Flight Management Navigation -
FMS Lateral Navigation (LNAV)

Amendment: Holding Patterns included.

LNAV provides steering commands to the next waypoint or the selected route intercept point. When selected after takeoff, LNAV engages when laterally within 2.5 nautical miles of the active route leg. FMC LNAV guidance normally provides great circle courses between waypoints. However, when an arrival or approach from the FMC database is entered into the active route, the FMC commands a heading, track, or a DME arc to comply with the procedure.

Waypoints

Waypoint (navigation fix) identifiers display on the CDU and HSI.

The CDU message NOT IN DATABASE is displayed if a manually entered waypoint identifier is not in the database. The waypoint can still be entered as a latitude/longitude, place-bearing/distance or a place-bearing/place-bearing waypoint.

FMC-generated waypoints contain a maximum of five characters assigned according to the following rules.

Navaid Waypoints

VHF - waypoints located at VHF navaids (VOR/DME/LOC) are identified by one, two, three or four character facility identifier. Examples:

NDB - PEGASUS
waypoints located at NDBs are identified by use of the station identifier. Example: FORT NELSON, CAN - YE.

NDB - Non-PEGASUS
waypoints located at NDBs are identified by use of the station identifier followed by "NB". Example: FORT NELSON, CAN - YENB.

Fix Waypoints

Waypoints located at fixes with names containing five or fewer characters are identified by the name. Examples:

Long Waypoint Names

Waypoints with more than five characters are abbreviated using the following rules sequentially until five characters remain. For double letters, one letter is deleted. Example:

Keep the first letter, first vowel and last letter. Delete other vowels starting from right to left. Example:

The next rule abbreviates names even further. Apply the previous rule, then delete consonants from right to left. Example:

Fixes with multiword names use the first letter of the first word and abbreviate the last word, using the above rules sequentially until a total of five characters remain. Examples:

Unnamed Waypoints

If an unnamed turn point, intersection, or fix is collocated with a named waypoint or navaid on a different route structure (such as low altitude routes or an approach), the name or identifier of the collocated waypoint is used. Example:

Identifier codes for unnamed turn points not coincidental with named waypoints are constructed from the identifier of a navaid serving the point and the distance from the navaid to the point. If the distance is 99 nautical miles or less, the navaid identifier is placed first, followed by the distance. If the distance is 100 nautical miles or more, the last two digits are used and placed ahead of the navaid identifier. Examples (NAVAID - DISTANCE - IDENT):

Waypoint located at unnamed flight information region (FIR), upper flight information region (UIR), and controlled airspace reporting points are identified by the three-letter airspace type identification followed by a two-digit sequence number. Example:

Unnamed oceanic control area reporting points in the northern hemisphere use the letters N and E, while points in the southern hemisphere use the letters S and W. Latitude always precedes longitude. For longitude, only the last two digits of the three-digit value are used.

Placement of the designator in the five-character set indicates whether the first longitude digit is 0 or 1. The letter is the last character if the longitude is less than 100° and is the third character if the longitude is 100° or greater.

N is used for north latitude, west longitude. E is used for north latitude, east longitude. S is used for south latitude, east longitude. W is used for south latitude, west longitude. Examples:

 

Procedure Arc Fix Waypoint Names

Unnamed terminal area fixes along a DME arc procedure are identified with the first character D. Characters 2 through 4 indicate the radial on which the fix lies. The last character indicates the arc radius. The radius is expressed by a letter of the alphabet where A = 1 mile, B = 2 miles, C = 3 miles and so forth. Example:

An unnamed waypoint along a DME arc with a radius greater than 26 miles is identified by the station identifier and the DME radius. Example:

When there are multiple unnamed waypoints along a DME arc with a radius greater than 26 miles, the station identifier is reduced to two characters, followed by the radius, and then a sequence character. Examples:

DME step down fixes are identified by the distance and a "D". Examples: 138D, 106D, 56D, 3D.

Procedure Fix Waypoints

Marker beacons are identified by the marker type identifier followed by the runway number. Examples:

Waypoints located at unnamed runway-related fixes are identified by adding a two-letter prefix to the runway number. The following list is used to determine the applicable prefix:

 

Examples: OM25L, MM09, IM23, RW04, RW18L.

For airports with more than one approach to the same runway, the two-letter prefix may change to allow different identifiers for the same waypoint. The first letter identifies the type of fix and the second letter identifies the type of approach as follows:

Type of Fix Type of Approach
  • C ( ) Final approach course fix.
  • F ( ) Final approach fix.
  • P ( ) Missed approach point.
  • I ( ) Initial approach fix.
  • D ( ) Minimum descent altitude.
  • T ( ) Touch down point.
  • R ( ) Runway centerline intercept.
  • ( ) L - Localizer only.
  • ( ) B - Backcourse ILS.
  • ( ) D - VOR/DME.
  • ( ) V - VOR only.
  • ( ) S - VOR with DME points.
  • ( ) N - NDB.
  • ( ) Q - NDB with DME points.
  • ( ) M - MLS.
  • ( ) T - Tacan.
  • ( ) I - ILS.
  • ( ) R - RNAV.

Examples: CI32R, PV15, FN24L

Unnamed turn points that are part of a procedure are identified as a latitude and longitude waypoint. These include waypoints (except conditional waypoints) defined by flying a course or track from a waypoint (except conditional waypoints) to a radial or DME distance. These waypoints are automatically entered in a route by selection of a procedure using these waypoints, from the DEPARTURES or ARRIVALS page.

Airport reference points are identified by the ICAO identifier.

Duplicate Waypoints

Application of the abbreviation rules may create identical identifiers for different waypoints. When a duplicate waypoint identifier is entered, the page changes to the SELECT DESIRED WPT page. The page lists the latitude, longitude, and the type of facility or waypoint of all the waypoints with the same identifier. Select the latitude/longitude of the correct waypoint to enter the correct waypoint on the original page.

If more than one waypoint with duplicate identifiers must be entered in a route, the second and following waypoints must be entered as a place bearing/distance, place bearing/place bearing, or Latitude and Longitude.

Conditional Waypoints

Conditional waypoints may be displayed in the route when selecting a DEPARTURES or ARRIVALS page procedure. Usually, conditional waypoints cannot be manually entered on a Route or Legs page. These waypoints indicate when an event occurs and are not at a geographically-fixed position. The types of conditions are:

Altitude and course intercept conditional waypoints display on the CDU inside (parenthesis) marks. The diagram below shows conditional waypoints.

Manually Entered Latitude/Longitude Waypoints

Pilot defined waypoints entered as a latitude and longitude are shown in a seven-character format. Latitude and longitude waypoints are entered with no space or slash between the latitude and longitude entries. Leading zeroes must be entered. All digits and decimal points (to 1/10 minute) must be entered unless the latitude or longitude are full degrees. Examples:

Manually Entered Place-Bearing/Distance or Place-Bearing/Place-Bearing Waypoints

Waypoints entered as a place-bearing/distance or place-bearing/place-bearing are identified by the first three characters of the entry followed by a two-digit sequence number. Examples:

The two digit sequence numbers reserved for RTE 1 are 01 through 49. The two digit sequence numbers reserved for RTE2 are 51 through 99.

Manually Entered Airway Crossing Waypoints

Airway crossing fixes are entered as a five character waypoint name or by entering consecutive airways on the Route page. In the latter case, the display is an X followed by the second airway name. Example: entering J70 on the VIA line of the Route page causes box prompts to display opposite on the same line. Leaving the box prompts empty and entering J52 on the next VIA line, directly below J70, causes the FMC to calculate the intersection of the two airways and replace the boxes with the waypoint identifier, XJ52.

Manually Entered Latitude or Longitude Reporting Point Waypoints

Latitude or longitude reporting waypoints are entered as the full latitude or longitude followed by a dash, then the increment chosen for the following multiple waypoints. Example:

  • W060-10 adds waypoints starting at W060 in ten-degree increments from that point to the destination.
  • The entry must be made on a Legs page on any line before the first reporting point.
  • Usually, this entry is made on the active waypoint line and proper sequencing is performed by the FMC.

Manually Entered Along-Track Waypoints

Along-track waypoints are created on the active route and do not cause route discontinuities when they are created.

Along-track waypoints are entered using the waypoint name (the place), followed by a slash and minus sign, for points before the waypoint, or no sign for points after the waypoint, followed by the mileage offset for the newly defined waypoint. The created waypoint is then inserted over the original waypoint. The distance offset must be less than the distance between the originating waypoint and next (positive value) or preceding (negative value) waypoint. Latitude and longitude waypoints cannot be used to create along-track waypoints. Examples:

Engine Out SID

An engine out SID is a procedure developed by an airline for a particular runway to provide unique routing if an engine fails on takeoff. If the database contains an EO SID for the takeoff runway and an engine fails [EPR reduction of 0.1 or more before CLB thrust selected] while the flaps are extended, the active route is automatically modified to the engine out route. The modification may be either executed or erased.

Holding Patterns

The FMC computes holding patterns with constant radius turns based on the current wind and the FMC command speed (displayed on the command airspeed bug in VNAV mode). The computed pattern displayed on the HSI map is limited so as not to exceed the size of the FAA or ICAO protected airspace. In LNAV mode, the AFDS tracks the displayed holding pattern using up to a 30° bank angle. However, if airspeed or wind speed is in excess of the FAA and ICAO assumed speeds, the airplane may leave the protected airspace. This can be seen on the map by observing the airplane symbol or trend vector outside the magenta holding pattern.

The holding pattern entry method is not displayed on the HSI map. If LNAV mode is engaged before passing the holding fix, standard holding pattern entry methods (parallel, teardrop or direct entry) are used with the following differences:

  1. The entry method used (parallel, teardrop or direct entry) is a function of actual airplane track as the holding fix is crossed, not a function of airplane heading or the direction from which the active route approaches the holding pattern.

  2. The initial outbound leg is maintained until a distance from the fix is reached, rather than maintained for a specific time. This distance is a function of the command airspeed and wind speed as the holding pattern becomes active.

  3. Teardrop entries use a 40° angle.

  4. Parallel and teardrop entries may take the airplane slightly beyond the outside end of the pattern shown on the map. However, the airplane will remain in protected airspace if the airspeed is within FAA or ICAO Limits.

If LNAV mode is not engaged until after passing the holding fix, standard holding pattern entries are not used. Rather, the initial turn is in the shortest direction toward a track equivalent to the holding pattern inbound course. If this is in other than the desired direction, Heading Select mode should be used to complete the desired entry procedure.

Heading Hold Submode

A submode of LNAV exists which is designed to maintain the airplane on current heading during the following conditions:

  • When flying into a route discontinuity. The CDU message DISCONTINUITY is displayed.
  • When flying past the end of a lateral offset. Approaching the end of an offset, the CDU message END OF OFFSET is displayed.
  • When flying past the last route waypoint. The CDU message END OF ROUTE is displayed.
  • When executing an INTERCEPT LEG/COURSE TO procedure while the airplane is outside the LNAV capture band of the active leg.
    • If current airplane heading intercepts the active leg, LNAV maintains heading pending leg capture.
    • If current airplane heading does not intercept the active leg, LNAV maintains heading and the CDU message NOT ON INTERCEPT HEADING is displayed.

HSI Map Displays

The route is displayed on the HSI in MAP, and PLAN modes. The display color and format represent the following status:

The HSI displays the FMC position at the apex of the airplane symbol. All HSI map data displays relative to this apex.

When adequate GPS or radio updating is not available, the HSI map may display a shift error.

When adequate radio updating is not available, the HSI map may display a shift error.

This error results in the displayed position of the airplane, route, waypoints and navigation aids being shifted from their actual position. An undetected, across track map shift may result in the airplane flying a ground track that is offset from the desired track. An undetected, along track map shift may result in the flight crew initiating altitude changes earlier or later than desired. In either case, an undetected map shift may compromise terrain or traffic separation.

Map shift errors can be detected by comparing the position of the airplane on the HSI map with data from the ILS, VOR, DME, and ADF systems.

VNAV