A320 Autoflight System (DSC-22)

The A320 autoflight system consists of three main components: autopilot (AP), flight director (FD), and autothrust (A/THR). These systems work together with the Flight Management and Guidance System (FMGS) to automate aircraft flight path and speed control. Understanding managed vs selected modes, autopilot engagement rules, and autoland operations is critical for A320 type rating.

Autoflight Components

Flight Management and Guidance System (FMGS)

FMGS consists of Flight Management (FM) and Flight Guidance (FG), operating together as an integrated autoflight and navigation system.

FMGCs (Flight Management and Guidance Computers)

• Quantity: Two FMGCs, each capable of independently providing full Flight Management and full Flight Guidance
• Flight Management: Computes and updates lateral/vertical path with time and fuel predictions
• Flight Guidance: Provides autopilot and flight director commands

NAV ACCUR (Navigation Accuracy)

• NAV ACCUR HIGH: Navigation error sufficiently low to support managed NAV modes
• NAV ACCUR LOW: GPS PRIMARY lost and radio updating cannot maintain required accuracy

Takeoff and Initial Climb Modes

SRS (Speed Reference System)

• Purpose: Commands pitch ensuring safe speed margin above stall and adequate climb gradient
• Activation: Automatically when thrust levers in TOGA or FLX detent

RWY Mode (Runway Mode)

• Function: Provides localizer-based lateral guidance during takeoff to keep aircraft aligned with runway centreline
• Availability: When ILS localizer available for departure runway

RWY TRK Mode (Runway Track Mode)

• Function: Maintains ground track that existed at engagement, following initial flight path after liftoff

Flight Management and Guidance System (FMGS)

The FMGS consists of two major components: Flight Management (FM) and Flight Guidance (FG), operating together as an integrated autoflight and navigation system.

FMGCs (Flight Management and Guidance Computers)

• Quantity: Two FMGCs, each capable of independently providing full Flight Management and full Flight Guidance
• Flight Management: Computes and updates lateral/vertical path with time and fuel predictions for every segment
• Flight Guidance: Provides autopilot and flight director commands

NAV ACCUR (Navigation Accuracy)

• NAV ACCUR HIGH: Estimated navigation error is sufficiently low to support managed NAV modes throughout the phase
• NAV ACCUR LOW: GPS PRIMARY lost and radio updating cannot maintain required navigation accuracy

Takeoff and Initial Climb Modes

SRS (Speed Reference System)

SRS provides pitch guidance during takeoff ensuring safe speed margin above stall and adequate climb gradient.

• Purpose: Commands pitch for safe margin above stall with adequate climb gradient
• Activation: Automatically engaged at takeoff when thrust levers in TOGA or FLX detent
• Speed Target: V2 + appropriate margin based on configuration

RWY Mode (Runway Mode)

RWY mode provides localizer-based lateral guidance during takeoff to keep aircraft aligned with runway centreline.

• Availability: When ILS localizer available for departure runway
• Function: LOC-based lateral guidance on takeoff

RWY TRK Mode (Runway Track Mode)

RWY TRK maintains the ground track that existed at engagement, following the initial flight path after liftoff.

• Function: Holds ground track from liftoff
• Purpose: Maintains initial departure path in crosswind conditions

Managed vs Selected Modes

A320 autoflight philosophy centers on managed (FMGS-guided) vs selected (pilot-commanded) modes. This is fundamental to understanding A320 automation.

Mode Type	Color on PFD	Control	Usage
Managed	Magenta	FMGS flight plan	Normal operations, follows programmed route and profile
Selected	Blue/Cyan	Pilot FCU inputs	ATC vectors, tactical changes, manual speed/altitude control

FCU (Flight Control Unit) Operation

The FCU is the primary interface for autopilot, flight director, and autothrust:

• Pull Knob: Engage managed mode (magenta) - FMGS controls that parameter
• Push/Turn Knob: Engage selected mode (blue) - pilot controls that parameter
• SPD/MACH knob: Speed target (managed: FMGS, selected: pilot sets value)
• HDG/TRK knob: Heading/track target (managed: NAV follows route, selected: HDG/TRK to set value)
• ALT knob: Altitude target (always set by pilot, managed vs selected affects V/S or FPA)
• V/S or FPA selector: Vertical speed (ft/min) or flight path angle (degrees)

Autopilot Modes

Lateral Modes (What direction?)

Mode	Type	Function
NAV	Managed	Follows FMGS lateral flight plan (magenta line on ND)
HDG	Selected	Maintains heading selected on FCU
TRACK	Selected	Maintains ground track (heading corrected for wind)
LOC	Approach	Captures and tracks ILS localizer for precision approach
LAND	Autoland	Dual-channel LOC and G/S tracking down to FLARE and ROLL OUT engagement

Vertical Modes (What altitude/climb/descent?)

Mode	Type	Function
CLB	Managed	Climbs following FMGS vertical profile and speed schedule
DES	Managed	Descends following FMGS vertical profile and speed schedule
ALT	Both	Maintains selected altitude (ALT* during capture, ALT when captured)
V/S or FPA	Selected	Climbs/descends at selected vertical speed or flight path angle
G/S	Approach	Captures and tracks ILS glideslope for precision approach
FLARE	Autoland	Adjusts pitch smoothly near ground to achieve correct landing attitude and touchdown (~40ft RA)

Alpha Protections and A.FLOOR

Alpha Protection (Alpha Prot)

Limits pitch-up demand to prevent angle of attack from exceeding protected threshold.

Alpha Max

Maximum allowable AoA before stall. Aircraft cannot increase AoA further.

Alpha Floor (A.FLOOR)

• Trigger: Low-energy condition when AoA approaches protected threshold
• Action: Autothrust commands TOGA thrust automatically
• TOGA LK: Keeps thrust at maximum until pilot sets thrust levers to matching detent

Low Energy Warning

"SPEED SPEED SPEED": Energy state insufficient, requires immediate thrust application.

FAC Functions (Flight Augmentation Computer)

Two FACs provide critical yaw damping, rudder limiting, and speed indication functions.

FAC Primary Functions

• Yaw Damping: Automatic yaw-damping to stabilize aircraft and improve Dutch-roll characteristics
• Rudder Travel Limiter (RTL): Reduces maximum rudder deflection at higher speeds to prevent structural loads
• PFD Speed Indications: Provides VLS, VMAX, and F-speed markings for managed-speed operation
• Turn Coordination: Automatic during roll inputs

ROLL OUT Mode

ROLL OUT provides automatic lateral guidance along runway centreline during initial ground roll after touchdown.

• Engagement: Automatically after touchdown in LAND mode
• Function: Keeps aircraft on runway centreline using LOC signal

Autopilot Engagement and Disconnect

Autopilot Engagement

To engage autopilot:

1. Flight Director must be ON (FD button on FCU)
2. Aircraft in stable flight (not excessive pitch/bank/speed deviation)
3. Press AP1 or AP2 button on FCU
4. AP engages, controlling pitch and roll per armed/active modes

Minimum engagement altitude: Typically 100ft AGL after takeoff (company policy may vary).

Autopilot Disconnect

Autopilot disconnects by:

• Press AP button on FCU: Normal manual disconnect
• Press instinctive disconnect button on sidestick: Quick disconnect (red button on sidestick)
• Automatic disconnect: Excessive deviation, dual input, system failure

Autothrust System

Autothrust automatically manages engine thrust to maintain speed or climb/descent profile.

Autothrust Modes

Mode	Function	When Used
SPEED	Maintains selected or managed speed	Climb, cruise, descent (level flight)
MACH	Maintains selected or managed Mach number	High altitude cruise
THR CLB	Applies climb thrust (not maintaining speed)	Climb phase, AP in CLB mode
THR IDLE	Applies idle thrust (not maintaining speed)	Descent phase, AP in DES mode
THR LVR	Thrust levers manually positioned, A/THR armed but not active	Takeoff, go-around
A.FLOOR	Alpha floor protection - TOGA thrust automatically applied	Approaching low speed protection (alpha floor)

Thrust Lever Position

With autothrust active, thrust levers typically remain in CL (climb) detent:

• Takeoff: Levers in FLX or TOGA detent, A/THR armed (THR LVR mode)
• After takeoff acceleration: Reduce levers to CL detent, A/THR becomes active (SPEED or THR CLB mode)
• Cruise/Descent: Levers remain in CL detent, A/THR manages thrust automatically
• Landing: Reduce levers to IDLE at 30ft (automatic call "RETARD"), A/THR disconnects at touchdown

Autoland (CAT II/III Approaches)

A320 can perform fully automatic landings in low visibility using autoland system.

Autoland Requirements

• Both autopilots serviceable
• Autoland capability on aircraft (certified for CAT II or CAT III)
• ILS approach to suitable runway (CAT II/III certified)
• Both flight directors ON
• Approach mode armed (LOC and G/S)
• LAND mode armed (shows in white on FMA)

Autoland Sequence

1. Approach Phase: Single AP active (AP1 or AP2), LOC and G/S captured
2. Below 400ft: LAND mode arms (white on FMA)
3. Below 400ft: Second AP automatically engages (both AP active)
4. LAND mode active: Green on FMA, both APs guiding aircraft to runway
5. 40ft RA: FLARE mode activates, automatic pitch up for flare
6. 30ft RA: "RETARD" aural call, reduce thrust levers to IDLE
7. Touchdown: Main gear touchdown, autothrust disconnects
8. After touchdown: Automatic rollout guidance (nose gear steering), pilot takes over at low speed

CAT II vs CAT III

Category	Decision Height	RVR Minimum	Notes
CAT II	100ft	300m	Requires autoland, pilot must see runway at DH to continue
CAT IIIA	50ft or none	200m	Autoland required, lower visibility than CAT II
CAT IIIB	None	75m-200m	Autoland, very low visibility operations
CAT IIIC	None	Zero	True zero visibility landing (rare certification)

Related A320 Systems

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