FMC basics – Part I

Intriguing mysteries around the FMC use are abundant. It is therefore the topic of this second post of the “Tutorial for Flight Simmers, Young Pilots, and Aviation Enthusiasts“.

What follows is based on Boeing’s terminology and systems, more specifically on the 737NG. There are many differences with Airbus (and other aircraft manufacturers), which will be the topic for another day.

First of all, here are some abbreviations to not mix up :

  • FMC = Flight Management Computer;
  • FMS = Flight Management System;
  • CDU = Control Display Unit;
  • MCDU = Multifunction Control Display Unit;
FMS00-600x918FMC stands for Flight Management Computer, the flight computer. We can access its parameters through an interface called CDU or MCDU, which consists of the keyboard and the screen as shown on the picture on the left.

The MCDU is an evolved CDU, with a « M » standing for « Multifunction », since it also gives access to ACARS, DFDAU, ALT NAV, etc. This is the reason why it contains a  MENU  key (nope, this isn’t here to know what is there to eat in the plane!).


The FMC is only one of the many elements composing the FMS, the Flight Management System. The FMS contains, among others, the AFDS (Autopilot and Flight Director System), the A/T (Autothrottle System), the IRS, and the GPS.

General functioning : keyboard, LSKs, scratchpad


The CDU consists of a keyboard and a screen for which preliminary explanations will now be given.

The inferior part of the keyboard is quite clear: a numerical pad, an alphabetical pad and some specific keys such as  SP  for space,  DEL for delete (used to erase a whole line in the route for example), and CLR  for clear (erases the last character on the scratchpad, then the complete line if we keep pressing it).

There are 12 keys for direct access to the FMC pages between the screen and the two pads ( INIT REF   RTE   CLB   etc. – their name is usually straightforward but they will be explained in more detail later on anyways). We can also find there the  PREV PAGE  and  NEXT PAGE keys, which allow to navigate in the different sub-pages of a section.

In addition, the 12 LSKs are located on each side of the CDU screen. These keys are called « LSK » for Line Select Key. A standard convention is used to designate each of them: first the line number, then « L » or « R » for Left and Right, respectively. For instance, LSK 3R refers to the 3rd key starting from above on the right side, and LSK 1L refers to the uppermost key on the left. Click on the picture on the left to enlarge it (opens in a new window).

The scratchpad is the line at the bottom of the screen (circled in green in the picture on the upper left). It’s a zone where the data that we want to assign to a parameter is temporarily written. When we type a number with the keyboard, it appears on the scratchpad. Then, we can press the needed LSK key to assign that number to the parameter of interest. It is on this line as well that the system communicates simple messages such as “DRAG REQUIRED“, “ENTER IRS POSITION“, and “STEEP DESCENT AFTER XXX“.

We should also mention the  EXEC key and its light indicator : when the light switches on, this key allows to confirm/validate the data entered in the sensitive pages.

For our example, we’ll use the EBBR-LSGG route used in the previous tutorial.

Flight data : all the data is arbitrarily chosen for our example: weight, loading, weather, route, runways used, procedures, etc. are chosen to be realistic but are not to be followed as advice. In other words, prepare the simulation with all the available tools (FuelPlanner, RouteFinder, TopCat, PFPX, ActiveSky,…) and use the following as inputs for your situation :

  • AIRCRAFT: B737-800 NG – Engine Rating 26k
  • POSITION: gate 140 at EBBR
  • DEPARTURE: runway 25R – SID ROUS7C
  • CRUISE ALTITUDE: FL 330 (= 33.000 feet standard barometer – see a coming tutorial on altimetry and the QNH subtlety…)
  • ISA DEVIATION: the cruise temperature is -48°C in our case, so approximately +2.4°C with respect to the standard ISA temperature
  • AVERAGE WIND: 285° / 19 KT
  • WEIGHT: 56 tons, without fuel (ZFW = Zero Fuel Weight)
  • FUEL: 5 tons, including 2 of reserve fuel


Since the MCDU is multifunction, its initial menu proposes many functions : we choose  <FMC  via LSK 1L.

After a quick verification of the  IDENT  page (airplane model, engine rating, database, software), we go to the  POS INIT  page via LSK 6R where we can input the airport and the gate where we are, as well as the IRS position (SET IRS POS). This means you are supposed to have positioned the IRS selectors on the « NAV » for them to be aligning.

Once  EBBR  is set in  REF AIRPORT  and  140  in  GATE , we can see the position given to the latter : N50°54.1 E004°28.8; a rapid cross-check with the position given by the GPS (page 2/3) confirms that the positioning data is coherent (i.e. is the same). We can thus copy paste this position safely (e.g. by pressing LSK 3R then LSK 4R).

Let us now move on to page  RTE , either by pressing on  ROUTE>  via LSK 6R, or by pressing directly the  RTE  key.

EN ROUTE – Here we go!

In RTE, page 1 asks us to input the airports of departure in  ORIGIN  and arrival in  DEST , the flight number in  FLT NO.  and the runway in  RUNWAY . The  CO ROUTE  line can be used to set a Company Route which would be stored in the system. We won’t use it here though.

We now press on  NEXT PAGE  to go to page 2 and start encoding the route, segment by segment. Note that we won’t use SID nor STAR at this point, our route goes thus directly to ROUSY, then AKITO passing through the points listed on the complete route. We therefore encode the following data :


Next, we press  DEP ARR  to go to the  DEP/ARR INDEX  page where we can choose first our departure via LSK 1L  <DEP EBBR .

The page  EBBR DEPARTURES  lists all departures available at EBBR : the runways in the right column, and the SIDs in the left column. We want to select the  25R runway that we set by pressing  NEXT PAGE  then LSK 1R. The left column shows now only the SIDs connected to 25R.

Pressing  NEXT PAGE  a few times leads us to our desired choice :  ROUS7C .

We now press the  DEP ARR  key again in order to go back to the  DEP/ARR INDEX  page where we can now choose our arrival with LSK 2R  LSGG ARR> .

The page  LSGG ARRIVALS  lists the arrivals on LSGG: the runways and procedures on the right column, and the STARs in the left column. We select  ILS 23 with LSK 4R. The left column now shows only the STARs connected to that approach.

We press LSK 1L and our STAR on AKIT2R is now selected!

Usually, it is preferable to go through the different segments of the encoded route in the  LEGS  page in order to ensure no discontinuity is present. If pieces of the route are separated by empty boxes preceded by  THEN  and followed by  ROUTE DISCONTINUITY , we can join them if that is our plan. We only need to press  DEL  then the LSK key for the empty boxes to disappear and the segments to be joined.

The last step is to press  RTE  to go back to the main page where the option  ACTIVATE>  is now presented to us. As soon as we press LSK 6R, the  EXEC  light will turn on, waiting our confirmation for the constructed route.


Check next post for Performances!

One thought on “FMC basics – Part I

  • These are great posts. I’ve been having a difficult time learning the FMS. This was really helpful. I got Chrome to translate Part II.

    I hope you keep on posting this quality information.

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