A320 Flight Control Computers: The Complete Architecture Guide

The Airbus A320 is flown by seven flight-control computers. There are 𝘁𝘄𝗼 𝗘𝗟𝗔𝗖𝘀 (Elevator Aileron Computers), 𝘁𝗵𝗿𝗲𝗲 𝗦𝗘𝗖𝘀 (Spoiler Elevator Computers), and 𝘁𝘄𝗼 𝗙𝗔𝗖𝘀 (Flight Augmentation Computers). Two additional computers, the 𝗙𝗖𝗗𝗖𝘀 (Flight Control Data Concentrators), sit alongside the primary seven and concentrate flight-control data for the displays and the maintenance system without flying the aircraft themselves. Most pilots fly the airplane for years and never properly learn who is who. This guide answers the architectural questions a pilot actually needs to know - both for the type rating oral and for the operational reality of flying a fly-by-wire airplane that degrades along specific cascades.

This is the hub of a three-part Foundation Series. The series goes deeper into each subject in dedicated spokes: the Foundation overview, the ELAC deep-dive, and the SEC and FAC deep-dive. Each spoke pairs with a published article on LinkedIn that frames the architecture for a working pilot rather than a textbook reader.

Every architectural claim on this page is anchored to the FCOM. The full examiner-grade walkthrough, with citations attached to every consequence and every trap, lives in the A320 Oral Exam Prepper at customgptsolutions.ai.

The architecture distributes flight-control work across three computer families, each doing a job the others do not.

The 𝗘𝗟𝗔𝗖𝘀 are primary for elevator and THS pitch control and for aileron lateral control. In normal law, lateral control combines aileron deflection with spoiler deflection (except spoiler 1), so the SEC-controlled spoilers are part of the normal roll path alongside the ailerons - the ELACs are not the sole roll computers. The 𝗦𝗘𝗖𝘀 run the spoilers on a fixed mapping, the ground lift-dumping logic, the speedbrake function, and a backup pitch path for the case where both ELACs fail. The pilot's rudder input is mechanical. The 𝗙𝗔𝗖𝘀 provide the electrical rudder control functions on top of the mechanical pedal path: yaw damping, turn coordination, rudder trim, and rudder travel limitation. The FACs also control the PFD speed scale, computing the minimum and maximum speeds (VSW, VLS, VFE and VFE next, VLE, VMO/MMO), the maneuvering speeds (green dot, S, F), the speed trend, the alpha-floor command, the windshear detection, and the low-energy warning.

The architecture is 𝗹𝗮𝘆𝗲𝗿𝗲𝗱. The ELACs are primary for elevator, THS, and aileron control. The SECs are backup for pitch and primary for the spoilers, including the spoilers that participate in normal-law roll alongside the ailerons. The FACs provide the electrical rudder control functions on top of the mechanical pedal path, and they own the PFD speed scale and the envelope warnings. The protections that pilots associate with "normal law" come partly from the ELACs (load factor, bank angle, alpha protection in normal law) and partly from the FACs (alpha-floor as a command function, the PFD speed-scale data, the low-energy warning).

Failures cascade in specific, FCOM-documented ways. A single ELAC failure transfers function to the other ELAC and the airplane stays in normal law with reduced redundancy. A double ELAC failure puts the airplane in alternate law with direct law at gear extension, with specific landing penalties. A triple-SEC failure puts the airplane in alternate law with a configuration-driven direct law trigger that most candidates misstate. A 𝗱𝘂𝗮𝗹 𝗙𝗔𝗖 𝗳𝗮𝗶𝗹𝘂𝗿𝗲 - the cascade most candidates underprepare for - has an 𝗲𝗳𝗳𝗲𝗰𝘁𝗶𝘃𝗶𝘁𝘆-𝗱𝗲𝗽𝗲𝗻𝗱𝗲𝗻𝘁 law outcome. Some MSN standards revert to ALTN LAW (PROT LOST). Later standards show ALTN LAW indicated with mechanical yaw and no reversion to DIRECT LAW at landing gear extension. The single-answer mental model does not survive a real oral. The cascade also removes all FAC-provided functions across both effectivity groups, all at once.

The three deep-dives below walk each family in turn.

The two 𝗘𝗟𝗔𝗖𝘀 (Elevator Aileron Computers) are primary for elevator and THS pitch control and for aileron lateral control. In normal law, lateral control uses ailerons plus spoilers (except spoiler 1), so the SEC-controlled spoilers participate in the normal roll path - the ELACs are not the only computers involved in normal-law roll. Most pilots learn that the airplane has "two ELACs" and stop there. The role split between the two ELACs is counterintuitive: 𝗘𝗟𝗔𝗖𝟮 is the pitch master, controlling both elevators and the THS, and 𝗘𝗟𝗔𝗖𝟭 is the aileron master, owning the ailerons that participate in normal-law roll alongside the SEC-controlled spoilers. The names lie. The computer called 2 flies pitch, the computer called 1 owns the ailerons, and that is the 𝘀𝗶𝗻𝗴𝗹𝗲 𝗺𝗼𝘀𝘁-𝗺𝗶𝘀𝘀𝗲𝗱 𝗳𝗮𝗰𝘁 in A320 system orals.

A single ELAC failure retains normal law but loses CAT 3 DUAL autoland - the operational consequence is CAT 3 SINGLE only. The ECAM presents 𝗙/𝗖𝗧𝗟 𝗘𝗟𝗔𝗖 𝟭(𝟮) 𝗙𝗔𝗨𝗟𝗧 for a full failure or 𝗙/𝗖𝗧𝗟 𝗘𝗟𝗔𝗖 𝟭(𝟮) 𝗣𝗜𝗧𝗖𝗛 𝗙𝗔𝗨𝗟𝗧 for a pitch-channel-only failure. The transfer is automatic. The pilot does not lose handling, just redundancy.

A dual ELAC failure is a different magnitude of event. The verified ECAM consequence for 𝗙/𝗖𝗧𝗟 𝗘𝗟𝗔𝗖 𝟭(𝟮) 𝗙𝗔𝗨𝗟𝗧 (BOTH COMPUTERS FAILED) includes pitch and roll normal laws lost, THS motor 1 lost, both ailerons lost, load alleviation function degraded (uses spoilers only), 𝗙/𝗖𝗧𝗟 𝗔𝗟𝗧𝗡 𝗟𝗔𝗪 (𝗣𝗥𝗢𝗧 𝗟𝗢𝗦𝗧), and 𝗗𝗜𝗥𝗘𝗖𝗧 𝗟𝗔𝗪 at gear down. Landing penalties apply: Flap 3 configuration, VREF +10, and the landing-distance procedure. Underneath the law name, pitch falls back to SEC1 or SEC2, the THS uses motor No. 2 or No. 3, and the ailerons revert to damping mode. Saying "both ELACs failed, so we are in alternate law" is true but it is the answer of someone who memorized the law and missed the airplane.

A subtle but important point on dual ELAC failure: it does not mean "no roll at all." The ailerons are lost, but roll remains available through the 𝘀𝗽𝗼𝗶𝗹𝗲𝗿-𝗯𝗮𝘀𝗲𝗱 𝗽𝗮𝘁𝗵. The spoilers continue to operate through their assigned SECs, the same SECs that already participate in normal-law roll. Candidates who answer "both ELACs failed so we have no roll" miss the spoiler-based roll path entirely. The right answer names the law, names the consequences, and names what is still flying the airplane.

For the full ELAC walkthrough with every hydraulic detail, every motor reassignment, and every examiner trap, see the ELAC deep-dive.

The three 𝗦𝗘𝗖𝘀 (Spoiler Elevator Computers) control specific spoiler panels on each wing. The assignment is fixed in the FCOM and pays no attention to what would be convenient to memorize.

The numbering for the SECs does not match the numbering for the spoiler panels, and the mapping is not in ascending order. If a candidate's mental model has SEC 1 controlling spoiler 1, the mental model is wrong.

In normal law, lateral control combines aileron deflection with spoiler deflection (except spoiler 1). The SEC-controlled spoilers participate directly in normal-law roll - they are not just emergency, speedbrake, or ground lift-dumping surfaces. The ELACs command the lateral order through ELAC1 and the ailerons; the SECs execute the spoiler-side contribution. Candidates who treat the SECs as exclusively a backup or auxiliary system in normal flight do not get the architecture right.

Beyond the spoilers, SEC 1 and SEC 2 carry partial elevator authority as the backup pitch path if both ELACs fail. If neither ELAC is available, pitch control shifts to whichever SEC the circuit logic supports at that moment. SEC 3 does not participate in the pitch backup - it controls only its assigned spoiler panel. The SECs also independently compute ground spoiler deployment logic from landing gear compression and wheel tachometer inputs.

A single SEC failure triggers an amber ECAM caution and a panel fault light. Normal law and full envelope protections are retained. The specific spoiler panels assigned to the failed computer are lost, but roll authority through the surviving panels and the ailerons is preserved. The case that deserves specific attention is 𝗦𝗘𝗖 𝟭 failure. SEC 1 controls the No. 3 and No. 4 spoilers, which are the primary speedbrake surfaces, so SEC 1 failure brings the published operational restriction 𝗦𝗣𝗗 𝗕𝗥𝗞 𝗗𝗢 𝗡𝗢𝗧 𝗨𝗦𝗘 - not a slight reduction in authority, but an explicit prohibition that the candidate is expected to know and quote.

The triple-SEC failure is the first of two cascades that finish orals. 𝗙/𝗖𝗧𝗟 𝗦𝗘𝗖 𝟭+𝟮+𝟯 𝗙𝗔𝗨𝗟𝗧 reverts the airplane to 𝗔𝗟𝗧𝗡 𝗟𝗔𝗪 with 𝗣𝗥𝗢𝗧 𝗟𝗢𝗦𝗧. The direct law reversion that follows is 𝗰𝗼𝗻𝗳𝗶𝗴𝘂𝗿𝗮𝘁𝗶𝗼𝗻-𝗱𝗿𝗶𝘃𝗲𝗻, not autopilot-state-driven.

The common wrong answer in orals is "direct law on AP disconnect." That answer is incorrect. The trigger is tied to landing configuration, not autopilot state. The strategic question for the pilot is how to manage the approach knowing the configuration change will trigger direct law, not how to time an AP disconnect.

For the full SEC walkthrough with every consequence and every examiner trap, see the SEC and FAC deep-dive.

The two 𝗙𝗔𝗖𝘀 (Flight Augmentation Computers) do entirely different work from the SECs. The first thing to get right about them is the 𝘁𝘄𝗼-𝗽𝗮𝘁𝗵 𝗿𝘂𝗱𝗱𝗲𝗿 𝗰𝗼𝗻𝘁𝗿𝗼𝗹 𝗺𝗼𝗱𝗲𝗹. The pilot's rudder input is mechanical - the linkage from the pedals to the rudder surface does not pass through any computer, and the rudder is hydraulically actuated by three servojacks. The FACs provide the electrical rudder control functions on top of the mechanical path: yaw damping, turn coordination, rudder trim, and rudder travel limitation. The mechanical pedal authority and the FAC electrical functions coexist. This is a two-path architecture, not an either-or.

FAC work falls into two categories. The first is electrical rudder control: yaw damping, turn coordination, rudder trim, and rudder travel limitation (the variable mechanical limit that reduces maximum rudder deflection at high speed). The pilot retains direct mechanical pedal authority to the rudder surface; the FAC electrical functions add the stabilization layer that makes the rudder behave the way pilots expect it to behave.

The second category is the PFD speed scale and flight envelope computation.

The air data itself - raw airspeed, altitude, angle of attack, temperature - comes from the 𝗔𝗗𝗜𝗥𝗨𝘀. On newer effectivity (𝗠𝗦𝗡 𝟬𝟲𝟬𝟬𝟯-𝟬𝟳𝟬𝟳𝟳), the FCOM states the FAC computes operating speeds based on FMS gross weight, with FAC GW/CG computation available as backup. The FAC is the processing layer that combines air data with weight and configuration information to display the speed scale and the warnings the pilot sees.

This distinction matters because it changes the right answer to one of the more common examiner traps. "If both FACs fail, can we still get airspeed?" Yes - raw airspeed comes from the ADIRUs and remains on the PFD. It is the FAC-computed speed-scale data that disappears, not the airspeed itself.

A single FAC failure (𝗔𝗨𝗧𝗢 𝗙𝗟𝗧 𝗙𝗔𝗖 𝟭(𝟮) 𝗙𝗔𝗨𝗟𝗧) transfers all FAC functions to the surviving FAC. The pilot does not feel a handling change. The PFD still shows the FAC-computed speed-scale data and the rudder still has its travel limiter. But the status message tells the pilot what just changed: 𝗕𝗢𝗧𝗛 𝗣𝗙𝗗 𝗢𝗡 𝗦𝗔𝗠𝗘 𝗙𝗔𝗖 and 𝗖𝗔𝗧 𝟯 𝗦𝗜𝗡𝗚𝗟𝗘 𝗢𝗡𝗟𝗬. The CAT 3 DUAL autoland capability is gone. Any approach to minima below CAT 3 SINGLE requires the redundancy that no longer exists.

The dual FAC failure is the second cascade that finishes orals, and the FCOM is more nuanced than candidate intuition tends to allow. 𝗔𝗨𝗧𝗢 𝗙𝗟𝗧 𝗙𝗔𝗖 𝟭+𝟮 𝗙𝗔𝗨𝗟𝗧 produces a law outcome that is 𝗲𝗳𝗳𝗲𝗰𝘁𝗶𝘃𝗶𝘁𝘆-𝗱𝗲𝗽𝗲𝗻𝗱𝗲𝗻𝘁.

The single fleet-wide answer does not exist - the candidate has to know which standard the operator's fleet falls into and quote the corresponding law table. What is constant is that the candidate-intuitive answer of "dual FAC failure leaves us in normal law as long as the ELACs are healthy" is wrong for every standard. Some form of law reversion applies in all cases.

The consequence set is comprehensive across both effectivity standards. Yaw damping, turn coordination, rudder trim, the PFD speed-scale data, alpha-floor (inhibited because alternate law is active), windshear detection, and low-energy warning are all lost. The rudder travel limiter is locked at the value reached at the second failure, with the ECAM showing 𝗥𝗨𝗗 𝗪𝗜𝗧𝗛 𝗖𝗔𝗥𝗘 𝗔𝗕𝗩 𝟭𝟲𝟬 𝗞𝗧. Full rudder travel authority is recovered at slats extension. The PFD speed-scale data are lost, but 𝗔𝗣𝗣𝗥 𝗦𝗣𝗗 continues to display as 𝗩𝗥𝗘𝗙 + 𝟭𝟬 𝗞𝗧 on ECAM, and on some standards ECAM also shows 𝗢𝗣𝗘𝗥𝗔𝗧𝗜𝗡𝗚 𝗦𝗣𝗗 𝗖𝗢𝗠𝗣𝗨𝗧𝗘. 𝗡𝗢 𝗗𝗜𝗦𝗣𝗔𝗧𝗖𝗛 applies under MEL.

For the full FAC walkthrough including the alpha-floor examiner trap and the FAC/FMS gross weight relationship, see the SEC and FAC deep-dive.

The connective tissue across all three computer families is the flight control 𝗹𝗮𝘄 𝗰𝗮𝘀𝗰𝗮𝗱𝗲. Every failure eventually expresses itself as a step down this ladder, and understanding the ladder is what lets a candidate answer follow-up questions in an oral.

In 𝗮𝗹𝘁𝗲𝗿𝗻𝗮𝘁𝗲 𝗹𝗮𝘄 with reduced protection, pitch attitude protection is lost, bank angle protection is lost, high-AOA protection is replaced by low-speed stability, high-speed protection is replaced by overspeed stability, and alpha-floor is inoperative. Load factor limitation remains. Lateral control becomes roll direct law. Yaw is alternate fly-by-wire or mechanical depending on the failure case.

In 𝗱𝗶𝗿𝗲𝗰𝘁 𝗹𝗮𝘄, pitch is direct stick-to-elevator, roll is direct stick-to-surface, yaw is mechanical, no automatic trim, no protections, alpha-floor inoperative. Overspeed and stall warnings remain. The PFD shows 𝗨𝗦𝗘 𝗠𝗔𝗡 𝗣𝗜𝗧𝗖𝗛 𝗧𝗥𝗜𝗠.

𝗠𝗲𝗰𝗵𝗮𝗻𝗶𝗰𝗮𝗹 𝗯𝗮𝗰𝗸𝘂𝗽 is the continuation architecture, not a finesse law. Its purpose is temporary total electrical loss, temporary loss of five fly-by-wire computers, loss of both elevators, or total loss of ailerons and spoilers. What the pilot has left is pitch via manual THS trim and lateral and yaw via rudder pedals. The PFD shows 𝗠𝗔𝗡 𝗣𝗜𝗧𝗖𝗛 𝗧𝗥𝗜𝗠 𝗢𝗡𝗟𝗬 in red. This is survival and continuation, not refined handling.

The pilot technique by law is part of what separates a good answer from an excellent one. In alternate law, stop assuming protections will save the aircraft. Respect 320 kt / M.77 limits where stated. Expect Flap 3 / VREF +10 / landing distance procedure on many degraded approaches. In direct law, manual pitch trim is mandatory, inputs should be smaller and more measured, manual thrust is strongly recommended, and speedbrake should be used with care. In mechanical backup, think trim and rudder, not sidestick fly-by-wire.

The temptation in studying flight-control computer architecture is to treat it as oral exam material - memorize the consequences, quote the FCOM, pass the test. The deeper reason to know it is operational. An airplane that has just degraded along one of these cascades is an airplane the pilot now has to fly, and the technique for flying a degraded-law airplane depends on knowing what was actually lost.

A 𝗱𝘂𝗮𝗹 𝗙𝗔𝗖 𝗳𝗮𝗶𝗹𝘂𝗿𝗲 on approach is an airplane with the PFD speed scale gone, the rudder travel limiter locked, no yaw damping, and no alpha-floor backstop. The ECAM status carries APPR SPD VREF + 10 KT, and on some standards OPERATING SPD COMPUTE, so the approach speed is published rather than improvised. The technique is to fly the published approach speed with restrained rudder input, and remember that aggressive pedal work in a yaw-damping-free airplane invites the Dutch roll oscillation the FAC was designed to suppress. A 𝘁𝗿𝗶𝗽𝗹𝗲-𝗦𝗘𝗖 𝗳𝗮𝗶𝗹𝘂𝗿𝗲 is an airplane that will revert to direct law at a configuration change, not at an autopilot disconnect, so the approach planning is configuration-aware rather than autopilot-aware. A 𝗱𝘂𝗮𝗹 𝗘𝗟𝗔𝗖 𝗳𝗮𝗶𝗹𝘂𝗿𝗲 is an airplane in alternate law that becomes direct law at gear extension, with landing penalties published in the QRH.

The candidates who study the architecture at depth tend to be the line pilots who handle these cascades better in the simulator and, if it ever comes to it, on the line. The architectural knowledge is not separable from the handling. Knowing what is left is the first step in flying what is left.

Quick answers to the highest-leverage cross-cutting questions on the A320 flight control architecture.

This hub presents the integrated architecture. Each subject has a dedicated spoke that goes further into the specifics, the examiner traps, and the FCOM citation depth that finishes orals on a strong note.

Each spoke pairs with a published article on LinkedIn. The full Week 3 closing article in the series - "The Quiet Ones in the Corner: The Five Computers That Do the Unglamorous Work" - frames the SEC and FAC architecture for a working pilot rather than a textbook reader.

The complete examiner-grade walkthrough of the entire flight-control architecture, with FCOM citations attached to every claim and a conversational format that lets candidates practice thinking out loud before the real oral, is available in the A320 Oral Exam Prepper at customgptsolutions.ai. The airline-specific variants are tuned to each operator's syllabus, FCOM section structure, and OMA-specific operational restrictions, including the MSN-specific FAC law tables.