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Hi i was fly pic and turned off the disconected the L Gen Disc and was wondering why the Gen Cont switch still showed on. if u disconect it shouldnt it be turned off? whats the diff in if u turn off the gen driv disc switch and the gen cont switch?http://events8.tripod.com/pilot_tag.JPG

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Guest Ian_Riddell

"whats the diff in if u turn off the gen driv disc switch and the gen cont switch?"Each engine electrical generator is mechanically driven by the (respective) engine's "N2" rotor (N3 on an RB211). When the engine spins, so does the generator rotor. When the rotor spins fast enough, the generator is capable of providing electricity (Actually, the generator is not attached directly to the N2 rotor... The N2 drives a gearbox to which the generator is attached).When you push the Drive Disc switch, not only are you sending an electrical signal to the aircraft computers to trip (open) the Gen Cont Breaker stopping power getting to the airplane, you are also physically disconnecting the generator from the gearbox. If you push the Gen Cont switch, the generator rotor still turns c/o the spinning engine."I would have thought that the Gen Cont light OFF light would also have illuminated if you pushed the Drive Disc switch."Me too :-)I may have to do some more reading in the Boeing Maitenance Manual to see what puts on the Gen Cont light (whether it is simply looking at the position of the Gen Cont Breaker or something more), but I believe the end result should be that the light illuminates.The GCB light is not like the Bus Tie lights which are inhibited from illuminating (when airplane automatics dictate that the BTB's should be open).Hope this helps.Cheers.Ian.P.S. Perhaps the experts can chime in here, but the APU "OFF" light seems to be triggered not by the position of the Aux Power Breaker, but by the state of the APU generator field relay(?).

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Guest Ian_Riddell

BTW, whilst I was checking out the electrical system in PIC, I noticed that the DRIVE lights didn't seem to be illuminating at the appropriate times in PIC either.The DRIVE lights should illuminate when:1) the oil-out temperature of the IDG (generator) reaches too high a value (The IDG is full of oil to help keep it cool)2)the internal IDG oil pressure is too low (When the IDG is turning, the oil pressure inside the IDG builds up). This means that when an engine isn't spinning, the DRIVE light illuminates. I can't remember the approximate N2 rpm which extinuishes the DRIVE lights, but I recall it can be done, if the conditions are right, simply by spinning the engine with the starter motor (this is probably something for our resident pilots to check out ;-)).Of course, electrical power is required to illuminate the bulbs (The pic below shows the lights even illuminate during Standby Power operation... There may be, however, other power sources which illuminate them also).http://www.ozemail.com.au/~iriddell/767/St...erheadPanel.jpgSorry for the poor quality of the pic... It was relatively dark.. and if I used the flash on my camera, the lights may not have been visible.Incidentally, the operation of the DRIVE lights is much the same from one Boeing aircraft type to another (767, 757, 747-400, etc).Cheers.Ian.

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