A320 LOC and APU bleed
 

Hi all,

New to the bus, and I have a couple questions.

1. Why is it that when LOC is armed, the managed dot appears in the FCU heading window? If I understand the philosophy, the dot means that the parameter is under FMS control. Why then, would the dot appear if we're tracking an external signal?

2. Is there anything wrong with starting the APU with the APU bleed selected on?

Thanks for the technical insight.

1. It's true that you're tracking an external signal directly in LOC, but you can't select a different value. For example, you can select speed 250 and adjust it, you can select a heading/track and adjust it, but with localizer you can only capture it. Additionally, LOC* mode includes "Enhanced LOC capture function", which uses FM position in order to make anticipate LOC capture and makes a smoother turn to the localizer. So it's not just external data.

2. Nothing wrong, the APU bleed air valve won't open until APU N > 95% anyway, just not the way this things are done.

I still give the apu 3 minutes after I have started it to purge it before I put the bleed on ,so it could fill the cabin with oil fumes or de-icing fluid, if you start it with the bleed on.

1. I suppose that makes sense. I do remember reading about the enhanced loc capture. Quite sophisticated, this machine.

2. Got it.

Thanks

That mirrors a policy on my old airplane, and our current policy during deicing. I was thinking more about the hot summer day when the ground crew opts not to connect the air hose.

Airbus explained it in a document that the APU bleed is automatic and it's OK to put bleed ON even before start is complete as it won't open unless there is pressure as FlyingStone rightly said, but you may not do it during cockpit prep because you need to check that there is no external air is connected as it is forbidden to use both together. You don't do it after landing after starting the APU either because it will suck kerosene fumes. System wise there nothing wrong. APU will create smoke in the cabin only if during last shut down the batteries were switched off before APU flap had closed which disrupts the de oil sequence.

Thanks Vilas. Do you happen to remember where that was written, or any other info to steer me in the right direction?

Smoke during start was mentioned in Instructor Support Flight FAQ and use of bleed was in a instructor standardisation meeting Toulouse.

Hey Villas, if you have External A and B connected fully powering the ship, shutdown the APU and then immediatly switch off the batteries will that still interrupt the de-oiling and shutdown sequence for the APU?

I suspect the answer is no it won't matter then.....

Cheers

Thanks. I'll go look them up.

We have tried turning the batteries off with external power on before the APU flap was indicated closed. Eventually it did close as it would with the batteries on, but I'm still very interested to know if by doing this changes anything.

ACM and 747
The flap will close as long as there is power and as long as flap closes de oil sequence is completed so it shouldn't matter whether batteries or external power closed it and shouldn't cause any problem in next start. This problem is basically while securing the aircraft when there is no external power and you switch of the batteries before flap closing.

Why would you WANT to start the APU with the bleed on? What is the advantage?

down here A/C is not hired for costs and APU usage is restricted to 5min before push, the cabin is hovering around 30c, i put the bleed on while the APU is staring but instructors don't like it.

Putting the bleed ON before pushing start button doesn't serve any purpose but if you are sure there is no external air is connected then it is possible to push Master, start and put bleed on in that order. But this is best left to to the Flt. Ops. They have access to Airbusworld to get clarifications. It takes about 40 to 45 seconds for APU to start fully, even if you put the bleed before it is not going to open so when it makes no difference to cabin cooling raising instructor's temperature doesn't make sense.

Then don't do it in front of said instructors. My motto in training has always been "cooperate and graduate".

I've seen countless people start the APU and get distracted letting the APU run with ground power connected but without bleed engaged.

In the relentless and distracting world of the Low cost turn I see it as good airmanship to select the bleed on whilst starting the APU. As in the air, the aircraft will look after you on the ground too.

For the sake of your passengers and crew *please* give the APU some time to blow out any contaminants before pumping your cabin full of potentially harmful air.

Summertime close to the equator- you arrive to the plane to find the ground crew has (again) the connected the GPU, but no air. Go into the cockpit and start the APU, with the bleed selected. Once your external inspection in done, the cabin is approaching a reasonable temperature.

Interesting technique. Though I prefer the packs off as well as the APU for walkaround. You only get hit by a ground servicing van once - I hope!

I do envy the tropical layovers with non-morning departures :).