B747-400 APU Door Question

[dontdoit]

Observation...many of my colleagues after landing switch the APU switch to "ON" (positions are OFF-ON-START) as they say "it opens the APU door", then select "START" as we approach the stand.

When I flew the 747-200, I'm sure this was correct, but can anyone tell me if this is the case with the 747-400...I'm almost sure that if "ON" is selected from "OFF", nothing happens (until it is "START"ed, if you see what I mean).

Thanks in advance.

[lefthanddownabit]

Switching to ON from OFF opens the APU door and the fuel valve, arms the APU bleed isolation valve and activates the fuel pump (DC or AC). If you put the switch to START then release it to ON, all the preceding will happen and the APU starter will engage when the door is fully open.

On the Classic you have to wait until the APU door is open (blue in-transit light out) before selecting START. Selecting START before then does nothing.

Maybe there's an operational benefit in opening the door, thus ventilating the APU (which will be cold soaked), followed by a rapid start when approaching the stand. Or it might be old habits taking time to die out.

[spannersatcx]

Next time before starting the APU you can have a look at what it's doing by selecting ground test enable swith then select the CMC on CDU select EICAS Maintenance pages, go to 49 APU select display and on the lower EICAS you will see Note the door is shown closed in the first pic.

Then when you select the APU master switch to on will see after a few seconds Door will be open (this pic shows the APU at 100%, it is for illustration of the door position)

[dontdoit]

Spanners - Thanks for that, but when you say

"Note the door is closed, then when you go to on you will see after a few seconds Door will be open (this is at 100%)"

Don't you mean "...then when you go to START and back to ON you will see, etc"

So, totally confused now, any benefit from selecting "ON" before going into the start sequence, or is it a waste of time/a case of old habits die hard ???

[Hotel Mode]

My company does it as standard to allow some ambient air to get into the duct and hopefully warm things up from cruise temp.

[spannersatcx]

I've edited the text slightly. Sorry to confuse you. Actually the confusion is caused by your collegues. There is no reason to do what they are doing, unless it is SOP's although I can't see why!
When you go from 'OFF' to 'ON' the door will show open after a few seconds, you can then select 'START' anytime you like, 1 min or 1 hr later doesn't matter and the starter will engage and ign/fuel etc will come on and hopefully it'll start.
If you go from 'OFF' through 'ON' to 'START' the door has to open first before anything else happens, as soon as the APUC sees the door open it will then start the starter and fuel/ign etc.
I was doing this on an A/C a few days ago as there was a problem with the APU, I was calling up the EICAS maintenance page to see what was happening, I went from OFF to ON and after the door showed OPEN then went to START (the START position is momentary and the switch will (should) always return to ON) this was only because I was trouble shooting a fault, normally I would go from OFF, then to ON, then through to START and then back to ON.
Hope that helps and has not confused you more.

[kuobin]

Checked schematic,The whole start sequence is fully automatic,you hit "START" and let go,the start relay will be latched until APU stable,if there is anything wrong, you will get message.
It is your choice to saty "ON" before hit "start".