Michael wrote:

What does the POH say? The instructions (and posssibly some insight)
will be there.


The instructions will be there, They may or may not be optimal. They
are unlikely to offer any insight.


As a rule, the ALT is left off until the engine is running to avoid
power spikes.


Correct procedure (field off until engine start), wrong reason. You
are not going to hurt your alternator field with a voltage spike
produced during starting - it's not at all delicate. Neither will the
alternator produce much in the way of a spike at starting RPM's. I've
heard that reason passed on from CFI to student for years, and it's
just an aviation OWT.

The real reason is far more prosaic - the alternator field coil draws
3-6 Amps (depending on the alternator) that are better used driving the
starter motor. Aviation batteries are sized to be just barely big
enough to start the engine to keep the weight down, and in terms of
quality and design they are the worst batteries available, far inferior
to any reasonable quality lawn tractor battery. As a result, available
starting power is often marginal and anything done to reduce it is a
bad idea.

It is the avionics master is off during engine start to keep spikes
from getting to the avionics. I suspect that's the cause of the OWT.
In airplanes lacking an avionics master, optimal procedure is to shut
down the avionics prior to engine shutdown and turn them on only after
engine start. In cars this is accomplished by means of an automatic
contactor, but in the aviation world we're not up to that level of
technology.

But the _real_ reason is that when the field winding is energized
(current flowing), the alternator is bogging down the airplane engine.
Generating electricity requires lots of torque. And, the total amount
used to start the plane must be pumped back into the battery. That is
the "short term spike" that people confuse with some sort of static
discharge. Once the battery has been recharged, the torque required
tapers down to just enough to run the plane, but not the starter.

Turning off avionics master, the Pitot heater, and excess loads of that
nature also decrease the mechanical load during starting.