Air speed D10a

[skyhi]

I am flying an RV8 with the D10a and EMS. On approach I have a disparity between my standard Vans ASI and my EFIS.
Typically if the Standard ASI is reading 70kts my D10a is reading 70mph ie. approx 10mph low.
They are both plumbed into the same pneumatic line - and appear to read correct values at cruise.
Any suggestions please, Rgds Nic

 

[dynonsupport]

Hi Nic,

Differences among airspeed instruments are hard to figure out for a few reasons. First, it could be that both instruments are off by a few mph in opposite directions, and figuring out would require than you fly against a known correct airspeed. There are procedures that involve flying multiple legs in different directions (to account for wind) while referencing GPS. I think there may even be one of these procedures in the latest AOPA Pilot magazine.

Next, is altitude reading right? If the EFIS's static measurement is off, you'd also expect to see incorrect airspeeds.

If there is a leak in the plumbing near the EFIS, you might see a similar effect as well.

Finally, we're always willing to have a unit back here under warranty if you'd like us to have a look at it.

 

[khorton]

I think the most likely scenario is instrument error in the standard ASI and/or the EFIS.  You could either have an avionics shop hook up a pitot-static test set, or do your own test using a water manometer.  

You can get info on how to make a water manometer - http://www.eaa1000.av.org/technicl/instcal/instcal.htm.

I created an Excel and OpenOffice spreadsheets that converts water manometer height to ASI reading - http://www.kilohotel.com/rv8/filemgmt/visit.php?lid=3.

Typical instrument errors are fairly small (2 or 3 kts), but I have heard of instruments with much larger errors.  The results of my calibration with a water manometer are at http://www.kilohotel.com/rv8/article.php?story=ASI_Calibration_2

You say that "They are both plumbed into the same pneumatic line" - I'm not sure exactly what that means, as there are actually two lines.  Are both units hooked to the same pitot and static sources?  If not, there may simply be a position error problem on one of the sources, or a leak.  I've got info on checking position errors at http://www.kilohotel.com/rv8/phplinks/out.php?&ID=31.

 

[skyhi]

Thanks for the feedback -  I will have a closer look at the options for calibration, thanks to Kevin for his links and suggestions. The Vans ASI and the EFIS are plumbed in series into the same pitot and static sources.
At 120 Kts they read within 1-2Kts and even at 170Kts the disparity is small (2-3Kts), however on approach ie. at the bottom end of the range, the difference increases markedly and the EFIS will read for example 62Kts whilst the new Vans ASI will read 70Kts.
Is the Dynon more likely to read correctly across its full scale (as it is peizoelectric).

Rgds, Nic

 

[khorton]

Is the Dynon more likely to read correctly across its full scale (as it is peizoelectric).

I would expect the error was probably in the ASI, as it is mechanical.  But it is possible that the Dynon could have some strange failure, perhaps.  If you check the calibration with a water manometer, you can do it with the instruments in the aircraft, and test both at once, which will show which unit is causing the problem.

 

[dynonsupport]

Skyhi,
We calibrate and test every unit here at the factory, over different airspeeds and temperatures, so I'd expect that your EFIS unit is correct. The best test for you to do is the manometer test, and to check it at low airspeeds.

We'd also be happy to check your unit for you here at our factory, but you'll obviously need to return it for us to do that, so it's up to you how you would like to proceed and what is easiest/fastest for you.