What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Brake Line Flambé

PandaCub

Active Member
Something to watch for on your RV-14 main wheels:

Yesterday I did a normal landing at my home airport (KBJC) and taxied back to the hangar. About 15 minutes later brake fluid began to drip from the left main. I popped off the wheel pant, only to behold the zip tie(s?) holding the brake line away from the rotor was missing and [what was left of] the brake line had been rubbing against a piping-hot disc!

I now plan to replace the brake lines on both sides and secure them with clamp(s) rather than zip ties.
 

Attachments

  • IMG_7331.jpg
    IMG_7331.jpg
    220.9 KB · Views: 451
Last edited:
If you like you brake hoses well done, without some sort of steak sauce then do this.
YES--this is one of ours--installed by the previous owner/builder. Seems there was a tire tube issue on the same wheel recently, so there is no way of knowing exactly. The other side was secured. OP reported it to us, but from the pics, I think the liner is intact and would hold pressure. OP did NOT report a brake failure---although now with a caliper leaking, we suspect a severely overheated piston O'Ring. He's changing both calipers ( was going to upgrade anyway) and new hoses---non crispy option.

I'll get the original back and do an autopsy, and share the findings with all.
Tom
 
^^ Tom is one of the most helpful guys I've encountered in the industry and it'll be interesting to get to the bottom of this. As he said, the brake hose itself didn't fail and I'll wager it still works fine (the leak was coming from the caliper and not the hose).

My current theory: I'd recently landed on a grass strip (Gaston's 3M0 - highly recommended) and noticed the RV-14A's wheel pants make a great "scoop" for the dead grass underlying the turf. If enough of that dead grass got into the wheel pant and dried out over the course of a few days, it's entirely plausible that a hot brake disc could ignite it, causing a fire inside the wheel pant after landing and taxi.

I'm currently awaiting new brake lines from Tom and an upgraded brake set from Grove (https://www.groveaircraft.com/rvwheel.html). Will update next week after installation.
 
Last edited:
The plans I have show 3 wraps of friction tape around the brake lines and they do appear to be aluminum tubes (U-01414-L and U-01414-R).

(By way of background, I am not the original builder of this beautiful bird. Just discovering more about her as I go along).
 

Attachments

  • RV-14A-brake-detail.jpg
    RV-14A-brake-detail.jpg
    323.5 KB · Views: 319
If you look closely to at a Matco master cylinder, you will see the travel from off to full braking is very short, any tiny bit of pressure on the top of the rudder pedal will make it drag. I know of several instances of fires or near fires in the brakes of a -14 and I would bet they are most or all from either pedal pressure while taxiing or a caliper not moving smoothly and dragging. The return spring master cylinder mod is a good idea plus hyper focusing on keeping your heels on the floor and not letting them put any pressure on the pedals, especially on long taxi runs.
 
My brake lines (from TS Flightlines) came with an adel clamp to hold the line in that position rather than a zip tie.
 

Attachments

  • IMG_0068.jpg
    IMG_0068.jpg
    389.7 KB · Views: 327
I would think that a much closer inspection of mounting systems of plumbing, hoses, etc is warranted, especially in hot places like under the cowl. Where else were zip ties substituted for proper fasteners? And other places something wasn't plumbed/wired/rigged per the plans.

 
If you look closely to at a Matco master cylinder, you will see the travel from off to full braking is very short, any tiny bit of pressure on the top of the rudder pedal will make it drag. I know of several instances of fires or near fires in the brakes of a -14 and I would bet they are most or all from either pedal pressure while taxiing or a caliper not moving smoothly and dragging. The return spring master cylinder mod is a good idea plus hyper focusing on keeping your heels on the floor and not letting them put any pressure on the pedals, especially on long taxi runs.

Thicker discs will solve all of your problems...
 
Thicker discs will solve all of your problems...
I recall Danh doing some math that showed that the disk mass is the main factor in how much capacity the braking system has. What I took away is that with the standard brakes in the RV-8 I get one full braking cycle - aborted takeoff or aggressive landing - and then I need to let the brakes cool.

Brakes are essentially converting your kinetic energy to heat, and that heat is generated by the friction between the brake pad and the disc. If the disc gets too hot, it will cause lots of bad things to happen.

I can't find the thread I'm thinking of, but here are some others.

https://vansairforce.net/community/showthread.php?t=153627

https://vansairforce.net/community/showthread.php?t=104995
 
Gee guys the hose WASNT the question OR the problem. Jeff just showed it to show what happened. Whether it was a teflon hose, or an aluminum rigid tube isnt the issue---its to show what can happen, and to be careful of install, even after a seemingly easy procedure as a tire tube install. Obviously a mistake here.

MY view on this was to show that a teflon hose COULD SURVIVE a fire and at least that part of the brake system integrity. OH and page 40A-10 show the line secured by 'friction tape'

Tom
 
Last edited:
Gee guys the hose WASNT the question OR the problem. Jeff just showed it to show what happened. Whether it was a teflon hose, or an aluminum rigid tube isnt the issue---its to show what can happen, and to be careful of install, even after a seemingly easy procedure as a tire tube install. Obviously a mistake here.

MY view on this was to show that a teflon hose COULD SURVIVE a fire and at least that part of the brake system integrity.

Tom

Typical topical wander, Tom - but I do have a question - - since that gear is not readily going to accept some adel, will you be working on a metal part (gear friendly) that can hold the hose as a standard solution for your products? - -that could provide all future customers with a good solution.
 
If you like you brake hoses well done, without some sort of steak sauce then do this.
YES--this is one of ours--installed by the previous owner/builder. Seems there was a tire tube issue on the same wheel recently, so there is no way of knowing exactly. The other side was secured. OP reported it to us, but from the pics, I think the liner is intact and would hold pressure. OP did NOT report a brake failure---although now with a caliper leaking, we suspect a severely overheated piston O'Ring. He's changing both calipers ( was going to upgrade anyway) and new hoses---non crispy option.

I'll get the original back and do an autopsy, and share the findings with all.
Tom

An overheating problem was described.

The picture looks like when the line was installed, it was leaning on the disc so the installer pulled it back to the leg and held it there with a pull tie. When the tie finally failed, the natural tension on the line forced it to it's natural position against the disc.

Is there a smaller line that can be used in that location that would have more flex?
 
Just had an old C172 in the other day for IFR cert, no wheel pants on it, rigid alum lines all the way to the caliper same as what Van's specs (except I would suggest using 5052).
 
Just had an old C172 in the other day for IFR cert, no wheel pants on it, rigid alum lines all the way to the caliper same as what Van's specs (except I would suggest using 5052).

Plenty of Pipers (including my old Tomahawk) were done the same way. The service loop solves the fatigue issues.
 
Plenty of Pipers (including my old Tomahawk) were done the same way. The service loop solves the fatigue issues.

Ummm... from experience... the service loop delays the fatigue issues.
The last aluminum hard lines I replaced were installed on a certified aircraft (Grumman AA5A). Yup, fatigue cracking of the flare on a factory-made hard line, a line which has proper clamps installed at the bottom end of the gear legs. Happened at about 2700 hours time in service.

Hard line vs flex lines is a bit like the holy wars, or the primer wars. There's no right answer, only personal biases built up from a history of personal experience. My personal experiences with hard lines at the calipers cause me to install flex lines and recommend others do the same. I kinda like my brakes to work EVERY time I need them! :D
 
An overheating problem was described.

The picture looks like when the line was installed, it was leaning on the disc so the installer pulled it back to the leg and held it there with a pull tie. When the tie finally failed, the natural tension on the line forced it to it's natural position against the disc.

Is there a smaller line that can be used in that location that would have more flex?

No flex line should be used unless engineering plans call for it. The rigid aluminum line does not move appreciably during use unless the disc is severely warped. Fatigue is a non-issue--spend the $5 to replace with every engine you wear out. :D
 
To the OP -- consider switching to Viton o-rings (If Grove doesn't use them already) and switching to the higher temperature, fire 'resistant' brake fluid.

Royco 782, Mil-Spec: MIL-PRF-83282 fluid. The fluid is compatible with Buna-N (nytrile) o-rings, but you want the higher temperature tolerance of the viton o-rings to go along with the higher temperature fluid.

Oh - and I am firmly in the camp of using a flex line to connect to the caliper. Make a metal bracket that clamps to the gear strut and holds a bulkhead AN fitting for the transition from solid tubing to flex line. Make sure the flex line forms a natural half-loop that doesn't bias the caliper motion - it would eventually cause wear on the guide pins for the caliper if there was a torque applied to the caliper.
 
Last edited:
Back
Top