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FAA Glossaries

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Split Master

If your airplane has a split master switch, one half provides current to the alternator field windings and that side can be turned off during engine start to reduce the load on the battery. Once the engine is running, the alternator field side of the master switch can be turned back on to provide electricity to the rest of the electrical system. This procedure will not be found in the Pilot’s Operating Handbook.—Bob Gardner

It’s common sense, when you think about it. With both halves of the switch ON, the alternator field windings are connected across the battery, creating a drain in addition to that drawn by the starter…and the alternator can’t make electricity until the engine is rotating anyway. So why keep that load across the battery? Turn off the alternator side, directing all battery voltage to the starter, and only then put the alternator field windings into play.—Bob Gardner

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

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