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The fuel shutoff valve in the Zlin Z-142 has four different positions: Off, Left, Right and Both (L+R).
In the POH, the procedures related to normal flight operations, suggest that during taxiing and takeoff the selector is on Right and to switch to Both 5 minutes after takeoff.
I was wondering: why is that the case? Why not starting with Both? And also, why is it not needed to switch to Left after taking off to balance the fuel?
Section 4, part 4.10 of Zlin Z-142 POH presents a following caution:
Caution:
At 1.L position (LEFT) and fuel quantity drop in this tank below 30 l it is necessary to increase attention to the right control of the aircraft. At long time yawing - the ball of turn and bank indicator being out of the centre on the left side - limitation of the fuel supply can appear and be followed by irregularity of engine run.
In case of fuel pressure fluctuation or irregularity of engine run change over to 2.R (RIGHT)
This leads me to believe that the fuel system design is such, that the left tank is more susceptible to interruptions in fuel flow than the right tank. Since taxiing often imposes high slip forces (while turning), and takeoff is a phase of flight with all sorts of "floating around" regardless of how skilled the pilot is, it is best to ensure the fuel source is as stable as possible. Note that selector position 2.R (RIGHT) is also commanded for approach, and is to remain there all the way to engine shutoff. This would support the theory of right tank being the most reliable source of fuel.
The POH also commands startup and subsequent procedures to be executed with tank selector in 2.R (RIGHT) position. In addition to the right tank being (probably) more reliable when it comes to fuel delivery, not changing the tank until later in flight would keep the fuel flow coming from one single source, and possible problems relating to this source would likely present themselves before lifting off. Should problems arise after liftoff, there is then the possibility to try the left and both positions of tank selector to troubleshoot (I did not check the POH for these procedures).
Depending on fuel system design, selecting both -position during ground maneuvers may lead to fuel transferring inadvertently to either tank due to turns and/or slanted ground. This would cause imbalance during takeoff, which would be, if nothing worse, uncomfortable. More importantly, as StephenS quite correctly pointed out, selecting both tanks for approach and landing might prove catastrophic if, for some reason, extended period of approach was flown in sideslip. This might lead to problems described in POH in a very critical phase of flight.
One should also keep in mind that having 30 l of fuel in left tank you most probably have about the same in right tank too. That would be 60 liters of fuel in total, which is quite a lot, about two hours of flight time (rough estimate). So this this fuelling condition reffed to in POH of Z-142 is present in very "normal" fuel loads, not just when running on fumes.
Common feature in all procedures in POH's generally is keeping thing as simple as possible (this actually is not entirely true, but that's how things should be). This may also be a reason to stick to one fuel selector position until in cruise.
It is simply because in the Z142 the fuel pump is delivering more fuel to the engine than the required and this excessive fuel is transferred back always to the right tank. If during taxi/take off it is full and you would use the left one or both tanks, there would be a possibility to leak this excessive fuel out from the aircraft via the right overflow valve.
