A320 Flight Controls Explained | Normal Law, Alternate Law, Direct Law

The Airbus A320 flight control system is a fly-by-wire design where pilot inputs are interpreted by computers that command control surfaces. Understanding the different control laws (normal, alternate, direct) and their associated protections is critical for A320 type rating exams and safe operations. This guide covers all aspects tested during type rating.

Flight Control System Architecture

The A320 flight control system consists of primary flight controls (ailerons, elevators, rudder), secondary flight controls (slats, flats, spoilers), and multiple redundant computers that provide different levels of control law sophistication.

🖥️ Computers

2 ELAC (Elevator Aileron Computer), 3 SEC (Spoiler Elevator Computer), 2 FAC (Flight Augmentation Computer), 2 FMGC

✈️ Primary Surfaces

Ailerons, spoilers (roll), elevators, THS (pitch), rudder (yaw) - all hydraulically powered

🛡️ Control Laws

Normal law (full protections), Alternate law (reduced protections), Direct law (no protections), Mechanical backup

⚡ Hydraulics

Green, blue, yellow hydraulic systems provide power to all flight control surfaces with redundancy

A320 Normal Law

Normal law is the default control mode providing full envelope protection. The flight control computers interpret pilot inputs and command surfaces to keep the aircraft within safe flight parameters. The aircraft cannot be stalled or overstressed in normal law.

Normal Law Pitch (Longitudinal)

In normal law pitch, sidestick commands a load factor (g-load), not pitch rate or angle. The aircraft maintains the commanded load factor automatically.

Load Factor Demand: Sidestick aft = 2.5g, sidestick neutral = 1g, sidestick forward = 0g to -1g
Auto-Trim: Aircraft trims automatically to maintain selected load factor, no manual trim needed
High Angle of Attack Protection: α-floor activates at high alpha, TOGA thrust applied automatically
Pitch Attitude Protection: +30° nose up, -15° nose down (clean config), prevents excessive pitch
High Speed Protection: Prevents exceeding VMO/MMO, automatic nose-up command near limits

🛡️ Alpha Floor Protection

When angle of attack reaches α-floor threshold (typically around 15° AoA depending on config), the autothrust system automatically applies TOGA thrust to both engines. This is an automatic stall prevention feature. The pilot can override by retarding thrust levers to IDLE, but this is rarely appropriate except in specific training scenarios.

Normal Law Roll (Lateral)

Sidestick commands a roll rate. When released, aircraft maintains current bank angle (up to bank angle protection limits).

Bank Angle Protection: Maximum 67° bank angle, soft stops at 33° and 67°
Automatic Turn Coordination: Computer adds yaw input automatically for coordinated turns
Roll Rate: Up to 15° per second depending on sidestick deflection

Normal Law Yaw (Directional)

Yaw damping and turn coordination are automatic. Rudder pedals provide direct rudder control, mainly used for crosswind landings and engine-out operations.

⚠️ Normal Law Available When...

Normal law requires:
• At least 2 ADR (Air Data Reference) working
• At least 2 IR (Inertial Reference) working
• All flight control computers nominal
• Slats/flaps position sensors working
• Green + Blue OR Green + Yellow hydraulics available

Loss of any critical component degrades to alternate or direct law.

A320 Alternate Law

Alternate law occurs when certain flight control system components fail, removing some but not all protections. There are two main versions: Alternate Law 1 (with some protections) and Alternate Law 2 (fewer protections).

What Triggers Alternate Law?

Multiple ADR or IR failures
Loss of slat/flap position sensors (SFCC failure)
Significant hydraulic failures (e.g., dual hydraulic loss)
Flight control computer failures (ELAC, SEC degradations)
Certain abnormal configurations detected by FMGCs

Alternate Law Characteristics

In alternate law:

NO Alpha Protection: Aircraft can be stalled - pilot responsibility to avoid stall
Stall Warning: Aural "STALL STALL" warning replaces alpha floor protection
Reduced Load Factor Protection: +2.5g / -1g limits may still apply but not guaranteed
High Speed Protection: May or may not be available depending on alternate law version
NO Bank Angle Protection: Pilot can roll beyond 67° (unusual attitudes possible)
Manual Pitch Trim Required: Auto-trim lost, pilot must manually trim with pitch trim wheel

🚨 Alternate Law: Pilot Responsibility

In alternate law, the pilot is responsible for maintaining safe flight parameters. The aircraft will NOT prevent you from stalling or overstressing. Fly carefully, respect V speeds, and manually manage pitch trim. ECAM will display "USE MAN PITCH TRIM" reminder.

Alternate Law at Landing

If alternate law persists to landing, the system degrades further:

Below 100 ft RA: System reverts to DIRECT LAW
Flare Mode Lost: No automatic pitch-up tendency in flare
Landing Technique: Requires more active flare input from pilot, similar to conventional aircraft

A320 Direct Law

Direct law provides a direct relationship between sidestick position and control surface deflection - no computer interpretation, no protections. This is the most degraded control mode.

When Does Direct Law Occur?

Severe flight control computer failures
Below 100 ft radio altitude when already in alternate law
Multiple critical system failures (triple hydraulic loss, etc.)
On ground with certain faults

Direct Law Flight Characteristics

Pitch: Sidestick commands elevator deflection directly, requires manual trim
Roll: Sidestick commands aileron/spoiler deflection directly
NO Protections: No alpha, load factor, bank angle, or speed protections
NO Auto-Trim: Pilot must trim constantly using pitch trim wheel
Speed Stability: Aircraft less stable in speed, requires active pitch management

⚠️ Flying in Direct Law

Direct law requires conventional aircraft flying techniques: active pitch control, continuous trimming, careful speed management. Treat the aircraft like a non-fly-by-wire airplane. Follow all ECAM procedures and consider declaring an emergency.

Mechanical Backup

If all electrical flight control fails, mechanical backup provides pitch control via cables connecting the pitch trim wheel to the THS (Trimmable Horizontal Stabilizer).

Pitch Control: Via manual rotation of pitch trim wheel (very slow, heavy)
Roll Control: Via rudder-induced roll (very limited)
Extremely Rare: Requires total electrical failure, never occurred in airline operations

Control Law Comparison Table

Feature	Normal Law	Alternate Law	Direct Law
Alpha Protection	✅ Yes (α-floor)	❌ No (stall warning)	❌ No
Load Factor Protection	✅ Yes (+2.5g/-1g)	⚠️ Reduced	❌ No
Bank Angle Protection	✅ Yes (67° max)	❌ No	❌ No
High Speed Protection	✅ Yes	⚠️ Maybe	❌ No
Auto-Trim	✅ Yes	❌ No (manual)	❌ No (manual)
Can Stall?	❌ No	✅ Yes	✅ Yes
Flare Mode	✅ Yes	❌ No	❌ No

Frequently Asked Questions

What is A320 normal law?

Normal law is the default flight control law providing full fly-by-wire protections including load factor limitation (+2.5g/-1g), pitch attitude protection (+30°/-15°), high angle of attack protection (α-floor), high speed protection (VMO/MMO), and bank angle protection (67° max). The aircraft cannot be stalled or overstressed in normal law as the flight control computers prevent exceeding safe flight parameters.

What causes A320 to degrade to alternate law?

Alternate law occurs when certain flight control computers fail (ELAC, SEC), multiple ADR or IR failures occur, loss of slat/flap position sensors (SFCC failure), or significant hydraulic failures. Protections are reduced: no alpha protection (stall possible), reduced load factor protection, no bank angle protection, and manual pitch trim required. Stall warning replaces alpha floor protection.

What is the difference between alternate law and direct law?

In alternate law, flight control computers still provide some protections and envelope management, though reduced from normal law. In direct law, there is a direct relationship between sidestick deflection and control surface position with no computer protections whatsoever. Pilot has full manual control but no flight envelope protections - the aircraft can be stalled, overstressed, or put in unusual attitudes.

Does the A320 have autotrim in alternate law?

No. Auto-trim is lost in alternate law. The pilot must manually trim the aircraft using the pitch trim wheel on the center pedestal. ECAM will display "USE MAN PITCH TRIM" as a reminder. This is one of the key differences from normal law where the aircraft trims automatically to maintain commanded load factor.

What is alpha floor in A320?

Alpha floor is an automatic stall prevention feature active in normal law. When angle of attack reaches the α-floor threshold (approximately 15° AoA depending on configuration), the autothrust system automatically applies TOGA thrust to both engines regardless of thrust lever position. This prevents the aircraft from stalling. The pilot can override by retarding thrust levers to IDLE, but this is rarely appropriate in normal operations.

Can you stall an A320 in normal law?

No. In normal law, the aircraft cannot be stalled. High angle of attack protection prevents reaching critical AoA. At approximately 15° AoA, alpha floor activates applying TOGA thrust. At maximum AoA (around 18-20° depending on config), the flight control computers prevent further nose-up pitch regardless of sidestick input. The only way to stall is if the aircraft degrades to alternate or direct law where alpha protection is lost.

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A320 Flight Controls (DSC-27)