I’ve seen on videos online the startup process on the A320.

I notice the first buttons pressed whilst cold and dark are the battery 1&2 switches, now I understand these need to switched on the enable the APU to be started (Is this the only reason?) But what I don’t understand is why does the batteries need to be switched on when there is ground power available because surely once the EXT power button is pressed this will enable the APU to be started and batteries won’t be needed.

Depends on how long it has been powered off for, what is checked is the battery voltage level, if it is too low it will require a 20 minute charge cycle.

After the batteries are on, the external power is selected on. I think the battery needs to be on to close the contactor to enable external power to connect to the aircraft network. Once the contactor the external power can continue to power the whole aircraft is the batteries are then turned off.

There is a switch in the cabin that allows cleaners etc to turn on the lights etc in the cabin from external power without powering the whole aircraft up or having batteries on.

You need DC for the external power control unit (which checks voltage and frequency). Without it the contactor won't close as that'd mean putting an unknown source into the system.

Batteries probably also act as capacitors and stabilizers of the GPU power. Same idea as on a car, you can unhook the battery while its running to see if the alternator is putting out, but you wouldnt want to drive around like that without the battery to smooth out the alternator output. GPU is just a diesel generator, would be a shame to lose your FMC init and flightplan if it runs out of diesel or starts running like shit and voltage drops, or fry the entire airplane if the governor on it fucks up and it overspeeds and overvolts, all because the whole plane is running on GPU only with no battery buffer

I guess, would be for fire detection and protection. Most a/c, the fire protection and detection system is on the DC side of an electrical system.

You can have aircraft powered just connecting GPU, but You can not start APU. It is common practice for pilots to disconnect bateries before leaving cockpit. And offcourse some systems are on hot baterry bus.

One quirk of turning on the batteries in a dark cockpit is that when you press the BAT 1 pushbutton, the BAT 2 pushbutton will go from dark to showing "OFF". The battery was actually off before, but there was no power for the pushbutton light.

bigb wrote:

I guess, would be for fire detection and protection. Most a/c, the fire protection and detection system is on the DC side of an electrical system.

Not sure about that, the A330 has a Mel for A/C only start of the apu.

The GPU connects to the AC bus like the engines would on the A320, DC BUS 1/DC BUS2/DC ENT are powered from the two AC busses via a transformer rectifier.

I know the APU year on the A320 cannot be started if a battery charge limiter has failed.

The APU can’t be started unless the batteries are on and also it will auto shutdown if the batteries are turned off. The reason for this is indeed the fire protection, which is not available with the batteries off. It’s quite common however to have the aircraft parked at the gate with batteries off and external power on.

Flow2706 wrote:

The APU can’t be started unless the batteries are on and also it will auto shutdown if the batteries are turned off. The reason for this is indeed the fire protection, which is not available with the batteries off. It’s quite common however to have the aircraft parked at the gate with batteries off and external power on.

This is incorrect, APU can be started from batteries, the normal electric system, or ground power.

See this attached this old FCOM 1.49.10 P1 https://www.smartcockpit.com/docs/A320- ... r_Unit.pdf

It will shut down if the batteries are the only source of DC power and turned off.

Battery 1 and battery 2 switches are the first two switches to be pressed because that is the procedure. But turning the batteries on is not the first step in powering on the aircraft. There are the steps of verifying radar, parking brake, landing gear, wipers, then you get to the electrical panel to check the battery voltage. Only then you get to turn the batteries on if everything is satisfactory up to that point.

While you can start the APU with batteries off, doing so means you are making up your own start up procedure. You can get away with it for a long time or even your entire career, but at some point if anything happens, the first thing the investigation looks at is were you following procedure?

If not, there better be an emergency going on... it’s hard to imagine an emergency occurring that requires you to deviate from the start up procedure if you’re starting up an aircraft from cold and dark.

The batteries can’t be charged when they’re off.

You want to confirm the battery voltage of a minimum of 25.6v before turning them on or applying external power. After you apply external power, the meter reads the Bus voltage of 28v and not the battery voltage.

you can bypass all of that and step one just press the external power button to power up the aircraft from cold and dark. But now you can’t check your battery voltage to see if they’re sufficiently charged or if they require charging.

There is a separate check where we do turn the batteries off. And then turn the batteries back on. You’re checking to see that the battery amperage drops to a minimum within 10 seconds of being powered back on.

zeke wrote:

Flow2706 wrote:

The APU can’t be started unless the batteries are on and also it will auto shutdown if the batteries are turned off. The reason for this is indeed the fire protection, which is not available with the batteries off. It’s quite common however to have the aircraft parked at the gate with batteries off and external power on.

This is incorrect, APU can be started from batteries, the normal electric system, or ground power.

See this attached this old FCOM 1.49.10 P1 https://www.smartcockpit.com/docs/A320- ... r_Unit.pdf

It will shut down if the batteries are the only source of DC power and turned off.

I can assure you all day long on the baby bus, without the batteries on, you will can never start the APU.

I can not speak to the bigger buses but on the A320 family, this is the case.

Its been a long time since I worked the 320s, but the 300/310 and others (Boeing, MD etc..) will start on GPU, if the GPU can handle the amp draw. We have one GPU that will start the APU without the batteries, but it isn't happy about it and the other GPUs we have can't provide enough amps and the aircraft avionics shut down and the APU fails to start. We always turn on the batteries just for that reason. I don't remember on the 320s if its the same deal or not.

Mr AirNZ wrote:

I can assure you all day long on the baby bus, without the batteries on, you will can never start the APU.

I can not speak to the bigger buses but on the A320 family, this is the case.

I just don’t know, the A320 AMM task 49-00-00-860-008-A is for “APU Start by External Power”. I know the batteries can definitely be off while the APU is running as long as the DC bus is powered.

Looking at this a little deeper the BCL connects the APU start contractor, and without that contractor being closed by the BCL the ECB will abort the start, the BCL uses this logic to prevent a start during RAT deployment.

zeke wrote:

bigb wrote:

I guess, would be for fire detection and protection. Most a/c, the fire protection and detection system is on the DC side of an electrical system.

Not sure about that, the A330 has a Mel for A/C only start of the apu.

The GPU connects to the AC bus like the engines would on the A320, DC BUS 1/DC BUS2/DC ENT are powered from the two AC busses via a transformer rectifier.

I know the APU year on the A320 cannot be started if a battery charge limiter has failed.

I was answering the op question regarding why the Bat switches must be on prior to starting the APU. You need the DC system for fire protection/detection. Unless the fire detection/protection is on some kind of hot battery bus... But having those batteries switch in the ON position gives you a back up to power the necessary DC busses for your fire detection system in the event of T/R malfunctions.

bigb wrote:

I was answering the op question regarding why the Bat switches must be on prior to starting the APU. You need the DC system for fire protection/detection. Unless the fire detection/protection is on some kind of hot battery bus... But having those batteries switch in the ON position gives you a back up to power the necessary DC busses for your fire detection system in the event of T/R malfunctions.

That is not the reason. The batteries need to be in auto position so the BCLs are connected, when the BCLs are connected they enable the APU start contactor to be energised.

You can attempt to start the APU without the batteries in auto it will not go above 20%. The BCLs also control the APU start in the air by the same way to prevent a start with the rat being deployed.

This is highlighted by way of the MEL where APU start is prohibited with an inop BCL.

Once the start is completed the APU will continue to run from GPU, APU, ENG generators without the batteries in auto as long as the DC bus is powered.