my 3 hours of flying...so I am sure I had enough warmup time. Additionally, the ACK A-30 does not transmit the code until it is warmed up (according to the manual).
To debug this, I built an encoder test box so I could read what was being reported. I then hooked up a suction pump to the altimeter/encoder part of my static system (disconnecting the rest of the static system). Sure enough, all the way up to 20k feet read 300' low.
If the reading were 500' low, I can see possibly one of the bits of the gray-coded output having issues, but at 300', this is not possible as all bits are being toggled. I confirmed this with my test box.
Before I simply recalibrate it and have it checked again, I would really like to figure out what the real issue is. I find it odd that it would jump a consistent 300' with no apparent changes to the system. The best guess I have is that we checked the system in the heat of the AZ summer, and when winter hit, things started acting differently. Unfortunately this does not seem likely either because 1) the error does not increase/decrease as the encoder warms up, and 2) it is consistent throughout the altitude range. The only other thing that I can see as a possibility is maybe the adjustment pots were somehow vibrated loose. Apparently it does not take much to tweak the calibration, and maybe the dust/dirt affected it by that minisucle 300'. I just find it odd that it would "jump" like that and not be a gradual progression.
BTW, yes, the altimeter does read correct at ground level both with the correct pressure set as well as what I would expect with 29.92.
Can anyone think of something that I may have missed here?
Thanks
