737NG airstair logic

[172_driver]

I realize the following question has little practical use .. but I am actually quite interested. I also understand it's dangerous to draw conclusions from incomplete electric diagrams such as the FCOM… but;

About the forward airstair, exterior control - standby mode. My FCOM says that holding the switch in STANDBY provides DC power from the battery bus for airstair operation. The BAT switch on the flight deck does not need to be ON when operating the airstair on standby from the exterior control panel.

I was under the impression that the BAT switch and the STANBY POWER SWITCH (in BAT) were the two switches that could power the Battery Bus while on the ground, cold & dark, before TR3 is available.

http://i307.photobucket.com/albums/nn287/patrikcertain/StandbySystem.png (http://s307.photobucket.com/user/patrikcertain/media/StandbySystem.png.html)

Is there any easy explanation to this?

Thanks in advance

[AerocatS2A]

I don't know the B737 but wouldn't the refueling panel also use power from the battery bus?

[mainwheel]

Not real sure what your question is.

However, consider this.

If aircraft is operating, APU is on etc.....then the airstairs can be operated without the use of the external stby switch.

Now consider the aircraft is locked up, dead ship. If you haven't got mobile steps, like alot of outports and the reason airstairs are fitted, how do you get to the cockpit and turn anything on?

[Ediboy]

the airstairs are usually left out overnight with the handles in the stowage position in all the cases ive come across with the 737.

[172_driver]

Now consider the aircraft is locked up, dead ship. If you haven't got mobile steps, like alot of outports and the reason airstairs are fitted, how do you get to the cockpit and turn anything on?

That's exactly what I mean, I understand they do work when the ship is completely dead. That's how I get into cockpit. What I wonder is how the battery bus gets power with BAT switch OFF.. still assuming a dead ship here no Ground Serv. Bus on either.

Sometimes we're lucky and mx have left the stairs out for us, but "security reasons" these days they're apparently not supposed to.

[Capn Bloggs]

I don't fly 737s and am not a sparkie but it seems to me that when you flick the Standby Power switch on, it pulls that "Alternate" thingee over which connects the battery to the Hot Battery Bus and the Battery bus; stairs are then powered.

No need for the Battery switch itself to be on by the looks of that diagram?

[172_driver]

Capn Bloggs,

The standby power switch on the diagram is located in the flight deck, it's not the same switch that's operated from the exterior airstair control. But i'd say you are right you can bypass the battery switch via the "alternate thingee"

Had a quick look in the 737 cockpit companion (or if it was the MRG) together with my colleague today. It said something different to the FCOM, if we read it correctly:

a) The airstair standby-mode is powered by the Sw. Hot Bat Bus, and

b) Holding the exterior airstair switch to standby will power the Sw. Hot Bat Bus until released, even if Bat switch is off.

That would explain my question, but the "official" source, i.e. the FCOM says something else.

[Bob Lorentz]

This would not be the first case where the FCOM tries to explain something in a physically impossible way. I find the explanation of the companion pretty plausible.

[172_driver]

Thanks Bob,

Having a word with an engineer confirmed that operating the standby switch on the exterior control panel does actually light up some displays in the cockpit, if you were there to see it. Supposedly the standby switch can sometimes be a bit sticky and he said it's happened more than once that it has completely drained the battery, despite everything being off on the deck.

[Bob Lorentz]

Thanks for that !