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Airbus A320 Flight Control Laws

Oct 3, 2025

Overview

This lecture explains the Airbus A320 flight control laws: Normal, Alternate, Direct, and Mechanical Backup. It details their protections, control characteristics, and operational procedures, focusing on how each law affects pitch, roll, and yaw control, as well as the transition between modes.

Normal Law

  • Indicated by double green symbols on the Primary Flight Display (PFD).
  • Provides full flight envelope protection, preventing excessive maneuvers and keeping the aircraft within safe limits.
  • Controls all three axes: pitch, roll, and yaw, with computers moderating pilot inputs.
  • Ground Mode:
    • Side stick movement directly controls elevator deflection; no auto trim.
    • Trimmable Horizontal Stabilizer (THS) is set to zero degrees, within the green band.
    • During takeoff roll, maximum up elevator deflection reduces from 30° to 20° at 75 knots.
    • Ground mode transitions to flight mode within the first five seconds after liftoff.
  • Flight Mode:
    • Side stick controls elevator and THS to maintain a load factor proportional to stick deflection, independent of speed.
    • With the side stick neutral and wings level, the system maintains 1g in pitch; no manual trim needed for speed or configuration changes.
    • Automatic pitch trim is active except when:
      • Manual trim is selected via THS.
      • Radio altitude is below 50 feet (or 100 feet with autopilot engaged).
      • Load factor is below 0.5g or above 1.25g.
      • Angle of attack or high-speed protections are active (THS setting is limited).
      • Bank angle exceeds 33° (THS setting is limited).
  • Autopilot:
    • Limits pitch and roll commands.
    • Side stick has a restraining force; overcoming it disconnects autopilot.
    • Autopilot can also be disconnected by rudder pedal input or excessive pitch trim movement.
    • All normal law protections remain active with autopilot engaged.
  • Flare Mode:
    • Activates below 500 feet radio altitude.
    • THS is frozen; system memorizes pitch attitude as a reference.
    • At 30 feet, pitch attitude is reduced to -2° nose-down over 8 seconds, requiring gentle nose-up input to flare.
    • Flare mode transitions to ground mode over the next 5 seconds if pitch attitude is less than 2.5°.

Alternate Law

  • Indicated by amber "ALTN" on the PFD and ECAM alert.
  • Flight control law changes from normal to alternate, with reduced or no protections.
  • Pitch Control:
    • In flight, follows load factor demand law similar to normal law, with auto trim for speed/configuration changes.
    • No pitch attitude protection.
    • In flare mode, pitch control becomes direct law with manual THS trimming.
  • Ground Mode:
    • Becomes active 5 seconds after touchdown.
    • Functions identically to normal law's ground mode: direct side stick to elevator relationship, no auto trim, THS set to zero degrees.
  • Lateral (Roll) Control:
    • Follows roll direct law: side stick directly controls ailerons and spoilers 4 and 5.
    • If spoiler 4 fails, spoiler 3 replaces it; if ailerons fail, all spoilers become active.
    • Roll rate is limited to about 30°/sec (clean) and 25°/sec (slats extended), higher than in normal law, making the aircraft more sensitive.
    • No bank angle protection.
  • Yaw Control:
    • Turn coordination is lost; pilot uses rudder pedals only.
    • Yaw damping remains, but authority is limited to ±5° of rudder deflection.
  • Protections:
    • Load factor limitations: +2.5g to -1g (clean), +2g to 0g (other configurations).
    • Artificial low-speed stability replaces normal angle of attack protection; active from about 5 to 10 knots above stall warning, depending on weight/configuration.
      • System introduces a gentle nose-down signal to prevent speed from dropping too low; pilot can override.
      • Bank angle compensation maintains constant angle of attack.
      • PFD speed scale shows a black/red barber pole below stall warning; audio stall warning activates at a safe margin.
      • Upper alpha floor protection is inoperative.
    • High-speed stability: above VMO/MMO, a nose-up demand is introduced to prevent overspeed; pilot can override.
      • Oral overspeed warning remains available (VMO+4 knots or MMO+0.06 Mach).
  • Alternate Law Without Protections:
    • Only load factor limitation remains; no low-speed or high-speed stability.

Direct Law

  • PFD displays "USE MAN PITCH TRIM" (flashes for 5 seconds, then steady); ECAM shows "protection loss."
  • All axes (pitch, roll, yaw) have a direct relationship between side stick and control surfaces.
  • Pitch Control:
    • Direct stick-to-elevator relationship; elevator deflection is proportional to side stick movement.
    • Maximum elevator deflection varies with aircraft CG for controllability.
    • No automatic trim; pilot must trim manually using THS.
    • No pitch attitude protection; overspeed and stall warnings are available, but no protections are active.
  • Lateral (Roll) Control:
    • Same as alternate law: direct stick-to-control surface movement using ailerons and spoilers 4 and 5.
    • If spoiler 4 fails, spoiler 3 replaces it; if ailerons fail, all spoilers become active.
    • Roll rate is limited to about 30°/sec (clean) and 25°/sec (slats extended); aircraft is sensitive, with no bank angle protection.
  • Yaw Control:
    • Controlled by rudder pedals only; yaw damping and turn coordination are lost.
  • Landing:
    • Landing in direct law is similar to a conventional aircraft; careful manual trimming and small control inputs are required.
    • Avoid large thrust changes or sudden speed brake movements.

Mechanical Backup

  • Used during a complete loss of electrical power and computers; extremely rare.
  • PFD displays "MAN PITCH TRIM ONLY."
  • Only manual pitch trim via THS and rudder pedals are available for control.
  • No protections; only basic control is possible.
  • Thrust changes affect pitch (increased thrust = nose up, decreased thrust = nose down) and are more responsive than trimming.
  • Differential thrust can induce yaw but is not recommended due to difficulty.
  • This mode is intended only to maintain control until systems are restored.

Key Terms & Definitions

  • Normal Law: Full flight control protections via computers; shown as double green bars on PFD.
  • Alternate Law: Reduced protections; some automatic functions lost; indicated by amber on PFD.
  • Direct Law: No protections or auto trim; pilot directly controls surfaces.
  • Mechanical Backup: Minimal control using only manual trim and rudder due to electrical failure.
  • THS (Trimmable Horizontal Stabilizer): Adjustable surface for pitch trimming.
  • PFD (Primary Flight Display): Cockpit display showing attitude, airspeed, and flight mode indications.
  • ECAM: Electronic Centralized Aircraft Monitor for system alerts and warnings.

Action Items / Next Steps

  • Review company manuals for specific procedures, limitations, and law transition cues.
  • Practice identifying flight control law changes on the PFD and ECAM.
  • Familiarize yourself with manual trimming and control inputs for alternate and direct law scenarios.
  • Understand the effects of thrust changes on pitch and yaw, especially in degraded modes.
  • Stay alert for system messages and warnings indicating law changes or protection loss.

Full transcript