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TAS Correction

Postby lr172 » Wed Jan 04, 2017 10:32 pm

My IAS is reading about 5 MPH high and I would like to use the TAS correction table to address it. The instructions say to set up into the wind and enter the IAS, then repeat with the wind. I understand that it will interpolate that actual TAS. Do I enter the IAS figure that I want to be correlated with my actual TAS at that moment.
lr172
 
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Re: TAS Correction

Postby Bobturner » Thu Jan 05, 2017 6:07 pm

I would suggest you fix the problem. If you're showing 60 kias but really at 55 kias you are very close to a stall. IAS errors are usually due to pitot-static problems, particularly static port placement and design. Little things, like the shape and thickness of the port, matter.
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Re: TAS Correction

Postby lr172 » Thu Jan 05, 2017 7:17 pm

Bobturner wrote:I would suggest you fix the problem. If you're showing 60 kias but really at 55 kias you are very close to a stall. IAS errors are usually due to pitot-static problems, particularly static port placement and design. Little things, like the shape and thickness of the port, matter.


Bob,

I have the standard Van's static rivet in Van's specified location and have confirmed no leaks. I have a steam ASI, as well, and my issue is that the GRT reads 5 MPH higher (IAS) than the ASI at cruise speeds. At pattern speeds, the difference is only 1-2 MPH. I have confirmed that the ASI gauge is accurately represent TAS when verified with 4 point GPS runs at cruise speed. The Dynon I just replaced had the same error/difference from the ASI, so it's possible the steam gauge is reading low and the Dynon/GRT are accurate/standard. However, The ASI's readings are a perfect fit for my pitot / static arrangement. I would prefer a calibration vs. moving around the static port from the proven location.

Larry
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Re: TAS Correction

Postby jbasol » Thu Jan 05, 2017 11:17 pm

The TAS correction procedure corrects TAS only, not IAS. There is no correction procedure for the IAS. If the IAS is inaccurate you have either a pitot/static problem or an ADAHRS problem. Given you noted the same issue with another EFIS, I suspect the pitot/static system. First step would be to build a manometer and test IAS on the ground. If it all checks out on the ground you likely have an issue with static port or it's plumbing.

-John
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Re: TAS Correction

Postby Bobturner » Fri Jan 06, 2017 4:38 am

I agree. Test with a water nanometer and find out if the EFIS or mechanical gauge is correct.
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Re: TAS Correction

Postby lr172 » Fri Jan 06, 2017 11:14 pm

I had my IFR cert done today no static or pitot leaks and the GRT IAS indication was dead on with the test equipment. I guess that I goofed up the 4 GPS test that I was benchmarking against or the older Dynon had an issue with the OAT and/or TAS calculations. I will do more testing today. With our single digits temps today the TAS was the same or lower than the IAS.
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