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Flight Management System (FMS) & Flight Director System (FDS)

Flight management system

Almost all modern passenger transport aircraft employ a computerised flight management system, the purpose of which is to reduce crew work- load, thereby enabling a reduction in crew numbers, and to achieve the best possible fuel economy with the overall result that operating costs are minimised. 


The system may function purely as an advisory unit, providing the flight crew with advice on the control settings required to achieve optimum fuel economy in each of the various flight conditions such as take-off, climb, cruise, descent and approach. In most cases this is done by displaying the necessary Engine Pressure Ratio (EPR) or torque settings and the recommended climb/descent rates. In this type of flight management system all the control adjustments are made manually by the crew in response to the FMS advisory messages. 


Associated with this type of FMS will be a discrete Flight Director System (FDS), providing the flight crew with command and advisory information regarding the vertical and lateral flight path of the aircraft through an Attitude Director Indicator (ADI) and Horizontal Situation Indicator (HSI).

More sophisticated flight management systems interface with the aircraft automatic flight control and automatic thrust control systems to achieve fully automatic control of all flight phases with the exception of take-off and initial climb-out.


In these systems the management of the aircraft planned flight path is divided into lateral and vertical profiles. The FMS guides the navigation of the aircraft vertically (VNAV) to achieve the planned altitude at each stage (waypoint) of the planned flight and laterally (LNAV) to arrive overhead each geographical turning point (waypoint) of the planned flight. At each stage of the flight the FMS instructs the automatic thrust control system as to the power setting necessary to achieve maximum fuel economy. 

In addition to the control functions performed by the FMS, it continuously provides information to the flight deck displays such as EFIS, EICAS or ECAM. Flight director commands to the flight crew, particularly necessary during take-off and initial climb-out, are transmitted through the EADI and EHSI functions of the EFIS.

In order to perform its multitudinous functions, the FMS must be provided with navigational data from all the navigation systems such as INS/ IRS, DME, VOR and Doppler and from all the engine and associated systems monitoring equipment.

Flight management system
FMC Interface.jpg


Multi-purpose Control and Display Unit (MCDU)

The link between the flight crew and the FMS is the multi-purpose control and display unit. It provides the crew with the means to make inputs to the system in order to obtain required displays, or information to assist with decision-making in respect of the aircraft's flight progress. The CDU display is in paged, digital format on a small, typically 2 6 3 inch, CRT screen.

The MCDU is primarily used for long-term actions, such as monitoring and revising the flight plan, selection of operating mode and insertion of data such as aircraft weight, wind speed and direction, various temperatures and performance data. It provides the Flight Management Computer (FMC) with readout capability, together with verification of the data entered into the computer memory. Flight plan and advisory data are continuously available for display on the MCDU.

The MCDU panel has a full alpha-numeric keyboard, along with mode, function and data entry keys. The keyboard includes advisory annunciators, display light sensors and a manual brightness control.



Display screen

The display screen of the MCDU has 14 lines with a total of 24 characters per line. The page format of the screen is divided into four areas, these are:

  • Title field : Which contains the title of the page of subject data displayed and the page number (e.g.1/2, meaning page 1 of 2).

  • Left and right fields : Each containing six pairs of lines of 11 characters per line. The pilot has access to one line of each pair through the line select keys on either side of the unit.

  • Scratchpad : Which forms the bottom line of the display screen. Scratchpad entries may be pilot-inserted, unless an FMC originated message is displayed in this field, and they are independent of the page displayed.

Line select keys

Momentarily depressing a line select key affects the line adjacent to the key on the respective side of the MCDU for entry, selection or deletion of data.

Brightness control

The light intensity of the MCDU display may be adjusted by rotating the BRT knob. Brightness of the illuminated keys is automatically adjusted by a remote flight deck control.


There are two annunciators on each side of the keyboard. These display the following messages when appropriate:

  • DSPY (Display (upper left)) : The white display light illuminates when the active lateral or vertical leg performance mode is not displayed on the current MCDU page.

  • FAIL (Fail (lower left)) : The amber light illuminates when there is a fault in the FMC.

  • MSG (Message (upper right)) : The white light illuminates to indicate to the pilot that an FMC-generated message is displayed on the scratchpad, or is waiting to be displayed when the scratchpad is cleared.

  • OFST (Offset (lower right)) : The white light is illuminated when lateral navigation (LNAV) is based on a route parallel to, but offset from, the active route.


Alpha-numeric keys

These keys allow the pilots to enter letters and numbers onto the scratchpad successively from left to right. They include space (SP), delete (DEL), slash (/), and plus/minus (+/7 ) keys.


Function keys

The function keys control the MCDU field displays, accomplished by the execution of pilot inputs and requests. The purpose of each of the function keys is briefly described below:

  • EXEC : The execute key is the command key for the FMCS. Whenever a modification or activation is pending, a white light bar illuminates. Depressing the key will activate the flight plan, change the active flight plan or change the vertical profile, as appropriate.

  • NEXT PAGE : Depressing this key causes the display to page on to the next higher page number in multi-page displays.

  • PREV PAGE : Depressing this key causes the display to page back to the next lower page number in multi-page displays.

  • CLR : The clear key extinguishes the MSG annunciator light and clears any message from the scratchpad. Where more than one message is displayed, each momentary push clears a single message; multiple messages are cleared by repetitive momentary pushes or a single long push.

  • DEL : Pressing this key inserts the word DELETE onto the scratchpad, provided that the pad is clear. Line selection by means of a line selection key deletes the entered data on that line, but is only available for specific pages.

Mode keys

The mode keys control the type of page displayed on the MCDU and are therefore the means by which the pilots gain operational access to the flight management system. There are twelve keys, as described below:

  • NIT REF (Initialisation/reference) : The initialisation/reference key selects the first of a series of pages used to initialise the position of the FMCS and the inertial reference system (IRS).

  • RTE (Route) : The route key provides access to planned routes and selects the page for entering or changing the point of departure, destination or route.

  • DEP ARR (Departure/arrival) : Depressing this key calls up an index listing all terminal area procedures.

  • ATC (Air traffic control) : This key selects the ATC automatic dependent surveillance status page.

  • VNAV (Vertical navigation) : Depressing this key provides access to the climb (CLB), cruise (CRZ) and descent (DES) pages for evaluation and modification.

  • FIX : The fix key provides access to the fix information pages, which are used for creation of waypoint fixes.

  • LEGS : The legs key provides a page for evaluating or modifying lateral or vertical details of each route leg.

  • HOLD : The hold key calls up the page for entering, exiting or amending a holding pattern.

  • FMC COMM (Flight management computer communications) : In most current systems this key is non-operational.

  • PROG (Progress) : This key is used to select current dynamic flight and navigation data, such as ETAs and fuel remaining at a given point (e.g. next two waypoints, destination or alternate).

  • MENU : The menu key provides access to other aircraft subsystems and to the alternate control for the EFIS and EICAS control panels in the event of failure.

  • NAV RAD. (Navigational radio) : Depressing this key selects the page for monitoring or modifying navigational radio tuning.

The MCDU is duplicated so that each pilot has access to the system and the two units are usually located on either side of the central console. Operation of the FMS is fully described in the Aircraft Operating Manual for each aircraft type.

¥light direztor systems

Flight director systems

A flight director system (FDS) integrates the display of aircraft attitude, in terms of roll and pitch, with radio navigational data to provide a complete directional command function for both vertical and lateral navigation.


The vertical gyro unit instead of providing attitude information directly to a display it transmits attitude-related electronic signals to the flight director computer. In aircraft not equipped with a flight management system and EFIS the outputs from the computer are fed to an attitude director indicator (ADI) and a horizontal situation indicator (HSI).



Attitude director indicator (ADI)

The attitude director indicator display closely resembles that of the gyro attitude indicator, but with vertical and lateral deviation indicators and a radio altitude readout added. A fixed aircraft symbol, typically in the form of a flattened triangle, is positioned centrally against a background tape that is able to scroll up or down to indicate aircraft pitch attitude. The tape is coloured to represent sky and ground, with a horizontal dividing line representing the horizon. The tape is driven by a servomotor, which receives signals from the pitch channel of the vertical gyro unit.

The background tape and its roller drive are mounted upon a ring gear and this is rotated by a second servomotor that receives signals from the roll channel of the vertical gyro unit, to indicate roll attitude. The tape has +908 freedom of movement in pitch and 3608 in roll. Bank angle is indicated by a pointer attached to the ring gear, which moves against a fixed scale on the instrument casing.

Deviation from the ILS localiser beam is indicated by a pointer and lateral deviation scale at the bottom of the display. Below this is a conventional `ball and tube' slip and skid indicator. Deviation from the glideslope is indicated at the left-hand side of the display by a pointer and vertical deviation scale. A radio altitude readout of height agl is displayed, typically during the last 200 ft of descent. This may take the form of a scrolling digital readout, or a `rising runway' directly beneath the fixed aircraft symbol.



It will be noted that the ADI includes command bars. These are driven in response to signals from the flight director computer and they indicate pitch and roll commands to the pilot. The two bars are not physically connected to each other, but they normally move as a pair and the aircraft must then be flown to position the triangular aircraft symbol in the `vee' formed by the bars, in order to satisfy the command. 

When the command bars have moved to demand `fly up' and `fly left'. In order to satisfy the flight director command the pilot must therefore pitch the aircraft up into a climb and bank it to the left.

The ADI display when these manoeuvres have been carried out and the aircraft is satisfying the climbing left turn command.

Some ADIs are equipped with a pitch command facility with which the pilot can select a given climb or descent in certain modes of operation of the flight director system. This will position the command bars to indicate the required pitch attitude.

ADI Indications.jpg


Horizontal Situation Indicator (HSI)

The HSI presents a display of the lateral aircraft situation against a compass rose. The compass rose is driven by a servomotor receiving signals from the aircraft magnetic heading reference system. In aircraft fitted with an inertial navigation system the servomotor may also receive signals from this, enabling either magnetic or true heading to be selected. Aircraft heading is indicated by a fixed lubber line on the instrument casing.

The aircraft symbol, in the form of a miniature aircraft, is fixed at the centre of the display and points toward the aircraft heading lubber line. Control knobs at the bottom of the display allow a VOR/localiser course and a desired heading to be selected. Rotation of the course selection knob rotates the course arrow to indicate the selected course on the compass rose. At the same time, a digital course counter at the upper right of the display provides a readout of the selected course.


The centre section of the course arrow is a movable lateral deviation bar, which is deflected left or right of the course arrow to indicate deviation from the selected VOR radial or localiser centre line. It thus indicates fly left or right to intercept the localiser beam or VOR radial and the command bars of the ADI will move to direct roll in the appropriate direction. A to/from arrow indicates whether the selected course is to or from the VOR.


The dots of the deviation scale represent displacement of approximately 1.25 deg and 2.5 deg from the localiser centre line or approximately 5 deg and 10 deg from the VOR radial. Once a course is set, the course arrow rotates with the compass rose as aircraft heading changes. In VOR mode of operation the deviation bar begins to indicate when the aircraft reaches approximately 16 deg from the radial; in ILS mode, movement of the bar begins at approximately 4 deg from the localiser beam centre.

The heading select knob, at the lower left side of the display, is used to set a desired heading. When it is rotated, a triangular heading bug moves against the compass scale to indicate the heading selected. In the heading mode of operation the ADI command bars will move to direct roll in the appropriate direction until the desired heading is attained.

Vertical deviation from the selected ILS glideslope is indicated by a pointer moving against a deviation scale. Range to a selected DME is displayed digitally at the upper left side of the HSI.



Warning indications

In the event of a fault developing it is vital that the pilot should be aware that the display may be in error. The command signals generated by the flight director system are continuously monitored and, should they weaken to the point that the information provided is unreliable, small red warning flags appear at the relevant part of the ADI or HSI display.

The ADI has three warning flags to indicate failure of the ILS glideslope receiver, the vertical gyro and the computer. These are labelled

  • GS 

  • GYRO 



When the GS flag is activated it obscures the glideslope pointer and deviation scale; the GYRO and COMPUTER flags appear at the bottom of the tape display.

The HSI also has three warning flags, labelled

  • GS


  • VOR/ LOC.

The GS flag operates in the same manner as that on the ADI, whilst the COMPASS flag is activated in the event of failure of signal from the magnetic heading reference system (MHRS) and the VOR/LOC flag indicates failure of the VOR or localiser signal.

Modes of operation

The flight director system may be operated in a number of different modes and these vary with the various system manufacturers and aircraft types. Similarly, the method of mode selection by the pilots varies between systems, but is usually achieved through push button selector switches.

The basic operating modes common to most systems are 



With the system switched off the command bars of the ADI are retracted from view and the indicator is used purely for attitude reference.


With heading selected, the ADI command bars provide guidance in roll to reach and maintain the compass heading selected with the heading select control knob and indicated by the heading bug. Where a pitch command facility is fitted this will be enabled in this mode.


The ADI command bars provide guidance in roll in order to capture and maintain a selected VOR radial or ILS localiser beam and lateral deviation is indicated on both ADI and HSI displays. Pitch com-

mand facility is also enabled in this mode.


With glideslope selected the ADI command bars provide vertical and lateral guidance to capture and maintain the ILS glideslope and localiser beams. Lateral and vertical deviation indicators are activated on both displays.


This mode is basically the same as GS mode, except that interception and capture of the glideslope is automatic once the localiser beam has been captured.


In this mode the ADI command bars provide guidance in pitch to maintain a preselected altitude.


This mode is used when making an approach to a Category I ILS. Lateral and vertical guidance for the capture and tracking of glideslope and localiser beams is provided by the ADI command bars.


Guidance is the same as for APPR I, but beam tracking is of a higher standard to meet the requirements of Category II ILS.

GA (Go-around)

After a missed approach using one of the approach modes, this mode may be selected for the execution of a go-around pro- cedure. The ADI command bars will order a wings level, pitch-up atti- tude. Once the required power and speed settings have been achieved, HDG and IAS modes may be selected.


This mode is used when it is necessary to maintain a given airspeed during climb-out or descent. The ADI command bars provide guidance in pitch.


This mode is selected when guidance is required for a specific rate of climb or descent (vertical speed). The ADI command bars provide guidance in pitch.


This mode is for use at higher altitudes. Guidance is the same as for the IAS mode.

Gain programme in approach mode

The system response is optimised during an ILS approach by means of a programmer in the gain control section of the FD computer. Following capture of the localiser and glideslope beams the pitch and roll signals and the deviation signals are reduced to allow for the convergence of the beams during descent. The programmer is selected automatically at predetermined positions on the localiser and glideslope.

Lateral and vertical beam sensors

The task of these sensors is to supply input data to the FD computer to assist with the task of stabilising and adjusting the aircraft attitude as necessary. Disturbances about the aircraft's lateral and longitudinal axes are sensed and signalled to the FD computer, which generates pitch and roll command bar movements in response. The commands are computed to reflect the rate of change of deviation due to disturbance.

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