"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