A320 Emergency Electrical Configuration
This page covers the emergency electrical configuration: the state the aircraft enters after loss of AC BUS 1 and AC BUS 2, when no other main AC source is supplying them.
Part of the A320 Electrical System Foundation Series.
Frequently Asked Questions
What triggers the A320 emergency electrical configuration?
The ELEC EMER CONFIG alert appears when the AC 1 and AC 2 busbars are not supplied. That is the published trigger. It is not phrased as both generators failing, and the distinction matters, because the aircraft has GEN 1, GEN 2, and the APU generator, and the trigger is about the busbars not being supplied rather than about any particular generator.
If both AC BUS 1 and AC BUS 2 are lost and aircraft speed is above 100 kt, the RAT extends automatically.
What is the RAT, and does it power the aircraft directly?
No. The RAT does not power the aircraft directly, and this is the single most common misconception about the emergency configuration.
The Ram Air Turbine extends into the airstream and is turned by the passing air. It powers the blue hydraulic system. The blue hydraulic system drives the emergency generator through a hydraulic motor. The emergency generator is what supplies electrical power. Four links, and only the last one is a generator.
So the RAT is not the emergency generator. It is three links upstream of it. A candidate who says the RAT powers the airplane has skipped the chain, and an examiner will pursue exactly that gap, because the failure modes live at different links: a RAT that will not extend is a different problem from a blue system that will not pressurize, which is a different problem from an emergency generator that will not couple.
What does the A320 emergency generator supply, and at what rating?
The emergency generator supplies 5 kVA of three-phase 115/200 V 400 Hz AC power. In the emergency configuration, the emergency generator supplies the AC ESS BUS directly and supplies the DC ESS BUS through the ESS TR. While the emergency generator is running, it also supplies the AC SHED ESS BUS directly and the DC SHED ESS BUS through the ESS TR.
The 5 kVA figure is the AC output rating of the generator. It is not a rating on the DC buses, which are supplied through the ESS TR.
What are the three RAT speeds a candidate should know?
Three different figures apply, and they are easy to conflate. Above 100 kt is the automatic RAT extension threshold when both AC buses are lost. The procedure publishes a MIN RAT SPEED of 140 kt. And the procedure cautions that the RAT is capable of supplying the emergency generator down to 125 kt, except during flare.
A candidate who answers 100 kt as the published minimum RAT speed has confused the automatic-extension threshold with the procedural MIN RAT SPEED of 140 kt. The RAT is nevertheless capable of supplying the emergency generator down to 125 kt, except during flare. They are different numbers.
What powers the aircraft during the coupling interval?
The emergency generator does not connect the instant the RAT extends. It connects only once its electrical parameters are within tolerance, and reaching tolerance takes a moment.
For about 8 s during RAT extension and emergency generator coupling, the batteries power the emergency generation network. Battery 1 supplies the AC STAT INV bus and, above 50 kt, the AC ESS BUS through the static inverter. Battery 2 supplies the DC ESS BUS. If the RAT is extended manually, coupling occurs 3 s after the RAT begins supplying the emergency generator.
What is the static inverter's role in the emergency configuration?
The static inverter converts DC power from Battery 1 into 1 kVA of single-phase 115 V 400 Hz AC for part of the AC essential bus. It is a converter, not a generator.
When only the batteries are supplying the aircraft, its activation depends on speed. Above 50 kt it activates automatically, regardless of BAT pushbutton position. Below 50 kt it activates only if both BAT 1 and BAT 2 pushbuttons are on. That activation rule is about speed and the pushbuttons, and it is separate from the rule that sheds the AC ESS BUS below 50 kt on the ground.
What are the two ground thresholds in the emergency configuration?
As the aircraft decelerates on the ground, the emergency configuration sheds in two distinct steps at two distinct speeds. Keeping them apart is the central point.
First step. If the RAT stalls, or if the aircraft is on the ground below 100 kt, the emergency generation network automatically transfers to the batteries and static inverter. The AC SHED ESS and DC SHED ESS buses are automatically shed. On the ground below 100 kt, the DC BAT BUS is automatically connected to the batteries.
Second step. On the ground below 50 kt, the AC ESS BUS is automatically shed, and this leads to the loss of all display units.
The two steps are not interchangeable. The below-100-kt step is a transfer to batteries and static inverter with the shed-essential buses lost. The below-50-kt step is the loss of the AC ESS BUS and every display unit. Quoting one speed for both events is the error an examiner is most likely to probe.
Can the APU be started in the emergency configuration?
Yes, subject to published conditions. APU start is not available for 45 s after the loss of both engine generators. The FCOM states that this delay prevents interference with emergency generator coupling. In the air, the APU may be started below FL 250. On the ground, when only the batteries are powering the emergency generation network, APU start is available only below 100 kt.
That is three separate conditions on one action, and each is the kind of qualifier an examiner listens for.
Does the emergency configuration depend on which aircraft I am flying?
The core system behaviors published as applicable to all are the automatic RAT-extension condition, the RAT and blue-hydraulic-system and emergency-generator drive chain, the roughly 8 s battery-supplied interval, and the two ground thresholds. Those do not change by tail number.
However, the applicable ELEC EMER CONFIG procedure and the SYS REMAINING and inoperative-system status are aircraft-effectivity dependent. In the Wizz Air FCOM, the SYS REMAINING table is split into six applicability variants and the ELEC EMER CONFIG procedure into five; the two counts do not map one to one.
Effectivity differences include, but are not limited to, the approach speed, published as VREF +10/140 KT or VREF +15/140 KT, the center-tank fuel treatment, which in one variant is unusable and in another can be gravity transferred to the wing tanks except the last two tons, and the inoperative-system status, where an item such as STEEP APPR appears in some variants and not others. Because remaining-system availability is delegated to the aircraft-applicable table, crews must use the aircraft-applicable FCOM pages rather than a generic list.
The landing configuration does not materially differ; all variants keep Flap 3, and only the wording changes between USE FLAP 3 and FLAP LVR 3. These are aircraft-effectivity differences within the A320 and A321 type, captured in the operator-tailored manual, not properties of a different airplane.
Where This Sits in the Series
This is the Spoke 3 page of the A320 Electrical System Foundation Series, covering emergency electrical configuration. The normal architecture is on the hub page. Source priority and external power indications are on the Spoke 1 page. Generator fault and IDG disconnect are on the Spoke 2 page. The full narrative walkthrough of the emergency configuration is in the companion article.
