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GBWFH2010
Joined: 31 Jul 2011
Posts: 38
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 Posted: Wed Apr 22, 2015 12:40 pm Post subject: Sight Gauge problem |
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I have just completed the 5 year rubber fuel line replacement on my Classic, including renewing the sight gauge clear tubing.
When I was replacing the sight tube I discovered the previous owner had at some time installed a non-standard fuel gauge system (which never worked, but the sight gauge did!) using an 'external' sender unit plumbed in from the sight gauge link from the LH tank. I am embarrassed to say I had never noticed this set up as it was well hidden from view. I was not happy about this situation and decided to remove unapproved kit and restore the connections per the build manual.
The sight gauge on my aircraft runs from the LH tank, through the tunnel and is securely fastened to the bulkhead on the left hand of the passenger seat. It then routes through through the passenger headrest and follows the filler pipe up to the vent on the fuselage. The system had worked well before and given a reasonably accurate indication of the contents.
I thought I had done everything correctly and filling the tank the 'new' gauge seemed to be giving what appeared to be logical indications. However when I flew the aircraft for the first time the level in the tube went way past maximum (I had a total of 55 litres on board), particularly in the climb. There was also a smell of fuel which may have ben caused by the fuel venting.
My question is does anyone have an idea what could be causing this? There is a restrictor in the tube similar to that found in the fuel return line. This was in the previous set up so I re-installed it, but I can not find any reference to this in the manuals. Could this be causing the problem?
Sorry for the lengthy post and any help would be greatly appreciated.
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Gordon Grant
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Fred Klein
Joined: 26 Mar 2012
Posts: 503
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| Posted: Wed Apr 22, 2015 9:26 pm Post subject: Sight Gauge problem |
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Gordon…you wrote:
| Quote: | The sight gauge on my aircraft runs from the LH tank, through the tunnel and is securely fastened to the bulkhead on the left hand of the passenger seat.
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Could you please identify the “LH” tank as either port or starboard and give additional information on “the bulkhead”? Is the referenced bulkhead the vertical setback?
F.
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GBWFH2010
Joined: 31 Jul 2011
Posts: 38
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| Posted: Thu Apr 23, 2015 11:16 am Post subject: Re: Sight Gauge problem |
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Fred,
The sight gauge feed is from the PORT tank.
My use of the word bulkhead was completely wrong, the vertical section sight gauge tubing is attached to the passenger seat back adjacent to the tunnel. Sorry for the confusion.
Gordon
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Gordon Grant
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JonSmith
Joined: 21 May 2010
Posts: 110
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| Posted: Thu Apr 23, 2015 12:03 pm Post subject: Re: Sight Gauge problem |
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Hi Gordon, there is a bit more about the sight gauge in the build manual chapter 17 (Bonding Cockpit Module Into Fuselage).
On page 17-3 part way down is a paragraph titled 'Fuel Sight Gauge' and this states that the restrictor IS required in the sight gauge pipeline. Just to act as a damper I should imagine.
My own sight gauge has always worked pretty well. Mine is set up exactly as per the manual, even with the routing up the front and behind the instrument panel - as long as one understands the readings relevant to the aircraft attitude its ok. The only difference to standard is that I have plumbed the sight gauge and fuel tank vents into the top of the filler cobra (XS style filler tube) and then vented the cobra to the standard s/steel tube on the top of the fuselage.
Sorry to ask but if you are suddenly getting these strange readings after working on it are you COMPLETELY sure you haven't made an error with the hose connections? There's quite a lot of pipes in a small space and several of them look rather similar!
Also, on your port tank outlet does it have two separate outlets coming out of it, one dedicated for the feed to the engine and the other dedicated to the sight gauge (combined with the drain if fitted)? The early version had just one outlet to feed to the engine with the sight gauge Tee'd off and apparently with the engine running that DID give weird readings set up like that, hence they split the outlet into two separate outlets. (The starboard side tank outlet should also have two ports, one for the reserve engine feed and the other for the return line).
Another thought, could it be you have a low point in the sight gauge line somewhere which has got a slug of fuel trapped in it preventing pressure equalisation to the vent thus duff readings? Blowing down the sight gauge vent might clear this? Also is the vent to the outside correctly orientated and not subject to some strange pressure influence that would mess up the reading?
Just random thoughts, all the best, Jon
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Fred Klein
Joined: 26 Mar 2012
Posts: 503
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| Posted: Thu Apr 23, 2015 12:22 pm Post subject: Sight Gauge problem |
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Gordon…thanks for the clarification…I’m wondering, where does your sight gauge “T” into the fuel tank vent line?…and have you assured yourself that there are no fuel pockets in possible low points as the sight gauge line snakes towards the fuel filler which may block the line? From your routing description, I’d look for kinks in or collapsing of your sight gauge line as it (presumably) angles from the vertical across the flat area of the CM, and into and out of the PAX headrest. As I recall, a constrictor in the sight gauge line is required.
Hope this helps,
Fred
| Quote: | On Apr 23, 2015, at 12:16 PM, GBWFH2010 <gaxuk2001(at)gmail.com> wrote:
Fred,
The sight gauge feed is from the PORT tank.
My use of the word bulkhead was completely wrong, the vertical section sight gauge tubing is attached to the passenger seat back adjacent to the tunnel. Sorry for the confusion.
Gordon
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Gordon Grant
G-BWFH
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GBWFH2010
Joined: 31 Jul 2011
Posts: 38
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| Posted: Fri Apr 24, 2015 3:12 am Post subject: Re: Sight Gauge problem |
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Fred and Jon,
Thanks for your advice and also clarifying the situation regarding the restrictor. I was looking in the Classic manual and I did not notice it on the fuel system drawing, although it is shown quite clearly. That being said I am sure I have everything connected correctly. My Classic has the dual connectors at the tank so the sight gauge tube is not connected by a T-piece.
I agree it is looking as if a blockage, or air bubble, in the system is the cause of the problem. I will go over on Monday, have a really thorough inspection and let you know how I get on.
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