Normal Law Protections in the A320

The A320 has various flight envelope protections, which protect the aircraft from entering certain critical situations while Normal Law is active.

The guides in this section shall cover some main envelope protections of the A320's Normal Law.

Control Laws

The fly-by-wire principle of the A320 uses several "Laws" on how to control the flight control surfaces in relation to the pilot's input to the side stick.

Normal Law:
- normal conditions even after single failure of sensors, electrical system, hydraulic system or flight control computer
Alternate Law:
- activated after certain double (or triple) failures
Direct Law:
- Mainly after double or triple IRS failure
Mechanical Backup (after loss of all electrical power):
- Trim wheel
- Rudder pedals

The different laws will support different types of protections and change the relationship between pilot's stick input and flight control interfaces.

High-Speed Protection

High-Speed Protection (HSP) aims to protect the aircraft from overspeed situations and activates latest when V_MO+6 kt or M_MO+0.015 speeds (maximum operating speeds in knots or mach) are reached.

Engagement Conditions

HSP is activated at or above V_MO or M_MO speeds (maximum operating speeds in knots or mach), depending on flight conditions.

Disengagement Conditions

HSP is deactivated when speed is reduced below V_MO/M_MO.

Indication and warnings

The overspeed limits are shown on the speed band on the PFD as a black and red strip and a pair of green lines.

1: V_MAX = Lowest of V_MO, M_MO, V_LE, V_FE
2: V_MO + 6 kt or M_MO + 0.01 Overspeed Protection becomes active
3: Actual Airspeed Reference Line
4: Current Mach speed

High-Speed Protection is usually accompanied by the Overspeed Warnings:

Continuous repetitive chime
Master warning light
Overspeed red message on ECAM
Red and black strip along the PFD scale

Protective Actions

Automatic AP disconnection (V_MO + 15 kt or M_MO + 0.04)
When V_MO + 6 kt or M_MO + 0.01 is reached, a positive load factor demand is automatically applied (pitch up action)
When full nose-down stick is maintained, speed is limited to around V_MO + 16 kt and M_MO + 0.04 (pilot nose-down authority is reduced)
When sidestick is released at HSP activation, the airplane will slightly overshoot V_MO/M_MO and fly back towards the envelope.
When sidestick is released, the aircraft's bank angle will return to 0° (instead of 33° outside HSP)
Bank angle limit is reduced from 67° to 40°
Pitch trim is frozen

Recommended Action to Recover

Extend Speed Brake
Increase pitch
Reduce thrust and/or activate A/THR

High Angle of Attack Protection

The Angle of Attack Protection protects against stalling the aircraft.

Angle of Attack

"The Angle of Attack is the angle at which relative wind meets an Aerofoil. It is the angle formed by the Chord of the aerofoil and the direction of the relative wind or the vector representing the relative motion between the aircraft and the atmosphere."

Based on the article Angle of Attack (AOA), Source: www.skybrary.aero.

The angle of attack is commonly called α (alpha) which we will use in the following sections.

Engagement Conditions

The High Angle of Attack Protection is engaged when:

Current angle of attack is greater than α_prot (in Normal Law) when above 100 ft RA
Below 100 ft RA during the landing, when α_max is reached

Indications and Warnings

1: Green Dot Speed is the best lift-to-drag ratio speed in the clean configuration.
2: V_LS is the lowest selectable speed and provides an appropriate margin to the stall speed. The autopilot will not go below this speed if autothrust is active.
3: Selected speed in the FCU
4: α_prot limit
- this speed is maintained when side stick is neutral
- if sidestick is deflected aft this will eventually activate α_floor A/THR protection - see Alpha Floor Protection
5: α_max is the speed with the maximum angle of attack (AoA) the aircraft will allow
- this speed is maintained when side stick is deflected fully aft
- it has a small margin before reaching the stall AoA

Protective Actions

Automatic AP disconnection
If α becomes greater than α_prot then the angle of attack will become proportional to stick deflection. Autotrim will stop, which results in a nose-down tendency.
If α reaches α_floor the autothrust system will apply go-around thrust. See Alpha Floor Protection.
α_max cannot be exceeded even with the pilot pulling the stick full backward. In other words, the aircraft cannot be stalled in Normal Law by the pilot's pitch up stick input.
Bank angle limit is reduced from 67° to 45°

Recommended Action to Recover

Push sidestick forward to reduce pitch and gain speed.
If Alpha Floor Protection (α_floor) is activated, see next chapter Alpha Floor Protection

Alpha Floor Protection

The Alpha Floor Protection automatically sets TOGA thrust when a very high angle of attack is reached.

Engagement Conditions

Alpha Floor Protection engages after lift-off until 100 ft RA before landing, when:

α is >α_floor
- 9.5° in CONFIG 0, 15° in CONFIG 1 and 2, 14° in CONFIG 3, 13° in CONFIG FULL
Sidestick deflection is > 14° when either
- High Angle of Attack Protection is active, or
- the Pitch Attitude Protection for pitch is active.

The above values for α_floor and sidestick deflection are not modelled realistically in Microsoft Flight Simulator.

Alpha Floor Protection is inhibited, when:

speed is above 0.6 Mach
TCAS mode is engaged

Indications and Warnings

If the α_floor (A.FLOOR) protection is triggered, the Autothrust FMA shows this symbol with a flashing amber border.

Protective Actions

Alpha Floor Protection signals the autothrust system to set TOGA thrust. Thrust lever positions are ignored.

As α_floor also implies α_prot see High Angle of Attack Protection for additional actions.

Recommended Action to Recover

See our specific guide on how to recover from A.FLOOR: A.FLOOR and TOGA LK

Load Factor Protection

Maneuvre Protection, also called Maneuvre Protection, enables immediate PF reaction, by pulling the sidestick to full aft without any risk of over stressing the aircraft.

The load factor limit is:

- 1.0 to + 2.5g load factor for clean configuration
0 to + 2.0g positive load factor for other configurations

Indication and warnings

The lower ECAM displays the load factor (G LOAD) in amber, when the value is above 1.4g or below 0.7g for more than 2 s.

Pitch Attitude Protection

To protect the aircraft from excessive pitch attitudes, this protection limits pitch angels in the following ways:

Nose up limited to 30° in CONF 0 to 3
- Progressively reduced to 25° at low speed
Nose up limited to 25° in CONF FULL
- Progressively reduced to 20° at low speed
Limited to 15° nose down

If these limits are approached, the aircraft's pitch rate decreases and will stop at the limit.

The flight director bars disappear when pitch exceeds 25° up or 13° down. They appear again when the pitch returns to 22° up or 10° down.

Pitch Attitude Protection supports high-speed protection, high load factor protection, and high AOA protection.

PFD Attitude Indicators

1: Bank limit indicator at 67°
2: Pitch down limit indicator at - 15°
3: Pitch up limit indicator at 30°

Bank Angle Protection

Bank angle during normal conditions is limited to 67° if the pilots hold the sidestick fully deflected laterally.

If the sidestick is neutral and the bank angle is no greater than 33°, the system will hold that bank angle.

If the bank angle was greater than 33° and the sidestick is released to neutral, the system reduces the bank angle automatically to 33° and holds it there.

If these limits are approached, the aircraft's roll rate decreases and will stop at the limit.

If High Angle of Attack Protection is active, the bank angle is limited to 45°.

If High-Speed Protection is active, the bank angle is limited to 40° and will roll back to 0° when the sidestick is released to neutral.

The autopilot disconnects and the flight director bars disappear when the bank angle exceeds 45°. They appear again when the bank angle is less than 40°.

See PFD Attitude Indicators.

Windshear Protection

Not yet available in the A32NX on Microsoft Flight Simulator

Windshear Protection is available during take off and approach phases when:

at takeoff 3 s after lift off up to 1300 ft RA
at landing from 1300 ft RA to 50 ft RA
as least CONF 1

Indication and warnings

Visual “WINDSHEAR” red message displayed on both PFDs for a minimum of 15s.
Aural synthetic voice announcing “WINDSHEAR” three times.

Recommended Action to Recover

Flight Director pitch order based on the speed reference system (SRS).
Pilot must set TOGA thrust immediately and follow the FD pitch order to execute the optimum escape maneuver.

Low-Energy Protection

Not yet available in the A32NX on Microsoft Flight Simulator

If the aircraft’s energy level is going below a threshold, an aural low-energy “SPEED SPEED SPEED” alert warns the pilot to increase thrust, to regain a positive flight path angle through pitch control.

It is available in Configuration 2, 3, and FULL. The FAC computes the energy level with the following inputs:

Aircraft configuration
Horizontal deceleration rate
Flight path angle.

The aural alert is inhibited when:

TOGA is selected, or
Below 100 ft RA, or
Above 2000 ft RA, or
Alpha-floor, or
the ground proximity warning system alert is triggered, or
in alternate or direct law, or
if both radio altimeters fail.

During deceleration, the low-energy aural alert is triggered before alpha floor (unless alpha floor is triggered by stick deflection). The amount of time between the two alerts depends on the deceleration rate.