Menu Sign In Contact FAQ
Banner
Welcome to our forums

The right shutdown sequence? (electrical considerations)

As a consequence of a recent alternator failure I came across an article in the Cessna Pilots Association technical resource detailing a preferred engine start procedure on aircraft with a split master/alternator switch. It advocated starting up with the alternator switch left in the off position, only turning it on once the engine has started and settled around 1000 rpm.

My question is not about the starting procedure but the shutting down procedure. This technical article never mentioned shutting down so I’m intrigued to know if turning the alternator off prior to shutdown has any benefit. My normal procedure is to shut down all avionics, pull the mixture then once stopped turn off the ignition switch then turn off both master and alternator. Any thoughts?

EGNS/Garey Airstrip, Isle of Man

Bringing the alternator on line after start, is correct. Most checklists also show taking it off line before shutdown, but am not sure what the technical reason might be. On start, I presume it is to avoid a voltage spike due to engaging the starter. On shutdown less sure why it is recommended practice.

Enstone (EGTN), Oxford (EGTK)

This one has been debated many times. I agree with Robert; there is a good case for having the field current OFF during start, to guard against alternator+regulator misbehaviour and blowing up equipment which is powered before the avionics master switch. But there is no apparent case for doing that at shutdown. I do it anyway for the aforementioned reason: the voltage regulator doing weird things as it finds the alternator output dying and it goes to max field current trying to maintain the bus voltage.

Administrator
Shoreham EGKA, United Kingdom

Is there something about modern aircraft electrical systems which requires us to faff about like this? My Citroen seems happy for me to just turn the key: clockwise to start, anti-clockwise to stop – despite everything from suspension to seat positions being controlled by electronic wizardry…

I’ve never managed to destroy any airplane stuff by ignoring this OWT, but supposing that switching alternator field on after start-up was “kinder” (which I doubt) who wants to give panel space to any instrument or avionics which could be killed by a mere voltage spike? Arguably, electronic [email protected] of that order is fit only for target practice.

Glenswinton, SW Scotland, United Kingdom

There once was an epic thread on Beechtalk about Alt on vs off during engine start. At the end of the thread, there was no result.

Frankfurt (EDFZ, EDFE), Germany

All cars made since many decades ago turn off all electrics (except obviously the ECU on modern ones) when you turn the starter key into the starting position. This is arranged via the angular contact spacing inside the switch into which the starter key goes. In this way they avoid damage to the radio etc.

Some planes do this too. For example all (or all the more recent) TB20s do this. The avionics master system, 3 buses, is done with 3 relays and a 4th relay gets energised when the starter motor is energised and this 4th relay disconnects the other 3. This is what I have. So there “should” be no need to turn off avionics master during starting. But, as Clint said, “do you feel lucky?”.

As regards ALT ON (i.e. enabling the field current to the alternator, thus enabling it to generate more than a volt or so) this is going to be highly component dependent. I reckon most of the time it does nothing, and does even less at shutdown. But if you have a crappy voltage regulator which goes berserk at the limit(s) of its control range, you will get spikes on the bus, and these can blow stuff up. In a 24V system, more than about 40V, for even 100ms, will blow up something expensive. One bloke blew up the whole stack in a early 1980s TB10 (I knew about this only because I was looking at buying a share; he said the insurance paid for a new stack, under the ground risks cover).

And not all stuff is behind the avionics master. Typically, things like EDMs are before it (mine isn’t) so are vulnerable. Fuel gauges, strobe inverters, electronic tachos, etc likewise. Sandel also recommend their EHSIs and AHRSs to go before it, so they are warmed up and ready to go by the time you are (they say they have really good spike protection) but I still put mine behind the avionics master.

The big issue in GA is that very few of the people designing stuff are or have for many years been competent. King had very good people in the 1980s and 1990s (all left c. 1999). Much of the rest in GA was crap. The “electrical” stuff (voltage regulators etc) are mostly 1950s ex-car crap which used to pack up all the time in cars. In the 1970s when I was first on the road, the roadside of any motorway was littered with broken down cars, and mostly due to electrical failures.

So, the answer will be very system dependent, and very few people will know their electrics that well. I read e.g. Beechtalk too and there are about 3 people on there who really know their stuff in this area. The rest are just guessing…

The other issue with ALT OFF during starting is that it gives you a much cleaner rise on the bus voltage. If you have ALT ON during starting, you get a load of spikes, gradually increasing, as the engine coughs and splutters into life. This could play havoc with various loads. Whereas if you go ALT ON at say 1200rpm, the bus rises nice and fast to its normal voltage. Much better. And at shutdown, having ALT OFF prevents the same crap going onto the bus. There is no question about this – the only question is whether it affects anything, and nobody will know that until they try.

Administrator
Shoreham EGKA, United Kingdom

At the end of the thread, there was no result.

It’s so easy: Do what the book says. (In this case, the aircraft flights manual). Why do people (pilots especially ) always think they are smarter than the guys who built their aeroplanes?

I fly both types of aircraft: Those who want the generators turned on before starting the engines (unless it is a ground power start in which case they must be off, otherwise some unaccessible current limiter might melt) and those whose generators are supposed to be off until the engines are idling.

All cars made since many decades ago turn off all electrics (except obviously the ECU on modern ones) when you turn the starter key into the starting position. This is arranged via the angular contact spacing inside the switch into which the starter key goes. In this way they avoid damage to the radio etc.

Not mine. It is only 16 years old and has a start button next to the key switch. Nothing that you turned on before pressing that button will be turned off during start. So far, it has not suffered any electrical/electronic failures.

Last Edited by what_next at 23 Oct 21:56
EDDS - Stuttgart

With most modern cars featuring auto start-stop, it’d be pretty annoying if they shed all the loads every time the engine started!

Not mine. It is only 16 years old and has a start button next to the key switch. Nothing that you turned on before pressing that button will be turned off during start. So far, it has not suffered any electrical/electronic failures.

Well obviously my comment doesn’t apply to cars that don’t have a conventional starter switch

I was going through possible reasons for the procedures.

Administrator
Shoreham EGKA, United Kingdom

I would think that you should read the POH. My old Comanche POH stated that the ALT field should remain “ON” at all times.

54 Posts
Sign in to add your message

Back to Top