Sunday, December 1, 2013

Taking a Brake

The RV-12 hose/line replacement kits are flying (heh!) out of the collaborative effort between Steve from AircraftSpecialty and Tom from TS Flightlines - you probably still remember the replacement fuel lines I installed just a week or two ago, and here they are back again with a kit to replace the aluminum brake lines that run from a fitting up in the belly of the plane down to the brake calipers on the wheels.

The standard aluminum tube lines don't suffer from the onerous five year life limit that the fuel lines did. In fact, chances are that these lines would last the life of the airplane.  That said, there is a chance that they won't - through vibration and wear against the tie wraps that hold them, or from metal fatigue/work hardening near the fittings, they could eventually become brittle. That alone may or may not be a compelling enough reason to replace them, but for builders that want to install the optional wheel pants, not having to work around the loop in the aluminum line that gets added at fabrication time to provide a little "slack" in the line to move the caliper out of the way when working on a wheel (this loop itself could also get brittle or damaged with time) is a boon.

The replacement lines are far more robust and easy to work with because they are made of the same type of strong, flexible hose as the fuel lines, although these lines have an outer abrasion-resistant cover rather than the fire sleeve that encases the fuel lines. They also have the same high quality fittings as the fuel lines, and as with the fuel lines, they have been pressure tested to ensure their integrity.

You will note that one hose is longer than the other. The kit components that I received were, just as with the fuel lines, intended to test lines sized based on anecdotal measurements. Sometime back in the early days of the RV-12, conventional wisdom stated that the left side hose needed to be 34" long, while the right side should be an inch shorter. That seemed as good a place as any to start testing, although no one could definitively state the reasoning behind the need for differing lengths on what to all intents and purposes is a symmetrical airplane.

I decided to test two methods for replacing the lines.  While it seemed that it would theoretically be possible to do the replacement using only the access provided by the inspection ports located under each of the pass-through blocks up in the belly of the plane, it seemed that access to the fittings up in there would be much easier if I drilled out the rivets that hold in the panels that close the gap around the landing gear legs as they exit the sides of the fuselage.

I laid out the tools that I thought I would need.

  - Drill with #30 bit (Method #1 only)
  - LP4-3 rivets (Method #1 only)
  - Modified center punch (Method #1 only)
  - Phillips head screwdriver
  - 9/16" stubby open end wrench. Stubby wrenches are essential, and are thankfully dirt cheap at Harbor Freight, Home of Approximately Sized Wrenches. This one is approximately 9/16".
  - Side cutters.
  - Two AN caps sized to fit the fittings currently installed on the plane.  I think they're AN-3, but as with the gender of cats, I have no idea how to tell for sure.
  - Eight 12 - 14" long tie wraps (not shown)

For Method #1, which is the removal of the panel, the first step is patently obvious: drill out the rivets.

During the build, I found that the easiest way to drill out a blind rivet is to punch the mandrel out with a modified center punch. Harbor Freight has a $5 punch that is perfect for modification, primarily due to its most important quality: five bucks.

The grinding wheel meets yet another Harbor Freight tool.

Grinding the tip down allows it to fit into the hole of the rivet and make direct contact with the mandrel.

In many cases, a couple of "pops" with the center punch will push out the mandrel. Sometimes it won't.  It doesn't matter - the next step is to drill the head off of the rivet with a #30 bit. This is much, much easier than drilling out a solid rivet because the hole in the rivet will hold the bit where it needs to be. After just a few seconds of light drilling, the "cap" of the rivet will typically come right off, leaving either an open hole, or the remains of the mandrel if the first attempts with the punch didn't remove it.  With the cap gone, another punch at the remaining mandrel should pop it loose.

These are rivets with the caps gone but mandrels that resisted the punch still in place:

A couple more tries with the punch popped them loose.

Step Two of Method #1 is the same as Step One of Method #2: remove the inspection plate.

The fitting on the left is the one that will be removed, but DON'T DO IT YET.

There are a couple of things to do first that will serve to keep brake fluid from dripping all over your hands, paint, and hangar floor.

You're going to want to be able to move the brake line fitting away from the pass-through once you loosen the fitting in order to cap the fitting on the mount. This will keep brake fluid from draining out.

To allow the line to move enough to get the fitting out of the way, you have to first use the side cutters to cut away the tie wraps that hold it in place.

Now the fitting can be removed and the cap put in place.  This is far, far easier with the side panel removed, but it can be done even with the panel in place. With the panel in the way, it required quite a bit of digital dexterity (which I lack) to get the cap on, resulting in a lot more brake fluid coursing its way down my arm.  This also made the cap very slippery and visibility of the operation nearly nil.

So, it's your choice, but if I were doing it over again, I think I would opt to remove the panels on both sides.

The fitting at the caliper can also be removed.  Brake fluid will drain out of the line, so it is advisable to have a towel ready.

The line will be easy to remove from the airplane, although the loop might have to be opened a little to get it past the gear leg.

Remove and retain the plastic tubes used to protect the line from wear induced by the tie wraps.

Attach the new line to the caliper.

Slide the plastic tubes onto the new line.

This next step is an area where I stray from the by-the-book approach. Rather than introduce a line full of air into the system, which would then require a full purging of the brake system (which seems to invariably introduce a little air into the system), I pre-fill the new line with brake fluid before attaching the fitting at the pass-through block.

Do this at your own risk!! It has worked flawlessly for me, but it is a non-standard practice.

Just as the brake fluid starts coming out of the hose at the top fitting, I screw in finger tight onto the fitting. Again, this was far more difficult on the side that I was working exclusively through the inspection port.

With the fitting finger tight, I pumped just a little more brake fluid in, then tightened the fitting.

Repeat on the other side.

Tie wrap the brake line to the landing gear leg. I used four tie wraps rather than the two that were on there originally to better control the path of the line.

This is the right side, which was the shorter hose. This length looks just about right, although it would still create a potential problem for fitting wheel pants. I discussed that with Steve - he had heard the same from another builder. He is working on a solution that will provide a differently angled fitting that will remove the need for the semi-loop in the line.

He is also going to shorten the left side hose. The extra inch was clearly not needed or desired.

Here you can see how a different fitting would help keep the line right up against the gear leg, allowing it to exit the smallish opening in the side of a wheel fairing.

Because of the non-standard process of keeping the lines full of fluid while installing them, I pay particular attention to testing the brakes for firmness - they were perfectly fine.  I have flown the plane for roughly three hours since the installation of the lines and tested the relatively hard braking used in a short field landing with no symptoms of sponginess at all.


  - Products described were provided from the vendor at a substantial discount. 
  - Methods and processes described in this posting are experimental in nature and are not to be taken or construed as technical direction. Use your own judgment.

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