On May 3, 10:30 am, "Phil" wrote:
Thanks to ALL for the useful information , I think my friend has decided
against using a small tube for crankcase venting , your suggestions probably
saved him a lot of grief and possible danger , Thank You All Again , Phil
Lohiser EAA 12873


Over the years (since 1972, EAA 89913) I have read of a few
accidents arising out of fuel tank venting issues. The "venting of Lyc
crankcase" reminds me that sometimes homebuilders don't have adequate
frame of reference when making changes to systems and end up getting
hurt or dead, or at least busting up a nice airplane.
A classic problem involves two or more fuel tanks, perhaps one in
each wing, or maybe a main and a header tank. For certified airplanes
having more than one tank, and the possibility of feeding from both
tanks simultaneously, certification requires that the tanks have a
common vent. They word it this way:

"Airspaces of tanks with interconnected outlets must be
interconnected." (FAR 23.975(4))

Howcome? Well, imagine a high-wing airplane with a tank in
each wing, with the fuel flowing from each tank through aluminum
tubing down through the fuselage to a tee where they join, then
through a shutoff valve, and from there to the strainer and carb.
Imagine, too that each tank has its own vent sticking out from under
its wing.
If there is any difference in pressure inside each tank, fuel
will flow faster from the tank with the higher pressure. If the
difference is high enough, the high tank will empty completely while
the low-pressure tank will not flow at all, or even worse, fuel from
the high tank will flow through the tee and into the low tank,
overfilling it and spilling from its vent. When the high tank is
empty, the engine gets nothing but air, and the flight is finished
even though there's still a full tank on board.
It's very easy to get uneven pressures from two separate
vents. Vent shape, angle of tip, any minor disturbance if the air
around it, will all affect its pressure. The Glastar had this separate-
vent system and uneven flow was the order of the day. Interconnecting
the top of the tanks of the one we had here fixed it. The Cessna 150
also has this system, but being certified, it has a single vent that
feeds both tanks. The 172 has a "Both" position on its selector, and
because of that it also has a single vent source.
Low-wing airplanes with two tanks do not normally have a "Both"
position because we're not relying on gravity flow, and if one tank
happened to run a little faster than the other (flying one-wing-low,
for example) the pump would be quite happy to suck air from the dry
tank rather than fuel from the one with fuel in it. The low-winger has
very little "head" between the tank outlets and the lowest point in
the system (maybe even no head at all) where the high-wing airplane
might have three feet or more. Sucking air from the tanks in the low-
wing airplane become a problem.
Vented fuel caps sometimes work well, sometimes don't. With wing
tanks, the low pressure atop the wing can reduce tank pressure
dangerously. Cessna uses check-vented caps in case of the main vent
plugging up with bugs or ice, but those caps have specially-designed
static ports on them to somehow reduce the suction. I flew an Aircoupe
that had a similar setup, without the check valves and without any
other tank vent, and fuel could be seen streaming off them when the
tanks were full. They also had those specially-designed non-sucking
cap vent ports but they obviously didn't work too well. Taylorcraft
used a cap with a forward-facing scoop to use ram pressure. The cap
could be installed backwards, though. Citabria has the single-vent two-
tank tee system like the 150, with totally unvented caps that *look*
like older auto or truck fuel caps, except that those older auto caps
were vented. I found one of those auto caps on a Citabria we bought,
and of course it had uneven fuel-flow issues. One cap was sucking, the
other not, and the low-pressure tank was a little slower. The
interconnected tank vent kept things from getting too far out of hand.
Parking an airplane with full tanks in a hangar can be asking
for trouble. If the day is cold, the fuel will be too, and it will
expand considerably in a heated hangar and run out of the vents,
creating an awesome fire hazard. If the airplane has the
interconnected outlets (or the selector is on "Both") and the hangar
floor is sloped and the vented wing is lower, fuel will cross-flow
through the system and run from the vent. As the higher tank drains,
the lower wing gets heavier and droops lower and the flow increases. I
get VERY annoyed when I find full tanks in our hangar, even after all
the prohibitions against it. We've had some very close calls with
fire.

Dan