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A320 Auto Flight System Notes

ATA 22 Auto Flight

  • Auto flight systems (AFS) calculates orders to automatically control flight controls (EFCS) and engines(FADEC)
  • AFS is built around 4 computers,  two interchangeable Flight Management and Guidance Computers (FMGCs) and two interchangeable Flight Augmentation Computers (FACs). Each  consist of command and monitor channel.
  • It is a FAIL OPERATIVE system.
Basic operational principles
 In normal operations
               • Automatic
                        - A/P or A/THR
    • Manual
            - Pilot activates as side sticks or thrust levers

Navigation principles:
  • Calculated position of the A/C using several sensors
    • Inertial system
    • Radio navigation system
  • Flight plan
    • Stored in the memory predetermined by the operator
    • Describe complete flight from T/O to landing
    • Vertical information
    • Intermediate way points

Operation:
Normal way
            • AFS uses flight plan automatically
                - Position and desired flight plan (chosen by crew), system will computed     orders then send to the surfaces and engines to follow the flight plan
                - During AFS side stick and throttle levers will not move
AFS fly by wire
            • When A/P system engaged, apply force on the side stick causes A/P disengage, after normalize the force on the side stick A/C will maintain actual attitude with manual flight

The Auto Flight System (AFS) is divided into four main parts:
  1. Flight Management (FM),
  2. Flight Guidance (FG),
  3.  Flight Augmentation,
  4.  Fault Isolation and Detection System (FIDS).
1. FM (Flight Management )
- It can be made by different manufacturer than the FG part, and can be replaced separately
- Controlled/interfaced from the MCDU
Its responsible for:
  • Flight planning
  • Lateral Navigation
  • Vertical Navigation
2. FG (Flight Guidance)
- Consist of 2 parts Command and Monitoring
           - Controlled/interfaced from FCU
    Its responsible for:
  •  AP
  • FD
  • A/THR

3. Flight Augmentation
The basic functions of the Flight Augmentation Computer (FAC) are:
    a) yaw damper,
    b) rudder trim,
    c) rudder travel limitation,
    d)flight envelope protection,
- Fault Isolation and Detection System (FIDS).
a) The yaw damper provides:
        - dutch roll damping,
        - turn coordination in cruise,
        - engine failure compensation in auto flight,
        - yaw guidance order execution.
b) Rudder Trim
  • Manual trim from the Rudder trim control panel
  • Auto trim
  •  Through FMGC via FAC to rudder trim actuator with A/P engaged
c)  Rudder Travel Limitation
  • Limits rudder travel according to the A/C speed for structure Integrity (Speed information comes from ADIRUs)
  • Prevents excessive deflection which penalize the A/C performance
d) Flight Envelope Protection:
  • Speed
  • AOA
  • Wind Shear
4. FIDS (Fault Isolation and Detection System)
  • Used as a system Bite to concentrate maintenance information and compare with all information from
    other AFS computers to filter unwanted gives most accurate faults to CFDS
  • Receives commands from CFDS and sent to other AFS system Bites and receives malfunction report from
    Bites if applicable reports to CFDS
  • If FIDS fails Bites continues to work and the result can be read in the shop or after FAC 1 change 

Navigation database

  • Provides necessary information for flight plan construction and follow up
  • Database updated every 28 days

AP and FD modes
Guidance
Managed modes
Selected modes
LATERAL
NAV, APP NAV LOC *, LOC RWY, RWY TRK GA TRK
ROLL OUT
HDG - TRK
VERTICAL
SRS (T.O and G.A) CLB, DES
ALT CST, ALT CST * ALT CRZ * , ALT CRZ G/S *, G/S
FINAL, FINAL APP FLARE
OPCLB, OPDEC V/S, FPA
ALT*, ALT EXPEDITE
SPEED
FMGC REFERENCE (ECON auto SPD LIM) Expedite
FCU REFERANCE

A/P engagement

  • Possible only 5 sec after lift off and only one at a time
  • To select land mode allows both A/P to engage
  • After T/O to rollout A/P remain engaged to control A/C on the runway center line
  • A/P disengagement by pilot at low speed taxing
  • On Gnd. can be engaged if engines are not running

Flight Director

  • Displays Guidance commands on both PFD to fly manually according to FMGC demand
  • Two cases to be considered
• A/P not engaged
  • F/D symbols display on PFD, gives orders to pilot to maintain the desired parameters
    (crew fly A/C as per the orders acting on flight controls)
• A/P Engaged
• FD function displayed symbol on PFD represent A/P orders to be monitored by pilots 

FD(Flight Directors)
Engaged:     
  • with the Pushbutton at FCU
  • During power Up(Automatic Engagement)
  • During TCAS RA situation operation
Dis-engaged:
  • with the Pushbutton at FCU
  • When there is Failure
AP(Auto-Pilot)
Engaged:
  • with the Pushbutton at FCU
  • No Automatic Engagement
  Dis-engaged:
  • with the Pushbutton at FCU
  • Side Stick Disconnect switch
  • Side stick Movement
  • Not all conditions are met
  • When there is Failure
A/THR(Auto-Thrust)
Engaged:


  • with the Pushbutton at FCU
  • When A/THR is moved to TO/GA
  • During Alpha floor protection(Automatic Engagement)
  • During TCAS RA situation operation
Disconnected:
  • with the Pushbutton at FCU
  • With the Pushbutton at the Thrust lever
  • At IDLE power
  •  When there is Failure

Some FAC functions:
  • W/S Windshear
  • Low Energy Warning (SPEED, SPEED, SPEED)
  • ROPS(Runway Overrun prevention System)
  • SRIW(Stop Radar Input)

  • PWS(Predictive Windshear)
- Becomes active below 2300ft
- Visual warning becomes active below 1500ft: W/S AHEAD
- Aural warning becomes active below 1200ft: MONITOR RAD DISPLAY, WINDSHEAR       
  AHEAD  GO AROUND




Credit :    Rakesh Singh

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