C-172 Fuel

In article .com,
says...


I fly a 172 F.

The fuel selector says to select single tank operation at or above 5000
feet.

If you leave the selector in the "both" position, under 5000 for any
prolonged period of time (couple of hours) the right tank appears to go
down more quickly than the left. This is confirmed at fillup.



Are you sure?
I ask because in the 172's I fly it's just the opposite. The left tank
always goes down faster than the right, and I frequently switch to RIGHT
tank only during flight, to even them out (not that it matters overmuch).
The 172' are of course placarded for use of "both" during TO/landing, but
I have never seen the 5000' placard.

I have heard that the left tank becomes slightly pressurized, due to the
forward-facing vent on that tank, which is why it drains faster than the
other. I do not claim this to be a fact - just something I've heard.

G Faris

G Farris wrote:



I have heard that the left tank becomes slightly pressurized, due to the
forward-facing vent on that tank, which is why it drains faster than the
other. I do not claim this to be a fact - just something I've heard.

The left tank becomes pressurized because the vent tube is slightly out
of rig. It's supposed to be directly behind the strut. If it gets a
clear view of the incoming slipstream it forces a lot more air into the
tank. That forces fuel from the left to the right tank via the vent
line above the headliner in the cockpit. You will never solve the
problem until the vent tube under the left wing is positioned correctly
and then you only mitigate the problem to a certain degree.

Newps wrote:
G Farris wrote:



I have heard that the left tank becomes slightly pressurized, due to the
forward-facing vent on that tank, which is why it drains faster than the
other. I do not claim this to be a fact - just something I've heard.

The left tank becomes pressurized because the vent tube is slightly out
of rig. It's supposed to be directly behind the strut. If it gets a
clear view of the incoming slipstream it forces a lot more air into the
tank. That forces fuel from the left to the right tank via the vent
line above the headliner in the cockpit. You will never solve the
problem until the vent tube under the left wing is positioned correctly
and then you only mitigate the problem to a certain degree.

Seems to be a popular misconception as to how that fuel gets
across to the other side. It can flow through the FUEL lines, at any
fuel level, while the selector is on both. If, for instance, the
airplane is parked left wing low, the fuel will run down through the
fuel line from the right tank, through the selector valve, and up the
other fuel line into the left tank. No mystery at all. If the tanks
have enough fuel, it will run out the vent onto the ramp. Stopping it
requires placing the selector on L or R or OFF. The old Cessna 150s,
with their on/off system, had the lines teed together before the valve
and would do this anytime, valve on or off.
The fuel will run across the VENT line only if the tanks are
full.
The fuel venting system applies the same pressure to both tanks.
The vent, whether behind the strut or below it, leads to the left tank,
and the vent line from that tank across the right tank makes sure that
the pressures are equal. Unequal pressures are a result of leaking fuel
caps, either at the gasket or through a defective vent check valve, and
air will flow through the cap into the tank and cause airflow one way
or the other through the vent line, upsetting the pressure balance and
causing one tank to drain faster than the other. Pressures are usually
negative if the check valve leaks, and possibly could go positive if
the gasket had a forward-facing leak. I've seen split gaskets on fuel
caps.
The vent is located aft of the strut to minimize icing blockage,
and one or both caps has a silicone check valve to let air into the
tanks in case of vent icing. That valve deteriorates over time and lets
air out of the tank. The valve disc isn't available as a separate part;
the cap must be replaced. Thankfully, they aren't expensive like
Piper's.

Dan

I should have quoted a couple of sources regarding fuel tank venting.
For the US, here's the FAR, in part, that deals with it. Note that
(a)(1) deals with the icing problem of vents, hence Cessna's locating
it behind the strut, and (a)(4) deals with interconnected tanks such as
the 172/182's, so a vent system common to both tanks is required to
keep tank pressures equal.

23.975 Fuel tank vents and carburetor vapor vents.
(a) Each fuel tank must be vented from the top part of the expansion
space. In addition -
(1) Each vent outlet must be located and constructed in a manner
that minimizes the possibility of its being obstructed by ice or other
foreign matter;
(2) Each vent must be constructed to prevent siphoning of fuel
during normal operation;
(3) The venting capacity must allow the rapid relief of excessive
differences of pressure between the interior and exterior of the tank;
(4) Airspaces of tanks with interconnected outlets must be
interconnected;
(5) There may be no point in any vent line where moisture can
accumulate with the airplane in either the ground or level flight
attitudes, unless drainage is provided. Any drain valve installed must
be accessible for drainage;
(6) No vent may terminate at a point where the discharge of fuel
from the vent outlet will constitute a fire hazard or from which fumes
may enter personnel compartments; and
(7) Vents must be arranged to prevent the loss of fuel, except
fuel discharged because of thermal expansion, when the airplane is
parked in any direction on a ramp having a one percent slope.
snip

Canadians can find exactly the same thing in CAR 523.975.

Dan

wrote in message
oups.com...

Newps wrote:
G Farris wrote:



I have heard that the left tank becomes slightly pressurized, due to
the
forward-facing vent on that tank, which is why it drains faster than
the
other. I do not claim this to be a fact - just something I've heard.

The left tank becomes pressurized because the vent tube is slightly out
of rig. It's supposed to be directly behind the strut. If it gets a
clear view of the incoming slipstream it forces a lot more air into the
tank. That forces fuel from the left to the right tank via the vent
line above the headliner in the cockpit. You will never solve the
problem until the vent tube under the left wing is positioned correctly
and then you only mitigate the problem to a certain degree.

Seems to be a popular misconception as to how that fuel gets
across to the other side.

(Snip)

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). Ram air pressure continues to force
fuel to the right side, until the left tank empties. The indicator for this
is when the right fuel gauge stays put, and the left goes rapidly toward
empty. Fuel running off the right flap in flight is another hint.

That's how my 182 managed to "burn" 32 gallons/hr on one leg of a long
trip.

Cessna fixed the problem in later years by adding a vent behind the right
strut. For the single vent planes, the location is so critical that it is
specified to the hundredth of an inch. That precision is related to
airflow, not ice.

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

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