Ever since we went to have a go in G-YURO all those years ago both
Dave and I have had severe problems with the fuel system as
recommended. A decent fuel supply is essential to flight, as we saw
it then there were three very serious problem areas:
1) Once in, it is impossible for evermore to inspect for debris in
the tank.
2) The sight gauge is likely to be severely affected by pitch.
3) There is a potentially disasterous flaw in that the sight gauge
is taken off the main line to the engine.
We have addressed all these problems and will not have any of then
on our bird. In the meantime however, If you are wise, I believe
your Europa SHOULD BE GROUNDED pending rectification of a
fundamental problem which manifested itself on Peter Kember a few
weeks ago and about which he told me at Popham the weekend before
last.
While returning from Ireland a few weeks ago in rain over the sea,
suddenly most of his fuel disappeared off the sight gauge. Fearing a
severe leak he declared a fuel emergency but made it safely to
Cardiff. Upon filling up he discovered that there was in fact still
some 25 - 30 litres in the tank.
So what happened? He had (sensibly) rigged a couple of plastic
filters in his vent lines (the one from the tank, the other from
the top of the sight gauge) to stop crap being drawn into the
system. The vents, as you all know, protrude from the top of the
fuselage and are bent forwards to slightly pressurise the system.
They also collect water if its raining.
This water had soaked the tank vent filter, partially blocking it and
creating a slight vacuum in the tank as fuel was used. The weak
point is the sight gauge which dropped in level.
It doesn't take much imagination to consider what would happen if
the vent had become slightly more blocked than it did, fuel would
be drawn from the sight gauge in preference to the tank, which
would not last long. A restricted orifice makes not an iota of
difference as air passes through small holes like this some 30 times
faster by volume than a liquid of the viscosity of petrol. A bee,
ice, water, any number of trivial things could be the downfall of
your pride and joy.
So, what to do?
If you vented the sight gauge to the tank rather than to the
outside, then at least the situation described above would not
have occurred. (think about it) The pump(s) probably have
enought suck to partially collapse the tank if the vent was
completely blocked (so long as it did not vapourise in the engine
compartment lines first, but that is another problem). However, if a
leaf or other such debris (how are you ever going to know there
isn't such things in the tank?) ever restricted the outlets of the
tank then a preference to draw fuel from the top of the tank via
the sight gauge would still occur.
You could fit a tap in the sight gauge vent line to switch off if
you noticed something unusual going on. This is definitely a last
resort fix, if it all went quiet on climb out, would you remember
to switch it? Possibly not, shortly followed by a loud crunching
noise.
Equally, if the sight gauge was put upstream of a booster pump,
would you remember to switch it on? Could it actually empty the
tank thru the sight gauge vent when on? Probably. Even if not it
would certainly affect the reading in a negative way when the pump
was on.
There is only one idiot proof fix. Do away with any possible source
of air ingress into any low pressure part of the fuel system.
Fitting the sight gauge via a separate route into the tank might be
one way of doing it, but there is no available fitting into the
tank and anything other than the present arrangement for tank
bottom fittings would be courting disaster in itself.
So. What have we done?
A) We have fitted an inspection hole in the top of the tank. This
fixes problem 1) above. As experienced microlighters we KNOW debris
gets into fuel tanks however careful you are, and we HAD to have
some way of looking inside and getting it out if necessary. In due
course this will be an approved mod and I will FTP it to AvNet.
(All the work has been done, we are using a special type of PTFE
tape as the seal - it is simply a matter of completing the
submission).
B) Our inspection hole cover is big enough to fit two fuel gauges:
The first is a capacitance type made by Avalec Instruments Ltd
(call Eddie Jelonek, 01296 682465). His gauges are unique in that
they are electronically calibrated to linearise the gauge to the
shape of any tank. We have been working with him and he will have
one available for the Europa tank shortly. (They have been used as
standard in Pegasus Microlights for some time and have proven
reliable in principle).
The second is a purely mechanical one (Aircraft Spruce 05-16700).
It has had to be modified to get a range thru full to empty,
without contacting the tunnel but now works just fine. It will live
under a little window in between the headrests in the inspection
hole cover. Not ideal, but is meant as a last resort in case of
failure of the electrical one.
We will have NO sight gauge. Problems 2) and 3) above are solved.
Further to the above it would also seem a good idea to vent the
tank to the underneath of the fuselage, via a loop to the top
of the fuselage rather than just coming out the top.
This would:
1) Prevent the possibility of water ingress in rain. It seems to us
that it would be possible with the present vent position to collect
enough water to displace all the unusable fuel (0.9 L) plus
gascolator in only one flight in heavy rain. Engines do not run
well on water.
2) A hot day, full tank, aircraft resting on Stbd. outrigger will
probably mean fuel pissing all over the top of the fuselage. (Unless
you have one of the very early tanks when it would be the Port
outrigger).
Of course pipes running up and down again can syphon, in the case
of 2) it could syphon quite a few litres out. A simple way to stop
this is to have the vent dividing into two at the highest point.
We will have to wait until ours is flying however to find a sutable
high pressure area underneath.....Unless someone else does it
first.
Regards
-------------------------------------------------------
Richard Meredith-Hardy TEL + 44 (0)1462 834776
rmh@flymicro.win-uk.net FAX + 44 (0)1462 732668
CIS: 100071,2422
Check out the British Microlight Aircraft Association at
http://www.avnet.co.uk/bmaa/bmaa.htm
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