An aircraft’s electrical system is composed of many separate buses. Some of these buses are maintenance buses. Systems that are needed to service the airplane while all the rest is powered down, are often connected to these maintenance buses.
(Just behind the cockpit door is a panel on the ceiling. The clear cover protecting it usually has a hole in it just over the commonly used “maint bus on” switch, so you don’t have to open the whole cover to use that switch.) For example the refueling of the airplane. If that switch isn’t set to “maint bus on” then the fueling system gets its power from the batteries, draining them. That may not be a big deal on a turnaround flight with a short ground time, but if the plane is going to be parked for quite awhile, it’s better that an external power source is connected and systems get their electrical energy from outside, saving the batteries. I haven’t flown the A320 series in many years, but it will be similar to the A330 I still fly. And on the A330 the switch also guarantees power from the GPU (ground power unit) to these:
- passenger compartment lighting
- galley lighting
- entrance area lights
- lavatory lights and service
- vacuum cleaner outlets
- flight compartment service outlets
- flight compartment flood lighting
- fuel quantity indications
- refueling
- lower deck cargo compartment lights and power outlets
- main and nose landing gear compartment lighting
- hydraulic compartment lighting
- landing gear compartment lights and service outlets
- navigation lights
- escape slide locking mechanism ice protection
- parking brake
- cargo door hydraulic pump
As you can see, it just guarantees that ground personnel like mechanics, cleaners, loaders… all have light and electricity to perform their jobs, while the rest of the plane is powered down.
Some of the main buses on the airplane you can see here, together with generators and switches. The maintenance buses would be connected to AC BUS 2 and DC BUS 2 on the A330, which has a slightly different but very similar blueprint. Here the A320’s electrical system is shown.) (A GPU or Ground Power Unit can be connected to this receptacle at the bottom of the nose of the plane. Sometimes you will see a separate cart next to the plane, connected to it, and other times the cable can just come from the gate itself.) You can also let the APU (Auxiliary Power Unit) run on the ground, with the engines shut down. That’s a jet-like generator in the very tail of the aircraft. But it makes that loud sharp noise so typical at airports that a lot of people who live nearby don’t like. So for noise abatement we usually have to switch the APU off as soon as possible (typically within 5 minutes after shutting down the engines at most airports I fly to). Also, the APU on an A330 will use about 150 to 200 kg of fuel per hour, so the pollution and cost are not welcome either. Since we need the APU to provide air for starting the engines, and because it provides air conditioning that’s usually superior to the external hoses plugged in for that, we need to start the APU before we start the engines (except in rare cases, and then you usually get sweaty). If you see the pilot operating that “maint bus on” switch, it means that they likely would park the plane for awhile and didn’t want to use the APU longer than necessary. Having the external power plugged into the plane, he would have preferred the ground staff to use that electricity, instead of draining the batteries unnecessarily. If he used the switch before your flight, then he will have wanted to switch it off and resume normal operations from the APU or battery. Of course, there are some other switches and circuit breakers there too. They mostly have to do with toilets and galleys. So perhaps he wanted to reset a system. That could be for something like trying to get a toilet to flush again. (In a previous airline, while parked for awhile, I’ve played the trick on a colleague & friend who was in the toilet, obviously for a number 2. Pulling the circuit breaker for the flush guaranteed a certain embarrassment, especially with the flight attendants.)。These other “buttons” would require the clear cover to be opened, since that little hole isn’t over them.
Pull the top left circuit breaker here on an A310 while your unsuspecting colleague* is taking a dump, and watch the shame when he has to explain to the flight attendants that he can’t flush. * only perform when parking time is long, when you’re in the right airline, and when you know the colleague also as a friend; the author is not responsible for unpleasant consequences or odors.)
