The Chain You Cannot See
A320 Electrical Foundation, Week 3 companion
TL;DR
The A320 Ram Air Turbine is the part of the emergency electrical system you can point to. It is not the part that makes your electricity. When both AC buses are lost above 100 kt, the RAT extends into the airstream, drives a coupled hydraulic pump, and that pump pressurizes the blue hydraulic system. Blue-system pressure then drives a hydraulic motor, and the hydraulic motor drives the emergency generator, which is rated at 5 kVA of three-phase 115/200 V 400 Hz AC. Five links, and only the last one is a generator. Pilots collapse that chain into the RAT because the RAT is the part they can see, and the collapse quietly costs them the ability to reason about which link failed. The lesson generalizes past this one device: the visible part of a system is rarely the working part, and the habit of asking what actually does the work here, rather than what is easiest to point at, is most of what separates a candidate who understands the airplane from one who has memorized its silhouette.
Ask a pilot what saves the airplane after both AC buses are lost above 100 kt, and most will say the RAT.
It is the natural answer. The Ram Air Turbine is the dramatic part. It springs out of the belly into the slipstream, it spins, and the lights stay on. If you had to draw the emergency electrical system on a napkin, you would draw the RAT, because it is the part with a picture.
It is also not the part that makes your electricity.
The part with a picture
Here is the actual chain, and it has five links.
The airstream turns the RAT. The RAT drives a coupled hydraulic pump. That pump pressurizes the blue hydraulic system. Blue-system pressure drives a hydraulic motor. And the hydraulic motor drives the emergency generator, which is rated at 5 kVA of three-phase 115/200 V 400 Hz AC. When it runs, it supplies the AC ESS and AC SHED ESS buses directly, and the DC ESS and DC SHED ESS buses through the ESS TR.
Read that again and count the components. The RAT. A hydraulic pump. The blue hydraulic system. A hydraulic motor. The emergency generator. Five links, and the thing that finally makes electricity is the last one, not the propeller in the wind. The generator is turned by hydraulic pressure, which was made by a pump, which was driven by the turbine.
The RAT is the part with a picture. The generator is the part that does the job. They are not the same part, and most of the difficulty in this corner of the airplane comes from pilots treating them as one.
Why the collapse costs you
You could argue this is pedantry. The RAT comes out, the lights stay on, who cares which link in the chain is technically the generator.
Here is who cares: anyone trying to diagnose a failure.
The failure modes live at different links. A RAT that will not extend is one problem. A blue hydraulic system that will not pressurize is a different problem. An emergency generator that will not couple to the network is a third problem. Three different failures, three different places, and if you have collapsed the whole chain into the RAT in your head, you cannot tell them apart. You are left staring at a dark panel knowing only that something in the emergency system did not work, with no framework for which something.
Here is the humbling part, and it is the thesis turned on itself. Even the RAT OUT memo only tells you about the visible part. It confirms the RAT is not fully stowed. It does not tell you whether the pump is pressurizing, whether the motor is turning, or whether the generator has connected. If the emergency generator is not supplying, RAT position alone does not identify the cause; you go to the ECAM indications and the applicable procedure. The indication you can see reports the link you can see, and the working links stay silent.
This is also why the examiner asks. When someone probing the emergency configuration asks you what the RAT powers, they are not testing whether you know the RAT exists. Everyone knows the RAT exists. They are testing whether you know it powers the blue hydraulic system rather than the airplane, because that answer reveals whether you have the chain or just the picture.
The eight seconds the picture hides
There is a second thing the visible-part habit hides, and it is a good one.
The chain takes a moment to come together. The RAT extends, the blue system pressurizes, the hydraulic motor spins up, and the emergency generator begins to turn, but the generator does not connect to the network until its electrical parameters are within tolerance. Reaching tolerance takes time.
During RAT extension and emergency generator coupling, about eight seconds in total, the batteries power the emergency generation network. Battery 1 supplies the static inverter and the battery-supported part of the AC ESS BUS. Battery 2 supplies the DC ESS BUS. The AC SHED ESS and DC SHED ESS buses are not powered before the emergency generator becomes available, so this is a reduced network, not the full one.
Nobody draws the batteries on the napkin. They are the least visible part of the emergency system and, for those eight seconds, the most important. That is the visible-part trap in its purest form: the drama is at the RAT, and the actual work, in that moment, is being done by two boxes nobody thinks about.
The same trap, one component over
Once you start looking for the visible-part error, it appears again a few inches away on the same panel.
The static inverter gets called a generator. It is not one. It converts DC power that Battery 1 already supplies into a small amount of AC, one kVA, for part of the AC essential bus. The word is converts, not generates, and the difference is about what kind of input each one needs. A static inverter needs a DC electrical input; it transforms Battery 1 DC into AC. A generator needs a mechanical input; the emergency generator turns hydraulic mechanical power into AC. Both depend on something upstream, but on entirely different somethings, and calling the static inverter a generator hides its dependence on Battery 1 specifically, which is exactly the fact you need when you are reasoning about what happens as the batteries drain.
The RAT gets miscalled a generator because it is upstream of one. The static inverter gets miscalled a generator because it produces AC. Opposite reasons, same error, and both are cured by the same discipline: ask what the component actually does, not what it superficially resembles.
What this has to do with passing an oral
None of this is exotic knowledge. It is all in the manual, in plain language, and none of it requires a feat of memory. The chain is five links. The batteries bridge the coupling. The inverter converts. A candidate could learn all three facts in an afternoon.
The reason it separates candidates anyway is that the visible-part habit is strong, and it survives a casual reading. You can read the emergency-configuration section, come away with the RAT saves the airplane, and feel like you have understood it, because the sentence is not wrong exactly. It is just collapsed. And a collapsed answer holds up fine until someone asks the one follow-up question that requires the chain to be uncollapsed, at which point the whole thing comes apart in a way that tells the examiner precisely how deeply you read.
The fix is a habit, not a fact. When you meet any system, the useful question is not what is the impressive part. It is what actually does the work here, and what is each part depending on to do its job. Ask that of the emergency electrical system and the RAT quietly steps back into its real role, first link of four, and the generator, the hydraulics, and the humble batteries step forward into theirs.
That habit is worth more than any single fact it uncovers, because it is the one that keeps uncovering them.
Where this sits
This is a companion piece to Week 3 of the A320 Electrical System Foundation Series, which covers the emergency electrical configuration. The full walkthrough, from the RAT extending into the airstream to the airplane on batteries at the gate, is in the companion article. The emergency-configuration questions are set out in question and answer form on the Week 3 page.
Week 1 covers the normal architecture and the source priority hierarchy. Week 2 covers single source loss and the reconfiguration that follows.
Prepare with citation-grade answers
The Custom GPT Solutions A320 Oral Exam Preppers walk the emergency configuration one link at a time: the RAT and the hydraulic pump it drives, the coupling interval the batteries bridge, the static inverter that converts rather than generates, and the on-ground speed thresholds. Below 100 kt, loss of RAT drive transfers the emergency network to the batteries and static inverter and sheds the AC SHED ESS and DC SHED ESS buses. Below 50 kt, the AC ESS BUS is shed as well. Every answer cites the exact FCOM, FCTM, AFM, or QRH passage so it can be verified against your own operator library.
The tool is an interactive study partner rather than a drill machine. Operator-specific variants are live for Wizz Air, Air Arabia, Hong Kong Airlines, Ryanair, and flydubai, at six dollars per month. Generic A320 and B737 tools sit alongside them. First week free.
