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