"Tanks on both" checklist item

"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