"Tanks on both" checklist item

Craig,

The geometry of the fuel tanks and the locations of the fuel ports in the
tanks cause the actual amount of usable fuel in each tank to vary depending
on aircraft attitude.

By way of example, let's say that you've only got 5 gallons in the right
tank, and the wind conditions on landing require a pronounced slip with a
bank to the right. If the fuel port is inboard and all the fuel goes
sloshing outboard, and only the right tank is selected, you've just starved
your engine of fuel. But as long as you have some fuel in both tanks and
Both selected, you ought to be fine even in a prolonged uncoordinated
condition.


1. Is the fuel port inboard on the C172?
2. Does selecting "both" draw fuel from both tanks, even if one is empty?




The only time that fuel is set to one particular tank is on the
ground. I've never used fuel from only one tank in flight. Why would
someone do that?

Cessnas have the notorious characteristic of not drawing fuel evenly from
both tanks in the Both position. This can result in fuel imbalance. In a
Cessna 172 you might never notice any difference in the flight
characteristics, but in a 182 which has more fuel capacity in each wing than
the 172 has in total, you can develop an imbalance which becomes fairly
uncomfortable. By selecting one tank or the other, you can burn fuel
(mostly) from the full tank in order to reestablish lateral balance.

Read your autopilot limitations carefully as well. My POH prohibits
operation of the autopilot when fuel imbalance exceeds 90 lbs. That's only
a 15 gallon difference between left and right, so I try to stay ahead of the
situation.


Why does is operation of the A/P prohibited when there is a fuel imbalance?




Also, why is the fuel selector set to one tank during refueling? Is
it to minimize crossfeeding?

If you're parked not quite level, and start refueling with the fuel selector
in the Both postion, if you start by topping off the high wing, some of that
fuel may crossfeed into the low wing while you switch sides and go about
filling the low wing. If you don't recheck the high wing, you might not
notice that the first tank is no longer full, and that you are a few gallons
short of the fuel load you planned for your trip.


Thanks,
Alex

Therefore, on these longer flights and only during level cruise, the pilot
should be using the Left/Right selector knob to even out this imbalance. I
find myself adjusting the selector knob once every thirty minutes or so.

--
Peter



Peter,

Is the procedure of only using one tank only applicable to certain
C172 year/models? I've flown a variety of 172's from late 60's models
to brand new 2002's, and have always used "both" tanks during
flight...just wondering...

Thanks,
Alex

Also, why is the fuel selector set to one tank during refueling? Is
it to minimize crossfeeding?

Yep. In my Maule, the lower tank will overflow through the vents if this is not
done. My 150 (which didn't have the option of selecting tanks) would do the
same if the vented tank was the lower one. Note that my Maule will do this even
if the selector is set to "off". Dunno if that's the case with a Cessna.


Would you mind re-explaining the mechanics you just described? I
don't get the "overflow through the vents" part".

Thanks a bunch,
Alex

(Koopas Ly) wrote

2. Does selecting "both" draw fuel from both tanks, even if one
is empty?

Well...it can hardly draw fuel from both tanks if one is empty.
The tank with the heavest "stuff" in it does the feeding. Fuel
is probably heavier than air.

Bob Moore

On 5 Dec 2003 01:17:22 -0800, (Koopas Ly)
wrote:

1. Is the fuel port inboard on the C172?
2. Does selecting "both" draw fuel from both tanks, even if one is empty?

I have an oblique answer to this question. I'm building an airplane
that seats four and is a high wing monoplane. The fuel tanks are in
the wing root, just like most high wing Cessnas. The plans show that
the fuel outlets should be placed near the wingroot but in different
locations fore and aft from each other. One tank has an outlet near
the wingroot by the trailing edge, the other tank has the outlet near
the wingroot but towards the leading edge.

The wings have dihedral so the fuel wants to flow to the wingroot and
the two differently located outlets are specifically designed that way
for low fuel situations. If you are low on fuel and you nose down for
descent, the leading edge outlet will flow fuel while the trailing
edge outlet may be unported (dry). If you have to go around, the
trailing edge outlet will flow fuel while the leading edge outlet may
be unported.

This is a gravity feed system. An option is to install a header tank
ahead of the instrument panel to augment the somewhat small wing
tanks.

The designer implied that this is a standard aviation design (the
different locations for the fuel outlets).

Corky Scott

PS, in addition

Koopas Ly ) wrote:

Is the procedure of only using one tank only applicable to certain
C172 year/models? I've flown a variety of 172's from late 60's models
to brand new 2002's, and have always used "both" tanks during
flight...just wondering...

Switching tanks is not in the '02 172 CRUISE checklist, but it is something
that I do to maintain an even balance across both tanks. This is because
the difference (at least in the '02 172SP I fly) between the two after a
long flight can be substantial.

--
Peter












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"Frank" wrote in message
...
Tony Cox wrote:

Some models of 172 *require* you to select a single tank
above 5000', due (supposedly) to vapor lock problems.


No 'supposedly' about it. Ours locked at 7500' on a humid day because one
of
my partners didn't believe it could happen and didn't follow the
procedure.
Fortunately an airport was within glide range. Strangely enough, by the
time he landed the engine was running fine again....
--
Frank....H

Its happened to me too. I said 'supposedly' because I've not
heard a plausible explanation as to why selecting one
particular tank rather than both should affect the chance of
vapor lock. Is the fuel selector close to a heat source?

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

Dan Thomas wrote:

Lots of old pilots didn't trust fuel gauges, which is still a
wise attitude. They'd sometimes fly on one tank until it ran dry and
the engine quit, then switch to the other and know exactly how much
they had left and how much they'd burned. This doesn't work well if
the tanks are very far off the airplane's centreline, as the imbalance
can require increases aileron input, causing more drag and tiring the
pilot. It can also panic passengers and create unpleasant cabin odors
and extra janitorial work after the flight.

If you're paying attention and know your fuel burn you can predict the moment of
running out within plus/minus a few minutes. When the fuel pressure gauge drops
to zero, you still have some time while the engine continues to run smoothly.
Switch tanks when the fuel pressure drops, the pax will never know.

Of course, you've run some on the other tank previously, so you know the fuel in
the other tank is sweet.

This old pilot thinks everyone should do this at least once so that they know
how their plane behaves in this situation. It also provides a chance to make a
direct measurement of exactly what your tank capacity is.

Remove SHIRT to reply directly.
Dave

Koopas Ly wrote:

Would you mind re-explaining the mechanics you just described? I
don't get the "overflow through the vents" part".

On the Maule, each tank has two 1/8" tubes which run up through the bottom of
the tank and end just below the top, inside the tank. These are vents intended
to allow air to enter the tank as fuel is burned. If the tank is overfilled, the
excess will also run out the vents. My old 150 had only one of these vents, but
it also had vented fuel tank caps. The vent of the 150 is larger than those on
Maules.

The Maule has a fuel system in which there is a fuel line coming out of one
of the lowest spots in the tank. IIRC, the right tank line crosses the cabin
just behind the top of the windshield, and both left and right fuel lines then
run down inside the left front door post to a valve on the lower left side
panel. A single fuel line then runs from here to the engine. In the 150, the
valve is in the floor, and I'm not sure if the right line crosses above or below
the cabin. The 150 also had a separate line between the two tanks which ran
above the cabin to allow air flow between the tanks to try to equalize fuel
burn, and fuel flows through this if either tank is more than about 3/8 full.

In either case, if the valve is "on" in the 150 or on "both" in the Maule, and
you park the plane sideways on an incline, fuel will run downhill to the lower
tank through the valve. If both tanks are full, fuel will discharge through the
vent tubes on that tank. In my 150, this was only a problem if the plane was
parked so that the vent was on the downhill side.

Some aircraft have anti-syphon devices on the vent tubes to prevent this sort of
thing. Perhaps the 172 does.

This problem also occurs in my Maule when the valve is "off", but not when it is
set to "left" or "right". I do not understand why this happens, but apparently
it is not unique to Maules. I got the suggestion of setting it to one tank from
an article in either AOPA Pilot or Sport Aviation some time back. The author
was advising someone else who had the same problem.

George Patterson
Some people think they hear a call to the priesthood when what they really
hear is a tiny voice whispering "It's indoor work with no heavy lifting".

"CVBreard" wrote in message ...
Lots of old pilots didn't trust fuel gauges, which is still a
wise attitude. They'd sometimes fly on one tank until it ran dry and
the engine quit, then switch to the other and know exactly how much
they had left and how much they'd burned.

I'm an old pilot, don't trust fuel gauges and purposely ran a tank dry...only
once...scared the hell out of me and my wife. Never did that again. :-)

Some planes have more complex fuel management than others. We have 5
tanks on our plane. We've run the tips dry a few times. Yes, it's a bit unsettling,
but Margy and I have the tank switch drill down. She hits the pump and I reach
for the fuel selector. Of course the real fun one was when Margy switched from
the tips to OFF rather than MAIN on downwind one day.