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"Tanks on both" checklist item



 
 
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  #1  
Old December 4th 03, 11:57 AM
Koopas Ly
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Default "Tanks on both" checklist item

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
  #3  
Old December 4th 03, 12:34 PM
Craig Prouse
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"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.

  #4  
Old December 4th 03, 01:09 PM
Roger Hamlett
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"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


  #5  
Old December 4th 03, 02:05 PM
Jaap Berkhout
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"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.
  #6  
Old December 4th 03, 02:53 PM
john
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"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.


  #7  
Old December 4th 03, 03:26 PM
Peter R.
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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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  #8  
Old December 4th 03, 03:50 PM
COUGARNFW
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Default

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
  #9  
Old December 4th 03, 04:06 PM
Tony Cox
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"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/


  #10  
Old December 4th 03, 04:37 PM
Peter R.
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Default

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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