What's the best brake fluid for an RV airplane?
by Bob Collins
(August 30, 2009) -- By the time you're well along on your RV project, the choices really come flying at you. What engine? What prop? What instruments go in the panel? What color? But some of the most important choices aren't very sexy or very obvious. Take brake fluid, for example. You're not supposed to use the smae stuff you put in your car. Van's recommends a different type. But reports of brake fires have been traced back to the fluid recommended, it has a low "flash point" -- the point of which it turns from fluid to flame.
If there's a leak or crack in your lines, the fluid dripping on hot brakes is pretty serious business. What are some of the options out there? Several recent threads have broken out on the various bulletin boards and some of the answers go beyond the fluid.
It started with Brian Meyette on the RV Yahoogroup this week (he also launched a similar thread on the RV List):
My RV-7A construction manual says I must not use automotive brake, but instead "use only the aircraft brake fluid recommended by the manufacturer". But they don't say what this fluid is. Does anyone know what fluids are recommended for use with the standard Van's Cleveland disk brakes?
Dan Baier wants to change what he's using:
They're probably suggesting 5606 (the milspec) hydraulic fluid. That's what
I've got in mine now. However, I since came across a writeup at
http://www.groveaircraft.com/fluid.html that makes me think that going forward
I'll purge the system and replace the 5606 with MIL-PRF 87257.
Dan has since settled on ROYCO© 782 Synthetic Fire Resistant "Super Clean" Hydraulic Fluid, MIL-PRF-83282D , which he found for $8.14 per quart from Skygeeks.com.
Which brings up the question of mixing two types of brake fluid.
Larry Bowen says the answer may not be brake fluid at all:
I have it on good authority (another RV'er) that Mobile 1 Synthetic
ATF works fine, and with the stock seals. I'm planning to use it in
the RV-7.
The concern about brake fluid is based on fire, and reports of the fluid catching fire because of hot brakes. Mickey's RV-8 Web site details one such fire and a different approach to solving it -- changing O-rings.
He quotes a post years ago by Charlie Kuss:
Back in January, I started a thread on the main RV
List regarding alternate brake fluids. I have
researched this and have decided to use automotive DOT
4 style fluid. My main objection to the 5056 mineral
oil fluid used in certified small aircraft is that it
BURNS! And it's flash point is a mere 250 degrees
Fahrenheit. A number of RVs have burned because of
brake fluid being sprayed on a hot brake rotor (failed
caliper O-ring or failed line). This recently happened
to RV-6A owner Ed Anderson. Ed only lost his tire,
brakes and wheel pant, because his son had the
presence of mind to snuff the fire, using some
antifreeze he had in his car.
At least 3 other RVs
have burned to the ground over the years because of
this.
Automotive brake fluids do not burn and have much
better performance (hot and cold) characteristics when
compared to MIL 5056. However, automotive brake fluids
are not compatible with the Buna N O-rings in our
brake parts. Auto brake fluids require the use of EPDM
(ethylene propylene) O-rings. I recently purchased the
needed O-rings to convert my master cylinders, Matco
parking brake vale and Cleveland brake calipers. To
obtain the highest quality O-rings, I had to purchase
in quantities of 100.
Ed Anderson of North Carolina is one of the RVers who had a brake fire:
Having experienced a ruptured brake line and ensuing brake fluid fire while using the 5606 fluid (Flash point around 240F), I strongly agree – use the Mi.-PRF-83232 – it’s flash point is around 440F. The nice part is - it’s fully compatible with the 5606 fluid so you don’t have to flush your lines or replace type of seal. Naturally the more 5606 left in the system the lower the flash point from that of pure 83232. So using this fluid requires no changes to the installation – and since it is compatible with 5606 – you could always add 5606 (available at any FB0) if you find yourself needing brake fluid while away from your gallon of 83232.
I would also suggest considering stainless steel, Teflon lined or other “real” brake lines. My aluminum tubing apparently fatigued and blew a chunk of the tubing wall out after an aborted take off (hot rotors). This could have been a result of me “working” hardening it while curving it around the landing rod, unnoticed damage or just material failure – but whatever the cause, you want to avoid it. But, the spraying of the low flash point 5606 on the hot rotor caused the real excitement. The resulting fire ball rose above the wing (as seen in video made by nephew), the brake peddle went to the floor and an excursion off the runway followed. Upon stopping, I popped the canopy and smelled burning resin - yes, the wheel pant was on fire and burning just below the full wing tank!! Fortunately, the fire was quickly put out and no damage to aircraft thanks to a small shallow ditch which arrested further progress.
Kevin Horton, who works as a test pilot for Transport Canada, is the E.F. Hutton of RV builders (if you don't know what that means, go ask an older person).
MIL-PRF-83282 is a replacement to MIL-H-5606 that was developed by the
military, with a flash point of 470 deg F, or higher. It is fully
compatible with all the seals. It is not that much more expensive,
but it is a little harder to find. If you are on the road, and need
to top up your brake system, you can add some MIL-H-5606 fluid to the
MIL-PRF-83282, but the resulting mixture has a slightly lower flash
point.
I found Royco 782, a MIL-PRF-83282 fluid, at a local supplier (Leavens
Brothers in Toronto) for $12 a quart. It should be cheaper in the
USA. This fluid is specified for some corporate aircraft, so you may
find it at a local FBO.
A similar thread broke out on VAF a few weeks ago. My takeaway is to also be careful using your rudder. Even a modest amount of pressure on the rudder is putting pressure on the master cylinder, a problem that Jaimie Painter solved:
If you shorten the rudder cable 'links' and mount the rudder bars a little more forward there's no problem. I have had no problems so far in 160 hours and my pads look like they can go another 160 or more. I also installed auxiliary return springs on the master cylinders which I feel helps with brake life.
I'm interested in collecting your thoughts and tips and adding them here. Please send them to me at bob@rvbuildershotline.com |
