C-172 Fuel

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.

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).

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.