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S.71
1st Sep 2004, 11:08
I was wondering as to whether or not there is a signifcant difference between selecting alternator on before start or alternator on after start in Piston Aircraft. I have flown now at four different flight school's and three of them we selected ALT on prior to start and one we selected ALT on as part of the after start checklist. Is there any significant impact on the electrical system assides from the obvious fact it will be slightly harder to start the aircraft with ALT on but re charging commences immediately after start.

Thanks Heaps....

:cool: S.71

411A
1st Sep 2004, 16:54
With a single engine aircraft, generally there is absolutely no difference.
Would suggest you follow the aircraft flight manual/POH in this regard.


With piston twins, the situation is slightly different.
If you start with alternators on, the first engine...no problem.
The second engine started however might present a few difficulties.
Starters draw a rather large amount of current, and altho the battery will supply most of this, enough momentary current draw from the operating engine\'s alternator may be enough to separate the elastomeric drive coupling used, thereby rendering that alternator unable to supply any current thereafter...until the coupling is replaced.

Not only are these couplings rather expensive, it has been known that bits from the sheared elastomeric coupling to enter the oil system, and clogging small oil galleries which...ain\'t good, at all.

Would suggest therefore that twin engine aircraft be started with alternators off, for best results.

Interesting to note that the large piston engine airliners of yesteryear were started with generators off as well....selected on after all engines started. The Kelsey Hayes/Bendix/Eclipse generators used were really expensive, so the ships batteries used tended to be large.

GotTheTshirt
1st Sep 2004, 17:14
411A
I realise from past postings that my extensive aviation technical library pales into insignificancve against your personal knowledge, and you probably went to college with the guy who invented the alternator, but many light twins do not have direct drive alternators They have belt drive.
Also, you may not have met him, but Charley Limiter had already appreciated what overloading an alternator would do and invented the Current Limter. Still used today on many aircraft
:cool:

411A
I realise from past postings that my extensive aviation technical library pales into insignificance against your personal knowledge, and you probably went to college with the guy who invented the alternator, but many light twins do not have direct drive alternators They have belt drive.
Also, you may not have met him, but Charley Limiter had already appreciated what overloading an alternator would do and invented the Current Limter. Still used today on many aircraft
:cool:

S.71
1st Sep 2004, 17:16
Excellent!

Thankyou very much for the quick response!

:cool: S.71

411A
2nd Sep 2004, 01:48
Sadly, T shirt, you are not entirely correct.

Well over half of the larger light twin engine aircraft produced in the last forty years have gear driven alternators.
This includes all of the 400 series twin Cessnas, all of the later model Beech Barons/Cessna 310's, as well as QueenAirs, AeroCommanders (with TCM engines) etc.

And as for current limiters, few of the older twins had 'em.
They had alternator field fuses...which ain't the same.