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.

Here's my question, if the right tank runs out, but there is still fuel
in hte left tank, selector is on "both" what will happen.

As part of my pre-flight, I always check operation on each tank, then
switch to "both" before my run up.

Thanks,

Jamie A. Landers
PP-ASEL

wrote:

> I fly a 172 F.
>
> The fuel selector says to select single tank operation at or above 5000
> feet.

172's occasionally get a form of vapor lock above 5000.


>
> 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.

On my 182 it was always the left because the vent was on that side.


>
> Here's my question, if the right tank runs out, but there is still fuel
> in hte left tank, selector is on "both" what will happen.

Nothing.


>
> As part of my pre-flight, I always check operation on each tank, then
> switch to "both" before my run up.

I never moved the selector from both except when I needed to fill the
tanks and make sure I got all the gas in there that the tanks could
hold. To do that you had to move the selector to any position but both.
I once started the plane in front of the hangar and then turned the
fuel off to see how long the engine would run. It ran about 30 seconds.
After that I never turned the fuel off. I certainly never checked the
fuel selector at runup. You have no sure way to know when you are
drawing off the other tank after selecting it. Plus with full tanks the
gas will transfer from one tank to the other thru the vent line across
the top of the fuselage if the plane is slightly out of level.

I had the problem with the Cessna 140. Turned out to be bad fuel vents
(caps in this case). I remember flying at night and looking up and
noticing the left tank was 100% empty. Apparently the plane figured out
how to get to the right tank on its own (this plane had a "both"
selector, unlike earlier C140's).

My current plane doesn't have a both. I"m not sure I would use a both
if I did have it. If for some reason I run out of gas (either my own
fault of a leak), I'd like to have a back up tank to switch to.

> wrote in message oups.com...
>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.
>
> Here's my question, if the right tank runs out, but there is still fuel
> in hte left tank, selector is on "both" what will happen.
>
> As part of my pre-flight, I always check operation on each tank, then
> switch to "both" before my run up.
>
> Thanks,
>
> Jamie A. Landers
> PP-ASEL
>

Funny, the C-172A and the 1975 Skyhawk manuals I just looked at have no limitations on the 'both' position. They do
caution that right and left positions are for use in level flight only

I experienced that stumble in a 172H ('67) at about 7000 msl forgetting
to switch to a single tank on a moderate temperature climbout.

Our theory was intially a vaporlock in the fuel lines even though this
was in the days before autofuel. It quit completely with no warning
for maybe about 20 seconds & then came to life normally. We had
switched to single tanks and pulled the inside fuel drain at the same
time. Maybe if we'd done nothing it would have recovered as well. To
this day I don't know what made it quit.

There was another poster maybe a year ago suggested a fuel vent
problem. He may well be right. Cessna was unable to simulate the
problem but they added both front and rear down pipes to each fuel tank
in later years.

I wouldn't switch the fuel selector before runup since you have to take
off and land on both. Try them individually enroute unless you are on
an extended over water flight. Make sure on the ground every annual
that it will shut off though.

Personally since I use autofuel in a 172M now, I turn the fuel off
taxiing in so that the carb bowl is parked empty & then gets fresh fuel
to aid cold startups. Makes the starting much easier & more
predictable. The primer won't work with the fuel shut off.

wrote:
> 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.
>
> Here's my question, if the right tank runs out, but there is still fuel
> in hte left tank, selector is on "both" what will happen.
>
> As part of my pre-flight, I always check operation on each tank, then
> switch to "both" before my run up.
>
> Thanks,
>
> Jamie A. Landers
> PP-ASEL
>

The Cessna 172 fuel system is a bit bizarre. Lets describe it.
There are two tanks. Each tank drains into the fuel selector.
the left tank. There is a cross tank vent that runs from
the left tanks above the headliner over to the right tank.
The right cap has a relief vent that opens ONLY if negative
pressure occurs (i.e., the left tank vent and/or the cross
vent get plugged).

Now here's the interesting part. The cross tank vent will
not only vent air, but will push fuel across if the fuel level
is more than about half. When running in both, the fuel is
pushed across the vent line because the left tank is pressurized
by it having the sole vent until the vent line becomes unported.

The Cessna Pilots Association can send you a two page info sheet
on this and what to do about it.

To answer your other question, if one tank goes dry and you are
on both, the other tank will flow. This is why Cessnas can have
a "both" setting. Both gravity feed down to the selector.

The reason you are told to run in a single tank mode abouve
5000' is to get increased fuel flow through whatever line you
are using to avoid vapor lock. It's a relative new (to the
design) suggestion.

Switching the fuel selector around just before takeoff can be
a very bad idea. The fuel flow at idle is negligable...do this
experiment. Sit at idle and turn the fuel OFF. See how long
it takes the engine to quit? Never change the fuel after doing
the run up.

>>Sit at idle and turn the fuel OFF. See how long
it takes the engine to quit? Never change the fuel after doing
the run up.>>

I replaced the seals in the 172H fuel selector (the one noted above),
after finding that it wouldn't fully shut off. One o-ring on one side
had poked out of its groove such that it would maintained idle etc
indefinitely, but I'm certain it couldn't have supplied takeoff power.


I agree never change the fuel selector after runup but think under some
conditions there would be enough in the bowl given a leaky fuel valve
to do a runup. The genuine off function isn't a problem, but the leaky
off condition is really dangerous. That's why in my present 172M I'm a
little hesitant on general principle, to always turn the fuel off, just
to get an easier start.

I'm now in a partnership flying a C172N. No placard on fuel. On "Both",
the port (left) tank drains faster than the starboard (right) tank. Not a
problem other than unbalanced load due to more fuel in one tank than the
other if you don't balance the fuel burn manually. However, in my
experience when flying C172s "manual" is the game one must play. Nothing
fancy in the mechanical aspects of this machine (avionics are another
question, especially these days), but that is one of the primary reasons
that so many of us love the design.

Years ago, I was in a partnership with a C172B, which was placarded for
single tank operation above 5000 ft. One of our partners had the thing quit
while flying VFR on top over eastern Arkansas. Per published procedures, he
applied carb heat and changed tanks, but the engine did not regain power in
time for him to avoid an unplanned landing. He put it into a cotton field
after missing the NDB approach at Forest City, suffering no damage other
than collecting a bunch of mud and mowed-down cotton. I drove over from
Memphis with a few tools. The farmer that owned the cotton field helped us
pull the bird out of his muddy field with his tractor. On the hard stand
next to his barn, we removed the cowling, cleaned the cotton out of the
engine compartment, and test ran the engine. It ran fine, no problems.
Buttoned it up, took off on the country road, flew it back to our A&P in
Memphis who found no problems. A year so so later, while I was flying IFR
at 6,000 MSL on a clear spring day over more-or-less the same route from
Memphis to Little Rock, the engine stopped producing power. I applied carb
heat, switched tanks, turned to the nearest airport (which was in sight and
well within gliding range), and reported the situation to ATC. We glided
for about 5 or 6 minutes at best range speed when power returned. The
engine ran smoothly, so I turned back on course, told ATC that we were
returning to assigned course and altitude and proceeded on to Little Rock as
planned. No similar power loss situation ever reoccurred during the 5 years
we owned that airplane.

I learned to avoid flying over eastern Arkansas in that C172 without
substantial attitude to allow vapor locks to clear. I also learned to
appreciate the experiences which manifested in the requirement for that
placard.

Cheers,
Leonard

> wrote in message
oups.com...
>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.
>
> Here's my question, if the right tank runs out, but there is still fuel
> in hte left tank, selector is on "both" what will happen.
>
> As part of my pre-flight, I always check operation on each tank, then
> switch to "both" before my run up.
>
> Thanks,
>
> Jamie A. Landers
> PP-ASEL
>

Thanks for all the updates guys...looks like single tank over 5000 it
is....

I trained in and flew 172s for years out of an airport with a field
elevation of 5900 feet. No one on the field ever heard of this placard; no
one ever operated the planes on any fuel setting other than "both" (though
we filled the tanks with the selector on a single tank to prevent
cross-flow); and I don't recall anyone ever having a fuel starvation
problem.

Maybe it's the notorious Arkansas Triangle?

Seth

> wrote in message
oups.com...
> Thanks for all the updates guys...looks like single tank over 5000 it
> is....
>

On Mon, 21 Nov 2005 18:11:10 -0500, Ron Natalie >
wrote:

>The reason you are told to run in a single tank mode abouve
>5000' is to get increased fuel flow through whatever line you
>are using to avoid vapor lock. It's a relative new (to the
>design) suggestion.

This appears to be model-specific. There is no such recommendation,
placard, or instruction in the aircraft or POH for the R and S models
that I have flown/have access to, and except to actively balance the
tanks I don't operate with the selector out of the 'both' position,
regardless of altitude.

They've made some changes in the system with the newest models, the
newer (R,S) models have vented caps on both tanks, as well as the
normal one on the left strut (in addition to some minor things for the
injected engine).

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.

It IS model specific. WIthout loading up the CDROM with the type
certificates, it was the C model through the H model that had this required
placard on the fuel handle ... as my feeble brain recalls.

Jim



> This appears to be model-specific.

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

Seth Masia schrieb:

> I trained in and flew 172s for years out of an airport with a field
> elevation of 5900 feet. No one on the field ever heard of this placard; no
> one ever operated the planes on any fuel setting other than "both" (though
> we filled the tanks with the selector on a single tank to prevent
> cross-flow); and I don't recall anyone ever having a fuel starvation
> problem.
>
> Maybe it's the notorious Arkansas Triangle?
>

The placard was found on early model 172s and was there by AD. My
old '59 172 had it. I believe the problem was eventually solved with a
vented fuel cap on the left tank. I don't know if that terminated the
AD or not. My 172 had both the placard and the newer style vented cap
on the left tank.

John Galban=====>N4BQ (PA28-180)

"John Galban" > wrote in message oups.com...
>
> Seth Masia schrieb:
>
>> I trained in and flew 172s for years out of an airport with a field
>> elevation of 5900 feet. No one on the field ever heard of this placard; no
>> one ever operated the planes on any fuel setting other than "both" (though
>> we filled the tanks with the selector on a single tank to prevent
>> cross-flow); and I don't recall anyone ever having a fuel starvation
>> problem.
>>
>> Maybe it's the notorious Arkansas Triangle?
>>
>
> The placard was found on early model 172s and was there by AD. My
> old '59 172 had it. I believe the problem was eventually solved with a
> vented fuel cap on the left tank. I don't know if that terminated the
> AD or not. My 172 had both the placard and the newer style vented cap
> on the left tank.
>
> John Galban=====>N4BQ (PA28-180)
>

Just checked the C-172A fuel selector, and sure enough it says to select a single tank for operation when cruising at
and above 5000'. It also has vented caps on both tanks...

> 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

> 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.