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.


Disclaimers:

  - 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.

Fueling Around - Part Deux

I'm reasonably sure I've used the 'fueling around' pun before, simple because I can't imagine myself not using such an obvious groaner. So.... part deux.

To get this lengthy article started, I will give you a little background. It has to do with the fuel lines on the Rotax 912 engine that powers the RV-12. They are rubber lines wrapped in fire sleeve and Rotax "recommends" that they all be replaced every five years. Five years isn't as long as it sounds when measured in airplane years, and the cost and labor of this replacement is burdensome - the Rotax Rubber Replacement kit goes for around $1,200, although that includes more than just the fuel lines.

A somewhat onerous requirement like this invariably stirs the entrepreneurial spirit in people and sure enough, a collaborative effort between Steve from AircraftSpecialty and Tom from TS Flightlines has resulted in a snazzy alternative: they have created a fuel line replacement kit that addresses the life-limit issue of the Rotax rubber fuel lines by replacing them with state-of-the-art hoses. Here is how they are described on Steve's web site:

Our stainless braid conductive teflon hoses are among the highest quality hoses that exist. These hoses are impervious to all types of fuel without having to worry about hose integrity. In addition, our -4, -6 and -8 hoses, have a .030 thick layer of carbon black manufactured into the hose which provides an electrical path to the fittings on the end. Essentially, the conductive teflon hose is able to dissipate static electricity buildup that can exist when fuel flows through a hose. This is something that is found on high end certified aircraft hoses. We feel it is important, because it adds an extra margin of safety to your aircraft, and also prevents static discharges from eventually creating little pinholes in the hose. 

Please note that our -3 hoses are not conductive teflon, because there is not enough fluid flow through them to warrant the dissipation of static electricity. They are typically used as primer lines in aircraft or as brake lines on cars. For aircraft we recommend -4 hoses for brake lines to mate with the typical -4 hard lines that are used in the cabin. 

Finally, our hoses are covered on the outside with a stainless braid wire. The rated bursting pressure on these hoses is between 8,000 and 12,000 PSI depending on the diameter. Working pressure is between 2,000 and 3,000 PSI, and we test all hoses to their rated working pressure before they leave our shop to give you peace of mind. This is something that not all manufacturers do, as it is time consuming. However, we recognize that these hoses are the lifeblood of your aircraft, and so we check each and every one before it leaves.

He also has a couple of videos that describe the benefits provided from these hoses and the way they are manufactured:

All that aside, they are also far prettier than rubber hoses:

Of course, they don't look like that when they're installed on the airplane - because they are going to be located forward of the firewall and there is a lot of heat in that area, they will be covered in orange fire sleeve. Still, it's enough to just know they're gorgeous!

This is all pertinent to Schmetterling because the set of hoses custom designed for the RV-12 is relatively new. They have been flight tested on multiple airplanes and each installation has resulted in changes to the design. It would seem that the iterative design process has run its course and Steve is ready for someone to do a test installation of the hoses in their latest state of design. The idea is that the average builder should be able to do the installation him/herself. 

I was enlisted to prove the case.

I gave to confess that I was a little nervous about it - anything having to do with the engine and its accessories automatically triggers my 'CAUTION' mode - but after talking it over with Steve it seemed to be relative straightforward.

We shall see.

Because this is a product in development, albeit late-stage, there were no printed directions included with the hoses and fittings.   I should probably say a few things about the fittings, because they too are pretty darn nice. Again, in Steve's words:

Our fittings are another place that differentiate us from the competition. We use stainless steel fittings on our hoses instead of aluminum ones.

Stainless steel fittings have two distinct advantages over aluminum ones. First off, they are much stronger than the same size aluminum fitting.

Secondly, in the unlikely event of a fire, aluminum has a much lower melting point. The stainless fittings will protect against the fire much better. This will give you extra time to manage the situation without having a breached fitting that will dump extra fuel onto the fire.

The final item to be aware of is flare angle. Our fittings are all true 37 Degree flare fittings. This means that they are designed to fit perfectly with the AN fittings that are used in your aircraft. Some automotive hose manufacturers that do experimental aircraft hoses use a different flare angle. The truth is that the pressures that we run in these hoses are relatively low, so you might not see any leakage. However, we feel that it is important to have a product on the market that is specifically designed for aviation purposes.

They're nice looking too, and they have the additional benefit of being out in the open and easy to see.

I thought that the first thing to do should be to come up with a way to identify each part so Steve and I would have a common vocabulary, and to keep track of things while I was attaching them to the plane.

The fittings:

• A: fuel 'out' from the fuel pump
• B: fuel in to carburetor (B & C are identical)
• C: fuel in to carburetor
• D fuel 'in' to fuel pump
• E: fuel return line at firewall fitting
• F: fuel marshal - replaces funky block on top of engine

The fuel lines:

• 1: Gascolator to fuel 'in' on fuel pump
• 2: input to left carburetor (this is the lengthier of 2 and 3)
• 3: input to right carburetor
• 4: input to fuel pressure sensor
• 5: fuel return line (uses fitting E)
• 6: fuel 'out' from fuel pump

This is the 'before' picture:

After pondering the 'before' setup for a few minutes, I was able to position the fittings and hoses as I thought they would go onto the plane:

With a clear understanding of what went where, it became a matter of figuring out when and how.

I started with the assumption that the current lines still had fuel in them and that it might be a good idea to drain as much of it out in a controlled way as possible. 

So, Step 1: close the fuel line in the cockpit using the red fuel shutoff knob.

Step 2: remove the drain plug from the gascolator and let it drain into a bucket.

Step 3: remove the main fuel line from the gascolator.

Note that you are going to see a lot of old thread sealant on the AN fittings - using the Teflon sealant was a habit that I got into when doing the brake fittings - the Teflon gave me at least one more full turn without it feeling like I was going to break a fitting.  Once in the habit, I used it everywhere. This is NOT required, or perhaps even advisable.

Step 4: remove the main fuel line from the 'in' port of the fuel pump:

This proved difficult. The old fitting was metric (14mm), but that wasn't the problem. I have a cheapy set of metric wrenches from Harbor Freight, The Home of Easily Sacrificed Tools, but the wrench was too wide to get down onto the fitting. I would need a narrower wrench. Now, I do not recommend this as a common shop practice, but I solved that problem thusly:

Apparently 'Vanadium' is Latin for for 'really soft' because it didn't take too much effort at all to thin down the wrench to a point that would allow the removal of the fitting.

Step 4A: retain the copper washer from the old fitting. You will need it for the new fitting.

At this point, the main fuel line can be removed.

Step 5: Feed one end of Hose 1 down to the gascolator and attach it to the fitting:

With the large mail fuel line out of the way, it seemed a good time to remove the rest of the hoses. They all meet at the fussy junction box at the top of the engine.

Step 6: Remove the fuel transfer block at the top of the engine by removing the two bolts.

The old parts have no use in the new installation, so I stored them away.

Having removed the junction box, there are now hoses flopping around loose. Having started at the fuel pump, I headed back there to continue.

Step 7: remove the fuel 'out' line from the fuel pump.

Step 8: Install Fittings A and D to the fuel pump, using the copper washers retained from the old fittings:

The next line to remove ran from the junction box out to the carburetors. It is held in place with a pair of Adel clamps. These proved quite tricky to remove, but only because I was reluctant to remove the carburetors. It was possible on the right carb, but the left side was far more difficult. I ended up removing the carburetor on that side to get better access to the bolt head. The bolt is threaded into the intake manifold, but if you can get a wrench on the nut, you can remove the bolt out from under it.

Step 9: Remove the Adel clamps holding the carburetor fuel lines. Remove the carburetor(s) as required.

Step 10: Remove the carburetor fuel lines from the carburetors:

Access to the fuel line on the right side carburetor can be improved by loosening the oil tank clamps:

Step 10A: Ensure that the copper washers are removed; the new fittings have their own washers.

Step 11: Install Fittings 'B' and 'C':

I ran into a problem here. I couldn't get a wrench onto the fittings to tighten them down - there wasn't enough clearance around the flats of the fittings to allow either an open end or a socket wrench to get a grip. I called Steve and he suggested using a #4 sized AN cap by screwing it onto a fitting and using a wrench on the cap to drive the fitting in. I was concerned that loosening the cap would also remove the fitting, but he felt that the increased friction provided by the washer on the fitting would hold it in place.

He was right!

I was then able to attach Hose 3 to the fitting. Unfortunately, I did it WRONG!  You cannot allow the fitting to interfere with the operation of the engine choke arm!

The correct orientation is to point the end of the fitting downward. This explained why the hoses initially seemed to be longer than needed.

Step 12: Install Hose 2 (the lengthier of hoses 2 and 3) onto the left side carburetor. Ensure that the fitting does not interfere with the operation of the throttle and choke arms.

Step 13: Install Hose 3 (the shorter of hoses 2 and 3) onto the right side carburetor. Ensure that the fitting does not interfere with the operation of the throttle and choke arms.

Step 14: Install Hoses 1 and 6 onto the fuel pump:

Step 15: Install one end of Hose 4 onto the fuel pressure sensor.

Step 16: Attach the loose ends of the installed hoses to the appropriate locations on Fitting F:

I put off installing the fuel return line for last because I thought it would be very difficult to get at the fittings on the firewall shelf, but it turned out not to be all that bad. Every now and then, having arms as skinny as a 12 year old girl's comes in handy.  The other time is when I buy tight fitting muscle shirts: I get those cheap in the Boys 8 to 12 section at Walmart.

Step 17: Remove the fuel return like from the pass through fitting on the horizontal shelf of the firewall. Install Fitting 'E' - note that this is a very important step! Without the restrictor port in this fitting, fuel would flow unobstructed back to the fuel tank which would have an adverse impact on the fuel flow/pressure to the carburetors. 

Install the end of Hose 5 onto Fitting 'E':

Step 18: After ensuring that all fittings and hoses are fully attached and checked to make sure the gascolator drain has been replaced, open the fuel shutoff and turn on the Master switch. The electric fuel pump should be able to provide a fuel pressure indication. Visually inspect the fittings and hoses for leaks.

Step 19: Restrain the fuel lines using your preferred method:

I personally started by re-routing the main line to be below the metal tube that crosses between the carbs:

This allowed me to re-use the metal support block that had been orphaned by the removal of the old hoses. I initially tried to use a size 10 Adel clamp, but it was too tight. I didn't have a size 11 on hand, so I used a size 12. It's a loose fit, but I don't much care - it's purpose is really just to keep the hose from flopping around. That said, I'll likely replace it with an 11 at some point in the future.

I used a pair of vise grips and some safety wire to hold the clamp in place until I could get the hex bolt started back into the support block:

You can see that the size 12 clamp leaves plenty of slack in the hose, but still serves to keep it fixed in space:

I secured the right side carb line with a pair of size 8 Adel clamps, and AN3-4A bolt, and a lock nut.

Before final tightening, I snugged the Adel clamp combo right up next to the Tee fitting. This served to restrain pretty much everything all at once, but I also added a pair of tie wraps to protect the Tee fitting from rubbing on the support block underneath it:

The fuel return line seems to be half an inch to an inch too long, which is something I communicated back to Steve and Tom. The problem is that it gets awfully close to the edge of the antenna shelf. Steve tells me that they will shorten that hose by an inch from now on.

I used a couple more tie wraps to keep it from working against that edge:

The right side carb line also had the potential to rub against the rubber tube underneath it, so I used a tie wrap to fix them together. The orange tape underneath the tie wrap is the stuff I use to bundle wires. I used it here to protect the fire sleeve from any rubbing from the tie wrap.

I did the same thing with the left carb fuel line and the fuel pressure line:

And again and the fuel pressure line and the radiator hose:

Disclaimers:

  - 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.