"Tanks on both" checklist item

[Koopas Ly]

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

[Andrew Rowley]

(Koopas Ly) wrote:

>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?

One reason I was given was "so when the engine stops you can try the
other one."

This was from someone who had been test flying a homebuilt, and the
builder apparently hadn't done a good job of cleaning out his tanks
while building...

[Craig Prouse]

"Koopas Ly" wrote:

> 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 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.


> 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.


> 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.

[Roger Hamlett]

"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

[Jaap Berkhout]

> "Koopas Ly" wrote:
> > 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.

I rent the aircraft I fly. Several times the previous renter wrote a squawk
stating: "Fuel gets used only from left/right tank.". Thus far it turned
out that they could not fly coordinated...
Nevertheless, whenever I see this kind of squawk, I select left tank to
taxi, right tank for runup, both tanks for takeoff. After having reached
sufficient altitude, I fly for several minutes first on the left tank, then
on the right. If this works oke, there is nothing amiss with the fuel feed.
Of course, if it turns out there IS a problem, I can select the other tank
(or both) and return with sufficient fuel. Beter to find out early than
discovering later in the flight one of the tanks does not feed at a moment I
do not expect it and possibly at a point which is a bit awkward.

[john]

>"Koopas Ly" wrote:
>
>> 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?

>
>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.

On longer XC flights in my 172N, my SOP is to depart with selector on
both & remain there 1/2 hour. Next 1/2 is on right only. Then, back to
both for next 1/2 hr, continuing that sequence with prelanding
checklist calling for both. The drawdown of fuel is much faster from
the left tank than the right when using both.

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

I leave the selector on both when refueling. After topping the tanks,
I rock the wings before I go pay & pee. During the following
preflight, when I confirm the placement of the filler caps, the fuel
level is always topped off.
YMMV
John
>
>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.

[Peter R.]

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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[COUGARNFW]

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

[Tony Cox]

"Koopas Ly" > wrote in message
om...
>
> 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?
>


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

Check your POH.

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

[Peter R.]

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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[Frank]

Tony Cox wrote:

> "Koopas Ly" > wrote in message
> om...
>>
>> 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?
>>
>
>
> Some models of 172 *require* you to select a single tank
> above 5000', due (supposedly) to vapor lock problems.
>
> Check your POH.
>

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

[Craig Prouse]

"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.

[Bob Martin]

> 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?

Not really related to 172's, but our RV-6 has no "both" position... either
left, right, or off. You just have to remember to switch them occasionally.

Also, IIRC the 150 has just "on" and "off".

[Roger Hamlett]

"Bob Martin" > wrote in message
...
> > 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?
>
> Not really related to 172's, but our RV-6 has no "both" position... either
> left, right, or off. You just have to remember to switch them
occasionally.
>
> Also, IIRC the 150 has just "on" and "off".
There is a significant difference here between low wing, and high wing
designs. If you have a low wing aircraft, and have a 'both' selector, if one
goes empty, air is sucked into the system by the fuel pump, and fuel
delivery stops. Hence low wing aircraft with a 'both' position, have to have
a central 'sump' from which the fuel is drawn. On high wing aircraft, where
the fuel is delivered by gravity, if one tank goes dry in the both position,
fuel will still feed from the other. Hence 'both', is relatively unusual on
low wing designs, but more common on high wing planes. This is why your RV6,
doesn't have a 'both'.

Best Wishes

[G.R. Patterson III]

Koopas Ly wrote:
>
> 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?

It's basically to ensure that you will have fuel through the entire takeoff. The
reason it's done for landing is in case you need to go around. The idea is that,
even if one tank goes empty or has some other problem, you will have a steady
supply of fuel.

> The only time that fuel is set to one particular tank is on the
> ground.

Then you don't have to worry too much about it, do you?

> I've never used fuel from only one tank in flight. Why would
> someone do that?

I do it in my Maule to equalize fuel burn. It will draw down the left tank more
rapidly than the right if I don't. I believe most Cessnas are worse in this
regard than my aircraft; my 150 certainly was.

> 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.

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".

[Jay Somerset]

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

[Don Tuite]

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 here:

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/WebCurrentADFRMakeModel/2C7B32477D2A88A286256A340063355D?OpenDocument

Don

[Dan Thomas]

"Tony Cox" > wrote in message .net>...
> "Koopas Ly" > wrote in message
> om...
> >
> > 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?
> >
>
>
> Some models of 172 *require* you to select a single tank
> above 5000', due (supposedly) to vapor lock problems.
>
> Check your POH.


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.

Dan

[CVBreard]

> 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. :-)

(Longer story, but that's the short version.)

[Bob Martin]

> > Not really related to 172's, but our RV-6 has no "both" position...
either
> > left, right, or off. You just have to remember to switch them
> occasionally.
> >
> > Also, IIRC the 150 has just "on" and "off".
> There is a significant difference here between low wing, and high wing
> designs. If you have a low wing aircraft, and have a 'both' selector, if
one
> goes empty, air is sucked into the system by the fuel pump, and fuel
> delivery stops. Hence low wing aircraft with a 'both' position, have to
have
> a central 'sump' from which the fuel is drawn. On high wing aircraft,
where
> the fuel is delivered by gravity, if one tank goes dry in the both
position,
> fuel will still feed from the other. Hence 'both', is relatively unusual
on
> low wing designs, but more common on high wing planes. This is why your
RV6,
> doesn't have a 'both'.


Ok, that makes sense now... thanks

[Koopas Ly]

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

[Koopas Ly]

> 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]

> > 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

[Robert Moore]

(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

[Corky Scott]

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

[Peter R.]

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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[Tony Cox]

"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/

[Dave Butler]

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

[G.R. Patterson III]

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".

[Ron Natalie]

"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.

[Judah]

The flight school where I rent from has all late-model Cessnas, and there
is no doubt that they do not drain evenly.

I've heard several "urban-legend" style explanations for the uneven
drainage issue, and I don't claim to know which one is accurate. However,
they don't generally seem to drain unevenly enough to have any significant
impact on weight and balance...

Now I actually did all my PPL training in a low-wing Tampico that did not
have a "both" option. And a year or so later, when I started flying
Cessnas, I saw the "both" selector as a major convenience, and I took
advantage of it. But a while ago, I flew in a friend's Bonanza, a low-
winger, and we were talking about fuel management. And he reminded me of an
EXCELLENT reason to switch tanks during flight even if there is a Both
selector. I think someone on this thread mentioned briefly...

Basically, let's say your flying West, into a headwind. It's a little
stronger than expected, and so you're all ****ed off, and gunning the
engine up a bit to try to make up some of the ground speed. Maybe you're at
a different altitude than planned, and for whatever reason you're not
leaned out as well as you could be. Based on your "optimum" calculations,
you think you have 5 hours of fuel. But between the headwind, and not
flying optimally as planned, you use 10% more fuel than expected, and you
are going to use 10% more time than expected...

Bottom line, you now need a fuel stop, but you don't know it...

If your fuel selector is on Both, you get one shot. Your right tank may run
dry first, but you won't have any idea until your completely dry in both
tanks. You get to glide to the nearest golf course and call for help.

If you are switching tanks every 1/2 hour, and your right tank runs dry,
it's like an alarm. Your engine fails, and you do your ABC checklist,
switch tanks, and get your power back. You can take a moment to calm down,
figure out what happened, and find the nearest airport with services and
fuel up. You get a "second chance". Of course, it's not guaranteed - you
might only have a drop of fuel in the other tank. But flying on both
guarantees you to be a glider on empty...

Ever since that conversation, I fly "low-wing" style once airborne, and
switch tanks to both on taxi, takeoff, and landing only...



Peter R. > wrote in
:

> 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.
>

[Dennis O'Connor]

Rules To Fly By And Not Run Out Of Fuel:

I routinely run the outboards dry on long cross countries (know your
airplane on this issue).. This cross checks my fuel burn against the clock
since I know exactly how much was in the tank when I selected it... Running
it dry removes fuel gauge error from the equation... I'm usually within five
minutes...

I fly by the clock, not the fuel gauge... If the gauge gets too far from
what the clock says it should be (either full or empty), we put down and
refuel to see what the heck is going on... <this only happened once - a
sticky gauge>

Lastly, I never get into the last hour of fuel for any reason... 60 minutes
of fuel left on the clock is "bingo time" and it is put it on the ground.. I
have landed and refueled just twenty minutes from my destination because I
hit bingo before I got there... One of the locals back at the home field
thought I was nuts... But then, he has had an engine run out of fuel while
taxing in from the runway one time, so what do you think I feel about his
opinion...

Denny - an old pilot...

"Dave Butler" > wrote in message
...
> Dan Thomas wrote:
>
> > Lots of old pilots didn't trust fuel gauges, which is still a
> > wise attitude.

[Ron Natalie]

"Judah" > wrote in message ...

> I've heard several "urban-legend" style explanations for the uneven
> drainage issue, and I don't claim to know which one is accurate. However,
> they don't generally seem to drain unevenly enough to have any significant
> impact on weight and balance...

The crossfeed issue on the 172 (why the left tank drains faster) is not
an urban legend. If you are a Cessna Pilot's Association member they
will send you a brief on what it is and how to fix it by tweaking the tank
vent.

[Teacherjh]

>>
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.
<<

Since it's a homebuilt, you have some flexibility in how you do things. Why
not put in two ports for each tank - one fore one aft. That way you can slip
and nose down and get fuel no matter which tank is selected? Try talking with
the designer of the plane about this - it might be worth considering.

Jose

--
(for Email, make the obvious changes in my address)

[Dale]

In article >,
(Corky Scott) wrote:


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

The way your fuel system is designed is a good idea. On the 182 I owned
the fuel tank outlets were in the same position fore/aft for each tank
so it isn't an industry wide standard.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

[G.R. Patterson III]

Judah wrote:
>
> If you are switching tanks every 1/2 hour, and your right tank runs dry,
> it's like an alarm.

Right. You switched tanks 3 minutes ago. Hope you're right over an airport.

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".

[Big John]

Dan

From an 'old' pilot.

We used to fly 30 minutes on a tank and then switch to other tank
(back and forth). This manual switching kept the fuel inbalance to a
max of 30 minutes fuel which was very manageable. Trying to feed from
both tanks for any period of time always caused a inbalance.

Also knew fuel burn and could compare fuel used and remaining, to fuel
gauge.

For landing, fuel was switched to fullest tank (check list item).

In 'heavy iron' and max range, would run tank dry. In GA, never
planned that long a duration so never had a engine stoppage caused by
empty tank.

Safety is as safety does.

Big John

On 4 Dec 2003 17:19:07 -0800, (Dan Thomas)
wrote:

>"Tony Cox" > wrote in message .net>...
>> "Koopas Ly" > wrote in message
>> om...
>> >
>> > 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?
>> >
>>
>>
>> Some models of 172 *require* you to select a single tank
>> above 5000', due (supposedly) to vapor lock problems.
>>
>> Check your POH.
>
>
> 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.
>
> Dan

[Robert M. Gary]

I don't have a "both" on my Mooney and I DO NOT miss it. "Both" just
means that if you ever run out of gas, you're really out of gas. The
Cessna 140 I used to fly would drain all the gas from the left before
touching the gas in the right (its not the same as the C-150, there is
no interconnecting hose in the 140).


-Robert



(Koopas Ly) wrote in message >...
> 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

[Robert M. Gary]

(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. :-)
>
> (Longer story, but that's the short version.)

Danm good thing you didn't have a "both" selector or you may not have
been here to tell about it! :)

-Robert

[Robert M. Gary]

An an example, the Swift (low wing) is just "both". There are a lot of
examples of low wing planes with "both".

In my Mooney, if I ever run out of gas, I'll just switch tanks. Thank
God there isn't a "both"! :)



> There is a significant difference here between low wing, and high wing
> designs. If you have a low wing aircraft, and have a 'both' selector, if one
> goes empty, air is sucked into the system by the fuel pump, and fuel
> delivery stops. Hence low wing aircraft with a 'both' position, have to have
> a central 'sump' from which the fuel is drawn. On high wing aircraft, where
> the fuel is delivered by gravity, if one tank goes dry in the both position,
> fuel will still feed from the other. Hence 'both', is relatively unusual on
> low wing designs, but more common on high wing planes. This is why your RV6,
> doesn't have a 'both'.
>
> Best Wishes

[John Galban]

Dave Butler > wrote in message >...
>
> 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.

Bingo! Dave's right on the money. My plane's flight manual
cautions against running a tank dry (I believe there's an outside
possibility of fuel pump cavitation). Using fuel pressure method
works great. I do this on long cross-country flights where maximizing
my range is important. In my Cherokee, the fuel pressure will begin
to drop with about 1.5 gallons usable in the tank (close enough for
me). At this point, assuming no unusual fuel burn, I have 1.5 hrs of
fuel left in the other tank, which in the real world means I have 1/2
hr. to be on the ground.

>
> 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.

Agreed. I do this on every flight where I'll be flying at maximum
range. By running a fuel tank almost dry, I know if my fuel burn is
normal and can complete the flight as planned. If the tank runs out
before the clock says it should, I still have a bunch of gas to make
it to an alternate (assuming I'm not flying in some of the more remote
parts of the Northern Territories of Canada :-).

By using this method, I can usually guess the fuel burn on a long
cross-country flight to within 1 gallon. This kind of knowledge makes
flying in sparsely populated areas a lot more comfortable and allows
me to get better utilization of the amount of fuel my plane is able to
carry.

Note : Do not take the above to mean that I fly my tanks to
dangerously low levels. My personal limit is to land with a minimum
of 1 hr. in the tanks. In 16 yrs. I've never landed with less, but by
gauging my fuel burn precisely, I can often reach my destination with
1.2 hrs. remaining, thereby maximizing my range, yet minimizing my
risks.

John Galban=====>N4BQ (PA28-180)

[Mike O'Malley]

"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.

[Frank]

Tony Cox wrote:

> "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?
>

The explanation I got had to do with pressure (or lack of it) above 4500'.
This problem is exacerbated by high humidity. It seems that the plumbing
allows a condition to develop whereby there is not enough pressure from
gravity to push the fuel through the plumbing without forming "bubbles". It
has to do with the venting. Closing off one tank (ie selecting left or
right) solves this.

This admittedly vague explanation is only intended to point out that
pressure differences are the culprit. No one has ever shown me in great
detail just how it happens. Nonetheless, I'm a believer.

--
Frank....H

[Tony Cox]

"Frank" > wrote in message
...
> Tony Cox wrote:
> >
> > 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?
> >
>
> The explanation I got had to do with pressure (or lack of it) above 4500'.
> This problem is exacerbated by high humidity. It seems that the plumbing
> allows a condition to develop whereby there is not enough pressure from
> gravity to push the fuel through the plumbing without forming "bubbles".
It
> has to do with the venting. Closing off one tank (ie selecting left or
> right) solves this.
>
> This admittedly vague explanation is only intended to point out that
> pressure differences are the culprit. No one has ever shown me in great
> detail just how it happens. Nonetheless, I'm a believer.
>


My 'vapour lock' occurred at 7500' on a cold November day
with low humidity (ceilings were well above us). If anything,
switching to one tank rather than both should actually *lower*
the fuel pressure slightly - faster flow = greater friction loss.

So I'm not really impressed at all by the explanation. Luckily,
engine power was restored when passing through 3500',
shortly (as it happens) after fiddling with the fuel selector as
part of the emergency checklist. Incidentally, that was the last
time I assumed that the POH for one plane was the same as
another.

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

[Frank]

Tony Cox wrote:

> "Frank" > wrote in message
> ...

<snip>

>> The explanation I got had to do with pressure (or lack of it) above
>> 4500'. This problem is exacerbated by high humidity. It seems that the
>> plumbing allows a condition to develop whereby there is not enough
>> pressure from gravity to push the fuel through the plumbing without
>> forming "bubbles".
>> It has to do with the venting. Closing off one tank (ie selecting left or
>> right) solves this.
>>
>> This admittedly vague explanation is only intended to point out that
>> pressure differences are the culprit. No one has ever shown me in great
>> detail just how it happens. Nonetheless, I'm a believer.


> My 'vapour lock' occurred at 7500' on a cold November day
> with low humidity (ceilings were well above us). If anything,
> switching to one tank rather than both should actually *lower*
> the fuel pressure slightly - faster flow = greater friction loss.
>

Right, lower fuel pressure means less oomph to get it through the
pipes....Seems consistent with the 'theory'. I don't know enough about it
to say whether it's right or wrong.


> So I'm not really impressed at all by the explanation. Luckily,
> engine power was restored when passing through 3500',
> shortly (as it happens) after fiddling with the fuel selector as
> part of the emergency checklist. Incidentally, that was the last
> time I assumed that the POH for one plane was the same as
> another.
>

I know the explanation wasn't very satisfying, I'd love to hear it explained
better too. But, as you have apparently also experienced, the phenomenon is
real. So there is something going on and pressure differences due to
venting/plumbing seems to explain it better than temperature related ones.

--
Frank....H

[Tony Cox]

"Frank" > wrote in message
...
> Tony Cox wrote:
>
> > My 'vapour lock' occurred at 7500' on a cold November day
> > with low humidity (ceilings were well above us). If anything,
> > switching to one tank rather than both should actually *lower*
> > the fuel pressure slightly - faster flow = greater friction loss.
> >
>
> Right, lower fuel pressure means less oomph to get it through the
> pipes....Seems consistent with the 'theory'. I don't know enough about it
> to say whether it's right or wrong.
>

To get maximum oomph, it would seem that 'both' should be
selected. Which is just what they tell you not to do!

>
> I know the explanation wasn't very satisfying, I'd love to hear it
explained
> better too. But, as you have apparently also experienced, the phenomenon
is
> real. So there is something going on and pressure differences due to
> venting/plumbing seems to explain it better than temperature related ones.
>

I just draw a blank on this. Unless there is a heat source near
the selector. Then I can rationalize an explanation. If there's some
venting/plumbing explanation, I'd expect some admonition to
"select the fullest tank" or "don't fly above 5000' without more
than 10 gallons in a tank". Makes no sense as is.

tc

[Frank]

Tony Cox wrote:

<snip>
> I just draw a blank on this. Unless there is a heat source near
> the selector. Then I can rationalize an explanation. If there's some
> venting/plumbing explanation, I'd expect some admonition to
> "select the fullest tank" or "don't fly above 5000' without more
> than 10 gallons in a tank". Makes no sense as is.
>
> tc

As far as I can tell there is no heat source anywhere near the selector.

One day I'll get the real story, until then....I'll just select L or R above
5000' and chalk it up to one of those mysteries of the universe.
--
Frank....H