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RV-6A Brake Fire - What is salvageable?

WingnutWick

Well Known Member
Well,

The girlfriend’s RV-6A continues to give me headaches as I slowly learn more about this plane. I recently just finished doing her annual following a new paint job. New brake rotors, pads, and O-rings. I also had to remove the piston from the pedal do get the valve open to allow the fluid to come up through the bottom. I finally finished this (along with a lot of other things) so her plane would be all good to go before I left for a couple months. Did a few flights uneventfully. Then on the last flight before I left, naturally, as we were pulling off the runway she said the right brake felt weird. Sure enough it seemed locked up. I tried pumping the pedal to see if I could free it, which it did but then subsequently seemed to start dragging again. Shortly after that smoke was coming up and we jumped out.

Sure enough the right brake had caught fire. Fire was going inside the wheel pant for about a minute before we could get an extinguisher on it. Newly painted pant - toast. Smoke had traveled up the leg fairing which blackened the leg and wing root with soot but I believe (hope) that it’s limited to just that vice any real heat damage to the leg itself or wing root.

Brake line at the brake assembly burned through, and it looks like some burning of the fiberglass/carbon fiber around the wheel leg. See pics.

I am looking to get some guidance and opinions on what is salvageable from this fire and what to look for to make that determination. Also any ideas what caused the locking? When I put the pistons back on, I tightened them....can this cause binding?

What kind of temps can these wheel and brake parts withstand? Also, I am unfamiliar with the construction of the leg. Is this carbon fiber surrounding the metal leg? What is that for? Is the burn damage at the base ok? Also, unrelated, it looks like there is a crack at the top (red arrow in pic) that we noticed in the carbon fiber material. Is this an issue?

Finally, wheel pants, what’s the process to make new ones after you order them from Vans? A royal pain or pretty easy?

Her original post was here: http://www.vansairforce.com/community/showthread.php?t=173877

Any and all help is greatly appreciated!
Thank you, cheers!

Charles

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Someone will be along shortly to answer your questions, no doubt.

In the meantime, good to see that RV with the wood prop removed. It?s nice to see that the plane went to a home that can give it the TLC that it deserves.
 
Is that a crack in the gear leg, or just the powdercoating? I would definitely disassemble the gear leg and remove/inspect it. New powdercoating should be applied to alleviate any issues with future rusting of the gear leg.

The brake line looks to be coming into the bottom of the caliper. I think that it should be coming in to the top of the caliper, and the bottom should have the bleeder valve. At least that is what the RV-9A plans show.

You'll probably need a new tire, wheel pant, gear leg fairings and brake line at a minimum. The wheel pant might be salvageable. Hard to tell from the photos. You might be able to salvage some of the hardware involved, like nutplates and hinges. Definitely will need to rebuild the calipers/pads, and please check on doing the brake pedal springs and long bolt modification for the pedals. It makes the brake pedals work much easier.

Making a new wheel pant should be an easy job for an experienced builder. The fiberglass work can be a pain, just because it seems like an endless process of waiting for epoxy to harden, then sanding/filling repeatedly. Actual time is not that long.

Once you get the gear leg/brakes back in airworthy shape, you can just take the other wheel pant off and fly pants-less while you get the new one fabricated and ready for installation.
 
A couple things...

The cloth you see wrapped around the bottom of the axle is likely (burnt and blackened) fiberglass. Many early RV builders built the "gear leg stiffeners" from wood and fiberglass to try to alleviate shimmy. Later on, the practice has been to carefully balance tires and let a few PSI out rather than glass on the stiffeners.

With that in mind, I'd suspect the large crack you see right at the gear leg socket, is just a crack in the epoxy/fiberglass of the top end of the stiffeners. You could try picking at it a bit with something like a flat head screwdriver or a pick/scribe. If it comes off in flakes/chips, it's most likely just epoxy thats cracked.

Considering the mechanics of that joint, any attempt to re-epoxy from the gear leg to the socket, will almost certainly crack again (flexible gear leg, rigid socket, and a brittle material adhered to each). If the other side was glassed identically, I'm sure it will have identical cracks. Probably not reason for concern, but have a closer look to be sure. I'd clean up the area, smooth out the cracking with some sandpaper, and apply paint to any raw steel you can see.

Regarding heat damage to the wing root: Is that burnt paint, or just soot from the burning fairing bits? (Can it be wiped off to reveal paint underneath it?) I suspect and hope it's just soot. If there was real heat in that area, those fuel lines and wing tank could have made a bad day, a disastrous one. Check for any heat damage to the inboard wing tank rib and the proseal that seals it.
If the soot cleans off, leaving behind undamaged paint, you probably don't have any re-heat-treated aluminum to worry about there.

You might have some salvagable hardware from the wheel area... But I'd spend the extra 10 bucks for new nutplates, fasteners, and hinges.

Good luck with the repairs! Fly the thing naked for a while if you have to (the airplane that is), plenty of RV's running around without their pants.
 
Is that a crack in the gear leg, or just the powdercoating? I would definitely disassemble the gear leg and remove/inspect it. New powdercoating should be applied to alleviate any issues with future rusting of the gear leg.

The brake line looks to be coming into the bottom of the caliper. I think that it should be coming in to the top of the caliper, and the bottom should have the bleeder valve. At least that is what the RV-9A plans show.

difficult to flush those lines with the bleeder on top
 
difficult to flush those lines with the bleeder on top

Yeah, I noticed that when we first got the plane. Is it simply a matter of swapping the two brakes? Or even easier, swapping the position on the valve on the assembly?

What?s the recommended way to fix the brake line? Splice it high up with new line? What?s the process and hardware needed for this?

Thank you!
Charles
 
Aluminum tubing is cheap, so no need to splice it. I would buy a new coil of tubing from Van's or your local industrial hardware store.
You can simply unscrew the bleeder valve and the 90 degree elbow from the caliper body and swap them. You'll need to have access to a tubing cutter and an aviation flaring tool to make the ends of the tubing fit onto the AN fittings.
 
Consider using braided SS over teflon all the way from the calipers to the master cylinders with the external return springs as posted a few weeks past. You can make your own or get TS flightlines to supply professional made lines.

Viton Orings in your caliper pistons and Royco 782 or Aeroshell fluid 31 will raise operating temp limits to over 400F.

Somebody recently posted about installing thermocouples on their brake calipers. Several EFIS and engine monitors have extra channels available that would allow you to monitor brake temps.

Thicker discs have more heat capacity and will not heat as rapidly. I am not sure what make your calipers and brakes are but Bruce hill posted his retro upgrade to thicker discs with his Matco brakes.
http://www.vansairforce.com/community/showthread.php?t=172644
Thicker discs provide more braking ability with less fade as they do not heat up as rapidly.

Installing external return springs on your master cylinders at the foot pedals should prevent almost all dragging brake issues.
From Tom in Australia:

To answer the OP's question in post #10, I'm not sure how similar the RV-r brake pedals are to the RV-7, but it is likely you need to do the "long bolt" mod that puts the brake pedals on a common axis, then also the "spring mod" which is described below to stop binding if you have any. Unfortunately the pictures have evaporated because the hosting site I was using has disappeared, but I'll dig some more up and repost them. I've run this mod now for about 200 landings and they work great and feel they way brakes should. I am with the other guys regarding run-in of new pads being unnecessary. So long as your disks are clean and you've made sure they adequately stop the aircraft, it is really a non issue.:


"After a recent post in another thread from someone asking about brake return spring mods, I thought I should finally update this thread with my brake modifications. I assembled my Matco master pistons with the springs recommended in this thread and there was only a barely perceptible increase in return force. I subsequently took some measurements of the spring force in the matco's, ran a few calcs, and came up with an alternative spring solution that will double the current spring force. The original spring upgrade was a maximum of 6lb in compression. This one is 30lb. I've tested them out over the past week during my taxi tests and they feel perfect, with no risk of dragging brake, even if your toes are naturally leaning on the pedals, but not too much force that they become an incomberance when you want the brakes. I also modified the collar sleeve arrangement so that even at full brake compression, there is no way that the spring can rub on the ram. The sliding collar is also a self lubricating polymer, which should help with longevity. Just make sure when you assemble things that the collar is low enough to just clear the top mounting tab, but not so low that you restrict compression of the ram. My aircraft is an RV-7, and I would absolutely recommend the long-bolt (single axis) mod to the brake pedal to eliminate binding, in addition to the spring upgrade. You'll also see in the photos some another mod I did to make some standoffs. These are just aluminum split collars and a piece of tubing cut into thirds, then riveted in place with some long AN470AD4 rivets (the collars are countersunk on the inside and I back riveted it against an appropriate diameter bolt shank). I've also stacked two more short pieces of the cut tubing in between to give myself a little further distance.
Anyway the McMaster Carr parts list is as follows.
Spring 9657K435
Lower nylon sleeve bearing: 6294K441
washers (for seating the lower nylon sleeve bearing) 90295A470
Top nylon sleeve bearing: 6389K625
Set screw shaft collar: 9946K13

For the standoffs:
Two piece collar: 6436K136
Aluminum tubing: 9056K79"

Hope this helps.
Tom.
RV-7

free image hosting sites

upload image online and share link
 
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Perhaps worth a closer study

By your own description, " I also had to remove the piston from the pedal do get the valve open to allow the fluid to come up through the bottom." (when bleeding the brakes)

Normally, when a person takes their foot of the brake, the fluid should be free to flow back into the master cylinder and reservoir. There should only be pressure and checked flow when you press on the brakes. With feet off the pedals, there should be no pressure possible, other than the 0.4psi per foot of static head elevation from the reservoirs to the brake cylinders at the wheels.

Likewise when bleeding from the brake / wheel cylinder/ bleed valve back up through the system, logically the fluid should be free to flow to the master cylinder reservoirs with no need to take anything apart.

There have been various installations of master cylinders (upright and inverted) and different pedal arrangements, so there might be exceptions.

Perhaps check with the manufacturer of the master cylinders and their instructions ( And / or send them a photo ) to verify you have a normal installation, operation and bleeding procedures.- in their opinion

If the pedal pivots are binding or if the piston shafts are not fully extending it might prevent the master cylinders check valves from opening and preventing normal reverse flow. If that were the case it might explain the dragging brake and excessively hot disc.
 
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As an outside observer, I'd say that RVs really need a blast of air on their brakes. It would be easy to make a NACA vent on the centerline side of the pants where it wouldn't be visible. I understand that Cirrus's have had similar problems with brake fires. My Lancair brakes are out in the free air, yet I've measured disc temperatures of 700F after maximum braking.
 
Consider using braided SS over teflon all the way from the calipers to the master cylinders with the external return springs as posted a few weeks past. You can make your own or get TS flightlines to supply professional made lines.

Viton Orings in your caliper pistons and Royco 782 or Aeroshell fluid 31 will raise operating temp limits to over 400F.

Somebody recently posted about installing thermocouples on their brake calipers. Several EFIS and engine monitors have extra channels available that would allow you to monitor brake temps.

Thicker discs have more heat capacity and will not heat as rapidly. I am not sure what make your calipers and brakes are but Bruce hill posted his retro upgrade to thicker discs with his Matco brakes.
http://www.vansairforce.com/community/showthread.php?t=172644
Thicker discs provide more braking ability with less fade as they do not heat up as rapidly.

Installing external return springs on your master cylinders at the foot pedals should prevent almost all dragging brake issues.
From Tom in Australia:

I would suggest collar part number 6157K13 instead. It is the clamping type rather than set-screw so it wont damage the shaft.
 
As an outside observer, I'd say that RVs really need a blast of air on their brakes. It would be easy to make a NACA vent on the centerline side of the pants where it wouldn't be visible. I understand that Cirrus's have had similar problems with brake fires. My Lancair brakes are out in the free air, yet I've measured disc temperatures of 700F after maximum braking.

Hmmmm....10,000+ flying and I hear of a couple of hot brake events per year - not sure that they REALLY need any kind of modifications!
 
snip...
I can perhaps see NACA ducts as a solution to those with nose draggers who are doing long distance crosswind taxiing, but they're definitely not necessary for tailwheels with active steering. Performance of a NACA duct at these velocities would probably be questionable anyway, and to make them large enough to work at these speeds, they're going to be a good drag source at cruise.

If we look at the fires reported recently from brakes, they are predominantly from brake dragging (poor geometry/maintenance/no external springs) or people doing things like unnecessary "bedding in" the brakes under power. Both of these scenarios can be easily avoided.
I'm frankly more disturbed about this supposed requirement to have to disassemble my tailwheel fork every 25hrs to stop the pin sticking. Definitely need to engineer a solution for that one, but that's a whole other thread.

Tom.
RV-7
 
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Brakes

Having experienced hot brakes with the old plastic lines and doing a lot of formation long taxiing over the years best time and money spent was upgrading discs (good recent discussion here on VAF), using racing hoses from brakes to wing root (used sponsor here on VAF) and use Mobile 1 ATF fluid. Now some will say all this is overkill and I would never question Ironflight along with many others. For me it was peace of mind after it happening to me in a formation run up and seeing it happen to others in run up areas and short, hot landings. Find you a good tech counselor and you will find you will enjoy your RV even more as you repair your air machine.
 
Looks like the brake line is broken not burned through. Perhaps it sprung a leak causing the brake fluid to make its way to the hot rotor or the piston o-ring was leaking. I am only guessing but perhaps the builder thought the bleed nipple should be on the top because they perceived that as being the high point to avoid air similar to car brakes although we bleed from the bottom up not top down.

Brake fires on our RV's are pretty rare and my guess is most of it is pilot braking technique. No need to stand on the brakes when landing, let the plane roll out and slow naturally. If you need to stand on the brakes you probably landed at too high an airspeed. Do not drag the brakes which is a perfect way to overheat them. If you are in situations where brake heating could be an issue such as long taxiing or extended formation taxiing perhaps go with a higher wet boiling point fluid.

Going with SS brake lines is a luxury but really not needed. There are thousands of RV's perfectly happy without them.

I would inspect everything to see what is salvageable. The caliper can easily be cleaned and rebuilt with a Viton o-ring (do both sides at the same time). Gear leg will need to be either repaired or replaced, it may be easier to just replace it. New brake line, you can splice in a fitting up inside the gear leg fairing. If the wheel pant is salvageable just repair it which will probably be easier than replacing it.
 
I'm frankly more disturbed about this supposed requirement to have to disassemble my tailwheel fork every 25hrs to stop the pin sticking. Definitely need to engineer a solution for that one, but that's a whole other thread.

In my opinion, "supposed" is the proper statement. Apparently based on a few opinions?
Unless being operated in very harsh conditions and/or a huge # of hrs per year, there is strong evidence that for most people, a good cleaning and inspection at the yearly condition inspection is adequate. Surely not more often that every 100 hrs. Which for most people would not be more than twice per year.....
 
It's all good Scott, it's not your tailwheel. It's an aftermarket tail wheel that I installed because we have rabbits living on our airstrip and I was concerned that the stock tailwheel may drop down a rabbit hole and catch. After 60 flight hours I had the pin stick in the extended position, so I disassembled and found it full of grit and not much grease. I contacted the manufacturer to propose that I drilled/installed a grease zerk into the body of it so that I could always keep it pressurised with grease and expel any grit, and he said that I was better off doing 25hr disassembly/resulbrication. He said that this was a common procedure.
Seems a little crazy to me, so I contacted some other non RV tailwheel owners. None have these types of issues. One Kitfox owner I spoke with who operated in identical conditions has disassembled his tailwheel once in 1500hrs, but he as a grease zerk on it and always keeps it full of grease. He has the same locking pin setup to us, but a different bearing configuration.
As I said, it's worthy of another thread, I just hadn't got around to posting it yet. I'm just interested in the experience of others that are operating in similar conditions (grass/wet) and if someone has come up with a more ideal solution.
I'm sorry I didn't clarify that it was an aftermarket tailwheel earlier. I would say I have not experience with the Van's tailwheel assembly, however when I did my transition training with Mike Seager I positively sucked on the first day and had a tough time keeping things straight, especially on rollout. He told me the next day that he found the pin had stuck in the retracted position. I'm not sure if this is just a freak coincidence, as it was the first 3 hours I had ever spent in an RV, and here was the same issue. Mike operates off a similar stip to ours, but only 1 in 10 of our landings were at Veronia, the rest at sealed Scappoose. 95 percent of the landings I perform are on grass/dirt/gravel runways. I'm just looking for a solution to what will otherwise become one of the highest maintenance items on my entire aircraft. Perhaps Doug's going to chime in and tell me I should have built that RV-15 bush plane. I would, but I've decided to wait for the RV-16 electric version instead.

Tom.
RV-7
 
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I'm sorry I didn't clarify that it was an aftermarket tailwheel earlier. I would say I have not experience with the Van's tailwheel assembly, however when I did my transition training with Mike Seager I positively sucked on the first day and had a tough time keeping things straight, especially on rollout. He told me the next day that he found the pin had stuck in the retracted position.

Mike is a true gentleman and diplomatic instructor.....he was probably just trying to make you feel better...... :D
 
Excellent point. He did seem like a good bloke. I?m still not sold that we?ve got the ultimate configuration yet in terms of tailwheel maintenance in harsh conditions compared to our other taildragging counterparts. My quest for a solution will continue...
Tom.
 
Getting away from the thread drift, here's a list of actionable items for you.

It does look like the aluminum tubing cracked and the brake fluid caught on fire. Shimmying gear legs (the reason stiffeners were installed) and aluminum brake tubing is not a good combination. Call up Tom Swearengen and have him make you new brake hoses.
Make sure the masters don't stick, get rebuild kits for them to replace all the seals/pistons. Also install the above suggested return springs. Matco doesn't approve of those, because they score the shaft, but they work great, I have them on my plane.
Use Viton orings (McMaster Carr) in the calipers, and Royco 782 fluid. Those two bring up the heat tolerance of your brake system from around 250F to 400F+.
Definitely swap the fittings around on the calipers. The bleeder valve should be on the bottom. Refill with fluid from the bottom up (pressurized garden sprayer bottle works well). Put a fitting and clear hose in the top end of the system and keep pushing up the fluid until you see no air bubbles in the hose, while slightly tapping the brake pedals to release air bubbles. If you leave any air in the brake fluid it expands when hot and they brakes will drag.
New wheel pants are a pain to install, mostly the alignment is what takes a bunch of time, but you can fly the plane without them until you get them done.
 
Thanks!

She adores her plane so far and we?ve definitely been putting in a lot of work to get her all dolled up!

Someone will be along shortly to answer your questions, no doubt.

In the meantime, good to see that RV with the wood prop removed. It?s nice to see that the plane went to a home that can give it the TLC that it deserves.
 
Thank you all for the great information! I?ll be getting back to CA and getting under the plane next week. Will do the spring mod, and new lines.

It was mentioned that the leg will have to be replaced? Is this really necessary? My understanding is that a leg like that can withstand some heat, and the fiberglass that is charred is for damping shimmy and a little bit of charring shouldn?t effect the overall functionality really. Do people agree with this or am I off base?

Thanks again!

Cheers

Charles
 
The fire would have had be hot enough to completely vaporize the paint and fiberglass in order to be hot enough to affect the case hardening of the gear. So replacing it is a bit of a stretch in my opinion. New brake line, new o-ring in the brake caliper, flush the system of the old 5606 fluid (it is *rap, very low flash point, as your lady friend found out) and use Mercon synthetic automatic transmission fluid instead, which has a much higher flash point. New fiberglass components obviously.
 
The fire would have had be hot enough to completely vaporize the paint and fiberglass in order to be hot enough to affect the case hardening of the gear. So replacing it is a bit of a stretch in my opinion. New brake line, new o-ring in the brake caliper, flush the system of the old 5606 fluid (it is *rap, very low flash point, as your lady friend found out) and use Mercon synthetic automatic transmission fluid instead, which has a much higher flash point. New fiberglass components obviously.

And relocate the bleeders to the bottom for both sides. They are removable, although they could be tight.

Lots of discussion about cars/trucks etc, but the reverse flow (bleeding) for this airplane is perfect for it. The bubbles all go up hill and follow the direction of bleed flow. This is proven to work, therefore recommended.
 
I fly my plane pants-less, and I actually like the "look" better without the spats. Apparently I'm an exhibitionist.

It appears to me that the aluminum tubing burst, which enabled the brake fluid to spray onto the hot brake parts. The tubing could have cracked, been damaged during mx, and / or weakened by heat transferred by the fluid, finally giving up under pedal pumping. IDK.

I had the brake return spring mod installed for a time, but when I rebuilt my master cylinders, the new / "fresh" internal springs eliminated the need for the mod IMO.

Tom S. made up a pair of flexible lines for me that I have yet to retrofit. Mine are short extensions from the existing gear leg tube to the caliper. All the bending and contorting the aluminum tubing was subjected to make the connection really bothers me - it appears "flattened" in some areas, and I feel that it's an accident waiting to happen (although seems to be original to the o.g. build back in 2000.)
 
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