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Fuel Flow transducer damage

Aussieflyer

Active Member
My 14A is 17 months old with 164 hours on the hobbs. My red cube fuel flow meter has just been replaced. The fuel flow meter is mounted as per instructions, that is very close to the exhaust pipe. The transducer failed with burnt and damaged wiring at the tope of the cube. It indicates to me that the location for installation may be compromised by the location to the exhaust pipe. Albeit the heat shield is fitted to minimise heat affects, this appears not to be sufficently effective.

Has anyone else had this problem?

I am using Garmin G3X and the UMA red fuel cube in my aircraft. For those who are following, it may be useful to consider installing the fuel cube somewhere else further away from the exhaust pipes. This would mean changing the lengths of fuel hose but this may ensure longevity of your fuel flow transducer.

Alan
VH - XIV
 
The prototype RV-14A is approaching 1000 hrs using the original flow transducer in the plans specified location (and using the specified heat shield).

Have you look for evidence of a leaking exhaust flange gasket?
 
There are a LOT of RV14's with the transducer in the stock location, and this is the first I've heard of this happening. If this is correct, the wiring is farther away than the supply hose---so even though its firesleeved, there may be some heat indications on the very edge of the end of the sleeve (even though end dipped) or slight discoloration of the sleeve band.

Take a pic of the install so we can see whats going on.

Tom
 
Photos of damaged sensor and location

Posting photos for Aussieflyer
VH-XIV
RV-14A
Heat Damaged Fuel Cube,

3wBwfo_TioyhswKauFF8XKkdAq6HFoQBcE0JV3UZ4Dr6mwDWcaVBxHd3djBhgXPGdRXNx1t6y8zrXeDIi1FyQLydk178vtF2mc5enTn9asrz-mHwSX4_rDXaDweYC-_4rB1FgJxt1Qfn64GkaDgzXtZnuOV8VXCajTs-oEWU5FIENSh74jFk3_lMt4KOi3ttP4hSpUctBwUhEnZarwT7JJxsusWzh7f2zZrtjpnwCClpVvbM5oWAYa-qoVg2y0QjIiqPf5zllaezpUNYuIzxZzR5WxZ0QUnaocvvJx4r0LrYSmabQQDILxYywUy_1mV0a1uQcgZit2Km2TMJ7VIlGEnG1amxE4lmta8M4dqrp8XjjPHpWcHdjacx1qDeumURfysdMa_fOx91MVQUEgEtoSw3tbYzMw5ZsosNV8OYenXO2EXxMrccYbcAgN0LW62ZyDeagvXWlQKXpJdhxBHHEP5j_JOp6MOCt6v0oc5qq5GOzpunzXM9e4xRh7tKTfKS16lOmwzB5MGbAaHVcvD9NMLqzTv9Q0owX2xnKE_JjFatRhWQTst13MA9ZAPZf3L3UB0DMHtHTFVKTN3a11yHxvR9wHl6Ql9ReQQEBWq4YhImPcgxeWaL1YHfUbxI3ipw8X0rvrCto5DujMb5olc73R-g2Sh7cWGDzw=w503-h670-no


and installation as per factory instructions,

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Would be very interested to know the outcome of this thread? Looks on the face of it , that the cube has been hot and then melted the wiring coming out of it , unless there is an electrical issue.
 
Was interested to know outcome of the damaged transducer. Is it necessary to cover the transducer with high temperature fire sleeve or not? I know some people have but Vansaircraft have made no mention of it.
 
For those still building, I recommend not mounting the fuel flow sensor as listed in the plans.

I spent weeks chasing my tail on a weird fuel flow indication with the sensor mounted here on an RV-14. It worked perfectly until the airplane climbed above 7000?, then fuel flow would go to 14.3gph, regardless of mixture setting. Various calls with the associated support teams failed to produce a cause or a cure, but all agreed that the fuel flow sender was the culprit after they reviewed the engine data log. Replacing the cube and remounting it and such had no effect. I note however the fuel flow sensor mounting instructions specifically state to not mount it on the engine.

The fix was to mount the sensor between the engine driven fuel pump and throttle body on the engine mount using adel clamps. Now it works perfectly. A side benefit was this also moved it further away from exhaust pipe heat.

It turns out there have been other RVs that experienced the exact same issue (fuel flow reacting to altitude). I can only assume the unique engine vibration on these few affected planes is the cause. The RV-14 in question had the prop well balanced.

Carl
 
The red cube is not going to react well to elevated temperatures and reduced fuel pressure, it will absorb large amounts of heat in that location and quite effectively transfer that heat to the fuel passing through, while also reducing the pressure of the fuel passing through it.
 
For those still building, I recommend not mounting the fuel flow sensor as listed in the plans.

I spent weeks chasing my tail on a weird fuel flow indication with the sensor mounted here on an RV-14. It worked perfectly until the airplane climbed above 7000?, then fuel flow would go to 14.3gph, regardless of mixture setting. Various calls with the associated support teams failed to produce a cause or a cure, but all agreed that the fuel flow sender was the culprit after they reviewed the engine data log. Replacing the cube and remounting it and such had no effect. I note however the fuel flow sensor mounting instructions specifically state to not mount it on the engine.

The fix was to mount the sensor between the engine driven fuel pump and throttle body on the engine mount using adel clamps. Now it works perfectly. A side benefit was this also moved it further away from exhaust pipe heat.

It turns out there have been other RVs that experienced the exact same issue (fuel flow reacting to altitude). I can only assume the unique engine vibration on these few affected planes is the cause. The RV-14 in question had the prop well balanced.

Carl

Do you have photos of the new location? I have noticed that the fuel flow at altitude doesn't react properly to mixture changes as well (not a significant as you're describing though).
 
Reports of Red cube locations/installations and performance has varied from person to person. I personally had the red cube installed in my 7A in the cabin before the boost pump with very accurate fuel flow (within .3G) at each fill up.
In my 14A, I have it installed at the VANs recommended location but I have doubled up the heat shield (two sided by side) and insulated the wire. So far, only 80+ hours but accuracy is just as good as my 7 and no apparent heat damage. Considering my exhausts are ceramic coated and you can touch the exhaust right after shut down, I expect/hope no heat damage to the Red Cube.
 
The 3 company demonstrators have flown many hundreds of hrs with the flow transducer mounted in the kit documented location.

During kit development, stick on temp sensors were mounted on multiple sides of the flow sensor. During testing the indicated temps never got close to the manufacturers specified limit for the sensor.

The fuel flow has always been steady and accurate on the company demonstrators.

There are now 108+ RV-14's and 14A's flying. As far as I am aware, the majority of them likely have the flow sensor installed in the prescribed location and have not had any problems.

There are numerous factors that could be a variable from one airplane to another, that couldn't possibly be evaluated during the test program.

Some that come to mind are -
Engine vibration - This could be a factor regardless of the sensor location, if it is mounted on the engine.
Peak EGT - If a particular engine is for some reason operated at EGT's that are higher than typical, it could expose the sensor to temps that were beyond what was expected.
 
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From the pic it appears there is some corrosion on the wiring. Could the cause of the failure not be heat related?

I agree it looks like corrosion. Is the airplane parked outdoors? Could rain/dew have leaked onto the cable, and with the cable exiting the cube vertically and to the rear, could that cable have become a conduit to allow water into the cube? You can also see some black residue on the top of the cube which could have been dust on the cable which got washed down on top of the cube. Heat alone would not have caused that black residue.

And, what's with the missing/broken bolt? Did they take the cube apart to try to troubleshoot it?

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The 3 company demonstrators have flown many hundreds of hrs with the flow transducer mounted in the kit documented location. SNIP....

I hear you Scott. But as I?ve posted I have pages of data that clearly show the per plans cube mounting position did not work on this RV-14A, and I would not be surprised if others have had issues but have not shared them with the good folks at Van?s.

My point - while the per plans mounting location seems to work for most builders, why not look at the option of mounting the cube on the engine mount with adel clamps as I recommended? What is the downside?

I don?t have a photo of the final RV-14 cube mount that fixed the problem, but here is a photo of the tentative mounting spot I picked for the new RV-8 project.

Carl
28-C74074-E8-BD-4-D8-F-A1-FE-0545-C9753-F4-A.jpg
 
Keep in mind these units are very precisely made and because of that there are some tolerances that have to be maintained internally, particularly the distance between the jeweled bearings and the shaft the wheel rotates upon. Radiant heat will have an effect on overall expansion which affects how tight or loose these internal components are. Some may be manufactured on the tight side and work fine when exposed to elevated temperatures, and some may not.

The best way is to mount one of these IMO is directly at the flow divider, which avoids radiant heat exposure.
 
SNIP..
The best way is to mount one of these IMO is directly at the flow divider, which avoids radiant heat exposure.

Works well but recommend taking steps to avoid creating vibration fatigue points, such as a mounting plate attached to the engine. I note that some use a 90 degree fitting on the spider and then hang the cube to that. If the connecting lower hose is restrained and this is a steel fitting, I suspect all will be well. The only remaining item is the install instructions calling for no 90 degree fittings close to the cube - but I suspect this would be a minor issue.

On my first plane (IO-360 with vertical AFP injection) I installed the fuel flow sender in the line below the baffle cylinder plate, so there was straight line between the sender and both the spider and throttle body. I covered the sender with firesleeve and used adel clamps on the line as support. This has worked perfectly for 17 years. On the new RV-8 project (horizontal injection) I found the fuel line run between the spider and the throttle body not to support this set up, thus my choice to mount the sender on the engine mount. I briefly looked at the ?90 degree fitting on the spider? option and chose not to do that.

Carl
 
One measurement is worth 1000 opinions.

Put a temp sensor on the side facing the pipe, to quantify radiant heating. Strap one to the back with some heat transfer compound and see the resulting cube temperature.

Allergic to data? Rather experiment? The heat shield could be improved a lot.
 
Works well but recommend taking steps to avoid creating vibration fatigue points, such as a mounting plate attached to the engine. I note that some use a 90 degree fitting on the spider and then hang the cube to that.

Carl

Always use a bracket to bolt the cube to keep stresses low on fittings.

At the flow divider the vibration forces are going to be the lowest due to its close proximity to the centerline of the crankshaft. There's a reason why the flow divider is there to begin with.
 
Just for the record my Red Cube failed at 90 hours. On my 14A, it is installed at the VANs recommended location and plans. My G3X lost all fuel flow, no reading after it failed.

I installed the replacement Red Cube with these minor modification. Using the same clamps and location, I added 4 inches of fire sleeve over both the IN & Out metal fuel line fittings at the cube. I think those fittings collect a lot of heat from the exhaust pipes. I also added 10 inches of fire sleeve over the cube wire closet to the cube. I fastened the insulated wire away from the cylinder, exhaust and block. I also added another Van's heat shield on the exhaust under the Red Cube. Only the Red Cube is exposed.

The fire sleeved fuel line connectors, insulated cube wire and two exhaust shields provide far more heat protection. I have not measured the temps in that location and I currently have 55 hours on the second Red Cube. The modified insulation shielding is working well.
 
The only remaining item is the install instructions calling for no 90 degree fittings close to the cube - but I suspect this would be a minor issue.
Carl

Carl brings up an excellent point. I have spoken with EI about this very issue. 90 degree fittings actually will work fine but will probably cause a change in K factor. The factory uses straight inlet and outlet fittings when they set the initial K factor. So the units are usually very close to good accuracy for flight right out of the box. If the builder opts for 90 degree inlet and/or outlet fittings they will almost definitely have to adjust and optimize the K factor over a number of flights. No big deal just more iterations to get the flow accurate. That is a small price to pay compared to wrangling to use perfect straight lines within the tight spaces under a cowl. I always ratio the K factor over a few flights on every installation anyway. So I use whatever style of fittings that work best for the installation. Remeber the more cross country flights used to adjust the K factor, the better the precision of the sender.

Jim
 
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I've installed quite a few of these, always supported by the hoses between the servo and the divider as recommended by the manufacturer, can't remember any failures, including my own which has well over 1K hrs on it.
 
Let's not discredit a factual post . . . but

Why move the cube when you can install a longer heat shield? Honestly . . . stock or not, it looks sorta short and exposed to the upper exhaust pipe. And your experience is an indication it could use it.

So, Just use a bigger, better shield.

You just might have had more hours at higher temps than the masses. Just because 400 others did not have an issue does not validate the design for peak EGT, and high ambients.

Edit: I did not see this earlier, but Scott also points out the peak EGT. To Vans credit, it takes a lot of extreme field testing to yield a design with perfect reliability. Thousands of hours and thousands of engineers and nothing is perfect, or perfectly predictable.

Peak EGT - If a particular engine is for some reason operated at EGT's that are higher than typical, it could expose the sensor to temps that were beyond what was expected.
 
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I've installed quite a few of these, always supported by the hoses between the servo and the divider as recommended by the manufacturer, can't remember any failures, including my own which has well over 1K hrs on it.

I think EI has updated it's preferred mounting location so maybe the flow divider line location was from older versions of the manual. My instructions from EI specifically state to install the transducer on the firewall or similar structure separate from the engine and place between the engine-driven pump and the fuel servo (or between the gascolator (or firewall fuel fitting if no gascolator) and the carb for non-FI engines).

Here is a cut and paste from the instructions:

"1. Find a convenient location on the firewall (away from any hot exhaust pipes) and mount a bracket for the Fuel Flow Transducer. Check both sides of the firewall for clearance before drilling any holes."

Maybe they changed to this since some folks were having the high-altitude issues or the like.

Jim
 
The CGR30 manual (2015) gives you a couple of options, one hard mounted like you said but 2 others that recommend suspending:
(is there later manual guidance?)

Mounting Procedure:

1. Find a convenient location between the Fuel Servo and Flow Divider and away from any hot exhaust pipes to suspend the Fuel Flow Transducer. The Transducer must mounted within 6" of a hose support or fitting. The support or fitting may be on the input or output line of the Flow Transducer and the support may be to an adjacent hose.

2. Remove the fuel hose which goes from the Fuel Servo to the Flow Divider.

3. Purchase two new hoses: one to be used from the Fuel Servo to the Fuel Flow Transducer and the other to be used from the Fuel Flow Transducer to the Flow Divider. There must be flexible hose in and out of the Fuel Transducer.
 
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My CGR-30P installation manual dated 5/8/2015, page 17, has a matrix of drawings based on the effectivity of the fuel system configuration. For Lycomings on RVs with either fuel injection without a fuel return line (most) or carburetors, the applicable drawings are 1229932 or 1229931 offering two types of mounting locations (pages 20 and 21). Both approved options show the transducer between the engine driven pump and the carburetor or FI servo. So while I was wrong understanding the firewall was the only place allowed for mounting there is still no provision in the effectivity table for mounting the transducer in the flow divider line.

That being stated, EI does allow mounting the transducer in the fuel line feeding the flow divider only if the engine has a fuel return line from the servo, as in the case of Continental fuel injection (page 22 drawing no. 0415941). That drawing is where the text you quoted appears. The only instance I know of a Lycoming having a fuel return line is with a Simmonds fuel injection system, which is very old and rare and plain weird. They have the individual injector lines under the cylinders rather than over them. But even with this system there isn't a place for the transducer after the servo as the flow divider is integrated with the servo which is on the accessory case. I had some experience with a Simmonds fuel delivery system in an RV-4.

For an EAB aircraft like a RV the EI installation instructions can be treated as suggestions. In the case of an STC'd installation on a type certified aircraft this effectivity is supposed to be followed. Notwithstanding EI's recommendations, if the flow divider line location works it works.

Jim
 
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