Flight Management Navigation -
FMS Operational Notes

When operating in the LNAV and VNAV modes, system operation must be monitored for unwanted pitch, roll, or thrust commands. If unwanted operation is noticed, roll and pitch modes other than LNAV and VNAV must be selected.

The system must be carefully monitored for errors following:

When operating far off the route, the FMC may not sequence the active waypoint when the airplane passes abeam the waypoint.

LNAV can only capture the active leg. It cannot capture an inactive leg in the active route. The DIRECT TO or INTERCEPT COURSE TO procedures can be used to create an active leg for capture.

When a waypoint is in the route more than once, certain route modifications (such as DIRECT TO and HOLD) use the first waypoint in the route.

Some SIDs or STARS contain a heading vectors leg. VECTORS waypoints display on the HSI map as a magenta line without an end point leading away from the airplane symbol. If LNAV is engaged, the DIRECT TO or INTERCEPT COURSE TO procedure can be used to start waypoint sequencing beyond the vectors leg.

When entering airways in a route page, the start and end waypoints must be in the database. Otherwise, the route segment must be entered as a DIRECT leg.

If the engines remain operating between flights, entering a new cruise altitude before the next flight recalculates the proper vertical profile.

If a climb to cruise altitude is necessary after completing a descent, a new cruise altitude entry must be made. Cruise altitude can be entered on the CLB page.

Direct-to courses are segments of a great circle route. When entering a direct-to waypoint on the LEGS page, the course above the waypoint before execution is the arrival course at the waypoint. However, after execution, the course is the current course to fly to the waypoint. These courses may not be the same.

During split IRS operation each FMC uses a different IRS for position calculations. The FMCs use the two IRSs which have the closest present positions. The IRS positions are not averaged as during normal operation. This can result in a difference between the two HSI maps and descent paths when radio updating is not available.

Occasionally a procedure in the data base. contains a hidden discontinuity that appears on the LEGS page as --- for the inbound course.

When using LNAV to intercept the localizer, use raw data to verify localizer course interception and verify LOC mode engagement. If this does not occur, HDG SEL or HDG HOLD mode may be used to establish a localizer intercept heading.

FMS Terminology