Flight Management System
Introduction to the Flight Management System (FMS)
There is considerable pressure to make more effective use of the airspace in which we fly. We are, therefore, increasingly adopting the Area Navigation philosophy and the modern transport aeroplane is extremely well equipped to comply with the needs of an Area Navigation System. In simple terms, the fundamental needs of air navigation have not changed since ancient times.
We must know where we are – current position
We must know where we are going to – destination.
If we cannot see our destination, we must be able to compute the direction and distance to that point and we must continuously monitor our progress to ensure that we are following the correct path. The problem is the same but the tools that can be applied to create a solution are powerful beyond the dreams of navigators, even of 50 years ago. The heart of this power is the modern Flight Management System (FMS).
On modern aircraft, the FMS is an integrated automatic flight management system that provides, through precision control of engine power and flight path, optimum economy of flight. At the same time, flight deck workload is reduced and this can considerably enhance safety. FMS is interfaced with the Power Management Control System (PMCS) and the Automatic Flight Control System (AFCS) so that it manages both control of power and flight path, both vertically and horizontally, against a pre-planned flight path.
A flight management system (FMS) is a fundamental part of a modern aircraft’s avionics. A FMS is a specialized computer system that automates a wide variety of in-flight tasks, reducing the workload on the flight crew to the point that modern aircraft no longer carry flight engineers or navigators. A primary function is in-flight management of the flight plan. Using various sensors (such as GPS and INS) to determine the aircraft’s position, the FMS can guide the aircraft’s autopilot along the flight plan. From the cockpit, the FMS is normally controlled through a Control Display Unit (CDU) which incorporates a small screen and keyboard. The FMS sends the flight plan for display on the EFIS Navigation Display (ND) or MultiFunction Display (MFD).
The FMS consists of two units namely:
Command Display Unit (CDU) – The crew’s interface with the system
Flight Management Computer (FMC) Handles all the complex calculations and memory items required.
The FMC has a data storage capacity, rather like the hard disk on your PC, which is in two broad sections. One section is dedicated to aircraft performance data for take-off, climb, cruise, descent, holding, go-around and abnormal flight (e.g. engine-out) situations. This data has a comparatively ‘long life’. The Navigation database, as the second section is known, stores all the data relevant to the airline’s route structure. All FMS contain a navigation database. The navigation database contains the elements from which the flight plan is constructed. The navigation database (NDB) is normally updated every 28 days, in order to ensure that its contents are current. Each FMS contains only a subset of the ARINC data, relevant to the capabilities of the FMS. The NDB contains all of the information required for building a flight plan and information relevant to it. This will include:
Navigation Facilities Position, frequency, identification, type
Waypoints Latitude, longitude, type (en-route etc.)
Airports & Runways Designations, elevations, locations, etc.
Terminal Procedures SIDs, STAR, holds, etc.
Approach & Go-around Procedures
Routes Airway identifier, magnetic course
Radio navigation aids DME, VOR, and (NDBs)
And a variety of related and often installation-specific information.
Requirements For FMS
- Alphanumeric and symbology
- Selection of Waypoint and Profile
- System Database
Components of FMS
- Computer Sub Systems
- Caution and Warning Systems
- Autopilot and FDS
- Cockpit Control and Display
Flight Management System
- Flight Management Computer System
- Thrust Management System and Throttle System
- Electronic Instrument Display System
- – Electronic Flight Instrument System (EFIS)
- – Engine Indicating and Crew Alerting System in Boeing (EICAS)
- – Electronic Centralised Aircraft Monitoring System in Airbus (ECAM)
- Navigation and Guidance
- Performance Management
- Data Display
This data does need frequent up dating and must be renewed every 28 days. Using these two packages along with the variable inputs (such as current position, wind velocity from aeroplane’s navigation computer and Air traffic clearance) it is possible to generate, or modify, a flight plan to meet the current needs. Since the FMC has all the data required, the activities associated with following a precision RNAV (PRNAV) route in three dimensions can be easily accommodated. All it requires is that the system is told the route to follow, the preferred flight profile, and the ATC clearance. This is done through the CDU. The aeroplane’s navigation sensors feed information to the FMC and, from those sensors, the best position information can be derived.
The navigation sensors used will normally consist of a hybrid of inputs from facilities selected by the FMC. A hybrid combination provides the necessary Required Navigation Performance to comply with the needs of a precision RNAV (PRNAV). These sensors may include some or all of the following:
At this time, none of these sensors alone can be depended upon to provide the reliability, integrity and accuracy necessary.
- VOR/DME units are limited in range ability and the position accuracy deteriorates with range from the facility.
- IRS suffers from accumulative position errors.
- LORAN does not provide reliable coverage nor is it worldwide.
- GNSS, as we have seen, still requires augmentation.
The FMC will evaluate the data from the available sources to derive the “best position”. In addition, using inputs of altitude, airspeed, temperature and Mach number from the air data computer along with engine parameters and fuel data, a complete control of the flight profile can be exercised. This will ensure that the flight will be optimised in terms of fuel efficiency.
- Preparation Of Flight Plan
- Synthesis Of Nav Information
- Automatic Frequency Selection and Tuning
- Guidance by Coupling the Flight Management Computer System (FMCS) to the Automatic Flight Control System (AFCS)
- Optimisation of the Flight Path
- Prediction Of Flight Parameters
- Display Management
Flight Management Guidance System
- Predicts Flight Time, Mileage Speed, Economy Profiles and Altitude
- Reduces Cockpit Workload and Improves Efficiency
- Generates Optimum Vertical and Lateral Flight profiles
- One FMGS performs all Operations if Other Fails
- Modification by Pilot is Possible on Short Term Basis (Speed, Hdg etc)
- FMGS Guides the aircraft to manually select target
- A preplanned route can be fed using Multifunction Control and Display Unit (MCDU)
- Flight Management Guidance Computer
- Multipurpose Control and Display Unit
- Flight Control Units
- Flight Augmentation Computers
|The aircraft is guided along pre planned route||The aircraft is guided to selected target, modified by the pilot|
|Predicted targets are computed by FMGS||Guidance is always under the control of pilot|
- Selected Guidance has priority over Managed Guidance
Flight Management Guidance Computer
Navigation and Management (Flight Navigation Radio Aids)
Management and Flight Planning
|Flight Director Command|
|Management of Displays||Auto Threat Command|
Multipurpose Control and Display Unit (MCDU)
- Two MCDU’s for flight crew for loading and display of data
- Allows selection of a flight plan for a lateral and vertical trajectories and speed profiles
- Allows modification of selected navigation or performance data and specific functions of flight management
- Additional data from peripherals can also be displayed
Flight Control Unit (FCU)
- It is interface between the crew and FMGC
- It is used to select flight parameters or modify those selected in the MCDU’s
- Autopilot or Auto thrust maybe engaged or disengaged
- Different guidance modes can be selected to change speed, hdg, track, altitude, flight angle, vertical speed
Flight Augmentation Computer
- Controls rudder, rudder trimand yaw damper inputs
- Computes data for the flight envelop and speed functions
- Provides warning for low energy and wind shear detection if their functions are installed
Use of FMS The FMS is now such a critical item of equipment that it will normally be a dual fit. With a dual installation, there is flexibility in how it is used, as follows:
Master/Slave Operation (Dual Mode). In this, one of the units is used as the “input” unit. All data entered into that unit is shared with the second unit. The computers “talk” to each other and, as well as sharing data, they compare each other’s information. Each FMC retains control of its associated AFCS, auto-throttle and selection of radio navigation aids. The two FMGCS are synchronised, one is master and the other is slave. All the data inserted to any MCDU is transferred to both FMGCS and to all peripherals.
Independent use in which the FMS units operate totally independently. They are linked only to peripherals on their own side of the flight deck. This allows the pilots to operate with one unit displaying performance pages while the other displays navigation data. Alternatively, one may be used to revise or review activities without disrupting either the active flight plan or the commands of the other CDU. “Independent Operation” appears on the CDU Scratch pad.
Single use when only one FMS is operational. It is selected automatically when one FMGC fails. The other FMGC drives all the peripherals. When one FMGC fails, the corresponding MCDU displays “Opp FMGC in progress”
Back-up when the FMC is suffering from some failure but there is still a limited FMS function. Within those parameters the crew need to decide on how much control is given to the FMS. The options are normally Managed Guidance, in which the FMS performs the task of maintaining the pre-planned route, speed and altitude profiles, or the alternative is to elect to control some parameter, such as a heading or speed hold, through the use of the flight control panel. A typical CDU is shown in the picture above. Through this unit, the flight crew can:
- Construct a detailed flight plan
- Select data pages to view
- Respond to FMC requests for data entry
- Change displayed data.
FMS design varies slightly from manufacturer to manufacturer but all will have similar capabilities and systems. At the top of the CDU is the data screen. This is a flat CRT normally providing up to 14 lines of characters, each line providing space for up to 24 characters. Small sized characters are either default or predicted values. The crew can change these if the data originates from the computer. Large size characters show data entered by the crew. The bottom line of the data screen provides for three activities. The left hand half is a scratch pad on which data entered by the pilot is shown. The next ten characters are used by the FMS to pass messages to the crew, while the last two characters display “up” or “down” arrows to show the direction of any necessary scroll (movement) up or down the screen. At the end of each line on the CRT is a “line key”. When the FMC requires data, a question mark (?) will appear at the relevant line. For example, the FMC will need to know the start position of the aeroplane in terms of gate number, so a question mark will appear. You can now type the gate number in, using the alpha numeric keypad. The details you type will appear on the scratch pad and, once you have verified them, can be transferred to the correct line by pressing the adjacent line key. If an arrow appears against a line it normally indicates an optional activity for the flight crew. This could be either functional (e.g., aligning the IRS, deselecting a GNSS satellite) or display (e.g., selecting another page of data). If you choose the option all you need to do is press the “line key”. Function keys are also located below the CRT.
Output Information Information derived from the FMS may be linked directly to the AFCS so that the aeroplane may be flown through or by the FMS. Displays of the information will normally be presented on the aeroplanes EFIS system with a map shown on the nav section showing planned route, active sector, waypoints etc. Details of attitude, speed, vertical speed, and etc. will appear on the PFD as normal.
Flight Management System (FMS) Questions
1) A bounded error in an INS system:
a) will produce a constant track error.
b) will cause the ground speed to oscillate about a constant mean value, which in itself will be an error.
c) will not increase with time.
d) will result in all of the above being correct.
2) What is an FMC?
a) An auto throttle system.
b) A flight management inertial reference system.
c) An autopilot/flight director system.
d) A flight management computer.
3) Which of the following is the FMS normal operating condition in the cruise?
a) LNAV only
b) VNAV only
c) LNAV or VNAV
d) LNAV and VNAV
4) In the event inaccurate radio updating is exercised, what effect will this have on the FMS?
a) this will cause the FMS to shut down.
b) this FMS will automatically update the system.
c) this may cause the FMS to deviate from the desired track.
d) this will have no effect on the FMS.
5) What are the inputs to the FMS?
- Radio Aids
- Engine Parameters
- Air Data
- Route Data
- Terminal Data
- Operating Data
a) 1, 3, 4 & 6
b) 2, 3, 4, & 5
c) All of the above
d) 1, 2, 3 & 6
6) What is the correct order of modes on an INS MCU?
a) OFF, ALIGN, ATT, NAV.
b) OFF, STANDBY, ALIGN, NAV.
c) OFF, ALIGN, NAV, ATT.
d) OFF, STANDBY, NAV, ATT.
7) What are the primary navigation inputs used by RNAV system?
a) INS, Mapping Radar, FMC.
b) INS, Nav Aids, TAS and Drift.
c) Nav Aids, INS, FMC.
d) Nav Aids, Mapping Radar, FMC.
8) A rate integrating gyro is used in which of the following:
- inertial attitude unit
- autopilot system
- stabiliser servo mechanism system
- inertial navigation unit
- rate of turn indicator
a) 1, 2, 3, 4, & 5
b) 1 & 4
c) 2, 3, & 5
d) 2, 3, & 4
9) An IRS is aligned in order to:
a) calculate the computed trihedron with respect to the earth.
b) establish true and magnetic north.
c) establish position relative to true north and magnetic north.
d) establish magnetic north.
10) In an Inertial Navigation System (INS), the main causes of Cumulative Distance errors are:
a) misalignment of the accelerometers in the horizontal plane.
b) wander in the levelling gyros and integrator errors in the second stage of integration.
c) initial azimuth misalignment of the platform and wander of the azimuth gyro.
d) because the true value of the distance run is increasingly divergent from the apparent distance run.
11) All the last generation aircraft use flight control systems. The Flight Management System (FMS) is the most advanced system ; it can be defined as a:
a) global 3-D Flight Management System.
b) management system optimized in the vertical plane.
c) management system optimized in the horizontal plane.
d) global 2-D Flight Management System.
12) The computer of a north referenced Inertial Navigation System (INS) in flight, provides compensation for:
a) aircraft manoeuvres, real wander, apparent wander, transport wander.
b) Coriolis, real wander, apparent wander, transport wander.
c) earth rotation, transport wander, Coriolis.
d) transport wander, apparent wander, Coriolis, magnetic variation.
13) If an alert message is generated by the flight management system:
a) it appears in the middle of the CRT screen and a red light flashes
b) it appears at the top of the CRT and an amber light flashes
c) it appears in the scratch pad and the MSG Annunciator illuminates
d) it appears in the scratch pad and an amber light flashes
14) In an Inertial Navigation System (INS), the main causes of Cumulative Track errors are:
a) wander in the levelling gyros, which causes a Schuler oscillation.
b) integrator errors in the second stage of integration.
c) initial azimuth misalignment of the platform and wander of the azimuth gyro.
d) because recorded value of the distance run is increasingly divergent from the true distance run.
15) What are the advantages of an IRS compared to an INS?
a) Reduced spin-up time and a dither motor to prevent ‘lock-out’.
b) Reduced spin-up time and insensitivity to ‘g’.
c) Increased accuracy and a dither motor to prevent ‘lock-out’.
d) Insensitivity to ‘g’ and reduced wander of the gyroscopes.