Dual Alternator usage

[gsxrpilot]

I have two alternators in my 252. I'm curious as to the proper usage and procedures for dual alternators. It seems that one of them doesn't actually produce a charge at idle power or while just taxiing.  There are switches to turn each alternator on. Are both turned on prior to engine start, or after start? Do you fly with both on or just one?

I haven't found anything in the POH about this and wondered how those of you with dual alternator K's use them.

Thanks,

[pkellercfii]

Start the engine with both alternator field switches in OFF. After engine start, the important part: Turn the #2 alternator field switch to ON FIRST. The #2 Alternator is the belt-driven alternator, which doesn't much mind the jolt which comes from suddenly picking up the load of an at least partly discharged battery. Even if it does develop problems, the #2 alternator is a relatively inexpensive & common 24V belt-driven alternator which isn't as expensive to repair or replace as the #1. The #1 alternator of a 252 is another of those rare, expensive bits living forward of the firewall in a 252. In addition to being expensive itself, about $800.00 to replace as of five years ago, the #1 alternator, being gear driven off of the engine back case gearing, also has an extremely expensive coupler, about 1 amu+, that you don't want to jolt with the startup load.  After the #2 output has stabilized, usually just a few seconds, then turn the #1 field switch to ON, and then fly with both in ON. 

If your charging system is setup correctly, you should observe that most of the load is carried by the #2 under nearly all load conditions, even with both alternators online. The #1 is pretty much just a backup. This said, something I do at runup which I'd recommend to other 252 owners is to check the pickup of each alternator separately during the runup. To do this, at runup RPM, with the engine monitor on bus voltage, turn off each alternator separately, and check that each alternator alone maintains the expected bus voltage of about 28.5-29.0 volts. Return both alternators to on before takeoff, and then leave them there the remainder of the flight. If you don't check the alternators separately at runup, it's real easy to miss a dead #1 alternator, since it normally doesn't carry much, if any load.

Enjoy the 252--one of the best Mooneys built.

 

--Paul Keller

CFI-IA

'89 M20K @ RLD

 

 

 

[carusoam]

Nice writing, Paul!

Clear technical details. I am looking forward to going all electric one day.

Best regards,

-a-

[Bryan1016]

Ditto Paul. Thank you.
I have dual 12 volt alternators. Lost something last week that sent the volt meter dancing. Dropped plane off with Rickey Gilmer in Pell City yesterday.


Sent from my iPhone using Tapatalk

[Bryan1016]

Just to follow up...bolts we're backing out of alternator casing on the primary, geared unit. Rebuilding it and the coupling. Expensive repair mostly cause the thing is so hard to get to.


Sent from my iPhone using Tapatalk

[carusoam]

Great follow-up Bryan!

Best regards,

-a-