The Dash 8 has two different main electric systems installed.
Firstly, there is the variable frequency AC system that generally feeds high-load systems like probe and propeller heats, the galley ovens and also the #1 hydraulic system electric pump. The generators are connected to the propeller gearboxes and require the propeller to run unfeathered to provide power.
And secondly, there is the 28VDC system driving everything else ranging from avionics to cabin lights, and also the engine starter generators. This is powered by the two mentioned starter generators, when the respective engine is running, and the APU generator as well (usable only on the ground due to airflow issues inflight that prohibited the certification, or so I am told).
There are 4 hydraulic circuits installed, of which only the 2 main ones are relevant for taxi. Both have one dedicated engine driven pump which is found on the prop gearbox as well; in case of a pump failure, #1 is fed with the mentioned AC-driven pump and #2 leeches on #1 via a PTU if needed.
So the APU has no connection at all to the hydraulic systems and therefore is not required to be run for single engine taxi. Assuming #1 engine has been feathered (but running) during the case in question, its EDP would still provide nominal pressure, albeit at a lower flow rate. But even with #1 engine completely shut down, the electric pump would get its AC from the still working #2 engine and keep #1 hydraulic system and therefore the normal brake system fully serviceable. And #2 hydraulic is still fed by the unfeathered #2 engine and its EDP; typically the PTU is run during taxi as well to cover for contingencies.
At Tyrolean, we are encouraged to taxi with one engine in feather, as it keeps cabin noise down and also saves a drop of fuel. It comes with some operational caveats, but when one knows the pitfalls here, it is a reasonable procedure IMHO.
Last edited by Tu.114; 4th Feb 2013 at 13:24.