Re: Interesting article on Brake Fluid


Mike Dwyer <mdwyer@...>
 

That is an excellent article, thanks for posting it. I'm going to
change from the auto transmission oil I was using.
Mike Q-200


moehlencamp@... wrote:

There is some interesting reading on brake fluid and component compatability
on the Thorplist. This was post 7915 in case it gets stripped from this Email.

Bob MO

This was posted on the Biplane Hangar Mailing list recently:
Bret - Biplane Hangar http://www.gf24.de/biplane/
<<Gang,
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 ordeting 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.

Let's be careful out there.








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