Yes! – We Have Brakes!

Now that the eagle’s propulsion has been squared away, it’s time to work on the claws when returning to terra firma. The brake lines have been installed for quite some time, but the system has not had brake fluid installed yet. In general, aircraft do not use the typical DOT 3 or 4 brake fluids common in automobiles, instead, they use various forms of specialized hydraulic fluid. At DOG Aviation the decision has been made to use Amsoil’s full synthetic ATF fluid for the brakes … it has a very high flash point of 453 degrees F, which is a good thing and actually higher than a few of the recommended aviation hydraulic brake fluids. In a pinch, when on a trip, Moble 1 ATF can be added and it is available at any automotive store on a Sunday afternoon … aviation brake fluid, not so easy to procure on a Sunday afternoon. By no means is this a recommendation to use a non-aviation brake fluid … it is just a decision made for the DOG Aviation RV-12 based on a little research. If the decision turns out to a bad one, I’ll return from the future and edit this post accordingly … but that is not very likely to happen.


Purging the brakes on any RV airplane can be problematic because of the crossover brake lines that connect the four master cylinders (two pilot and two co-pilot) together for each of the wheel brakes. The crossover lines tend to be high spots and are prone to capturing air which can be difficult to remove by just pressing on the brake pedals to bleed them.


The ultimate method is to use a hand vacuum pump connected to the fluid reservoir on the firewall and pull fluid in from the wheel cylinders or a pressure pump to push fluid… but they are pricey. The second best way is to push fluid up from the wheel cylinders using a hand pump. I elected to try using the hand oiler approach and had acceptable results once I got the hose attached to the oiler to stop leaking. (Suppose a small pump-up garden sprayer would work even better). I was thinking of making a small pressure vessel out of a quart Mason jar if the oiler did not work. When using the hand oiler, care must be taken to not pump more than about half the fluid out of the container …. this allows for the top to be unscrewed and the pump moved aside so the tank can be topped off again without lifting the pump out of the fluid which will induce air into the pump.

Pumping fluid into the brake bleeder valve using a large hand pump oiler. It would have been nice if the oiler squirted a larger volume of fluid per pump, but it did work OK and got the job done.


At the brake fluid reservoir on the firewall a ¼" threaded fitting was connected to a vinyl hose to collect the excess fluid. I would have preferred using a barbed fitting but my local big box store did not have any with ¼" pipe threads so a hose was slipped over the threads, which of course leaked … but that is not a big deal at the reservoir end because the hose is only for collecting the fluid.

At the brake fluid reservoir on the firewall a ¼" fitting was screwed into the reservoir and connected to a vinyl hose to collect the excess fluid. The use of a ¼" NPT to barbed fitting would be preferable here as opposed to the ¼" NPT to ¼" NPT fitting that was used.


While performing the purge, the bleed valve on the caliper can be left open while vigorously pumping the oiler ... however, a hand should be kept on the vinyl tubing to keep it  from popping off the fitting while pumping and making a big mess (ask me how I know).

Carburetor Choke Controls Installed – Ready for Engine Start

Yesterday, did make it to the hangar for a short work session to finish up installing the remaining control cables on the carburetors which consisted of the two choke cables. Installing the choke cables is a snap … the control cables are routed to the carburetors and inserted into a curved cable mount tube attached to the carburetors. As with the throttle cables, the longer of the two choke cables is installed on the right carburetor. The attachment of the choke cables to the carburetor is much simpler than the throttle cables, in that, the cable slips through a CT-00100 wire swivel with set screw.

As can be seen in the photo of the drawings, the choke cable is attached to the carburetors using a CT-00100 wire swivel with set screw.


The choke cable instillation is simplistic … with the choke control lever on the instrument panel positioned all the way in (choke off), route the choke cable through the curved cable mount tube, insert the CT-00100 wire swivel into the choke lever on the carburetor, slip the cable through the tiny hole in the wire swivel and lock down the cable with an Allan set screw.

Routing the left carburetor's choke cable through the curved cable mount tube attached to the Rotax 912's carburetor.

After inserting the choke cable in the wire swivel, the slack was pulled out of the cable while the Allan set screw was snugged up. Final tightening of the set screw was done while holding the wire swivel with a wrench to prevent the cable from twisting.

Holding the wire swivel with a wrench while using an Allan wrench to tighten the set screw onto the choke cable.

Completed throttle and choke cable instillation on the right carburetor ... the excess ends of the control cables still need to be cut shorter.


Basically, at this point the RV-12 is complete construction wise. There are a few items left to do but for the most part it is just attaching things that have already been fitted such as the rear window, bolting on the canopy, bolting on the steps, screwing the ELT antenna in place, etc.


There are a few items left to figure out such as mounting the new voltage regulator (may just install the original Ducati on the firewall shelf for the time being), installing the parking brake and placing foil at strategic locations on the inside of the lower cowl. Oh, can’t forget about putting coolant in the engine, bleeding the brakes, not to mention performing the fuel flow test and fuel level sender calibration on the SkyView. So I’ll still be busy the next few days.

McFarlane Throttle Cables Attached To Carburetors

Yesterday, was a very late afternoon start and became mostly a cleanup day of sorts egged on by the fact I could not locate the throttle springs or hardware that came with the McFarlane throttle in the engine crate. After about a half hour into a long overdue partial shop cleaning, the springs were located in the general area I had hoped to find them. Once the springs were located, continued cleaning for a while until I got bored with that and decided to get some progress done … so completed hooking up the throttle cables to the carburetors on the Rotax 912ULS engine.


When using the McFarlane Vernier-Assist throttle control, the builder is instructed to remove the throttle springs that come with the carburetors from Rotax and discard them. The McFarlane throttle control assembly is accompanied by a package of hardware which includes two carburetor springs and two cable stop nuts. My suggestion is DO NOT discard the Rotax carburetor springs because the replacement springs have been known to break.

The carburetor springs provided by Rotax are to be removed and replaced with these springs that accompany the McFarlane Vernier-Assist throttle control.


Installing the springs is simple, as can be seen in the above photo of the instructions. However, the use of pliers should be totally avoided if possible so the spring metal does not get scratches that can lead to stress fractures from the vibrations. A rounded spring tool was used to remove the existing springs and install the weaker McFarlane springs.

Using a spring tool to install the McFarlane springs on the left carburetor.

After replacing the throttle springs, the Rotax cable conduit and nuts provided with the carburetors are removed and the hardware that comes with the McFarlane cables is used to attach the cable conduit onto the carburetor’s cable bracket. The cable conduit on the end of the McFarlane cable has two nuts and two star washers that will secure the cable tunnel onto each carburetor’s cable bracket.

Removing the Rotax cable conduit from the cable bracket … these parts will not be used.

Installing the McFarland cable tunnel onto the cable bracket on the left carburetor. The cable tunnel should be installed so an equal amount of threads can be seen on both sides of the nuts.


After the cable tunnel is attached to the cable bracket, the throttle cable is attached to the throttle arm on the carburetor. The carburetor is shipped with a bolt and nut that will be discarded … but the Rotax step washer and flat washer will be reused when installing the throttle cable using a Van’s provided bolt and locknut.

A photo of the assembly instructions. The upper nut and screw my finger is point to was removed from the carburetor’s throttle arm and will not be used. However, the step washer that is slid onto the Van’s provided VA-219 cable bolt and the flat washer below it that my other finger is pointing to will be reused as shown. The cable stop provided by McFarlane in the package will be used as an idle position cable stop.


At this point, the builder is just placing the hardware in a position for a successful engine start. Later there will be a procedure where the carburetors will need to be balanced and the cable hardware will need to be readjusted anyway. Prior to installing the throttle cables onto the throttle arms the McFarlane throttle is to be pulled out away from the lock nut 2 ¾". I made a measuring tool from a tongue depressor to easily set the gap correctly without using a ruler at an angle.

A 2 ¾" strip of tongue depressor was used as a gauge for setting the position of the throttle control in the cockpit for idle.

I was working by myself and the instructions say…  while gently holding the throttle arm on the idle adjusting screw stop and pulling the slack from the throttle cable, tighten the nut to 8-10 inch pounds to secure the cable … two much for two hands. I opted to use a wire tie to hold the position of the throttle arm on the idle set screw and was able to use one hand to hold a wrench on the bolt and pull the slack out of the cable while using the other hand to torque the nut to 10 inch pounds. Caution here: … The cable locking nuts should not be over tightened or it will flatten the cable causing it to fray. A frayed cable will make future fine adjustments very difficult when synchronizing the carburetors … plus, the cable will be weakened and prone to early failure.

Snugging up the VA-219 cable bolt to the point the throttle arm will still slide on the cable, but barely.

After the throttle arm’s VA-219 cable bolt is tightened to 8-10 inch pounds, the cable stop is slid all the way back to the cable tunnel and snugged down. The viewer can see how nicely the cable tie worked for keeping the throttle arm in position on the idle screw (screw with the orange paint on it).

Tightening the idle cable stop against the cable tunnel.

Completed throttle cable instillation … of course, the wire tie needs to be removed.

The throttle cables were installed on both carburetors and all that is left to do is install the choke cables and position and tighten the Adel clamps the control cables are passing through.