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On Fire

The last time I was in the RV-6, it was on fire. *That was 12/26/2019 about seven hours and three beers ago.

I headed out west to the Lake Michigan coastal airport of Frankfort to fly the RV a couple weeks ago. *There was a small, dried puddle of something around the right wheel pant and the right brake pedal was mush. *The right line from the master was visibly white, and empty to the reservoir. *I removed the right pant and inspected the caliper and brakes in place. *Line and fitting were dry - the caliper had fresh 5606 drops dangling, the pads were wet and there was 5606 mud in the bottom of the wheel pant. *But no real culprit at this point. *I clipped the safety wire and plucked the caliper thinking the piston o-ring must be misbehaving.

Incidentally, the return from annual inspection was the last flight, and more than a month ago, where we had installed new tires, tubes, etc., and packed wheel bearings. *At the time, the left brake pads looked new, with the right showing slight wear, but definitely serviceable.

Back to the shop a week ago, an inspection of the cleaned brake assembly showed absolutely no signs of wear on the piston, nor the bore of the caliper. *The original o-ring had no visible defects, or stiffness, but it was likely 20 years old. *While I was at it with a new o-ring, just for good measure, I installed new brake pads at the shop then took the clean, fresh package out to install in today's 50+ degree balmy weather. *With the hangar door open, the bright sun lighted my workspace and warmed the recently-frigid wrenches and pliers to the bone. *The classical music station (keeping the mice out, because mice have no class) still played Holiday music; even the day after. *Bing, and Nat. *I was loving life.

We know that our phones will answer any question on the spot. *This would be a good time to take a micrometer to the brake disc, and compare to minimum thickness from Cleveland's specs. *The mic in the box was metric, but translated to .175-ish on a couple spot checks with min. spec for the 500x5 in the .160's. *Installed, the pad-to-disc clearances looked like a new installation.

I gas-ragged the 5606 mud (dirt and hydraulic fluid) from the belly of the right pant, pumped new brake fluid into the right bleeder, tested the right pedal until rock-firm while letting the volatile blue 100LL flash off before final installation of the pant. *Many brake cycles, no leaks, so on went the pant.

Now, back to the phone for a reminder of the Cleveland brake lining conditioning sequence... Oh yes, taxi 1500 feet with engine at 1700 rpm and brake(s) applied. *Stop. *Let cool 3 minutes, then test the brake-hold at a full power run-up. *If it holds, go fly. *If not, repeat process.

So that I did. *Sort of. *The frisky wind on the 15-33 runway was 200-240 at 12G22, so I thought maybe I'd just break-in the brake today instead of fly. *AWOS was showing an enticing downward trend of wind however, and I was being tempted to fly as I taxied in the stiff-ish left crosswind. *Dragging the right brake for the prescribed 1500 feet I couldn't hold 1700 rpm. *That's nearly take-off power in an RV-6 with a 160hp Lycoming 320. *Sub-1400 rpm *was getting me there just fine. *And largely because I was only breaking one brake in, (the right) as the left pads were fine, remember? *Everything was splendid. *The right brake was holding my taxi straight in the stiff left crosswind. *And braking was firm and effective. *I probably *ran an extra 500 feet to the run-up area with the break in procedure to generate glazing heat because of the cold.

When I finally slowed, then stopped at the runway end I could, not surprisingly, smell a little heat from the right brake side. *Then maybe a glimpse of smoke? *Or was that my imagination? *Nah, couldn't be that hot. *Then, at this stopped position the right brake pedal suddenly went limp. *#*!$$@. *Now what's going on? *And I'm 3000 feet down the runway from the barn. *Well, the now-right crosswind was a blessing having no right brake, I was able to idle very slowly, cautiously and carefully back to the hangar-ish. *All left turns, and all very slowly, and deliberately, because once the tailwheel unlocks, any asymmetrical braking situation, as in this case, makes this plane spin about aimlessly like a dazed housefly. *I was actually wondering about my insurance coverage as I straddled this delicate situation.

I did seem to get an occasional whiff of heat on the way back, but certainly no additional heat could be generated by the now-dead right brake.

I taxied to within 50 feet of the hangar before I ran out of directional wind benefit, and it's maneuvering luck. *I stopped, and now saw another puff of smoke. *Or not? *Could it be? Fuel off! mixture lean! electrical off! *key off! I hopped out and rounded the beak only to see the right wheel pant of my favorite airplane burning like yesterday's Christmas hardwood in great-nephew Kenny's home-heating fireplace. *I mean really burning, and right under the wing tank. *Yes, very momentarily, but there was again denial on my part. *Then an immediate *sprint to the hangar looking for something wet - yet not frozen. *Nothing in the wash bucket... Windex won't be adequate... WD40? *McGuiars won't ever put a shine on this again... the 5 gallon can of 100LL avgas sure ain't gonna help this situation!

"You idiot. *You have a fire extinguisher in here somewhere." And there it was., by the door, where it's supposed to be. *Dated, but there. *I grabbed the extinguisher, pulling the pin as I sprinted to the burning airplane. *You know, an extinguisher about 4" in diameter and 18" tall isn't quite adequate to put out a wheel pant, and now, tire-fire. *In retrospect, I wish I had choreographed this moment ahead of time to include time for a quick photo, or video, or selfie of this persistent, olympic torch. So a sprint back to the hangar for the bucket hoping the frost-faucet at the terminal building is working. Then a sprint back to the rekindled campfire with five gallons of water. *Boom! went the new $350.00 tire and tube combo with the five-gallon surgical douse of cold water. *And it wasn't out yet. *So, another sprint to the terminal building and another wet five, which got things under control. *Then one more, walking this time in disbelief of what just happened, and what nearly happened.

Well, with a little neighborly help, Richard, the Airport Manager, and another neighbor were able to help get the right gear on a dolly and back into my hangar before dark. *I still have a plane, but also a boatload of redundant, needless work.

The way that fire-retardant 5606 hydraulic fluid and fiberglass combo burned has me re-thinking the aluminum vs. composite construction world. *My wheel pant was very difficult to extinguish, and turned into a literal dishrag that I simply ripped from the mounts before dollying.

What a day. *Now, I need to think about *the root cause of this event. The quick answer may be that there are times "to go the extra mile" but riding the brakes on break-in, or ever, ain't one of 'em. *Funny, it could even happen in a fiberglass sailplane with no fuel aboard. *Some have disc brakes, and of course, that flame-retardant 5606. Dang, I always felt safer with no gas aboard. Does that even make sense?

Parachutes, Baby!
Happy New Year.

Miketitan
 
Wow, sorry to hear of your misfortune there. Glad you still have a plane in the end!
 
Royco 782

OMG!
Mike two things come to mind after reading your story , ( I also have a RV6 100 miles east of you ) I?ve read of this happening before with the 5606 and the risk associated having a leaking Caliper or ruptured brake line onto the hot brake disc drove me to going with a set of manufactured brake lines & using Royco?s synthetic fire resistant 782 Hydraulic fluid On my build 4 years ago,
So far after 200 hours in three years I?ve had no issues with leaks at the calipers or anywhere in my system so I?m going to endorse its use here and It?s now available at Aircraft Spruce!!!
 
I had a similar experience a few years ago. In my case I suspect it was due to damage (probably a knick and subsequent leak) to the aluminum brake line; I regularly land on a gravel strip. I replaced the aluminum brake lines with braided steel lines and use the 782 hydraulic fluid now. I had exactly the same experience with the brake pedal going limp, and then smoke from my left wheel pant.

Mark
 
This is not the first time I've read about a brake fire in a RV.

I believe this break in procedure. is dangerous (obviously)

Over the years I've always used the taxi fast and stop method for breaking in new pads. I think the drag the brake for 1500 feet is bad advice from the manufacturer.

Sorry for your misery but at least the plane is saved.

PS: By the way, where was your cockpit fire extinguisher?
PSS: This is a very good lesson that when one has a malfunction of this sort, STOP and inspect, Before going on.
 
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"Now, back to the phone for a reminder of the Cleveland brake lining conditioning sequence... Oh yes, taxi 1500 feet with engine at 1700 rpm and brake(s) applied. *Stop. *Let cool 3 minutes, then test the brake-hold at a full power run-up. *If it holds, go fly. *If not, repeat process."

This procedure works fine for a Piper Warrior because they were designed with brakes powerful enough to dissipate the heat. It takes 1500 rpm to taxi a Piper with the brakes dragging. It takes less than 1000 rpm to drag the brakes on an RV. The thin discs used on 2 seat RV's are too thin to drag for that long.

You will find when you dig into the VAF search box that many have done this same thing. You can install thicker discs as I did. My disc bell has not changed color and there is no rust on it...... so it looks like it is running a LOT cooler.

With new pads, just go fly it if you have solid pedals. Or break them in by doing three hard stops from 25 mph with power to idle.
 
Wow that is a great learning experience for me and those others who still have new yet to be used brakes. I?m glad the outcome isn?t any worse than it is. It does make me think that an on board fire extinguisher is a requirement though.

I had a similar but slightly different experience. I have a small Kubota tractor. It had been sitting for a while and I decided I better go trim a little area where some weeds had grown. I climbed aboard half expecting the battery to be dead but it cranked right up. Unknown to me a squirrel or field mouse had decided the engine area was a nice place to build a nest. It took about 15 or 20 minutes and I began to smell smoke. I had to run a get a fire extinguisher so I know that feeling of panic. I was lucky to get the fire put out. It did take some of the wiring with it. Like they say experience is from making mistakes. I?m very experienced BTW.

I was planning on using Royco?s 782. This seems like a cheap way to buy a little safety factory.

Thanks for sharing.
 
You can install thicker discs as I did. My disc bell has not changed color and there is no rust on it...... so it looks like it is running a LOT cooler.

I just installed new pads and the new thicker discs two days ago. Bedded them in with a couple of hard, higher speed runs. Initial impressions are little or no brake fade after hard braking.

I used the Rapco thick discs RA164-09900 which are less than half the price of the equivalent Cleveland discs and longer calliper bolts, AN4H-16A all available from Aircraft Spruce. I made 1/8 shims for each calliper to accommodate the thicker discs.

Fin
9A
 
Puck direction?

My first thought was did you put the puck back in correctly? When the brake puck is removed it must be reinstalled with the o-ring side of the puck first. What I mean by that is that the o-ring sits closer to one side, it is not centered. If it is installed the other way it can go out too far when the brakes are applied and allow the fluid to come out.
Pictures posted in this post:
http://www.vansairforce.com/community/showthread.php?t=146575&highlight=Brake
 
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Yikes!

Oh, Mikey,
So sorry for you... and your impending rehab work. Thanks for sharing, in your usual creative, literary style. Wish I were there to help you along.
Happy? New Year! :eek:
 
WindX, Garry, Michael, Bobby, Jereme, Mark, Warren, Carl, Snope, Ray, Fin...

Good to hear from you all, and thank you for the responses, and insights regarding yesterday's excitement. The brake lines are Aeroquip inside the fuse below the masters, and aluminum down the gear legs to the calipers with a twist around the lower leg for flexibility. Above the masters is poly to the reservoir.

I will be changing out fluid and updating the Buna nitrile rubber o-ring to Viton synthetic rated at 400F vs. the original at 250F. The fluid will be Royco 782. I am also considering the heavy disc option.

Now, regarding an on-board fire extinguisher... darn right, this plane won't fly again without one. However, had I relied on an on-board extinguisher at the 15 departure pad where I sat, the aircraft would have been just a fond memory. Recall, that I emptied a Kidde 3.5 pounder from the hangar quite accurately on the source. That tamed the monster long enough to have the pant collapse, split, and allow the tire to flare - requiring another (2) five gallon buckets of water to regain control. None of the water options were within 2500 feet of the current position, and of course, headed into the wind, the 5" engulfing flames would have been squarely under the tank, nitrile sump drain seal, and fuel vents while the fireworks display ensued. Not having an extinguisher saved the plane.

That, along with my typical dumb luck of in this case, get-back-itis. You see, the decision to conveniently head back (run for cover) vs. an engine shut down, and inspection at that location again saved the plane. The propwash, crosswind, taxi velocity, etc., must have kept the flames in check, and horizontal. An engine shut down and inspection with no adequate fire management equipment at the aforementioned location would have immobilized me in the worst possible location, and again resulted in a fireworks show. I do realize the urge to apply reactive logic, however. Where I did luckily and finally run out of maneuvering control couldn't have been more of a gift. Between facilities with a fire extinguisher, and with adequate water. The tail was upwind allowing 5" flames to be blown forward, and away from fuel sources while I acted really silly, really fast - for quite a while.

Friends, things happened fast. Nobody was hurt. My favorite plane's problem area has been cleaned with Super Clean, the purple stuff, which took the soot off nicely. The gear is off, and ready for inspection and replacements when the weather turns - warmer. Until then... silent flight cross country in Vero Beach, FL.

Aloha.

M1
 
Bobby,
Indeed, the ring groove on the puck is assymetrically placed. And thanks for that. It is not impossible... but I do know the assembly. I am human, we will see.
 
Good luck!

I hope you find the out what actually caused this. Glad you were able to save the plane and yourself. Keep us posted.
 
avoiding brake fires

Very long and helpful thread on this topic here.

http://www.vansairforce.com/community/showthread.php?t=16685

I went with the DOT 4/DOT 5.1 solution, which seemed to be the least likely to start a fire.

The only negative is that it will eat paint.

IMG_9139-768x1024.jpg
 
Far be it for me to criticize Van's design, but those brakes really need some air on them. My brakes are out in the breeze and even so I have measured peak temperatures of 700F on the discs after hard braking, and 400F for a few minutes afterwords. If that were my plane, I would add a NACA vent on the face of the wheel pants blowing on the brake discs. The OP never answered my question but the type of brakes lines can be a problem area. Many Experimentals used (and some fools still use) nylon brake lines at the brakes. When the brakes get hot, the nylon lines can soften and pull out of the fitting, thus spraying brake fluid on the hot discs (not to mention loss of directional control). The bulk of my brake lines are nylon, but I converted to SS-Teflon flex lines for the last 18" at the brakes. I used Russell motorcycle brake lines from Amazon.
 
Far be it for me to criticize Van's design, but those brakes really need some air on them. My brakes are out in the breeze and even so I have measured peak temperatures of 700F on the discs after hard braking, and 400F for a few minutes afterwords. If that were my plane, I would add a NACA vent on the face of the wheel pants blowing on the brake discs.

Let's not get carried away here.

The brakes are rated by the amount of kinetic energy they can absorb in the form of heat. Maintain energy <= rating, and they are just fine.

Most RV's have 40-78B/30-9 Clevelands. Kinetic energy rating is 117,500 ft-lbs. Discs are the energy storage mass; installing a 199-93 upgrade kit (thicker discs) pushes the kinetic energy rating up to 155,000 ft-lbs.

The Brake Kinetic Energy equation (from Part 23.735) is:

KE = (.0443*W*V^2)/2,

where:

KE is Kinetic Energy in foot pounds (ft. lbs.)

W is aircraft Weight in pounds (lbs.)

V is the aircraft's speed in knots (KIAS). Note that speed is squared.

2 = number of brakes

Assume an RV7A/8A at a gross weight of 1800 lbs aborting a takeoff from 50 knots (rated stall at gross):

KE = (.0443 x 1800 x 50^2)/2 = 99,675 ft-lbs

Re-run the calcs with more gross weight or higher initial speed at commencement of braking (velocity squared is a big deal), and it becomes obvious the rating can be exceeded. Dragging the brakes under power can do it too, as we've seen.

So don't do that.

Cooling air vents would return hot discs to ambient temperature more quickly. They would not improve braking power in a single stop, or prevent the incident suffered by the OP.

Would this have occurred with ATF instead of 5606?

Hard to say. It's believed to have a higher flash point, but we can't know how hot the brake disc got in this example. The real issue is the caliper o-ring. Any fluid is good if it can't get out of the caliper.
 
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Glad you and your bird survived.

In the canard world, we tend to use most of the available runway before braking. Energy dumped into the brakes increases at the square of the speed and is dissipated as heat. Let drag and rolling friction reduce the speed as much as possible before getting on the binders. Our composite gear legs tend to melt. So we also either wrap the leg/brake line with fiberfax or install a heat shield. My brake lines are behind the heat shields.

After an A&P spilled 5606 in the nose of my long ez while he was helping bleed my brakes, he showed me that rubbing alcohol removes 5606. Using 100LL on fiberglass may soften the epoxy depending on the type used. Even if using Safe-T-Poxy (which we use for fuel tanks and structural layups) I always use something other than LL.

Glad you're okay.
 
It sounds like a check of the master cylinder plunger return on the right side is in order along with the other unplanned restoration. Brake pads should wear relatively evenly, anything else means something else should be checked. If the pedal is sticking and fluid is not returning, then it is likely the brake (self) dragging added to the heat, may have been the last straw.

Good the "lesson" went no further. Catch me sometimes and I'll tell you about pressurizing a fuel line that had water frozen in it and the accidental ignition of an 18" flame shooting out of the (open) gas tank onto the garage wall. Funny how your mind can run a 1000 scenarios of the natural progression in a second. All's well that didn't burn down, or something like that . . . :eek:

So how do you stop the flame while preventing it from chasing back into the nearly empty gas tank??? All while the A/F in the tank is moving rapidly to the explosive regime? At a later time . . .:D
 
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Let's not get carried away here.
Who's carried away? I've read at least two threads here about brake fires on RVs and you want to dismiss it as pilot error? Whatever... On my Lancair, when I read about incidents related to the poor design - yes, there were several - I did something about it. The great thing about Experimental aircraft is that builders can do what thy think it prudent (peace sign).
 
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New brakes/break in procedure

1st, a brake pad break in procedure is not really needed on our RV machines if the system is up to spec on final assy. If you feel like such a procedure is warranted, do it without wheel pants in place. The fire will be much easier to put out.

I repeat: 1st, If you feel like such a break in procedure is warranted, do it without wheel pants in place.

2nd, if you are worried about the brake fluid catching fire because you want to disregard #1, use Dot 5 fluid (it will not burn) tho it will boil.

3rd Dot 5.1 is NOT Dot 5. Dot 5.1 can ignite. Do not mix these two.

Dot 5 cannot be used in (some of) the new brake systems with anti-skid, and Dot 5 cannot be mixed with any of the petroleum based fluids, including 5606.

Dot 5 will keep your O rings in good shape, as long as you don?t cook the system to max temps. Dot 5 will not hurt your paint if spilled or splattered.

To change to Dot 5, flush the empty system with Isopropol Alcohol and blow out with air before adding Dot 5 to the system. Best you also change the O rings at the same time.

If you have a brake problem ?on the road?, you can find Dot 5 fluid at any Wal-Mart, and the O rings at Home Depot (plumbing dept). That eliminates any searching for 5606 if you are stuck, tho Synth ATF will work fine (Wal-Mart again).

If I had a trike RV, I would dang sure use Dot 5.

Please go back and read #1.
 
Hmmm
Looking up flashpoints I find references:
Dot 5: >248 F, >252 F and one listed as 274 F
Royco 782: 445 F
 
Who's carried away? I've read at least two threads here about brake fires on RVs and you want to dismiss it as pilot error? Whatever... On my Lancair, when I read about incidents related to the poor design - yes, there were several - I did something about it. The great thing about Experimental aircraft is that builders can do what thy think it prudent (peace sign).

I think the point was more along the lines of why put cooling fins on wheel pants when all of the heat is generated while stopping. Which kind of makes sense, to me at least.
 
Wow. What a story. I can't recall ever been so engrossed by a VAF post.

I know everyone is pointing to aluminum brake lines but I can't figure out why if there was a puddle under the brakes before you replaced everything, there wouldn't have been a soft pedal AFTER you replaced everything when you were testing it. Clearly something was wrong beforehand.

Please keep us updated when you find the cause.

And thanks for the reminder to buy a larger fire extinguisher.
 
Who's carried away? I've read at least two threads here about brake fires on RVs and you want to dismiss it as pilot error?

I dismiss the notion that cooling air scoops on the wheelpants would be useful. They would not improve braking power or safety margin during a typical single stop landing, nor prevent fire if dragging the brakes during taxi.

The reasons are fundamental. There is no significant dynamic pressure (a function of velocity squared) available at the relatively low RV braking speeds, and effectively none at taxi speed. As a result, mass flow would be limited. Even with mass flow, the little aircraft rotors don't incorporate any features to increase heat transfer by conduction...no fins or channels, and nothing to promote boundary layer turbulence. Laminar flow is ineffective for heat transfer.

Flowing air through the wheelpants would create aerodynamic drag in flight, when speed is high...and the brakes are cold.

The great thing about Experimental aircraft is that builders can do what thy think it prudent (peace sign).

Sure. My own brakes incorporate polyalphaolefin-based fluid (the MIL-H-83282 Royco 782 noted earlier) to increase flash point, and viton o-rings to keep the fluid in the calipers at temperatures which turn standard nitrile o-rings to rocks. The o-ring change is primary, as again, most fluids are safe if retained in the caliper. The fluid change makes it less likely to ignite if it does get out.

I could probably still induce a fire if I dragged the brakes far enough under power. Even viton has limits.

Speaking of limits, the only downside to common viton o-rings is a reduction in sealing at low temperatures, one reason Cleveland doesn't make them standard. That said, there are specific compounds available with ratings in the -40F/-40C range, worth investigating if operating in the Arctic.
 
I dismiss the notion that cooling air scoops on the wheelpants would be useful. They would not improve braking power or safety margin during a typical single stop landing, nor prevent fire if dragging the brakes during taxi.
Any you have the engineering data to back up that assertion?

A member the Lancair community (aerospace engineer) has actually instrumented his brakes and recorded the hard data on brake temperatures during taxi, landing, and rollout. I have his graphs here on my computer. So when I tell you that peak brake disc temperatures are over 700F on landing and 400F on rollout, I have data to back that up. Someone in your community should instrument his brakes and you could go from there.

Oh, and I've been running the Royco 782 for many years.
 
At the local airport, a practical joker put a few drops of brake fluid on the hangar floor under someone else's aircraft brake.
 
Wow, I'm glad you are OK and your plane can be repaired! That was a great story, thank you for sharing it.
 
....So when I tell you that peak brake disc temperatures are over 700F on landing and 400F on rollout, I have data to back that up.

I believe you John. I'm sure no one disputes that the disc gets quite hot.

The proven way to reduce maximum disc temperature for any given braking situation is to increase disc mass...see the previous kinetic energy examples.

At odds here is the rate of heat dissipation, not maximum temperature.
 
I'm going to contact Ferrari and tell them they're doing it wrong:

main-qimg-1e83d2eb166a0240bac892bd0435e314-c

Apples to oranges....vastly different operating environment (and brakes!).

If somebody wants to add draggy ducts to their RV wheel pants that is fine with me. I won't be putting any on my 20 year-old RV-6, however. :)
 
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Any you have the engineering data to back up that assertion?

A member the Lancair community (aerospace engineer) has actually instrumented his brakes and recorded the hard data on brake temperatures during taxi, landing, and rollout. I have his graphs here on my computer. So when I tell you that peak brake disc temperatures are over 700F on landing and 400F on rollout, I have data to back that up. Someone in your community should instrument his brakes and you could go from there.

Please, proceed to proceed. You have an opportunity to educate us all.
 
Fire.

Flow thru ventilation would be fine if there was airflow on the ground, but there isn?t much available for brake cooling. Probably not needed if flight. Louvers on top of the pants might do some good to vent off heat on the ground.


Don B
 
Louvers on top of the pants might do some good to vent off heat on the ground.
That (or a few holes) sound like a good idea to me. You would get some natural convection even it the plane were standing still. I got out my Heat Transfer book from engineering school and tried to calculate how much air flow one would need to keep the brakes cool. Reynolds Number, Nusselt Number??? Sorry, that stuff is just beyond me in my old age. (It was probably beyond me in my youth, too :confused: ) Anyway, I'm tired of discussing this; I think I'll move on...
 
Umm...

You may want to look up Convective Cooling to get some idea of how heat transfer occurs without forced air...it appears, from your post, that you are not familiar with it...
 
Your brakes will heat up during your landing roll to their maximum temp, so you're probably only concerned about the last half of that roll where the highest temps will be. That's when you're moving from about 40mph down to about 10mph. At those speeds, you won't be getting enough air through any ducts that would put out a fire... You'll get enough air to stoke one though, just like blowing on the embers of a campfire.

As for cooling, i'd expect the effect to be minimal. The temperature is maximizing as your ability to blow air into the space is diminishing rapidly (as you slow down). You'll be hottest just before you stop, and will have almost no airflow.

Keep in mind your wheel rotating creates airflow up into the rear of your wheelpant, that will swirl around and exit elsewhere. That alone would create more airflow than a dedicated duct. In fact, a duct may reduce the cooling provided now, by changing the pressure distribution in the wheelpant and reducing the effectivity of the air drawn in at the rear of the wheel.

Clearly, instrumented testing is needed.
 
You may want to look up Convective Cooling to get some idea of how heat transfer occurs without forced air...it appears, from your post, that you are not familiar with it...

That was.... kinda ... the point.

Yeah, nevermind... :rolleyes:
 
Since it seems to be a slow day to catch this much attention, here is a crappy vid on the 777 brake test. I have the 1 hour original recorded from a TV show, but this has the essentials of the validation test. Full load, rejected takeoff, 5 minute wait at the end, and pass criteria of having the ability to jack up the plane repair and return to service with only gear damage. Expensive test, required by the airline!

https://www.youtube.com/watch?v=f4LFErD-yls

For our RV technology level and specifics of mass, velocity and braking required, DanH nailed it, disc mass is a primary solution parameter. I imagine if we had glass wings, the tolerance for a wheel fire would be a lot less.
 
The last time I was in the RV-6, it was on fire. *That was 12/26/2019 about seven hours and three beers ago.

I headed out west to the Lake Michigan coastal airport of Frankfort to fly the RV a couple weeks ago. *There was a small, dried puddle of something around the right wheel pant and the right brake pedal was mush. *The right line from the master was visibly white, and empty to the reservoir. *I removed the right pant and inspected the caliper and brakes in place. *Line and fitting were dry - the caliper had fresh 5606 drops dangling, the pads were wet and there was 5606 mud in the bottom of the wheel pant. *But no real culprit at this point. *I clipped the safety wire and plucked the caliper thinking the piston o-ring must be misbehaving.

Incidentally, the return from annual inspection was the last flight, and more than a month ago, where we had installed new tires, tubes, etc., and packed wheel bearings. *At the time, the left brake pads looked new, with the right showing slight wear, but definitely serviceable.

Back to the shop a week ago, an inspection of the cleaned brake assembly showed absolutely no signs of wear on the piston, nor the bore of the caliper. *The original o-ring had no visible defects, or stiffness, but it was likely 20 years old. *While I was at it with a new o-ring, just for good measure, I installed new brake pads at the shop then took the clean, fresh package out to install in today's 50+ degree balmy weather. *With the hangar door open, the bright sun lighted my workspace and warmed the recently-frigid wrenches and pliers to the bone. *The classical music station (keeping the mice out, because mice have no class) still played Holiday music; even the day after. *Bing, and Nat. *I was loving life.

We know that our phones will answer any question on the spot. *This would be a good time to take a micrometer to the brake disc, and compare to minimum thickness from Cleveland's specs. *The mic in the box was metric, but translated to .175-ish on a couple spot checks with min. spec for the 500x5 in the .160's. *Installed, the pad-to-disc clearances looked like a new installation.

I gas-ragged the 5606 mud (dirt and hydraulic fluid) from the belly of the right pant, pumped new brake fluid into the right bleeder, tested the right pedal until rock-firm while letting the volatile blue 100LL flash off before final installation of the pant. *Many brake cycles, no leaks, so on went the pant.

Now, back to the phone for a reminder of the Cleveland brake lining conditioning sequence... Oh yes, taxi 1500 feet with engine at 1700 rpm and brake(s) applied. *Stop. *Let cool 3 minutes, then test the brake-hold at a full power run-up. *If it holds, go fly. *If not, repeat process.

So that I did. *Sort of. *The frisky wind on the 15-33 runway was 200-240 at 12G22, so I thought maybe I'd just break-in the brake today instead of fly. *AWOS was showing an enticing downward trend of wind however, and I was being tempted to fly as I taxied in the stiff-ish left crosswind. *Dragging the right brake for the prescribed 1500 feet I couldn't hold 1700 rpm. *That's nearly take-off power in an RV-6 with a 160hp Lycoming 320. *Sub-1400 rpm *was getting me there just fine. *And largely because I was only breaking one brake in, (the right) as the left pads were fine, remember? *Everything was splendid. *The right brake was holding my taxi straight in the stiff left crosswind. *And braking was firm and effective. *I probably *ran an extra 500 feet to the run-up area with the break in procedure to generate glazing heat because of the cold.

When I finally slowed, then stopped at the runway end I could, not surprisingly, smell a little heat from the right brake side. *Then maybe a glimpse of smoke? *Or was that my imagination? *Nah, couldn't be that hot. *Then, at this stopped position the right brake pedal suddenly went limp. *#*!$$@. *Now what's going on? *And I'm 3000 feet down the runway from the barn. *Well, the now-right crosswind was a blessing having no right brake, I was able to idle very slowly, cautiously and carefully back to the hangar-ish. *All left turns, and all very slowly, and deliberately, because once the tailwheel unlocks, any asymmetrical braking situation, as in this case, makes this plane spin about aimlessly like a dazed housefly. *I was actually wondering about my insurance coverage as I straddled this delicate situation.

I did seem to get an occasional whiff of heat on the way back, but certainly no additional heat could be generated by the now-dead right brake.

I taxied to within 50 feet of the hangar before I ran out of directional wind benefit, and it's maneuvering luck. *I stopped, and now saw another puff of smoke. *Or not? *Could it be? Fuel off! mixture lean! electrical off! *key off! I hopped out and rounded the beak only to see the right wheel pant of my favorite airplane burning like yesterday's Christmas hardwood in great-nephew Kenny's home-heating fireplace. *I mean really burning, and right under the wing tank. *Yes, very momentarily, but there was again denial on my part. *Then an immediate *sprint to the hangar looking for something wet - yet not frozen. *Nothing in the wash bucket... Windex won't be adequate... WD40? *McGuiars won't ever put a shine on this again... the 5 gallon can of 100LL avgas sure ain't gonna help this situation!

"You idiot. *You have a fire extinguisher in here somewhere." And there it was., by the door, where it's supposed to be. *Dated, but there. *I grabbed the extinguisher, pulling the pin as I sprinted to the burning airplane. *You know, an extinguisher about 4" in diameter and 18" tall isn't quite adequate to put out a wheel pant, and now, tire-fire. *In retrospect, I wish I had choreographed this moment ahead of time to include time for a quick photo, or video, or selfie of this persistent, olympic torch. So a sprint back to the hangar for the bucket hoping the frost-faucet at the terminal building is working. Then a sprint back to the rekindled campfire with five gallons of water. *Boom! went the new $350.00 tire and tube combo with the five-gallon surgical douse of cold water. *And it wasn't out yet. *So, another sprint to the terminal building and another wet five, which got things under control. *Then one more, walking this time in disbelief of what just happened, and what nearly happened.

Well, with a little neighborly help, Richard, the Airport Manager, and another neighbor were able to help get the right gear on a dolly and back into my hangar before dark. *I still have a plane, but also a boatload of redundant, needless work.

The way that fire-retardant 5606 hydraulic fluid and fiberglass combo burned has me re-thinking the aluminum vs. composite construction world. *My wheel pant was very difficult to extinguish, and turned into a literal dishrag that I simply ripped from the mounts before dollying.

What a day. *Now, I need to think about *the root cause of this event. The quick answer may be that there are times "to go the extra mile" but riding the brakes on break-in, or ever, ain't one of 'em. *Funny, it could even happen in a fiberglass sailplane with no fuel aboard. *Some have disc brakes, and of course, that flame-retardant 5606. Dang, I always felt safer with no gas aboard. Does that even make sense?

Parachutes, Baby!
Happy New Year.

Miketitan

Check the extinguisher that you used. ABC, the most common powder type are very corrosive to aluminum and electrical connections.
 
Your brakes will heat up during your landing roll to their maximum temp, so you're probably only concerned about the last half of that roll where the highest temps will be. That's when you're moving from about 40mph down to about 10mph. At those speeds, you won't be getting enough air through any ducts that would put out a fire... You'll get enough air to stoke one though, just like blowing on the embers of a campfire.

As for cooling, i'd expect the effect to be minimal. The temperature is maximizing as your ability to blow air into the space is diminishing rapidly (as you slow down). You'll be hottest just before you stop, and will have almost no airflow.

Clearly, instrumented testing is needed.

Maximum brake fluid temperature doesn't occur on rollout, it occurs after you stop completely. This is why the airlines require a 5 minute sit after the emergency stop test. The small area under the pad will heat quickly, it takes time for that heat to spread through the rotor and get to anything flammable. Mild steel can be run at over 1000 degrees before losing a significant amount of strength. It will melt and deform long before it spontaneously ignites. Things that will ignite are wheel bearing grease and brake fluid. It takes a while for the heat under the pad to make its way to the axle and wheel grease. If your pads retract correctly and dont drag it takes longer to reach the brake fluid. Most caliper pistons are stainless to minimize heat transfer through the Piston, and if the pad isnt touching the rotor while taxiing, the only transfer is radiative and minimal convective, no conductive.

Flashpoints of fluids only matter if there is already an open flame to ignite the vapor. When looking at fluid specs, autoignition temp and boiling temp are the primary concerns. At the boiling point you'll lose the pedal and braking ability. It's difficult to get beyond the boiling point once you can't brake anymore. This happens during sustained operation of the brakes above the boiling point but below the ignition temp. If you do get the rotor over the autoignition point, you then have to have a leak develope that hits the rotor and raises the fluid temp over the autoignition temp. It does happen, but if the system is sealed correctly, the chances are low. What normally happens is you exceed the maximum energy of the rotor, then while sitting still the heat makes its way to the o ring, which eventually breaks down and leaks onto the overheated rotor and ignites. If the rotor is large enough you don't exceed the maximum energy, you have very little chance of a brake fire.

Brake cooling ducts only help with repeated brake applications, that's why Ferrari and other racecars run them. There is a steady state point on a car that the rotor operates at once it's fully heated after multiple laps. The amount of airflow changes the steady state operating temperature. It does very little to lower the peak temp seen during an emergency stop and sit scenario. Cooling flow is also ineffective if blown over the rotor in the same plane of the rotor. The airflow really has to hit the rotor at 90 degrees to have a major impact to cooling.

Main takeaway: if you want a lower chance of a fire, put in bigger rotors, higher temp seals and a brake fluid with the highest auto ignition to you can. If you want to use your airplane brakes for repeated applications with no cool down period, add some cooling ducts.

Source: designed and built the brakes for the SAE racecar in college. Did landing gear and brake testing as a flight test engineer in the Air Force.
 
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Brake pads

I have never ?broke in? brake pads?
Never recommended it and never had
a issues or gotten negative feedback
from any of my customers over telling
them to just operate it normally.
 
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