Flight Management Navigation -
FMS Terminology

The following paragraphs describe FMC and CDU terminology:


Flight plan data being used to calculate LNAV or VNAV guidance commands.


The procedure to change an inactive route to the active route for navigation. It is a two-step procedure:

  • Select the ACTIVATE prompt
  • Push the execute (EXEC) key
Altitude constraint

A crossing restriction at a waypoint.


Using the delete (DEL) key to remove FMC data and revert to default values, dash or box prompts, or a blank entry.


A speed schedule calculated to minimize operating cost. The economy speed is based on the cost index. A low cost index causes a lower cruise speed. Maximum range cruise or the minimum fuel speed schedule may be obtained by entering a cost index of zero. This speed schedule ignores the cost of time.

A low cost index may be used when fuel costs are high compared to operating costs.


Put data in the CDU scratchpad and line select the data to the applicable location. New characters can be typed or existing data can be line selected to the scratchpad for entry.


Remove entered data, which has resulted in a modification, by selecting the ERASE prompt.


Push the illuminated EXEC key to make modified data active.


Data not being used to calculate LNAV or VNAV commands.


Entering data required to make the system operational.


FMC information displayed in the scratchpad.


To change active data. When a modification is made to the active route or performance mode, MOD displays in the page title, ERASE displays next to line select key 6 left, and the EXEC key illuminates.


CDU symbol that aids the flight crew in accomplishing a task. Prompts can be boxes, dashes, or symbols (< or >) to remind the flight crew to enter or select data.


A self protection function which causes an FMC to shutdown and restart when an error is detected. Current flight and performance data is automatically re-loaded from the other FMC during the reset.

The automatic process of one FMC loading data in the other FMC when a significant difference between the two FMCs is detected.


Pushing a key to obtain the necessary data or action, or to copy selected data to the scratchpad.

Speed restriction

An airspeed limit associated with a specified altitude entered by the flight crew.

Speed transition

An airspeed limit associated with a specified altitude entered by the FMC.


A point on the route or in the navigation database. It can be a fixed point such as a latitude and longitude, VOR or ADF station, or an airway intersection. A conditional waypoint is not associated with a land reference; it is based on a time or altitude requirement. An example of a conditional waypoint is "when reaching 1000 feet".

FMS Navigation Position