Menu Sign In Contact FAQ
Banner
Welcome to our forums

The right shutdown sequence? (electrical considerations)

Pilot_DAR wrote:

Once I pull off the runway, and turn off a few lights, I’d turn the alternator back on, and let it top off the battery as I taxied in.

Interesting method. Forget it once and you can call for ground power on the next engine start – at least at places with large distances to taxi. Or forget to turn it on again during a go-around and on the next approach your whole cockpit will go dark.
I have flown a lot on planes with gear driven alternators (Cessna piston twins and jets and turboprops) and never had any gear-related problems with the alternators. All the problems so far came from the electronics/voltage regulators/GCUs/paralleling units.

So I prefer to do things as you write in your first line: Follow the procedures in the manual. So far, all aircraft that I have flown – Cessna&Piper singles&twins, Metroliner, Citations – call for alternators on together with the battery and off together with the battery (apart from the Citation where they always stay on unless for ground power starts).

EDDS - Stuttgart

Usually I set idle, then lean to check small RPM rise, then dead cut check both mags, then lean cut. Avionics off before shut down, bat and alt off after shut down (no particular reason)

Also the point about slipping direct drives to the alternator would also apply to belt drives if slightly slack. Once it slips it will wear rapidly at high load/low RPM conditions.

According to the AFM for the DA40-180, the ALT switch is supposed to be turned ON only AFTER engine start. The shutdown procedure calls for switching OFF ALT/BAT after engine shutdown.

On the DA42 (diesel), according to the AFM, alternators remain ON at all time and need to be checked ON before engine start.

LFPT, LFPN

Isn’t it best to follow the POH. On the army Cub with wind driven alternator the problem solves itself. When the engine is hot, and you are going to have a short stop, it’s best to stop it with the mags, or else you will be very tired trying to start it again (armstrong starter).

The elephant is the circulation
ENVA ENOP ENMO, Norway

Switching the Alt off means only to cut the exciting current. Once Alternators are excited, they will continue delivering current by self excitement. Do pot pull the 50 or 80 Amp CB, it will ruin your diodes.

So it does not matter when you switch the ALT off.

EDTQ / Sarentino

Thank you! That sounds reasonable!

. Once Alternators are excited, they will continue delivering current by self excitement.

You mean you can switch off the field current and it continues to generate say 50A into a load?

An alternator with no field outputs almost no power.

Administrator
Shoreham EGKA, United Kingdom

Snowbirdxx wrote:

Switching the Alt off means only to cut the exciting current. Once Alternators are excited, they will continue delivering current by self excitement.

What kinds of alternators are you talking about? In every aircraft I’ve tried it, switching off the alt field immediately stops the alternator from delivering current.

How would this self-excitement work? An internal connection to the field winding? How then would you turn off the alternator in an emergency, e.g. an over-voltage situation or fire?

ESKC (Uppsala/Sundbro), Sweden

Self excitement works in shunt connected generators. A bit of residual magnetism in the iron gets it started.

But car and plane ones are not shunt wired. The field winding goes only to the regulator.

Administrator
Shoreham EGKA, United Kingdom

Pilot_DAR wrote:

I got into the habit of turning the alternator off as the landing light went on on a night final approach

This might be my conservative mind, but it sounds like one of those stupid habits that students make up when they discuss flight procedures without input from an instructor. From your profile you seem to have quite a bit of experience, so I’ll refrain from making a judgement. Just to say that when flying at night, low to the ground, I personally like to have backups in place and running…

Snowbirdxx wrote:

Once Alternators are excited, they will continue delivering current by self excitement.

I highly doubt this? What then does the regulator do in case of an overvoltage condition? How can it shut down the alternator to protect your 1000s of $$$ avionics? If you are talking about generators, you’d be right. They don’t require a field to generate power.

Anyways:
- Avionics OFF
- Mixture idle/cut-off
- Ignition OFF
- Masters (Alternator) OFF

Last Edited by Archie at 14 Dec 20:57
Sign in to add your message

Back to Top