>  I suspected a charging system failure immediately and checked the
>battery voltage in the SDS monitor. This read 9.5 volts so I knew then

>we had a charging system failure. At the time, I was not worried about
>making it to an airport, assuming that the alternator had failed just in

>last few minutes and believing that I had 20-25 minutes of battery power
>remaining. In fact, it is likely that the alternator failed soon after
>takeoff from Springbank but I did not notice signs of the failure until

>battery was well over half dead.

         At 10.5 volts, the battery is officially 100% discharged. At 9.5
volts, the battery was not "half dead" but completely dead. He was running
on the battery "fumes".

         The battery voltage drops very, very rapidly under 11 volts.

         If the battery voltage drops much below 13 volts while in flight,
an idiot light should come on, and you should head for the closest

You should turn off anything electrical that you don't really need to fly
the airplane, communicate, and navigate. A full-throttle climb might be a
good idea if you are at low altitude and far from the nearest airport.

         If the voltage drops below 12 volts, you should turn off
everything electrical but the systems absolutely essential to keep the
engine running.

         At 11 volts, you need to pick a place to land and do it.

         During and very shortly after starting, it would not be unusual
for the battery voltage to dip below 12 volts. However, I would not take
off with the system below 13 volts. Checking the battery voltage should be
on the run-up checklist. For an EFI or EI engine, the battery voltage
indicator should be as prominent and visible on the instrument panel as

oil pressure indicator.



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