Many builders avoided this nightmare and fitted a capaticance or other
similar gauge.
We originally figured that fitting a fuel flow meter would be sufficient
along with the factory sight gauge. Our reasoning was that as long as the
rear wheel was raised such that the aircraft was level (we use a spirit
level to check this) the sight gauge ought to be reasonably accurate - for
refuelling purposes only.
Last weekend we fuelled the aircraft with 40 litres of fuel on top of the 15
already in the tank (15 l is our minimum safety level - ie: 11 in the
reserve plus 4). The sight gauge indicated 15 l beforewe refuelled and 55 l
after which is what we expected. The estimated flight time was just over 2
hrs excluding arrival and departure.
As soon as we took off the sight gauge indicated the tank to be virtually
empty which caused a degree of worry (you can never be certain there isnt a
leak !!!). This lack of fuel continued throughout the flight and on the
return. Before we put the plane to bed we re checked the levels and as
expected there were 20 litres remaining (having used 35l according to the
Fuel Flow meter). So far so good !!!
Yesterday we planned for a similar trip and started to refuell with 2x 20
litre cans. About halfway through the second can the fller tube filled up
and we ended with fuel everywhere. Assuming this was nothing more than an
airlock we lifted the tailplane to shoulder height (it is a momowheel) in
the belief that this would clear the blockage - IT DIDNT !. The tank
appeared to be full whatever we did.
So we drained off 20 litres through the drain valve and rechecked the
levels - guess what - 45 litres !!! The tank holds at least 75 litres (it is
one of the originals). 75-20 should be 55 litres. So we put back all 20
litres without any problem.
Then as soon as we took off the gauge indicated empty!
We should point out that as part of our refuelling check we first blow
through the vent pipe (that enters at the top of the tank) to clear it and
then we blow back the fuel down the sight gauge till air bubbles into the
tank to clear any possible airlocks. In theory this should ensure an
accurate reading.
The problem seems to be that the filler pipe has what is in effect a U bend
which fills up with fuel and it seems that the breather pipe is prone to a
similar problem - despite blowing through both to clear them.
If this theory is true then as soon as the fuel pumps start delivering fuel
to the engine this will create a partial vacuum in the tank which would
explain the low sight gauge reading.
Has anyone else had a similar problem or are we the only ones using the old
style gauge. We are aware that some of the sight gauges are fitted to the
seat backs but according to the above reasoning this problem would still
exist.
Any suggestions ? (other than retrofitting another type of gauge). We have
enough on our hands with mod 74 !
One thing we were considering is replacing the filler pipe with clear
plastic fuel tube (it is available) - at least it would be easier to
diagnose the problem.
Carl & Dot
G-LABS
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