Altimeter off by 300 ft

[Ron_B]

I have a D-100 and when I'm on the ground at the local airport, and set the altimeter at their pressure, I get about 300 ft higher than the 114 ft field height. What and or how should I check it. I plan on having my transponder calibrated by an avionic shop soon and it uses the Dynon for the encoder, so will this altitude error mess up my plans?
Ron

 

[khorton]

On the D-10A, there is a setting that allows you to adjust the displayed altitude to solve exactly this type of problem. I assume the D-120 should have the same feature. Dig into the docs a bit more.

 

[Ron_B]

Thanks Kevin
Are you saying the barometric pressure will still be as the tower says and the correct altitude will be displayed? I've looked a fair amount, but haven't seen such an adjustment, I'll keep looking.
Ron

 

[dynonsupport]

Ron,

There is an ALTADJ under the setup menu. This adjusts the altimeter by this setting at all altitudes. I'd check this to make sure it isn't set to a large number, since it is allowed to go to +/- 500 feet.

We don't expect that you would need to adjust it by more than about 30 feet though. If your ALTADJ is zero and your altitude is off by more than 50 feet, we'd really like to re-calibrate that unit. You shouldn't have an error that large. Also check your static system since a blockage in the line could cause this problem. Checking the EFIS with the static line disconnected is probably the best way to start.

We're at 160 feet here and won't ship a unit that requires more than 30 feet of ALTADJ to get it within tolerance. It's one of the last tests we do before shipping.

 

[Ron_B]

Thanks for the reply. I had to set the ALTADJ to -50 in order to get to be acurate. I have not removed the static line yet but will do so.
Ron

 

[dynonsupport]

Ron,
Does this mean your ALTADJ was +250 feet before?

If it's correct at -50, I'd probably not worry too much.

 

[WestCoastOz]

My D100 has a similar problem (out by almost 300 ft) however will disconnect the static line and check it again and report back.

 

[Ron_B]

I'm not sure about any of my results. I'm consentrating on flying my new aircraft, a PA-18 replica and therefore am not focused on the altimeter. I do remember setting the altimeter at the airport which has an altitude of 114' and the dynon showed above 300'. My D100 was set at zero when I reset it to -50. I will go back to the local airport and check the settings and remove the static line next time I get a chance, and let you know.
Thanks Ron
PS flying a new aircraft is a hand full for low time pilots. While flying off the restriction time within 25 nm , altitude ,( living at sea level with no terain over 300 ft and no obstacles over 400 ft) isn't a large priority. Restrictions will soon be lifted so I'll need to focus and have this problem straightened out.

 

[Ron_B]

Today I left on a mission to check my altimeter. I set the altitude of my grass strip, 100ft. The baro setting read 30.03. Once airborn I contacted YQI about 18 miles away and recieved their current baro of 30.03, right on. This was with a -50 setting under the ALTADJ . When I landed the altimeter read 120ft and YQI is 114 ft, right on again. On the ground at YQI I removed the static line from the D100 , no change . I then reset the ALTADJ to zero and the altitude read 170 to 180 (jumping back and forth), instead of the 114ft. I set the ALTADJ back to -50 and the correct altitude was showing. I reconnected the static line and the correct height of 120ft was showing (still at -50). What is the next step? Will my transponder work correctly with the -50 setting? We are trying to make a fly-in the first of Sept. and will need a working transponder.

 

[dynonsupport]

Ron,
The ALTADJ changes the transponder output as well. If you are happy with the altitudes shown on the screen of the EFIS, the transponder will be based on the very same numbers, so you will be happy with the encoder results as well.