"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

(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...

"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.

"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.

"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.

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) 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

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

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

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