Not sure I understand the logic behind the SOURCE OFF lights on the overhead Elec panel...From a dead ship, I start the APU. I then select, say, the APU Gen switch on the left to ON. The SOURCE OFF light on the left extinguishes, but the one on the right doesn't (even though the Right Transfer Bus has power). There is only one APU Generator and one Aux Power Breaker on the 737NG.Is there some obscure logic behind this? The 767 has a similar electical system, but the 767 panel layout/indications seem way more sensible.Thanks.Cheers.Ian.

Hi Ian,The SOURCE OFF light means that the (in your example) the right transfer bus is feed by the other source in this case the left transfer bus.This is what written in the 737ng cockpit companion:- no source has been manually selected to power the related transfer bus or - the manually selected source has been disconnected.- if a source has been selected to power the opposite transfer bus, both transfer buses are powered.- does not indicate that the AC transfer bus is de-energized example: in flight, left SOURCE OFF light comes on when GCB 1 trips, however, the bus transfer function lets IDG 2 power AC transfer bus 1.I hope this explane the function of the SOURCE OFF light.Best Regards,Alex

Ian, that's just because the 737 electrical system was designed by Fred Flintstone. The 737NG theoretical course was easily the most frustrating course I've done. Especially after the 757/767 course, this aircraft has a total absence of logic in its system design and operation. Actually, some systems actually employ logic that's 180 degrees opposite from the other Boeings, for example the speed trim system, which works to aid you during manual flight (make it easier to fly) in the 757/767/777, yet the 737NG speed trim works against you, adding extra opposite trim so it becomes harder to fly.. Go figure.

http://forums.avsim.net/user_files/60548.jpgBruno Francescoli.

Easier to visualize.http://forums.avsim.net/user_files/60552.jpghttp://forums.avsim.net/user_files/60553.jpgBruno Francescoli.

Thanks for the details, Bruno and Alex, but what I was asking was more along the lines of why it should be that way (Iz answered that nicely with his Fred Flintstone comment :-))Cheers.Ian.

There are some interesting related comments on the SOURCE OFF light in this posting on the PPRuNe site (see link)... Not all the postings agree with what is being said here.... Perhaps someone should straighten them out ;-)http://www.pprune.org/forums/showthread.php?threadid=120071(I deregistered myself from the PPRuNe site so can't add my comments).Cheers.Ian.