A320 Flight Controls — Fly-by-Wire, Normal Law, Alternate Law, Direct Law and the Sidestick

The A320 was the first production airliner with a fully fly-by-wire primary flight control system and a sidestick instead of a control column. The pilot inputs a demand. Computers translate that demand into surface commands, while simultaneously enforcing the aircraft's structural and aerodynamic limits. When those computers degrade, the limits disappear — and the pilot's technique must compensate for what the system no longer provides.

1. 1. Hydraulic System — complete guide
2. 2. Autoflight System — AP, FD, ATHR, FCU and FMA logic
3. 3. Flight Controls — Normal law, Alternate law and Direct law
4. 4. Electrical System
5. 5. Pneumatics — Air conditioning, Pressurisation and Ventilation
6. 6. Engines
7. 7. APU
8. 8. Fire Fighting
9. 9. Landing Gear and Brakes
10. 10. Ice and Rain Protection

What fly-by-wire actually means

In a conventional aircraft — a Boeing 737, an ATR, any aircraft with mechanical flight controls — moving the control column physically moves cables, pulleys, and push-pull rods that connect to the control surfaces. The pilot has a direct mechanical connection to the aircraft. Push forward on the column and the elevator physically moves down. The relationship is direct and immediate.

In the A320, there is no such connection except in a condition called Direct Law. The sidestick is a force transducer — it measures the force and deflection the pilot applies and converts it into an electrical signal. That signal is sent to the flight control computers. The computers decide what surface movement should result, taking into account the current flight envelope, aircraft configuration, and any active protections. They then send computed commands to the hydraulic actuators that move the surfaces.

The pilot does not directly move the surfaces. The pilot inputs a demand. The computers fulfil that demand — within the limits of the active control law.

This distinction is not merely technical. It has profound operational implications:

• The computers can prevent the pilot from exceeding structural limits, regardless of how hard the pilot pushes the stick
• The computers can fly a more aerodynamically efficient path than a human can maintain manually
• When the computers degrade, the nature of what the pilot is flying changes fundamentally
• The pilot receives no direct physical feedback from the surfaces — there is no aerodynamic feel. The sidestick resistance is artificial, generated by a spring mechanism, not by aerodynamic loads on the surfaces

Comparison with conventional controls

The sidestick — design, controls, and priority logic

The A320 sidestick replaces the conventional control column with a short, side-mounted joystick located on the outboard side of each pilot's seat — on the left for the Captain, on the right for the First Officer. It controls pitch and roll only. Yaw (rudder) is controlled via conventional rudder pedals.

Physical characteristics

The sidestick is spring-loaded and self-centring. When released, it always returns to the neutral (detent) position. There is no feedback from the aerodynamic loads on the control surfaces — the spring resistance is constant and does not vary with speed, configuration, or g-loading. This is a fundamental difference from conventional controls: on an A320, there is no aerodynamic "heaviness" that increases with speed, no buffet feedback through the stick, and no change in feel that warns the pilot of approaching limits. The envelope protections are provided computationally, not through stick force.

Maximum sidestick deflection is approximately ±16° in pitch and ±20° in roll. Small deflections at the centre of the range produce proportionally small demands; full deflection commands the maximum demand in that axis for the current control law.

Controls on the sidestick

Priority logic — the takeover system

The two sidesticks are electrically independent but their outputs are summed algebraically by the flight control computers. This means:

• If both pilots apply forward pressure simultaneously, the aircraft receives the sum of both demands
• If both pilots apply equal and opposite inputs, the demands cancel and the aircraft receives zero net command
• If one pilot is applying full back stick and the other applies full forward stick, neither demand reaches the aircraft — the aircraft holds its current attitude

This algebraic summing is different from conventional aircraft where additive inputs are physically combined at the control column. On the A320, the crew must be disciplined about who is flying and who is not.

Normal Law — complete description

Normal Law is the standard operating mode of the A320. In Normal law, the sidestick inputs are processed as flight path demands, not surface commands.

Pitch axis — how Normal law works

In pitch, the sidestick commands a load factor (g) when manoeuvring, and a pitch rate at low speeds and during takeoff and landing. Pulling back on the stick commands a positive g, pushing forward commands a negative g. The computers determine what elevator (and stabiliser) movement achieves the demanded g while remaining within the protected envelope.

At 1g level flight with the stick released, the computers maintain level flight automatically. The autotrim system continuously adjusts the Trimmable Horizontal Stabiliser (THS) to remove any residual pitch forces. The pilot does not need to maintain a pitch input to hold altitude — releasing the stick leaves the aircraft in level flight.

This is fundamentally different from Direct law or conventional controls, where releasing the stick at 1g requires that the aircraft is correctly trimmed. In Normal law, the trim is automatic.

Roll axis — how Normal law works

In roll, the sidestick commands a roll rate. Deflect the stick laterally and the aircraft rolls at the commanded rate. Return the stick to neutral and the roll stops — the aircraft holds its current bank angle without the pilot maintaining a lateral input. This is called bank angle retention.

At bank angles above 33°, a gentle automatic pitch-up is introduced to maintain altitude (the aircraft compensates for the reduced lift component in a banked turn). At bank angles above 67°, the bank angle protection becomes active — the aircraft will not roll beyond 67° regardless of sidestick input. If the stick is released above 33°, the aircraft rolls back toward 33° automatically.

Normal law protections — comprehensive list

Alpha floor — the most important speed protection

Alpha floor is activated when the aircraft reaches a high angle of attack threshold (approximately 9.5° in clean configuration, higher in landing configuration — values vary by aircraft standard ⚑). When activated, the Autothrust commands TOGA thrust regardless of the position of the thrust levers. The FMA annunciates A.FLOOR in column 1.

After alpha floor activation, even if the AoA reduces and alpha floor deactivates, the Autothrust locks in the TOGA condition — the FMA shows TOGA LK. To recover from TOGA LK, the crew must:

1. Manually move the thrust levers to the TOGA detent
2. Then move them to a lower setting (CLB or lower)
3. This resets the TOGA LK and returns the Autothrust to normal operation
4. Alternatively: bring the thrust levers back from the CLB detent, adjust using the instinctive pushbutton on the thrust levers to disconnect TOGA LK. A/THR can be re-engaged later when needed by pressing the A/THR pushbutton on the FCU.

Alternate Law — the two variants

Alternate Law occurs when the flight control computers cannot maintain Normal law due to sensor failures, computer failures, or combinations of hydraulic and electrical failures. There are two distinct variants with significantly different implications.

What triggers degradation to Alternate law

Degradation from Normal to Alternate law can be triggered by:

• Failure of one ELAC (the other ELAC takes over, but protections may reduce)
• Disagreement between air data references (ADR voting)
• Loss of multiple flight control computer channels
• Certain hydraulic system failures affecting computer power supply
• Loss of some inertial reference inputs

The ECAM message distinguishes between the two variants: F/CTL ALTN LAW (PROT AVAIL) indicates Alternate with protections; F/CTL ALTN LAW (PROT LOST) indicates Alternate without protections.

Alternate Law with protections

In Alternate law with protections, the flight control system is running on degraded computer resources but retains most of the significant envelope protections. The aircraft continues to behave similarly to Normal law for most purposes. Key differences from Normal law:

• Alpha floor is lost — if the aircraft decelerates to a high angle of attack, TOGA thrust is not automatically commanded. The autothrust continues to operate but will not override the current thrust lever position to prevent a stall.
• Some speed protections may be reduced — high speed protection may be reduced in authority.
• Reduced computer redundancy — the system is operating on backup channels. Further failures could produce additional degradation.
• Bank angle protection, load factor limiting, and pitch protections remain in most cases.

The handling of the aircraft in Alternate with protections is close to Normal law. The crew must be aware that alpha floor is unavailable and monitor speed more actively, particularly during approach.

Alternate Law without protections

This is the more significant degradation, and it demands a genuine change in pilot technique. In Alternate law without protections:

• All envelope protections are lost. The computers no longer prevent the aircraft from being flown outside its structural or aerodynamic limits.
• The sidestick commands surface deflection directly, not a g demand or pitch rate. The relationship between stick position and surface movement is approximately linear.
• The aircraft is neutrally stable in pitch. Unlike Normal law where releasing the stick returns the aircraft to trimmed 1g flight, in Alternate without protections the autotrim continues to operate — it trims out whatever pitch force the pilot applied before releasing the stick. If the pilot pulls back and then releases the stick, the aircraft has been trimmed to that pitch attitude and will hold it. It will not return to level flight on its own.
• Bank angle retention is lost. The aircraft does not hold bank angle when the stick is released — it begins to roll under the influence of any asymmetry.
• Yaw damping is reduced but some authority remains from the available FAC.

Approaching a stall in Alternate law without protections

In Normal law, alpha floor prevents a stall by commanding TOGA thrust automatically, and the AoA protection makes the aircraft resistant to stalling at all. In Alternate without protections, neither of these applies. The aircraft can stall. The crew must:

• Monitor airspeed actively — the stick does not provide aerodynamic feel warnings of approaching stall
• Monitor the PFD speed tape — the VLS (Lowest Selectable Speed) and stall warning speeds remain valid
• At the stall warning (stick shaker activates on some variants, audio "STALL STALL" on others), apply conventional stall recovery: push, thrust, roll level
• Stall protection in the conventional sense (stick shaker) may still be available even in Alternate law, but alpha floor TOGA thrust is not

Direct Law

Direct Law is the most degraded flight control mode and requires the most significant change in pilot technique. It is rarely reached in line operations — it typically requires multiple simultaneous failures. On the ground, Direct law is automatically engaged after touchdown to provide conventional ground handling characteristics (normal braking, steering).

What changes in Direct law

In Direct law, the sidestick commands a direct, proportional surface deflection. A specific stick displacement produces a specific surface movement, with no envelope computations and no autotrim. The aircraft now behaves like a basic, non-augmented conventional aircraft.

Specifically:

• No autotrim. Any change in speed, configuration, power setting, or CG will produce a pitch force that must be manually trimmed out using the pitch trim switches on the FCU (not the THS trim wheel — the trim wheel is for mechanical backup only). If the pilot does not trim manually, stick force increases progressively with speed change.
• No yaw damping. Dutch roll tendency becomes apparent, particularly at high altitude. The pilot must apply rudder to damp oscillations. This is unusual and requires active technique.
• No turn coordination. Coordinated turns require explicit rudder input. The aircraft will sideslip in banked turns without it, producing a slip ball deflection.
• No load factor limiting. The aircraft can be structurally overstressed by aggressive control inputs. The pilot must manually avoid exceeding g-limits.
• No speed protections of any kind.
• No bank angle retention.

Flying technique in Direct law

Direct law is manageable but demanding. The key technique changes:

• Trim continuously. Every power change, speed change, and configuration change requires manual trim input immediately.
• Fly coordinated. Check the slip indicator and apply rudder in turns.
• Be precise with control inputs. There is no load factor limiting — an aggressive pull in a turn can exceed structural limits.
• Monitor speed tape very carefully. No protective systems remain. VLS and Vmax are guidance only — the aircraft will comply with any input.

Complete law comparison — quick reference

ECAM messages and active law identification

Key numbers for ATPL oral preparation