LNAV (Lateral Navigation) — flies aircraft along complex routes on great circle tracks; can fly constant heading, track, or DME arc as required
VNAV (Vertical Navigation) — calculates optimum cruise altitudes; best combination of autothrottle and speed during climb and descent
Fig 21.1 — Schematic layout of a typical FMS system. CDU = Control/Display Unit; FGS = Flight Guidance System; FMC = Flight Management Computer; ADC = Air Data Computer; IRS = Inertial Reference System; GPS = Global Positioning System. Source p.272
2. Control and Display Unit (CDU)
The CDU is the interface between the aircraft and crew. Two CDUs are usually fitted either side of the centre console, with the left CDU normally the master. On the B747-400 a third CDU is on the centre console for engineering use.
Each CDU has a monochrome or colour CRT/LCD display showing "pages" of selected data, plus a selector key panel.
FMC Operating Modes
Mode
Description
Dual Mode (normal)
Both FMCs independently process pilot entries and cross-check results. Same output passed to both FMCs. Provides safety cross-check.
Single Mode
One FMC has failed; the surviving FMC operates the aircraft on its own. Failed FMC may be selected out.
Independent Mode
FMCs decoupled — no safety cross-check between them. Not normal for standard flight profiles.
Fig 21.2 — FMS CDU display and keyboard. Source p.273
3. FMC Data Base
Two major sections:
Performance Data Base:
Aircraft drag and engine characteristics
Maximum and minimum speeds
Drag and fuel flow factors (maintenance refinable)
Replaces need to refer to Cruise Control Manual in flight
Provides data for pitch and thrust commands
Navigation Data Base:
Location of navaids, airports, runways
SIDs, STARs, approaches, company routes
All areas where the aircraft is normally flown
Updated by maintenance every 28 days (matches navigation chart revision cycle)
FMC contains two sets of navigation data, each valid for 28 days — crew selects which is active during preflight
4. Initial Actions — FMC Pages (Boeing sequence)
graph TD
A["Power ON → Self-test → IDENT page"] --> B["POS INIT page"]
B --> C["RTE page"]
C --> D["PERF INIT page"]
Page
Content / Action
IDENT
Confirm aircraft model, operational programme ID, drag/fuel flow factors, Nav DB identifier and cycle. Shows changeover date — if out of date, prompts to change programme.
POS INIT
Check FMS clock against aircraft clock (synchronise for FDR and ETAs). Enter airfield datum or gate position for IRS alignment. Often updated at take-off point for best initial position.
RTE
Enter origin and destination ICAO codes, flight number, and select/enter standard company route.
PERF INIT
Enter fuel weight, fuel reserves, cruise altitude, Cost Index. On B747 — enter fifth engine carriage if applicable.
5. VNAV — Climb
Cost Index = 0: Economy speeds = maximum range cruise (minimum trip fuel). VNAV commands:
Climb with climb thrust, respecting all airspeed and altitude constraints in the SID
Climb at economy speed to entered cruise altitude
Cruise at economy speed until Top of Descent (TOD)
If flying the climb speed profile would violate an altitude constraint, the message "UNABLE NEXT ALT" is displayed — pilot must select a different (steeper climb) speed.
Cost Index relationship:
Cost Index = Aircraft operating cost (£/hr) ÷ Fuel cost (£/lb or £/litre)
CI = 0: Minimum trip fuel (max range cruise, low speed descent)
Higher CI: Higher speed → more trip fuel cost but lower time cost → when time is expensive
Economy Cruise Mode gives lowest overall operating cost based on the CI entered
6. LNAV — Cruise
LNAV guidance outputs are normally great circle tracks between waypoints. For stored procedures, the FMC can command constant heading, track, or a DME arc as needed.
FMC position determination (in flight):
Uses inputs from IRS/INS, DME, VOR, and other fitted navigation systems
FMC present position = combination (average) of all IRS/INS positions
On the ground: FMC uses IRS/INS only (no DME/VOR available)
Requires position from at least one IRS/INS to function
Since inertial systems accumulate errors with time, position error builds up if on ground for extended period
If significant map error noticed after extended ground delay → realign IRS/INS and re-enter present position
LNAV accuracy: Radial error rates of less than 0.05 NM/hour are not uncommon. GPS input will further improve performance. Operator skill and careful monitoring always remain a deciding factor.
7. Descent
When a programmed arrival is entered, the FMC calculates a descent path based on procedure airspeed/altitude constraints and the End of Descent (E/D) — a waypoint with altitude and airspeed constraint coinciding with the final approach fix or runway threshold.
For VFR and non-precision approaches: FMC path built to a point 50 ft over the approach end of the runway. Crew responsible for not descending below DH without adequate visual contact.
During missed approach: LNAV guidance available to the missed approach point and altitude.
8. CDU Key Groups
CDU keys fall into three major groups:
Group
Function
Alphanumeric Keys
Characters appear in scratch pad for data entry
Function and Mode Keys
System initialization, flight planning, performance selection, active flight plan modification
Line Select Keys (LSK)
12 keys (6 each side of CRT) — transfer data from scratch pad into the selected data field; can also copy displayed data into scratch pad
Economy Cruise Mode finds the optimum balance based on the entered CI
CI modifications possible until passing Top of Descent (TOD): Long Range Cruise (LRC), selected speed cruise.
11. FMS Key Facts Summary
Parameter
Value / Fact
Nav database update cycle
28 days (matches chart revision)
FMC database copies
Two (current and next 28-day cycle)
FMC modes
Dual (normal), Single, Independent
Radial error rate
<0.05 NM/hr typical
VFR/non-precision path end
50 ft over approach end of runway
FMS inputs (navigation)
IRS/INS (minimum 1), DME, VOR, GPS (when fitted)
CI=0
Maximum range cruise (minimum fuel)
UNABLE NEXT ALT
Speed profile would violate altitude constraint in VNAV climb
Waypoint 0 (FMS)
Aircraft's present position (not operator enterable)
Master exam point: The FMC cannot detect operator errors (wrong waypoint lat/long entries). It will navigate precisely to the wrong place if given wrong data. Pre-flight cross-checking of all waypoints against the flight plan is essential.
