By watching the ammeter as you do this, you can verify the charging system
function and confirm that the starter relay has not hung up which can turn
the starter into a generator at higher RPM's and fry parts of the electrical
system and avionics. It also makes for easier starts in cold weather for
the reasons the other posters mention.

When you start you should see negative ammeter deflection and any alternator
warning lights should come on. After the engine is running. bring the
alternator on line and the ammeter should switch over to positive deflection
and taper back to zero within about a minute. The alternator warning lights
should go out. After you have done this a few times in a plane, you'll be
able to spot any change in charging system function quiet easily.

You can even get an insight into battery condition. If you've drained it by
having lights on for your preflight, running flaps ups and down, etc. You'll
see a larger ammeter deflection. If you see that deflection without a
reason, it may mean something is going south in the charging system.

The only reason I have heard not to do this all the time is that the
alternator has a sudden load thrown on it. This may be an issue for
alternators directly driven by expensive gear trains but I think the belt
driven ones have a pretty good shock absorber in the belt.
--
Roger Long

Robert Winn wrote in message
...
Rented a C172SP (Made in 1999) on 04/10/04 and was told to start the
engine
with the battery side of the master switch only on, then turn on the
alternator side after the engine was running. I was told that this was a
new recommendation for all C172SP's. Can someone tell me where to find
the
text of this recommendation?

Thanks in advance,

Robert Winn