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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 of the 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 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.

Envelope Protections Overview:

Overspeed Protection

This protections aims to protect the aircraft from overspeed situation exceeding the Vmo or Mmo speeds (maximum operating speeds in knots or mach).

See also V-Speeds

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.

Speedband Overspeed Limits

  • 1: Vmax = Lowest of Vmo, Mmo, VLE, VFE
  • 2: Vmo + 6kt or Mmo + 0.01 Overspeed Protection becomes active
  • 3: Current IAS (In Air Speed)
  • 4: Current Mach speed

Overspeed Protection triggers the following warnings

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

Protectiive Actions

  • Automatic AP disconnection
  • When Vmo + 6kt or Mmo + 0.01 is reached a positive load factor demand automatically applied (pitch up action)
  • When full nose-down stick is maintained speed is limited to around Vmo + 16kt and Mmo + 0.04 (pilot nose-down authority is reduced)

TODO: PILOT INPUT

  • Increase pitch
  • Reduce thrust and/or activate A/THR

Angle of Attack Protection

The Angle of Attack Protection in the A320 is in simple terms a protection against a too high angle of attack and in consequence 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 also shall use in the following.

Indication and warnings

Angle of Attack Protection Speeds

  • 1: Green Dot Speed is the best lift-to-drag ratio speed
  • 2: VLS is minimal selectable speed providing 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: αprot limit
    • this speed is maintained when side stick is neutral
    • if sidestick if deflected aft this will eventually activate αfloor protection. See below.
  • 5: αmax is the speed with the maximum angle of attack (AoA) without the aircraft stalling
    • this speed is maintained when side stick is deflected fully aft

A.FLOOR FMA

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

Protective Actions

  • Automatic AP disconnection / AP cannot be activated
  • If α becomes greater than αprot then angel of attack wiull become will become proportional to stick deflection. Autotrim will stop which results in a nose-down tendency.
  • If α reaches αfloor the autothrust system will apply go-around thrust.
  • αmax cannot be exceeded even with the pilot pulling the stick full backward. In other words the cannot be stalled in Normal Law by the pilot's pitch up stick input.

  • Push sidestick forward to reduce pitch and gain speed.

  • If in A.FLOOR (αfloor) protection see this guide: A.FLOOR and TOGA LK

Last update: November 26, 2021
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