912 Question

[Guest DavidH10]

Hope you get it solved easily David. Actually I think an "attention getter" every now & then is not a bad thing, as long as it doesn't turn out to be - a bad thing! You might like to let us know what your findings are as well thanks David.

Actually wasn't too dreadful, but forced me to pull out of the formation training that evening with a quick radio call to "lead" to say #3 returning to aerodrome with rough running engine! I was able to hold altitude ok, but not to maintain a sustained climb.

As I said to the CFI, later, I thought I'd return and land while it wasn't an emergency.

 

All fixed today...

 

I removed and cleaned the carby bowls and floats. Only the expected fine layer of dust on them. Nothing to cause the issue, and the floats didn't rattle :)

 

Cleaned and re-oiled both air filters. Quite dirty, but again ,not enough to cause the problem.

 

Disconnected the fuel flow sensor and blew through it. The little turbine whirred nicely, so not the issue either.

 

In reconnecting the fuel flow sensor, I wasn't happy with the extent to which the fuel hose pushed onto the sensor's pipe at the outflow end. The pipe at the outflow end is shorter that at the inflow, so not as much room for the fuel hose to push on. The fuel hose had a well developed step in the opening from being squeezed by the hose clamp, but the opening in the centre was, while narrower than it should have been, ok. I tried it a couple of times, but still wasn't happy with the fit. I finally decided to see if it would come off with the hose clamp tightened. It did, with a bit of levering and when I looked into the tube, there the centre hole could have been blocked with a match, it was so small.

 

I replaced the length of hose on both sides of the fuel flow sensor. With new hose, it pushed all the way onto the tube until contacting the body. I was also careful to ensure the hose clamps were as close as possible to the fuel sensor body to prevent a recurrence.

 

Here's the photos... One of the carby bowls with floats removed, before cleaning.

 

.[ATTACH=full]922[/ATTACH]

 

The fuel hose showing the constriction when removed without undoing the hose clamp.

 

[ATTACH=full]923[/ATTACH]

 

The fuel flow sensor with the top hose removed, shows how short the tube is to clamp the hose at the top end. The gap, seen here, between the body and the hose at the bottom end was about the same as that at the top, but the tube at the bottom is longer, so no issue. As stated, I replaced that tube also[ATTACH=full]924[/ATTACH]

 

A test flight this evening demonstrated that there was no issue after a full power climb lasting 2.5 minutes. The problem first appeared the other day after climbing for about 15 - 20 seconds.

 

* Photos are Copyright 2011 David Hunt.

 

 

 

[Guest DWB]

It's interesting David how these things manifest themselves over time. Just sitting there waiting to happen with the hose slowly fatiguing till one day says, not going to play the game no more. Good find & educational for the rest of us as well. I take it you can't use the larger spigot on the transducer?

 

 

[Guest DavidH10]

It's interesting David how these things manifest themselves over time. Just sitting there waiting to happen with the hose slowly fatiguing till one day says, not going to play the game no more. Good find & educational for the rest of us as well. I take it you can't use the larger spigot on the transducer?

Too true Dexter.

To use the larger spigot would require larger diameter hose, which then wouldn't fit the tap spigot at the other end. Now understanding this issue, appropriate care (to ensure the clamp is not partially over any unsupported part of the tube) can be used to ensure it cannot happen again.

 

 

[cficare]

no heat shield around the fuel lines i see.

 

 

[Guest DWB]

no heat shield around the fuel lines i see.

Nor do I CFI. Most of my lines are the Bing Blue although my main delivery tank to mechanical pump is Gates rubber as Bing doesn't come in 5/16"

BING ALCOHOL RESISTANT FUEL LINE

 

This fuel line is blue in color and does not rot or deteriorate from the inside, clogging the carburetor or fuel system. It withstands 50 PSI and tensile strength is 8000 PSI. It is resistant to temperatures of -65° to 220°F without deterioration, features exceptional resistance to gasoline, oil, oxygen, and ozone, and has high elastic memory allowing use of Delrin or nylon barbed fittings in low pressure applications without the need for external clamps.

 

 

[Guest Michael Coates]

features exceptional resistance to gasoline,.

I would hope so if you are using it for fuel hose.

 

 

[Guest Michael Coates]

gentlemen, where do I start...... we have sold about 500 of these fuel flow sensors over the years because they use to be supplied with the skydat instrument range when they were freely available. There are a number of really critical issues which you have identified.

 

Firstly it is clearly written in the instructions not to blow through the device to hear the wheel spinning, even worse is using compressed air and if you pull unit apart is very difficult to get back together again without it failing because it is assembled in a controlled manner with special bearings which are almost impossible to see without magnification. So please be warned on all of these issues.

 

I can see also that you are over tightening the hose clamps because of the distortion in the visible end of the fuel flow meter. Small hoses like this are not ideal candidates for traditional hose clamps and this is one of the reasons why the guts are falling out of your hoses. Hose clamps on small hoses do not have equal clamping pressure all the way around and this is what is causing some of the problem. You should be using this type of hose clamp on small diameter hoses and this is standard practice with a number of manufacturers like Mercedes, BMW etc.

 

I don't know if these are deemed to be suitable for aviation but I have installed some on an old X-Air which has been flying for about 10 years without any issue although he did replace the fuel line at five years and fitted new clamps. Please do your own investigation on the suitability of these for factory built aircraft

 

 

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the hose clamp shown below is used on a number of our factory built aircraft and if you look closely you can see that when crimped they support the complete diameter of the hose and this is what is required for small diameter hose because you lose a lot of crimping pressure under a standard hose clamp. These units must be connected with a special tool which can be purchased off eBay for around $15 and the claims themselves are not cheap at around three dollars each, they can be reused a couple of times without any problems but they will be damaged if you try to close the clamps with pointy nose pliers or side cutters. Hopefully a few others will jump in the conversation with a bit more experience and knowledge about hose clamp in small diameter tube but I can see from the distortion in a photograph of your fuel flow meter that there is a problem and you need to be careful because these units will crack and they need also to be away from heat source which means they are normally covered in fireproof sleeving unless in a really good airflow position.

 

 

that's my $0.16 worth ! and my total knowledge of hose clamps

 

 

[Scotty 1]

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the hose clamp shown below is used on a number of our factory built aircraft and if you look closely you can see that when crimped they support the complete diameter of the hose and this is what is required for small diameter hose because you lose a lot of crimping pressure under a standard hose clamp. These units must be connected with a special tool which can be purchased off eBay for around $15 and the claims themselves are not cheap at around three dollars each, they can be reused a couple of times without any problems but they will be damaged if you try to close the clamps with pointy nose pliers or side cutters. Hopefully a few others will jump in the conversation with a bit more experience and knowledge about hose clamp in small diameter tube but I can see from the distortion in a photograph of your fuel flow meter that there is a problem and you need to be careful because these units will crack and they need also to be away from heat source which means they are normally covered in fireproof sleeving unless in a really good airflow position.

 

 

that's my $0.16 worth ! and my total knowledge of hose clamps

I don't believe this is a big cost if you take it over the life of the hose which I think is 5 years. I am a firm believer of doing it by the book. After all people like Rotax have a lot more experience than me with their engines. I have only had 2 of their engines, how many have they had experience with?

 

Cheers Scotty

 

 

[Wayne T Mathews]

I don't believe this is a big cost if you take it over the life of the hose which I think is 5 years. I am a firm believer of doing it by the book. After all people like Rotax have a lot more experience than me with their engines. I have only had 2 of their engines, how many have they had experience with?Cheers Scotty

I like the way you think Scotty...

 

 

[Guest DavidH10]

I was aware of the caveat against blowing through the fuel sensor, but it wasn't reliable anyway, so if my action had caused it to be less so.. no matter. The accuracy seemed to vary between 60%-80% of actual fuel consumption, depending on flow rate. An estimate based on engine run time was more accurate.

 

Since blowing it through, I notice that it now reads a fuel flow rate when taxiing. Something that has hitherto read zero. It will be interesting to re-calibrate its readings against actual consumption again and see if it has become useful.

 

I am using the clamps provided by the aircraft manufacturer and as it is a certified aircraft, I cannot change them to another type. I agree that they were previously clamped too tight. My issue, and rectified when I replaced the two hose sections. All other fuel lines were checked at the same time, both for over-tighening of clamps as well as clamp placement.