C-172 Fuel

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....