737 APU ?
 

Can some one explain to me why, if you turn the battery switch off on the ground it shutdowns the APU but if you do the same inflight it dosn't?

Many thanks

You switch the battery off in flight..Why?

We don't, it's a technical question on the system I'm asking!

PS, you might consider it if the battery was your only power source left, to consevre it.
Bad day at the office!

Barbers Pole,

Thats because fire detection on the B737 comes from the battery bus thus switching off the battery switch will disable the fire detection therefore the APU auto shuts down.

HS

As a slight aside:
Would an unserviceable APU require fire service attendance inbound on an A321 if this unseviceability was brought about by a battery problem. Said aircraft was towed off stand with no lights later.
BTW fire cover was asked for by handling agent prior to landing and ATC found out from fire service as aircraft was approaching stand!

Thanks HS

So inflight if you turned the battery off, the bat bus would still be powered via the No.2 gen/TR3, therefore it wouldn't cause a APU shutdown? As you would still have fire detection. Logic would have to work via a air/ground sensor??

Looking at the Boeing electrical schematics for the 737 (Ref 49-62-11), I see that there are several 28v dc inputs going into the APU controller. If the APU fire handle is pulled, a 28vdc signal is sent to the APUC. If a fire is detected, a 28vdc signal is sent to the APUC. If the APU is switched off, a 28vdc signal is sent to the APUC. If the battery switch is switched off, 28vdc is sent to the APUC. Presumably all these 28vdc signals are shutdown commands.

In the latter example, the signal goes through an air/ground relay ("R38" on the particular schematic I have). The contacts of the relay are opened in the air, so the "battery off" shutdown signal cannot get to the APU.

This explains the how. I'm still not sure of the why. All that battery circuit seems to be doing is providing you with less ways to switch off the APU in the air.

Rgds.
Q.

Because Boeing wired it that way :) :)

But seriously:

The APU on the -200 shuts down on the ground when the battery is shut off because you have lost APU fire DETECTION capability which comes from the Battery Bus. The Battery Bus is powered inflight by TR3.

Why you would turn off the battery in flight I am not sure but on e situation that comes to mind would be a suspected overcharging of the battery or a looming thermal runaway.