faelkle

Thanks for the clarification. I guess this is a safety feature. Edit: NG - battery switch shuts down APU in air and on ground; Classic: shuts down APU on ground only. http://www.b737.org.uk/apu.htm refers. Interesting subject.

I'm not sure what you mean by MFS - maybe someone else can answer that. The APU is a small jet engine that is started electrically, so you'll need to have the battery connected to get it running. Once running, the APU can power (some? all?) of the electrical buses, so the battery at least isn't discharging during that time. I believe the APU can charge the battery if the load of the electrical buses is sufficiently low. As Patrick says above, there is no light that says "the APU is on"; you have to infer this from the guage information: (Low Pressure = starting; EGT rise, peak, stabilize = running; APU GEN OFF BUS = available but not powering buses). You can also monitor the APU generator using the electrical multimeter on the overhead upper middle quadrant). Disclaimer: I'm not a 737 systems expert, I'm sure somebody more knowledgeable than I will jump in with the specifics :) I don't have the sim in front of me, can someone confirm: it should be theoretically possible to disconnect the battery if the APU is powering electrical buses - or does that switch do more than take the battery offline?