Polyflow brake fittings loosening over time

[Battson]

During our 100hr check (630 hrs elapsed and 5 years in service) we identified that the Polyflow fittings which join the Nylaflow hose were all loose. The plastic had "relaxed" over time, equivalent to three turns of the fitting collars.

I tightened all but one of the Polyflow fittings, and then taxied around leaning hard on the brakes. After 5 minutes of braking the untouched fitting blew apart, leaving zero braking one side.

I wanted to share this for those who are using these fittings, suggest you need to add tightening them to your annual checklist.


brasspoly.jpg

[Archer39J]

Interesting, thanks for sharing. On mine I've cinched them down until the nut wouldn't turn anymore, looks like it bottoms out on the fitting. Hopefully that will be enough, but they were obviously something to check at least annually

[PaulSA]

I have these fittings on my RV-10 and have never been particular happy with them. Thinking of changing to braided steel - which is what I have fitted to my Bearhawk. Minimal weight cost but certainly considerably more expensive........ Worth it IMHO........

[Bdflies]

I’ve owned 2 airplanes with plastic brake lines and never had an issue. A Kitfox, which admittedly didn’t require much brake action and a Glasair taildragger. The Glasair landed pretty fast and was steered by differential braking only. It’s been way too many years since the build and I don’t remember the details, but I don’t recall fittings that looked like those. I think the nuts were hex shaped for the full length, like one would use with copper tubing. I used the term 'plastic' brake lines because I'm not sure if polyflow or nylaflow was used. The kit came with all the materials.
I know this is a bit vague on details, but it could be that different materials might yield better results. I put about 700 hrs on that Glasair and went through a LOT of brake pads, without a single brake line issue.

Bill

[EBPaul]

Hi, When I was helping install the brake system on a Patrol the pressure side fittings needed an internal piece that the tubing went into to make the pressure rating.
Paul Roy

[Battson]

To me they looked like a fitting which can be over-tightened. They are almost out of threads now, after being tightened the second time.
The fact the tube can slip through the collar is a design flaw in my view, they really need a flared nozzle or something to prevent that happening.

[Archer39J]

Maybe using nuts with brass inserts instead of plastic would mitigate this issue?

[Battson]

Maybe using nuts with brass inserts instead of plastic would mitigate this issue?

Something like that, something which physically pinches into the tube to hold it fast.

[yateselden]

At work we use an aluminum insert that keeps the poly tubing from being squeezed beyond inside dia. The insert allows the farrel something to bite against. There is a slight line size restriction. These are Parker fittings