What is the mercury switch between voltage regulator and alternator for?

Joined
Jun 12, 2019
Messages
4
Location
New England region
#1
Hi, slogging through some issues in my mid-90s-but-new-to-me aircraft. Working an electrical problem, my mechanic and I were puzzling over this. Does anyone know the purpose of the mercury switch listed in schematics between the voltage regulator and alternator in Super Decathlons, at least older years? Haven't traced wires, but I think it's the small blue cylinder on firewall next to Christen inverted oil unit. Thanks!
 

Bob Turner

Well-known member
Joined
Apr 4, 2018
Messages
499
#2
I think I took mine out when I went to Plane Power. No idea what it was supposed to do. Yeah, it was a blue cylinder.
 

Bartman

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Joined
Mar 27, 2018
Messages
767
Location
New Jersey, USA
#3
Isn't a mercury switch supposed to make or open contact relative to attitude? Would there be a reason to not charge the battery when inverted?
 

Sully

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Joined
Jan 5, 2019
Messages
55
Location
H75 Hernando, Ms
#5
Learn something new every day. From POH

The 60 amp alternator provides charging current and has sufficient capacity to operate all electrical equipment wiithout battery drain.
During inverted flight, the charging circuit is disconnected by a mercury switch.
 
Joined
Jun 12, 2019
Messages
4
Location
New England region
#6
Thanks, interesting. Those two sentences are not in my airplane-specific Airplane Flight Manual, or the Service Manual. But when I dug out the more complete but generic ACA-reprinted "Bellanca Pilot's Operating Manual" issued 1979, purchased from the flight school I did my first Decathlon training from in 90's, there it was. I don't think aircraft that spend a lot more time inverted than Decathlons have these, but guessing and will confirm that later. Maybe a holdover from older battery technology?