IAS adjustment on D10A

J

jreekree

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Oct 22, 2007
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I have an error in my indicated airspeed and was wondering if there is a way to put a permanent adjustment in the IAS. I am reading about 8-11 knots faster than I am really going and would like to adjust the D10 down. Is this possible in the menu set?

Jerry Morris
 
I have an error in my indicated airspeed and was wondering if there is a way to put a permanent adjustment in the IAS.  I am reading about 8-11 knots faster than I am really going and would like to adjust the D10 down.  Is this possible in the menu set?
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A better solution would be to find out where the error is coming from and fix it.  There is a good possibility that the cause of the error is static source position error.  If this is the cause, it will also be affecting your altimeter.  A static source position error that gave a speed error of 10 kt error at 200 kt CAS would give an altimeter error of about 190 ft.  If the speed where you see the error is 100 kt CAS, the corresponding altimeter error would be about 95 ft.  You can find more info on measuring static source position error at http://www.kilohotel.com/rv8/rvlinks/ssec.html.

Another possibility (if you are using the display TAS as a measure of airspeed accuracy) is that your OAT is reading too high, which would cause the TAS indication to be too high.  But, it would take a very large OAT error to cause this much TAS error (about 30 deg C error at 200 kt would be required), so it is unlikely that this is the sole cause.  It could be a contributor though.

A third possibility is simply that you might be using an inaccurate method to determine the errors in your airspeed system.  Some folks come up with test methods, that might look good on the surface, but don't stand up to close scrutiny.  For example, some folks believe that if you take the GPS ground speed on the four cardinal headings, and average them, that the average is equal to the TAS.  In fact, this is only true if the wind is zero.
 
Kevin's on a crusade to correct static source position error (SSPE) as it's a primary source of all kinds of instrumentation errors.

He even thinks SSPE is the root cause of Amelia Earhart’s disappearance, the Challenger Disaster, the Hindenburg Crash and some have suggested that SSPE was the reason for Steve Fosset's disappearance (not to mention Global Warming) ;)

Let's all join Kevin in his quest to eliminate SSPE during our lifetime!

In all seriousness, he's probably right.

PilotKris
 
Another thing to note, too, is that airspeed equals pressure, period. That relationship isn't linear, either, so adjusting it isn't trivial.

We do have a zero pressure adjust that is capable of adjusting the sensor value that is regarded as the zero airspeed. This adjustment is provided because at the lower end of our airspeed range (15-20 knots), the pressures that we're detecting are incredibly minute and hard to sense.
 
Jerry - What kind of aircraft do you have?  Did you make any changes from the designer's specified static system?  Some RV builders decide they can improve the look of Van's specified "pop" rivet static ports by using flush static ports instead.  Unfortunately, the shape of the static ports is critical to their function.  I've seen several reports from people who found that the flush static ports caused IAS values that were 5 to 10 kts too low.

Your problem seems to be in the other direction, if I understand your report properly.  But it could be due to small variations in the static port details, or the contours around the static ports.  
 
Thanks for the replies.
I have an RV8 with the stock vans rivet head static system. While doing my condition inspection I did find a static line that is "T'd" off the main static line. This extra line went absolutely nowhere and I think it is my culprit. I can either plug this line or put a valve and use it as my alternate static source. Probably will plug it to see if it corrects my problem.
 
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