The Panel Is Not the Network
A320 Electrical Foundation, Week 1 companion
TL;DR
Three indications in the A320 electrical panel report something narrower than what most pilots read into them. The AVAIL light on EXT PWR means external power is plugged in and its parameters are normal, not that it is supplying the aircraft. The blue ON light stays on even when the engine generators supply the aircraft, so it is not evidence of what is currently feeding the buses. And an APU generator described as available is not necessarily connected, because its line contactor closes only if the APU generator parameters are normal and the external power line contactor is open. The common thread is that the panel reports selections and local states, while the question you are usually asking is about network state. For network state, the FCOM points you to the ECAM ELEC page. One caution: do not turn this into a universal rule about pushbuttons, because the FCOM gives separate indication logic for the BAT, GEN, APU GEN, and EXT PWR controls.
There is a particular kind of error that survives an entire career without ever being caught.
It is not the error of not knowing. Not knowing gets found out quickly, usually in a simulator, usually with an instructor watching. The durable error is the one where you do know something, and the something you know is true, and you have quietly extended it about fifteen degrees past where the manual stops.
The A320 electrical panel is full of opportunities to do exactly that. Not because the panel is badly designed, but because it is designed to answer a narrower question than the one pilots are usually asking when they look at it.
What the panel is for
An overhead panel control has two jobs. It gives you a way to select something, and it tells you the state of that selection and of the equipment immediately behind it.
Neither of those jobs is the same as reporting what is currently powering the aircraft. That is a network-level question, and the network has its own display.
Once you see the distinction it is obvious. Before you see it, three specific indications will mislead you, and they will do it in a way that feels like confirmation rather than ambiguity.
Case one: the light that stays on
Pushback at the gate. External power connected, EXT PWR pushbutton selected, blue ON light glowing. Engines start. Somewhere in the next thirty seconds the aircraft transfers from external power to the engine generators, because the engine generators have priority.
You glance up at the overhead panel. The blue light is still there.
Nothing is wrong. The FCOM states that the ON light stays on even when the engine generators supply the aircraft. That is the published behavior, not a fault and not a lag.
What has happened is that the light answered its own question honestly and you asked it a different one. It is not reporting that external power is feeding the buses. The FCOM is explicit that the blue ON light must not be used as a real-time indication that external power is feeding the aircraft.
This one is worth sitting with, because the trap is not subtlety. It is habit. You have looked at that light on a thousand pushbacks and it has never once been wrong. It has simply never been answering the question you thought you were asking.
Case two: available is not connected
Same gate, same pushback, and now the APU is running with its generator available.
Ask most working pilots what the APU generator is doing at that moment and you will get some version of online. Sharing the load, or standing by hot, or backing up external power.
The published logic is narrower. The APU generator line contactor closes if the APU generator parameters are normal and the external power line contactor is open. External power is selected and supplying, which means its line contactor is closed, which means the APU generator line contactor is not.
So the APU generator is running. Its parameters are within tolerances. It is genuinely available, in the specific sense that it could take over immediately. And it is not connected to the aircraft network at all.
I would have said online for most of my career. It is the natural word and it is wrong in a way that matters, because online implies a connection that does not exist. An examiner who reads the manual carefully will pull that thread, and what unravels is not the one word. It is the candidate's whole account of the priority logic, because the word revealed that the account was assembled from impressions rather than from the contactor conditions.
Case three: the word that does too much work
The AVAIL light on the EXT PWR pushbutton means external power is plugged in and its parameters are normal.
That is the whole meaning. It does not mean external power is supplying the aircraft.
The difficulty here is linguistic rather than technical. Available is a word that in ordinary speech shades toward in use, and the shading happens without anyone deciding to do it. Nobody sets out to believe that AVAIL means supplying. It just settles that way over a few hundred turnarounds, and then it sits there unchallenged because nothing in normal operations ever contradicts it.
This is what makes these three cases a family rather than three separate facts. In each one, the indication is precise and the pilot's reading of it has drifted slightly wider. None of the drift is dramatic. All of it is the kind of thing that costs you an oral exam question, because an examiner probing this territory is probing for exactly that gap.
Where network state actually lives
If the panel is not the network, something has to be.
The FCOM answer is the ECAM ELEC page. To determine which buses are actually powered and which electrical source indications are normal, that is where you look. The panel tells you what has been selected and what each source is doing locally. The ELEC page tells you the state of the distribution.
This is not a criticism of the panel. It is a division of labor, and it is a sensible one. A pushbutton has room for a couple of lights and it sits under your hand where you need to operate it quickly. A synoptic display has room to draw the whole network. Asking the pushbutton to carry network state would make it worse at the job it actually has.
The practical discipline is small. When the question is what did I select, look at the panel. When the question is what is feeding this bus right now, look at the ELEC page. Most of the time in normal operations the two answers agree, which is precisely why the habit of conflating them survives so long.
The caution that keeps this honest
Here is where it would be easy to overreach in the other direction, and I did exactly that in the first draft of the Week 1 material.
The tempting move is to generalize. Every source pushbutton reports its own contactor state, none of them report network state, one rule covers the panel. It is clean and memorable and the FCOM does not support it.
The manual gives separate indication logic for the BAT, GEN, APU GEN, and EXT PWR controls. They are described individually because they behave individually. Learning one control's indication behavior and applying it to the others is the same error as reading a light too widely, just committed at a higher level of abstraction.
So the honest version of this article's thesis is narrower than the memorable version. Three specific indications mean less than pilots typically read into them. When you want network state, the ELEC page is where the FCOM points you. And each control still has to be learned on its own terms.
Which is less satisfying as a slogan and more useful in an oral.
Why this is worth the trouble
None of these three facts will save an aircraft. No emergency turns on whether you called the APU generator online or available. If that were the standard, none of this would be worth a page.
The value is in what the habit does to everything downstream. A pilot who has noticed that indications report narrowly starts reading all of them more carefully. The next system they learn, they ask what exactly does this light report rather than what does this light suggest. That question, asked consistently, is most of what separates a candidate who has read the manual from one who has read a summary of it.
And the examiners know which one they are talking to within about four minutes, because the difference shows up in the qualifiers. The candidate who has read the manual says the light stays on even when the engine generators supply the aircraft. The candidate who has read a summary says the light shows external power is on. Both sentences are about the same light. Only one of them survives a follow-up question.
One caution, so this does not curdle into cynicism about anything a colleague ever told you. Sometimes the folklore and the manual agree, and the only way to know is to have checked. There is a phrase about the batteries being disconnected most of the time that gets passed around like a rule of thumb, and it turns out to be a verbatim note in the manual. The discipline is not to distrust what you were taught. It is to go and read the line.
Where this sits
This is a companion piece to Week 1 of the A320 Electrical System Foundation Series, which covers the normal architecture and normal operations. The full source priority hierarchy and the external power indications are set out in question and answer form on the Week 1 page. The complete architecture walkthrough, including the bus structure and the three-case battery connection rule, is in the companion article.
Week 2 covers single source loss and the automatic reconfiguration that follows. Week 3 covers emergency electrical configuration, the RAT, and battery-only operation.
Prepare with citation-grade answers
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