I was setting up a flight last night from Cairns to Melbourne in the Qantas (old livery) 737-800WL, and ran into a problem when I disconnected ground power. Perhaps to start at the beginning, I'd flown the tutorial flight with no problems a few times over the past few days, and so felt confident enough to venture out in the world all alone - it has to be said that I am fairly new to FMC's etc as I usually fly GA. So armed with a route from vRoute, the PMDG Tutorial (for guidance) and the PMDG 737NGX from Cold and Dark to secure pdf (from Nyxx I think) I sat myself down to get set up. Everything went fine until I disconnected the ground power, then all hell broke lose - alarm bells ringing, warning lights etc etc. I quickly saw that my IRS alignment had been lost and the CDU was showing a position which was constantly changing along with a notification to "Verify position". The MCP had basically also shut down and things like A/T, VNAV and LNAV switches and buttons which had been armed during the course of the pre-flight check list were now off and could not be switched on again. So clearly I have screwed up somewhere along the line and done something wrong or failed to do something before I disconnected the ground power. I'm planning to try again tonight, and would appreciate if anybody has any input as to what I may have missed or screwed up so that I can watch out for that in the second attempt. Have to say that I am just loving this aircraft, absolutely fantastic! ThanksJack

Sounds like the APU wasn't running, supplying power until the engines are running.

Make sure that before you disconnect the GPU that you have your APU supplying power to the aircraft or one engine started so you have a generator to power essential items then disconnect the gpu. Good luck! Trent Smith

Either what Graham is saying, or the APU was running and you forgot to put it online.

Did you have the APU running and its generator online before disconnecting ground power? Without the APU generator connected you would lose all those items. Harry

Ahhh, that could be it. The APU was switched on but maybe the generator wasn't online - I'll check that tonight. Thanks for the replies.Jack

Just remember the 737 rule - you have to actively make a change for something to happen when it comes to supplying power. For example, you do no actually 'disconnect' ground power by using the ground power switch (unless you want to deliberately unpower the aircraft) - you must 'replace' ground power with something else (ie engine or APU generators). The rule being that 'oncoming power has priority'. So, to give and example, to go from using ground power for powering the buses, to using APU power: The APU must be running (you'll see the blue APU GEN OFF BUS light illuminated indicating the APU gen is available but not powering the buses) - use the APU gen switches to power both buses. This will then 'kick' the ground power off the bus (you don't need to switch it off as well).

It was the APU gens, I hadn't noticed that they were not correctly switched on. Simon, your tip about the APU GEN OFF BUS light was most helpful. Thanks for the help everybody. Jack

I run into this problem a time or two myself. I adhere strictly to the cold and dark check list. Sometimes after turning the APU generator switches to ON and APU bleed air switches to ON, when I disconnect the GPU I get the alarm bells, but it seems like the switch just didn't set all the way. No big thing as I just go back and pick up the checklist at the APU and all is fine. Charlie