This can be argued two ways:
The “100% known” bit is difficult to implement. You need one of
Obviously having accurate gauges is unbeatable, for this purpose.
IMHO, the most important question is: when is it too late ? .
Once in the pattern or established final, I try not to switch anything unless absolutely necessary, which means I forgot it previously …
The approach is the most complex workload time for any pilot – especially in IMC. Switching fuel tanks on approach is ill-advised and should be done in level flight either enroute or in the pattern prior to initiating the approach. Here is a good example why:
NTSB determined the probable cause as the pilot’s failure to position the fuel selector handle in a fuel tank detent while on approach: http://generalaviationnews.com/2015/12/21/fuel-exhaustion-fatal-2/
True, but switching tanks “late” is better than running out of juice at 300ft…
Switching tanks is very unlikely to stop the engine, but running out of fuel is awfully very likely to stop the engine 
Obviously the context here is not landing with 50 USG in each tank; you would not switch tanks then 
Finally, switching “late” is not switching when flying an approach. I am thinking of switching say 30nm out.
Or just stay on “Both” :-), which is one of the reasons in favor of the 182
Part of my approach checklist, always: fuel selector to fullest tank. It is not only about the landing but also about a possible go-around.
PetitCessnaVoyageur wrote:
Or just stay on “Both” :-), which is one of the reasons in favor of the 182
Amen to that!
;-)
On the DA40-180 my schedule is to feed from the takeoff tank until established at cruise altitude, then switch tank, and then switch tanks every hour. That will normally give a max imbalance of 4-5 USG.
I think about what tank I am will be landing on already at the start of the arrival, and when I will be switching. Then I check again before the approach. The DA40 has very accurate fuel gauges and fuel totaliser, so it is easy to do.
I usually switch to the fullest inner tank before commencing descent, verifying that I have enough for the descent, procedure, go around etc so I don’t have to think about fuel while I’m busy later. The inner tanks also cause less lateral trim change with fuel use.
On every aircraft that I fly or have flown the check of the fuel system and selection of the correct tank (which must not necessarily be the fullest on some types) is part of the approach items and subsequent approach checklist. This should be completed before the approach itself begins at an altitude which is sufficient to sort out minor problems which could result from these handlings.
My personal list of malfunctions and failures is long, but does not have anything on it that was caused by operating controls of the fuel system. And mind you, when flying something like a C340 one has to switch tanks and fuel pumps at least ten times per flight – meaning ten chances for a malfunction on each flight. None has occurred so far, so I personally consider switching tanks as perfectly safe
