Originally Posted by
HighFlight
As for the continued debate on power messing up the plane...
When you plug in ground power, it sits at the receptacle if the switch is out, going nowhere. If the switch is in (either because you push it now or it was left in prior), where does the power then go? To the GCU. Where it is tested and monitored. If the phase rotation is correct, and the voltage and Hz. withing required ranges, and if the dual current transformers inside don't detect a spike in amperage (feeder fault or "short"), THEN the GCU allows power to go to the plane.
Now, it's true that a split second of power application is required in order for the current transformers to detect the feeder fault; electricity HAS to flow for there to be a "short" detected. But again, whether you push the switch in after power is applied, or the switch is already in, the same thing happens.
Basically, all that switch does is keep ground power away from the GCU, not the airplane. If you plug in "bad" power, the GCU SHOULD, in most cases, do its job and protect the plane. But you as a button pusher have no idea if said power is good or bad when you press the switch, and the same thing will happen either way. The GCU works about 1,000 times faster than your finger can, and either saves the plane or fails.
And the only power that is applied to the plane is 115V, 3 phase 400 Hz. Enough so that if you DID get hit by it, it would get your attention. Having been shocked at least 10 times in my career, I can honestly say it's the most fun I never want to have again, but it's not the end of the world.
Back to my original issue with this new switchology... I have been taught since day one to push the switch out to agree with condition. If the company wants us to do something different, it needs to be in writing, not just coming from LCAs as we fly with them. Supposedly, this new switch thing has been out for months, and I only heard about it last month.
Power goes to the GCU to be tested whether the switch is in or out. That's why you get a green light. The power is not applied on aircraft until switch position is met and the GCU continues to show good power.
We've all had this happen before: power shows good, but the airplane won't take it. The avail light is on... why didnt that work? Let me do it again and see if I can see what's happening? Why is it showing green and it won't take it? Because the external AC can't produce any load, any current, any of that good sweet fry my nuts if i held on with both hands lovin. Ive seen guys conflate that with some sort of extra check the gcu is doing once pushed in.
On the 900 very little on the electrical has ever been about matching switch position. The gens stay on and are "dusty" regardless of power on ac. The 200 doesnt because that stupid system will keep power on clear below 18v, sometime after 10 or 5v it finally kicks the bad power off. The 900 ac has always been left in while plane is in use, only upon complete shutdown would it go off, which is different in the 200. In effect the 900 and 200 are the same on that one part, so to bring the fleet into conformity theyd like the switches to have the same procedure.
Nothing in the book has changed, but training has been told this is happening so get used to it. I wish they'd just change the books and then do it, rather than half the pilots doing one thing and the other doing another.