Alternators

[that chinese fella]

Does anyone know a technical reason why some piston engine start checklists (but not B47) call for the alternator to be switched on after the engine is started?

I would have thought (and Bell it would seem) that an alternator would pose no drain on battery during start and seeing as it happens when starting a car engine I wonder why it is so.

[Aesir]

I have wondered about it too! Many fixed wings i.e. Cessna & Piper are started with the alternator on.

My guess is it really does not matter, but it´s good practice since many turbines are started with the Generator off.

[Milt]

Alternators start to generate at fairly low RPMs which means that during the engine starting they will be introducing a drag on the spin up. Some engine starters need all the help they can get.

Perhaps this is a good reason to start with alternator/s off together with anything else not needed during the start.

[Three Blades]

There is a similar arguement for when to turn on radios etc. I have flow identical aircraft where one establishment's checklist has the avionics turned on before start and another has them turned on afterwards.
My understanding as to why it is best to leave them off until after the start is the change in bus voltage as the starter is engaged.
Could this also be an explanation for the alternator question ?

[sprocket]

When the starter is engaged, there would be an overall system voltage drop. Leaving the altenator (and other systems) off would prevent low voltage loading.
I notice in newer cars, that things like Aircon and lights automatically cut out when the ignition key is turned.

[IHL]

With regards to radios. Radios should be off before turning on generators because the Generator Control Units (GCUs) which guard against: overvoltage, undervoltage, off frequency, reverse current, and (can't remember what else) are not active until the generator comes on line. There is a risk of a current/voltage spike that could fry your avionics.

Not sure about alternators other than (if memory serves me)they generator AC current and use a transformer rectifier to turn it into dc.

[that chinese fella]

Hmmm, still not convinced that the Alternator would create that much load but I guess it would be foolish to not comply with published checklist.

Anyone who has read the multimeter on the batteries while a generator is switched on would not ever have avionics on before stabilised regulated voltage is on the bus. I have seen the volts spike to 40VDC before the regulator kicks in and brings it back to 27 ish. I am sure over time the poor radios etc much suffer.

[helipedro]

It seems a good procedure to start with avionics off, specially if your machine has been sitting outside all night at -15C. You would need all the power you could get.
Regarding generators I had a few drops on n1,s, specially with Bo-105, 206 and Hughes 500 trying to engage the gen before 70% n1