zuutroy wrote:
How long is it safe to leave the master on without the engine running to mess around with the GPS etc? During my training my instructor used to make out like you should almost never have the master on and the engine off.
On the DA40 G1000 and C182 G1000 I left it on long enough to enter the FPL, monitor ATIS and get a start-up clearance. That could be 15 minutes. Never had a problem. But on the G1000 when you turn on the master switch it only partly turn on the avionics (1 NAV, 1 COM, PFD). On the SR22 I did not have any NAV/COM bypass switch and there was no other way to turn on avionics than turning on the avionics switch, hence all avionics; both NAV/COM, XPDR, audio panel, avionics fans… So I got a handheld and entered the FPL only after having started the engine.
On the airplane I currently fly there is a NAV/COM bypass that turns on NAV/COM 1.
So the answer depends on what the draw is with the master switch on, i.e. how many consumers that turns on.
On the SR22 I did not have any NAV/COM bypass switch and there was no other way to turn on avionics than turning on the avionics switch, hence all avionics; both NAV/COM, XPDR, audio panel, avionics fans…
OT, but in the Avidyne SR22, there is a trick to power up only one GNS430. See here:
Cool!
Usually the AFM states the minimum time after alternator failure for which lighting the whole Christmas tree is supported, usually 30 minutes. I wouldn’t worry about tinkering 15 minutes for FPL and startup.
I have done that a couple of times, but it gives me a better a feeling to use the handheld radio for the startup clearance than to drain the small BAT 2. Maybe that’s not an issue, because it will be recharged in flight, but I wonder how long that takes.
Some of us don’t have electric mechanical gyros yet. 
Unless you flew the aircraft yourself last time, and it is not long ago, and you are sure the batttery(-ies) was/were fully charged after flight, there is no way of knowing how much you can drain the energy. Yes, it is supposed to run the Christmas tree for 30 minutes from fulle charged battery state, but that is unrelated to battery requirement for starting the engine. As that requirement is often somewhat unpredictable, you are always reducing your chances for a succesful start-up at least a little bit whenever you run things on the battery with the engine stopped. I avoid it if it is at all possible, except if I know the plane well, and even then I minimize the time and pull the breakers for things like avionic fans and turn gyros. Of course on aircraft having two separate batteries and being able to run things from one battery and start the engine on the other that helps a lot.
I normally only try to flatten a battery if it won’t fit in my suitcase….
Dont wan’t to try, so I bought a hand held :)
A few days ago, I had troubles starting a C152. I was surprised it started after at least 15 trials with the starter. The battery wasn’t weakening. I dont know if it was changed recently but that’s the only explanation I can find.
Quite easy with a very large sledge hammer.
Sorry just could not resist that.
