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davetosky
29th Nov 2018, 09:24
Dear all.

I have some questions about FMGC reset proc on ground.
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On ground:
‐ Apply external power or APU generator power
‐ Wait 2 min before resetting the FMGC circuit breakers
‐ FD 1(or 2) (OFF)
‐ Pull the C/B of the affected FMGC and reset it after 10 s.
The circuit breakers for the FMGCs are:
AUTO FLT/FMGC 1 B2 ON 49 VU (Overhead Panel)
AUTO FLT/FMGC 2 M17 ON 121 VU (Right Rear Maintenance Panel)

CAUTION Always wait 1 min after the "PLEASE WAIT" message
disappears from the MCDU, before engaging or
reengaging the FDs and the AP of the reset FMGC.

Note: Due to electrical transient, MANUAL FMGS RESET procedure
may be unsuccessful. If so, the flight crew may attempt the same
procedure with engines not running.
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Do we have to turn on APU even we have running engs? why?
"note" says "Due to electrical transient. What is electrical transient? Does it mean electrical source change from eng to APU or something?

Thank you in advance.

TolTol
3rd Jan 2019, 01:36
I would like to know the answer to this also;)

Escape Path
3rd Jan 2019, 19:06
You don’t have to turn on the APU for the reset if you have electrical power from any other source (engines, ground power. Batteries won’t work). In context, what they’re trying to say there is “supply the full aircraft electrical network with power”, whatever it may be for the particular scenario you’re in.

You are correct on the electrical transient: when the aircraft switches from any electrical power source to another

Goldenrivett
4th Jan 2019, 02:09
What is electrical transient?
"Electrical transients are momentary bursts of energy induced upon power, data, or communication lines. They are characterized by extremely high voltages that drive tremendous amounts of current into an electrical circuit for a few millionths, up to a few thousandths, of a second."
https://alltecglobal.com/articles/what-is-an-electrical-transient/

If some part of computer logic has been switched by a voltage spike during a "transient" (not simply changing a power supply source), then sometimes it takes more than a simple 10 second reset. I don't understand why shutting the engines down and attempt another reset whilst on ground power or APU power would make a difference - but if you are still on the ground - then that option is still available.

mcdhu
4th Jan 2019, 14:08
I take it to mean that we should not play with the electrical system during the reset - eg do not switch from EXT AC to APU GEN or do not attempt reset during engine start (or shutdown).

Cheers
mcdhu

TURIN
4th Jan 2019, 15:12
Alternatively, when your attempt doesn't work because you are not following an approved AMM procedure, call your maintenance provider and he or she will be delighted to do it correctly. It will also ensure they have continued employment and will be available to get you home/away when you have a more serious problem.