AoA Mystery ? fixed, but not solved?.

[Ironflight]

I love my AFS Pro AoA system. But about a month ago, I was beginning to think that “Betty” needed therapy – or at least needed to cut down on the caffeine. I was getting warnings on almost every base to final turn – and when I looked at airspeed, I was above the white arc! This didn’t make any sense. I traded notes with Rob at AFS, and he suggested checking for leaks, so I pulled the wingtip, pulled an EFIS so I could get at the brain box, and checked the hoses, What I found was that the upper (blue) hose was blocked – and no amount of compressed air would clear it.

I was trying to figure out how to clear this line without disassembling everything when I happened on Tricia Blank from AFS at the R-cubed BBQ a few weeks ago. She said she’d end me some new blue hose and some splice fittings to help me troubleshoot – or replace the whole line. Today I finally had a chance to spend a few hours, so I dove in. My first thought was that I had a water plug at the low point, which would be at the wing root. It was easy to pull the fairing and cut the line right there. What I found was that from there to the tip was clear (whew! No wing plumbing required!), but the line up to the instrument panel was still blocked (rats!! That meant pulling the floor).

Like everyone faced with such a maintenance dilemma, I hemmed and hawed awhile, trying to figure out how to figure out where the blockage was without pulling the floorboards (not fun in an -8…there is no place to stand while you do it, like in a side-by-side). I finally realized I had to bite the bullet, and out came the seat and floor. Lots of blowing and sucking, inspecting, etc. – with now joy. OK – let’s just replace all the line in the fuselage! I started to clip cable ties – I had run the two sense lines in a piece of snakeskin from the wing root, through the stick opening in the spar, and then around the left side with some other bundles to the left gear tower, then up to the back of the panel, where the brain box sits on the AHRS. Once I had clipped the cable ties, I tested the line again – and lo and behold, I could get air through it! OK – so it was being crimped by a tie – I can believe that. Reassembly and flight test took an hour – and everything was fine. So – problem solved.

But…not the mystery. The question is, why did it work for the better part of a year, then suddenly get blocked, when the cable ties that I clipped were in an area that sees no flexing, no movement from controls, had no contact with anything? The problem seemed to come on slowly – not all of a sudden. It was a subtle change in behavior that had me initially questioning if there was even a problem. I don’t know the answers – but I can tell you that this very tiny, very soft tubing can be crimped – so don’t pull anything tight around it.

I am once again happily flying the AoA here at altitude, and it is a nice margin enhancer. Thanks again to AFS’s Rob and Tricia for helping out (at no charge) as I first doubted what I was seeing, and then accepted that there was a problem. The little double-ended barbs were just what I needed to aid the troubleshooting!

Paul

[AlexPeterson]

Paul - seems like it was just material creep or relaxation under the constant stress of the tie wraps. It could have been 95% closed off fairly quickly at initial installation, and the last 5% finally, but slowly, closed off. Since the flow rate through it is almost zero, it would only take a tiny opening to work well.

[ifixf15]

As you are transitioning to a more 'dynamic' climate and awaiting permanent facilities the aircraft may be getting cold soaked more often than normal. Colder plastic lines tend to lose much of their flexibility. At least you were able to duplicate the problem. That is the only way to be assured of a good fix.

[Mel]

Hey Paul,
Maybe you were using the wrong color cable ties. (See other thread!)

[rv9av8tr]

Maybe it was all the yank'n and bank'n 8-)

[Ironflight]

Originally Posted by Mel

Hey Paul,
Maybe you were using the wrong color cable ties. (See other thread!)

The pink ones?!

[mike newall]

This took me back a while.

We had finished our 7, done the fly off and were tidying all the loose ends.

Ahhh - calibrate the AoA on the Dynon.

Book in hand, Brother by my side, he reads - I do.

Up and down, slow, stall, etc.

Then it says to do some full power stalls.

Say what Bro ?

Full power stalls.

OK........

I end up giggling, he ends up squealing like a girl - this thing is pointed up at nearly 60 degrees, still flying !

Did get a tad warm on the CHT front.

Result, I can sneak into 900' strips if I want