C-172 Fuel

wrote in message
oups.com...
Dan, your explanation is theoretically correct, but Newps wrote from
experience. With an incorrectly located vent, and both tanks full, fuel
is
forced from left to right. The right tank stays full, and usually dumps
fuel out through the cap, which has a vent that opens both positive and
negative (Monarch caps, at any rate).

I have been maintaining Cessnas for ten years now, and the
proper Cessna cap will not allow air out of the tank. By law, an owner
MUST use parts as per Cessna parts manuals, and any other cap that
isn't PMA'd for that airplane is illegal. There is no way that the
airplane is legal if it's doing that, since it doen't comply with the
manuals.

Dan

Unfortunately, the proper Cessna cap allowed water into the tank.

Are you trying to indicate that the highly regarded Monarch caps are
illegal?

As to the uneven fuel feeding, refer to Cessna Pilot's Association Tech Note
003. The location of the vent pipe behind the strut is critical, as I said
in my first post.

Dick Meade
(Flying Cessnas for 10 years now)

Perhaps when you get to twenty, or thirty, or forty, you will understand how
they work.

Jim


wrote in message
oups.com...

I have been maintaining Cessnas for ten years now,

There's an AD addressing the requirement for the vented caps on
Cessnas (79-10-14R1), and this refers to Service letter SE77-6, which
gives the cap part numbers. The R182 manual I have here also gives the
testing details for these caps, which requires that the check valve
open at 4 inches water column or less of negative pressure in the tank,
and it must not allow any leakage out of the tank at 0.7 PSI or less.
It stipulates that any fuel staining around the cap is cause for
investigation.
Please tell me, Jim, what's wrong with insisting that the caps
aren't supposed to allow air out of the tank (except at high pressures
as a safety release) and thereby upset the pressure balance between
them and allow fuel to cross over in flight, which often leads to fuel
loss in flight?

Dan

wrote in message
ups.com...
Please tell me, Jim, what's wrong with insisting that the caps
aren't supposed to allow air out of the tank (except at high pressures
as a safety release) and thereby upset the pressure balance between
them and allow fuel to cross over in flight, which often leads to fuel
loss in flight?

Dan

I'm not Jim, but the point that he, Newps, and I are trying to make is that
the pressure imbalance is not so much caused by the caps as it is evidenced
by them. The under-wing vent tube is the primary culprit, a very large
percentage of the time. The fuel flow imbalance would exist even if it were
not able to vent out the right cap.

You are also ignoring the fact that the inter-tank vent tube contains fuel
for quite a while, due to the shape of the tanks and the dihedral of the
wings.