Overview
This lecture covers the Airbus A320’s flight control systems, including fly-by-wire controls, computer architecture, side stick operation and priority, flight control laws and protections, slats/flaps operation, standard cockpit checks, and ECAM messages for various malfunctions.
A320 Flight Control System
- The A320 was the first commercial airliner to use a digital fly-by-wire flight control system. All flight control surfaces are electrically controlled and hydraulically actuated, reducing mechanical complexity, weight, and maintenance costs, while increasing reliability and performance.
- Pitch control is provided by two elevators mounted on a trimmable horizontal stabilizer (THS).
- Roll control is achieved through ailerons and five spoiler panels per wing. Spoilers 2, 3, and 4 act as speed brakes; all five serve as ground spoilers.
- Yaw control is managed by a single-piece rudder on the vertical stabilizer. Both the THS and rudder are also mechanically linked to cockpit controls, allowing manual control if electrical power is lost.
- Each wing has two trailing edge flaps and five leading edge slats, which serve as lift augmentation devices for takeoff and landing.
- The traditional control column is replaced by a side stick controller, which, together with an auto trim system, allows for smoother, more precise, and lower-effort flying, improving passenger comfort.
- Inputs from the side stick or autopilot are processed by flight control computers, which command the actuators to move the control surfaces as needed. The system provides protections against flight outside the normal envelope, including stall, wind shear, overstress, and overspeed.
Flight Control Computers & ECAM
- There are three types of flight control computers, each with redundancy and failure protection:
- 2 ELACs (Elevator/Aileron Computers): control elevator, stabilizer, and aileron.
- 3 SECs (Spoiler/Elevator Computers): control spoilers and provide standby elevator and stabilizer control.
- 2 FACs (Flight Augmentation Computers): control the rudder electrically.
- Different manufacturers, hardware, software, and electrical power sources are used to prevent common-mode failures.
- The ECAM (Electronic Centralized Aircraft Monitor) displays flight control status and faults. The flight control page appears automatically with certain faults and can be selected manually. The ECAM wheel page also shows ground spoiler status.
Side Stick Operation & Priority
- The A320 has two side sticks (one for the captain, one for the first officer), which are not mechanically linked. Simultaneous inputs are summed, but the total is limited to the maximum deflection of a single side stick.
- If both side sticks are moved at the same time, both green side stick priority lights flash and a continuous "dual input" aural warning sounds. It is essential for pilots to be clear about who has control.
- There is no artificial feel system, only a spring centering device that returns the side stick to neutral when released.
- On the ground, after the first engine is running, side stick position indicators appear on the PFD (Primary Flight Display) and disappear when airborne. White corner indices show the limit of travel; a white cross shows the combined order from both side sticks.
- The side stick has two switches: an autopilot disconnect/priority takeover push button and a push-to-talk switch for communications.
- When the autopilot is engaged, a solenoid detent locks the side stick in neutral. The detent can be overridden with force, which also disconnects the autopilot, but the normal method is to press the disconnect button.
- Priority takeover: pressing and holding the priority takeover button gives full control to that pilot and deactivates the other side stick after 40 seconds. An audio message ("priority left" or "priority right") is heard, and side stick priority lights indicate which pilot has control. A red arrow in front of a pilot means their side stick is deactivated; a green light shows who has control. To reactivate a side stick, press its priority button.
Control Laws: Pitch Normal Law and Protections
- The flight control computers use control laws to determine how surfaces should move. Normal law is active when all systems are functioning, even after some faults, and provides protections:
- Load factor limitation
- Pitch attitude protection
- Angle of attack protection
- High speed protection
- Bank angle protection
- Low energy warning
- Load alleviation protection
- Normal law includes three pitch modes:
- Ground mode: Elevator deflection is directly proportional to side stick movement; auto trim is not functional. Pilots set THS manually for takeoff. Max elevator deflection is 30° nose up, 15° nose down, reduced to 20° nose up above 70 knots to prevent tail strike.
- Flight mode: After takeoff, the system transitions to flight mode over five seconds. With the side stick neutral, the aircraft maintains a constant 1G (constant trajectory). Moving the side stick aft demands more than 1G (positive G), forward demands less than 1G (negative G). Auto trim operates to maintain the selected trajectory.
- Flare mode: As the aircraft descends through 50 feet, flight mode changes to flare mode. The system memorizes pitch at 50 feet and reduces it to -2° over 8 seconds, simulating ground effect. Auto trim deactivates, requiring the pilot to flare manually.
- Bank angle protection:
- Maximum bank angle is 67° (flaps up) or 60° (flaps extended). If the side stick is released, the aircraft returns to and maintains 33° of bank.
- Beyond 33°, no pitch compensation is provided; pilot must hold aft stick to maintain pitch.
- When high speed or angle of attack protection is active, max bank angle is reduced to 45°. If the side stick is released, the aircraft returns to wings level (high speed protection) or 33° (high angle of attack protection).
- Load alleviation function (LAF): Reduces wing loads in turbulence by deflecting ailerons and spoilers 4/5 upwards when load factor differs by 0.3G from demand. LAF is inhibited with slats/flaps extended below 200 knots or above VMO+10 knots. LAF degraded or inoperative is indicated on ECAM.
Slats and Flaps Operation
- Each wing has five leading edge slats and two trailing edge flaps, controlled by a single flaps lever with five detents (0, 1, 2, 3, full).
- Two dual-channel Slat Flap Control Computers (SFCCs) monitor and control slats/flaps. Each SFCC controls one motor for slats and one for flaps. Hydraulic motors, via shafts and gearboxes, move the surfaces.
- When slats/flaps reach the selected position, pressure-off brakes (POBs) lock the motors to prevent movement.
- If a hydraulic system or SFCC fails, the affected slats/flaps operate at half speed.
- SFCCs monitor position via symmetry and feedback pick-off units. Asymmetry, runaway, or uncommanded movement activates wingtip brakes (WTBs), locking slats/flaps in position. WTBs can only be reset on the ground.
- Two indication pick-off units (IPPUs) send slat/flap position data to the ECAM, which displays positions using green, amber, and cyan boxes. The selected position is shown in cyan; movement is indicated by the green bar.
- Flap lever detents have internal blocks to prevent excessive movement; a collar must be lifted to move past certain positions.
- Alpha Speed Lock prevents slat retraction at high angle of attack (>8.5°) or low speed (<148 knots). If the lever is moved from 1 to 0 under these conditions, only the flaps retract; slats retract when speed increases to VLS for config 0.
- Ailerons droop 5° when flaps are extended, indicated on ECAM.
- If both hydraulic systems fail or there is a slat/flap fault, ECAM shows amber indicators and "slats locked" or "flaps locked" messages.
Standard Cockpit Checks & Procedures
- Configuration Safety Check: Confirm on ECAM that flap position matches the lever; check speed brake lever. If not correct, contact maintenance before applying hydraulic power or moving controls.
- Cockpit Geographic Check: Ensure all flight control computer switch lights are out, side stick priority lights are out, pitch trim wheels are at zero, speed brake lever is retracted, flaps lever matches ECAM, and rudder trim is centered/reset.
- After Start/Before Takeoff: Captain selects takeoff flap setting and arms ground spoilers. First officer checks full control surface travel using ECAM and PFD indicators, including aileron droop. Captain checks rudder travel and side stick movement. First officer sets pitch trim and confirms rudder trim at zero.
- Takeoff: Control deflection is directly proportional to side stick movement. Apply half forward stick at power application, reduce to neutral by 100 knots. Lateral inputs for crosswind should be minimal. At VR, rotate smoothly to the pitch command bar. After takeoff, retract flaps/slats at specified speeds (F speed for flaps, S speed for slats).
- After Takeoff: Once flaps are up, PNF announces "flaps up" and requests after takeoff check. Disarm spoilers as soon as practical.
- Descent/Approach/Landing: In managed descent, use speed brakes if "more drag" is displayed. On approach, select flaps at green dot and F speeds as required. For config 3 landings, select config 3 on MCDU and GPWS panel to prevent nuisance warnings. Arm spoilers before landing. Flare by pulling aft on side stick; use rudder for crosswind alignment. After touchdown, ground spoilers deploy automatically if armed. If not, and landing was with flaps 3, flight spoilers can be extended manually.
- After Landing: Disarm spoilers, reset flaps as required for icing or contaminated runways.
ECAM Messages & Fault Procedures
- Single Flight Control Computer Fault: Switch off and on to reset. The other computer takes over, except for SEC failure, which results in loss of associated spoilers. Do not reset ELAC if uncommanded maneuvers occurred.
- Multiple Computer Failures: Reset all. If unsuccessful, aircraft reverts to alternate law with reduced protections.
- Flaps/Slats Locked (Wingtip Brakes): Due to asymmetry or runaway, WTBs lock slats/flaps in current position. ECAM shows "flaps locked" caution, master caution, and max flap extension speed. Status page advises config 3 landing, increased approach speed, and landing distance. Increased fuel consumption may be required for diversion.
- Stabilizer Jam: ECAM recommends checking manual pitch trim. If available, trim to maintain neutral elevator. Alternate law becomes active; autopilot is unavailable. If manual trim is not available, use config 3 for landing and delay gear extension until flaps are in config 3 and speed is at Vapp. Once gear is down, direct law is active and further changes require constant side stick pressure. Approach speed is Vref+10; landing distance factor must be applied.
- Pitch Trim Wheel Jam: May not be detected by ELACs; no ECAM warning. If elevators remain deflected, disconnect autopilot and check manual trim. If unavailable, use config 3 for landing.
- Side Stick Fault: Triggered by pitch or roll transducer failure; ECAM displays warning.
- Loss of Both Elevators: Only manual pitch trim is available. ECAM and PFDs display "manual pitch trim only." Do not use speed brakes due to pitch changes. Status page outlines approach procedures; pitch control is mechanical backup, roll is direct law.
- Protections Lost in Alternate Law: Attitude, overspeed, and alpha limit protections are not available.
Key Terms & Definitions
- Fly-by-wire: Electronic flight control system replacing mechanical linkages.
- ECAM: Electronic Centralized Aircraft Monitor; displays system status and faults.
- ELAC/SEC/FAC: Types of flight control computers—Elevator/Aileron, Spoiler/Elevator, Flight Augmentation.
- Side stick: Joystick-like controller for pitch and roll.
- Normal Law: Default, fully protected flight control mode.
- Alternate Law: Reduced protection/control mode after faults.
- Wingtip Brakes: Lock slats/flaps after asymmetry or fault.
- Alpha Speed Lock: Prevents slat retraction at high angle of attack or low speed.
- Load Alleviation Function (LAF): Reduces wing loads in turbulence by deflecting ailerons and spoilers.
Action Items / Next Steps
- Review ECAM messages, procedures, and FCOM references for abnormal and emergency operations.
- Practice cockpit checks and flight control procedures in a simulator or mock-up.
- Complete assigned reading in FCOM 3.02.10 (operating techniques) and 3.02.80 (miscellaneous procedures) for handling faults and failures, including landing, go-around, and diversion procedures.