"Tanks on both" checklist item

"Corky Scott" wrote in message
...

snip


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


Dunno about others but in the few I've had occasion to help build-

The PA-12 has outlets only at the aft end of the tank. This worked great
for our use, banner towing, as most our flying was at high AoA (though,
since the gauge was at the FORWARD end of the tank they didn't work after
30-45 minutes).

The PA-18 has pickups at both ends, then feeds into a Y fitting, that then
goes to a header tank (small 1 gal) then to the selector valve. Worked
even better.

"Koopas Ly" wrote in message
om...
Good day all,

With regards to fuel tanks in a C172, why does the takeoff and landing
checklists both call for the fuel selector handle to be in the "both
tanks" position?

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?

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

Thanks,
Alex
There are several 'parts' to the answer here. The first is that in certain
failure scenarios, the ability to switch tanks is useful. Imagine in flight,
you suddenly see a leak from the right tank. In this situation, you
obviously want to land quickly, but with the individual tank selection
ability, you can switch to burn fuel from the leaking tank, then switch to
the other when this runs out, and this tank will not be loosing fuel. In a
sense this is a 'left over' from larger multi-tank installations.
The second relates to a problem that Some versions of the Cessna have in
flight, where in certain atitudes, there can be fuel feed problems. Some are
placarded to use single tanks at altitude to avoid this, since when the
problem occurs, switching to the other tank cures it (at least temporarily).
This was to do with a low pressure area forming over the fuel cap, and the
fuel caps were redesigned to prevent it.
The 'cross feed' answer is correct on fuelling. If the selector is left to
both, especially if the plane is not level, and the upper tank is filled
first, fuel can drain into the lower tank, which is then filled, and the
result is a fuel load significantly below what is expected.
The same problem, can also "rear it's head" in flight. If a pilot flies the
plane out of balance (or it is not rigged quite square), there can be a very
significant tendency to cross feed. Manually using the selector to draw fuel
from the 'heavy' wing, can allow this to be compensated for.

Best Wishes

Koopas Ly ) wrote:

snip
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?

The C172 (at least the more current model years with which I am familiar)
does not draw fuel from both tanks evenly when the selector knob is set to
"Both." The reasons for this anomaly are numerous.

This unequal fuel draw tends to be more noticeable on longer XC flights
where one tank could potentially be five-to-eight gallons lower per hour
when compared to the other. Unequal fuel load translates to unequal
weight distribution.

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












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Amazing...all those answers by 172 owners and no one of them went to the net to
find the "truth".

If you go to Google and fill in "Airworthiness Directive for 172 fuel system",
you will get a string of answers and some really foolish conclusions (like this
string) that are worth reading.

One states...my plane has the placard to not fly on both above 5,000 feet, but
I did and the engine quit at 8500. Sigh (mine).

If you know how to get to rec.aviation.owning of the Usenet, the string is
there.

Note that the problem is model/version specific.

Neal

COUGARNFW ) wrote:

Amazing...all those answers by 172 owners and no one of them went to the
net to find the "truth".

If you go to Google and fill in "Airworthiness Directive for 172 fuel
system", you will get a string of answers and some really foolish
conclusions (like this string) that are worth reading.

It really must be painful for you to walk among mere mortals.

--
Peter












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"COUGARNFW" wrote:

Amazing...all those answers by 172 owners and no one of them went to the net
to find the "truth".

The question is why is the selector placed in the "BOTH" position for
landing, not why is the selector placed in some other position above a
certain altitude for certain models.

On 04 Dec 2003 14:50:32 GMT, (COUGARNFW) wrote:

Amazing...all those answers by 172 owners and no one of them went to the net to
find the "truth".

If you go to Google and fill in "Airworthiness Directive for 172 fuel system",
you will get a string of answers and some really foolish conclusions (like this
string) that are worth reading.

Just tried that -- the only one that came up was your post.


One states...my plane has the placard to not fly on both above 5,000 feet, but
I did and the engine quit at 8500. Sigh (mine).

If you know how to get to rec.aviation.owning of the Usenet, the string is
there.

Note that the problem is model/version specific.

Neal

On Thu, 04 Dec 2003 18:58:52 -0500, "Jay Somerset"
wrote:

On 04 Dec 2003 14:50:32 GMT, (COUGARNFW) wrote:

Amazing...all those answers by 172 owners and no one of them went to the net to
find the "truth".

If you go to Google and fill in "Airworthiness Directive for 172 fuel system",
you will get a string of answers and some really foolish conclusions (like this
string) that are worth reading.

Just tried that -- the only one that came up was your post.

Are you guys talking about AD 72-07-02?

The text is he

http://www.airweb.faa.gov/Regulatory...D?OpenDocument

Don

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

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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