Flight Management System

A flight management system (FMS) is the airliner's central navigation and performance computer. The FAA describes it as an integrated suite of sensors, receivers and computers coupled with a navigation database, providing performance and area navigation guidance to the displays and the automatic flight control system. Once the crew has entered the route and the aircraft's weight, the FMS knows where the aircraft is, where it is going, at what speed and altitude it should fly each part of the trip, and how much fuel it will have on arrival.

EASA training texts give its aims as better navigation accuracy, better fuel efficiency and a lower crew workload. In RNAV terms a modern airline FMS is a four-dimensional system: it manages the lateral path, the vertical profile and the time of arrival (see area navigation). How it navigates, updates its position and flies descents is covered in FMS navigation and position updating; this article deals with the system, the crew's interface with it, its databases and its initialisation.

FMS overview

The heart of the system is the flight management computer (FMC). It combines the flight plan entered by the crew, data from the aircraft systems and the contents of its navigation and performance databases to calculate the aircraft's position and the roll, pitch and thrust commands for an optimum profile. On the Boeing 737 the FMS comprises the flight management computer system, the autopilot flight director system, the autothrottle, the inertial reference systems and GPS. Airbus calls its equivalent the flight management and guidance system (FMGS): two flight management and guidance computers (FMGCs), two MCDUs, a flight control unit and two flight augmentation computers. Each FMGC has a flight management part and a flight guidance part, the latter producing autopilot, flight director and autothrust commands.

The FMS draws position from the inertial reference systems, GNSS and radio aids, speed and altitude from the air data computers, and fuel from the fuel quantity system. It sends the route and the aircraft's position to the navigation displays, steering and vertical commands to the autopilot and flight directors, speed targets and thrust limits to the autothrottle, and frequencies to the navigation radios.

A typical airliner has two FMCs and two control display units, one for each pilot; some widebodies have three FMCs, and the Boeing 747-400 a third CDU on the centre console. Boeing's dual FMC installation is certified as a sole-source navigation system, allowing operation outside radio navaid coverage. In the generic description of EASA training texts, if both FMCs fail the inertial systems feed the displays directly: basic navigation remains, but the performance functions, predictions and descent paths are lost. On the 737, LNAV and VNAV disengage and the navigation displays show failure information.

The 737 FMC works through flight phases in order: preflight, take-off, climb, cruise, descent, approach and flight complete. The flight complete phase, after landing, clears the active flight plan and load data.

FMC and CDU layout

The crew talks to the FMC through a control display unit (CDU); Airbus's multipurpose CDU, the MCDU, also serves other systems such as the datalink. The screen shows one page at a time: a title line, data lines each with a small label above, and at the bottom the scratchpad. Down each side of the screen runs a column of line select keys (LSKs), one beside each data line; Airbus documentation numbers them 1L to 6L and 1R to 6R. Below the screen are page keys, such as INIT REF, RTE, LEGS, DEP ARR and PROG on Boeing units or INIT, PERF, PROG and DATA on Airbus ones, an alphanumeric keyboard, and the CLR and DEL keys.

Page formats carry meaning. On Boeing CDUs, boxes on a line mean the entry is mandatory and dashes that it is optional. Magenta marks data the FMC is using for lateral and vertical commands, and cyan page titles mark an inactive route.

Scratchpad, line select and EXEC keys

The scratchpad is where every entry starts. What the pilot types appears there and changes nothing until it is inserted. Pressing an LSK moves the scratchpad data into the field beside the key; the same key can also copy data from the line into the scratchpad, select a page or procedure, or delete data when DELETE is in the scratchpad. The FMC uses the scratchpad for its messages too, announced by the MSG light. CLR erases one character at a time, or the whole scratchpad if held; DEL puts DELETE in the scratchpad so that the next LSK press clears that field, back to its default, where deletion is allowed.

Boeing adds a safety step. A change to the active route or performance data creates a modification: the page title changes from ACT to MOD, the changed data is shaded, and the white execute light above the EXEC key comes on. The aircraft keeps flying the active data. Pushing EXEC makes the modification active and extinguishes the light. Airbus reaches a similar result with a temporary flight plan, shown in yellow on the MCDU, which has to be inserted before it replaces the active one.

The two-step design supports crew procedure. In the 737 flight crew operations manual, the pilot monitoring makes any CDU entries during taxi, and in flight the pilot making an entry executes it only after the other pilot has verified it. Airbus has the pilot monitoring check the pilot flying's FMGS preparation page by page, in the same order in which it was entered.

Navigation database

The FMS navigation database holds most of what used to come from charts: VHF navigation aids, waypoints, airports and runways, and airline-selected items such as SIDs, STARs, approaches and company routes. It is revised on the 28-day AIRAC cycle. The FMC stores two sets, each valid for 28 days, the current one and the next; maintenance loads the new set in advance and the crew selects the active one during preflight. On the A320 the database can also be crossloaded from one FMGC to the other, which takes about five minutes.

The FAA requires a current database for IFR use and advises pilots to check its dates and any notices from the database provider. Approach procedures must be retrieved by name from the database, never built from manually entered waypoints.

Crew-defined items live in separate memory. The 737 FMC has a supplemental database, kept until the crew erases it, and a temporary one, erased automatically at flight completion; between them they hold 40 navaids and six airports. Each A320 FMGC stores up to 20 pilot-defined waypoints, 10 runways, 20 navaids and 5 routes.

Performance database

The FMS performance database replaces the performance manual in flight. It holds the aircraft's drag and engine characteristics, its maximum and optimum altitudes and its maximum and minimum speeds, and gives the FMC what it needs to calculate pitch and thrust commands. Maintenance can adjust drag and fuel-flow factors to match the individual aircraft.

From it the FMC derives the economy speeds for the entered cost index, a cost index of zero giving maximum range cruise; the thrust limits, expressed on the 737 as N1; optimum and maximum altitudes; and continuous fuel predictions. On the 737 the scratchpad shows INSUFFICIENT FUEL when predicted fuel at destination is 2,000 lb or less, and USING RSV FUEL when it falls below the reserves entered on PERF INIT.

IDENT, POS INIT and PERF INIT

On a Boeing CDU the preflight begins with the IDENT page, then POS INIT (position initialisation), then the route; the table follows the page contents given in EASA training texts. Boeing wants initial and navigation data complete before the flight instrument check, and performance data before the Before Start Checklist.

Boeing recommends a full IRS alignment before each flight (see IRS alignment and modes). On PERF INIT, the performance initialisation page, the boxed items are mandatory. Boeing warns against entering the zero fuel weight in the gross weight boxes, a mistake that corrupts every calculation; the fuel shown must agree with the dispatch papers and the gauges, and the gross weight and cruise centre of gravity with the dispatch papers. Winds are entered on separate pages; leaving out en-route winds gives errors in flight time and fuel. VNAV cannot be armed until a flight plan and performance data have been entered and executed.

Airbus splits initialisation differently: INIT A takes the city pair or company route, cruise level and cost index, and INIT B the zero fuel weight, its centre of gravity and block fuel, which start the flight plan computation.

View full schematic

Dual, single and independent modes

With two computers the question is how they share the work:

• In dual mode, the normal one, each computer processes the crew's entries and computes position independently; they exchange data and cross-compare, one acting as master and the other as slave.
• Single mode follows the failure of one computer: the survivor runs everything.
• In independent mode each computer works alone, with no cross-check. It is not a normal mode.

The A320 names four modes: dual, independent, single and back-up navigation. Independent mode is selected automatically when, for example, the two FMGCs hold navigation databases with different validity; the scratchpads show INDEPENDENT OPERATION and an IND light comes on until the crew crossloads the database. Single mode is automatic when one FMGC fails, and the survivor drives both flight directors. Back-up navigation is selected on the MCDU after both FMGCs have failed; it keeps a flight plan, map display and automatic sequencing, using IRS or GPS position, but no autopilot or flight director NAV mode.

On the 737 the FMC source select switch decides. In NORMAL the left FMC controls both CDUs and feeds the autothrottle, while the right FMC runs in synchronisation with it; BOTH ON L or BOTH ON R puts everything on one computer, and moving the switch disengages LNAV and VNAV. If the right FMC fails, both scratchpads show SINGLE FMC OPERATION. If the left one fails, the CDUs show the MENU page and BOTH ON R restores full operation. A power interruption of ten seconds or more on the ground means the whole preflight must be entered again.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.