Re: Brake Issue??


Larry Hamm <Patlar@...>
 

Lynn,

I've been trying to spread the word, but yours is the first comment to pierce the deafening silence.

I've got a quart sitting on the shelf ready to go, but I don't recall where I bought it. If I can find the source, I'll let you know.

Larry Hamm

LJFrench wrote:

My thanks as well to this good discussion. I was not aware of Mil-H-83282. I have always been using the 5606, but after this discussion am interested in switching. A gallon would last me a life time though - considering I have probably only used a couple of cups of this stuff to date.
L. French
----- Original Message -----
From: "Larry Hamm" <Patlar@...>
To: <Q-LIST@...>; <jon@...>
Sent: Monday, October 29, 2007 1:15 AM
Subject: Re: [Q-LIST] Brake Issue??

Jon,

Here's a bit more feedback. Maybe it will help!

This was posted on the Biplane Hangar Mailing list recently:

Bret - Biplane Hangar http://www.gf24.de/biplane/

Had a brake fire on an RV-8A last weekend. Tidy combination of
operator error and design issues, much of which is specific to the 8A
and/or castering nosewheel, steer-with-the-brakes airplanes in general.
However, research did turn up a few items perhaps everyone should know.

The fire started after an overheated caliper leaked fluid on a hot
disk. The fluid flashed and lit the resin in the fiberglass wheel pant,
as well as the tire sidewall. The brake worked fine, with only slightly
higher pedal pressure required even when on fire.
I've posted a photo to the vault (yep, a bystander had a digital
camera). In the photo, I'm holding pedal pressure while shutting down
for the fire crew. Note the fire on the ground under the pant, believed
to be fluid and dripping resin. I don't recall any additional pedal
travel.

When something like this happens I get curious. Why did the seal
leak at some temperature well below a failure temperature for the rest
of the brake? And why did the fluid catch fire?

Fast forward: It turns out the Cleveland piston seal for the
little 30-9 caliper is an ordinary MS28775-218 nitrile o-ring.
Nitrile's temperature rating is - 65F to +275 F. We found the seal to
be brittle and flaking when we dismantled the caliper. A caliper seal
with a 275 F temp limit is below automotive standards, but that's
another story.

As for fluid, Cleveland's tech manual specifies either Mil-H-5606
or Mil-H-83282 as acceptable. Both are listed in AC-43 and the A&P
texts. Turns out that Mil-H-83282 was created because the military was
tired of setting it's airplanes on fire. Mil-H-5606 is the standard red
hydraulic fluid sold by Spruce, Wicks, Chief, etc. It is a petroleum
base, and turns out to have a very low flash point. The Mil-H-83282 is
also red, and compatible with 5606 fluid as well as seals created for
5606. However, it is a synthetic, with much higher flash and burn
points, and is self-extinguishing when removed from the
ignition source.

You can download complete specs for Aeroshell Fluid 41
(Mil-H-5606) and Aeroshell Fluid 31 (Mil-H-83282) at:

http://193.113.209.166/aeroshell/aeroshellhydraulicfluids.pdf

Note the flash points of the two fluids. Aeroshell 41 is 104 C,
which is only 219 F. Aeroshell 31 is 237 C, or 458 F.

A flash point of 219 F means that when a Cleveland caliper seal
fails at something above 275, the fluid is already hot enough to light
when it hits a hot disk and vaporizes inside the pant. Makes for an
interesting combination.

Live and learn. I always assumed standard "mil-spec red brake
fluid" was something special, and I doubt I was alone in this
assumption. It's not.
It's just another one of those "always done it that way" things
prevalent with light airplanes. Note that the Shell literature declines
to even refer to it as brake fluid.

Spruce, etc, doesn't sell Mil-H-83282 fluid, but they should. I've
already ordered a gallon of Fluid 31 from the local Shell distributor.
Since the old and new fluids are compatible, switching is as easy as
draining the old, flush with new, refill, and bleed.

Jon-you don't seem to be the kind of guy that does things "just because
they've always been done that way".

Most Sincerely,
Larry Hamm


Jon Finley wrote:
Wow - lots of feedback! Thanks everyone.

I spent the day completely disassembling and cleaning my system. I then put
everything back together with aviation hydraulic fluid (MIL-H-5606). If
problems persist, I'll do the whole rebuild kit/new seals thing.

FYI: I reported that the DOT 3 that I used was red. I was wrong, I checked
today and it is clear.

Jon

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