Flight Management System (FMS)

Flight Management System (FMS) provides Continuous Automatic Navigation, Guidance and Performance Management.
FMS System comprises of Two flight management computers, Three inertial reference systems and Global Positioning System
Flight Management System can operate in Lateral Navigation (LNav) mode to provide Track, Heading and Radial maintenance
Vertical Navigation (VNav) in FMS helps in Optimum Climb, Cruise and Descent profiles

Lateral Navigation includes Take Off, Standard Instrument Departure (SID), En-route Way Points, Standard Arrival Route (STAR), Instrument Approach, Missed Approach and Flight to Alternate Airport
Vertical Navigation assists in Noise Abatement procedures, AFC Speed Requirements, Rate of Climb, Economical Climb Speed, Climb Settings, Economic Cruise Speed, Step Climb Profile, Top of Descent calculation and Descent Profile

Flight Management System consists of Flight Management Computer as well as Control and Display Unit
FMS integrates 2 or 3 Inertial Reference Systems, Air Data Computer and Radio Navigation Inputs from VOR-DME
Databases available with FMS include those associated with Navigation, Performance, Engine and Fuel consumption
FMS guides or controls the Flight Guidance System, Flight Director In EFIS and the Auto Pilot

CDU of FMS communicates with FMC using Plasma display and keypad for draft and final entries
Display unit also alerts pilot using alert and advisory messages
Pre-flight pages of FMS include IRS setting, Dispatch Information, Flight Plan Route, Zero Fuel Weight and Cruise Altitude pages
In-flight pages of FMS include Display Information, Advisory and Alert Message pages
Performance pages of FMS include Climb and Cruise Performance, Thrust, Max and Min Altitude and Expected Fuel Flow pages
FMS Display Unit warns of incorrect page selection using display lights and failure warning using Fail Light
Message Light warns of unread messages in scratch pad and Offset Flight Path using Offset Light

Main, Supplemental, Performance and Temporary databases form the basis for all FMS Calculations
The current and next cycle of Main Navigation Database with a 28 days validity are required during start-up
Main Navigation Database consists of navigational information like air-routes, instrument let-downs and standard instrument departures or arrivals
Pilots would not be able to alter the main database
Supplemental Navigation Database is a permanent database fed by the operator on ground depending on the flight plan
Temporary Navigation Database can be fed by the operator and is valid only for one flight
Performance Database contains information pertaining to climb, cruise, thrust requirement, all-up weights, fuel flow and altitudes
Maintenance personnel are required update the maintenance database for a particular aircraft

Flight management system provides for automatic and manual change over between Dual, Independent, Single and Back-up modes
In the Dual mode Pilot’s and Co-pilot’s flight management computer calculates independently and the synchronised results are displayed to both pilots
In the Independent mode exch flight management computer performs and displays independent calculations
Outputs on pilot’s and co-pilots display may be different in independent mode of operation
Single mode is used in the case of failure of one FMC where the serviceable FMC feeds both the Pilot’s and Co-Pilot’s displays
Back-up mode is used in the case of failure of both flight computers in which EFIS displays IRS information bypassing FMC
Auto pilot is decoupled from FMS in the back-up mode of operation

FMS Start-up sequence consists of Ident, Position Initialisation, Route, Performance and N1 Limit Pages
Ident page is used to enter aircraft model, engine rating and AIRAC cycle
Position Initialisation page is used to update position of parking bay
Route page is used to enter Departure, Destination, Route and Way Points
Performance page is used to update gross weight, cruise altitude, fuel required, forecast wind, isa deviation, temperature and cost index
N1 limit page is used to reduce engine wear during climb
Page navigation on FMS can be selected using the function, mode, previous and next keys

FMS provides for temporary creation of way-points using either Latitude and longitude or Bearing and distance or Distance along track
FMS automatically updates position using Take Off Go Around (TOGA) switch or Manually displaced threshold
Cost Index calculations in FMS include optimum flight speeds based on maintenance intervals, fuel cost, maintenance cost and trip time
Required time of arrival calculations of FMS include take off time, route speeds and cost index to arrive at the required time at destination
FMS provides alert messages for Thrust Required, Incompatible Standard Departures or Arrivals, ILS Frequency Error and Insufficient Fuel
Advisory messages are provided for conditions like maximum flight levels in the case of heavy aircraft

Flight Management Systems affected by Schuler and Ramp errors of Inertial Navigation System
Schuler error or bounded errors due limitations in in manufacture
Ramp or unbounded errors increases with flying time
FMS corrects these errors using a technique called Kalman Filtering

Kalman filtering is the process of statistically finding the Most Probable Position called the FMS Position.
FMS takes the IRS position as the basic value and compares it with GPS and DME using an advanced form of deduced reckoning
Kalman Filtering used in FMS Position fixing is not a simple average of all the navigation systems but is a position computed using an algorithm
FMS algorithm uses a mathematical model in which the available inputs are processed with their accuracy levels

FMC will de couple the IRS at high latitudes in order to avoid errors due to meridian convergence
Meridians converge at a high rate above 84° N/S hence each FMC will use the IRS only on its side for reference

Best of luck