D180 ASI error troubleshooting

[rjcthree]

My RV-9A has about 3 hours on it now, and consistently shows an 8 Kt difference between the D-180 ASI and GPS four-ways. The ALT is within about 20' through most of the range, suggesting it's not a static issue. Static and pitot were leak checked by the avionics shop without the pitot mast in the loop. Its a Dynon AOA unheated mast in the stock location. The zeroing procedure was done with an accurate baro reading, no wind, etc.

I'll go back in and check for leaks as best I can, can you suggest additional procedure? Thanks, Rick

 

[Robski]

Compare with a 'steam' instrument? 
I'm in the UK hence forced to have to have a steam backup to the EFIS for ASI, ALT and Compass so it's easy for me to suggest this.

This does mean a static or pitot line leak or position error will render both measurements out by the same amount, but at least that would rule out instrument error.

 

[dynonsupport]

So a few notes/questions:

1) Technically, a 4-way GPS average isn't accurate. It's a bit non-intuitive, but the correct way is a triangle with some math. There's a great thread on VAF about this, with spreadsheets and such:

http://www.vansairforce.com/community/showthread.php?t=59870

2) GPS is TAS, but IAS is indicated. Are you correcting IAS for OAT and altitude to get TAS before you check vs GPS?

3) GPS altitude and indicated altitude aren't supposed to match in many cases. The way to check for static error is to hold a GPS altitude and speed up and slow down and see if your indicated changes. The absolute difference between GPS and indicated isn't very relevant.

Just making sure your process is accurate before we delve a bit deeper.

 

[rjcthree]

Thanks. I'm looking for (hoping for) the big chunks at the moment with 8kts off.

1. Understood on a 4 way being sketchy, but it's fast. Once I get looking for less than 5kts, I'll be flying a triangle as described. Honestly, I don't know how much the two methods would differ in this case, as I don't have the data in triangular legs, and the winds aloft <10kts. But I will next flight!
2. TAS adjustment during the period, as computed by the D-180 was +3kts. 125 IAS = 128 TAS. I believe the OAT to be accurate.
3. I do recognize that indicated ALT and GPS alt may not be directly relevant, but all I'm using it for is to confirm the lack of wild swings within the speed range. I expect the higher angles of attack to be a little wonky. At this point I'm trying to get bracketed in the 100kt-140kt range, which is where I expect to live most of the time.

My plan of attack at the moment is to relook at the connections between the plastic tubing and the pitot, and the connection at the D-180 for pitot. Everything else was checked by the avionics shop, and the D-180 was very accurate from 0 to 22K ft, the pitot plumbing leak free with the pitot out of the circuit. I'll check for bugs too!

Its funny the idea of steam is ntoed, I have one, and I've got a couple tee's to tie it in temporarily if I get too far down the road.

Thanks, Rick