Fms

✈️ Flight Management System (FMS / FMGS) — Short Review

1. What the FMS Is

The FMS (Boeing) or FMGS (Airbus) is the central navigation and flight-path management system. It manages the aircraft’s lateral and vertical path from shortly after take-off to landing, with minimal crew input.

2. Main Purposes

The FMS:

• Determines aircraft position (IRS + GNSS + radio updating)

• Guides the aircraft in:

  • LNAV (lateral navigation)
  • VNAV (vertical navigation)

• Optimises performance and fuel

• Displays navigation/performance data on PFD/ND

• Allows crew to:

  • Build/modify routes (SID, STAR, approach, holds)
  • Select automation modes
  • Override with heading, speed, altitude, V/S via MCP/FCU

3. Main Components (Boeing / Airbus)

• FMC / FMGC – Flight management computer
• CDU / MCDU – Crew interface
• MCP / FCU – Mode selection (selected vs managed)
• AFDS – Autopilot + Flight Director
• A/T or A/THR – Speed/thrust control
• PFD / ND – Displays

Typical installation:

• Dual FMCs (mandatory for RNP)
• Cross-talk between FMCs
• Integrated with autopilot and autothrust

4. Inputs to the FMS

The FMS uses data from:

• IRS / ADIRS (position, attitude)
• ADCs (airspeed, altitude, temperature)
• GNSS
• Radio NAVAIDs
• Fuel & configuration sensors
• Aircraft clock (time accuracy for ACARS, FDR)

5. Pre-Flight Setup (Logic Flow)

Via CDU / MCDU:

1. Who am I? → IDENT (aircraft, engines, nav database)

2. Where am I? → POS INIT (alignment / GPS position)

3. Where am I going? → ROUTE INIT (departure, destination, alternate)

4. How will I get there? → SID, route, STAR, approach (DEP/ARR + ROUTE)

5. When will I get there? → PERF INIT (weights, CI, cruise FL, winds, TO data)

Then:

• ACTIVATE route
• EXEC to confirm
• Cross-check vs flight plan

6. In-Flight Operation

• FMS computes:

  • Optimum vertical profile
  • Fuel and ETA predictions

• LNAV/VNAV (or Airbus managed modes) can be engaged

• Crew monitors and intervenes when required

7. Integration with Other Systems

FMS exchanges data with:

• Autopilot / Flight Director
• Autothrust
• EFIS (PFD/ND)
• EICAS / ECAM
• ACARS, FDR, maintenance systems

8. Automation & Human Factors

Advantages:

• Accurate navigation
• Reduced workload
• Precise SID/STAR and constraint compliance
• Enables RNP and autoland

Risks:

• Mode confusion
• Automation complacency
• Reduced situational awareness

Golden rule: 👉 Always know what the FMS is doing now and what it will do next.

The FMS computes and manages the aircraft’s lateral and vertical flight path, optimising performance while allowing crew supervision and intervention.

✈️ Boeing vs Airbus Automation Philosophy

Core Philosophy (Big Picture)

Boeing

👉 Pilot in command of the aircraft

• Automation is a tool to assist
• Pilot normally commands the aircraft
• Aircraft does what the pilot selects

Airbus

👉 Aircraft in command within limits

• Automation is a manager
• Pilot requests, aircraft decides
• Aircraft protects the flight envelope

Flight Path Control

Boeing

• Pilot selects targets (speed, altitude, heading)
• FMS advises and flies if commanded
• LNAV/VNAV are optional tools
• Vertical path can be overridden easily

Airbus

• Managed modes follow FMGS

• Pilot selects managed or selected

• Aircraft maintains profile unless overridden

• Clear distinction between:

  • Managed (FMGS controls)
  • Selected (pilot controls)

Mode Awareness & Indication

Boeing

• Mode logic is generally simpler
• Fewer mode transitions
• Pilot sets targets explicitly

Airbus

• More automation states
• Greater reliance on FMA monitoring
• Understanding what mode you’re in is critical

Flight Envelope Protection

Boeing

• Warnings only
• Pilot can exceed limits
• Pilot always has full authority

Airbus

• Hard protections (normal law)

• Prevents:

  • Stall
  • Overspeed
  • Excessive bank / pitch

• Pilot inputs are filtered

Autothrust / Autothrottle

Boeing

• Autothrottle moves thrust levers
• Pilot can see thrust changes physically
• Thrust mode changes are intuitive

Airbus

• Autothrust does not move levers
• Levers set to detents (CL, TOGA)
• FMGS controls thrust electronically

Pilot Interaction Style

Boeing

• “I fly the airplane”
• Automation supports pilot decisions
• Manual intervention is straightforward

Airbus

• “I manage the airplane”
• Automation flies the profile
• Pilot supervises and intervenes as needed

Failure Philosophy

Boeing

• Degradation leads toward manual flying
• Pilot authority remains unchanged

Airbus

• Laws degrade:

  • Normal → Alternate → Direct

• Protections are gradually removed

Exam Comparison Table

One-Line Exam Memory Hooks

• Boeing: “Pilot flies, automation assists”
• Airbus: “Pilot manages, automation flies”

✈️ Boeing vs Airbus — FMS Pages Overview

Naming Convention

• Boeing: CDU pages (often multiple FMCs)
• Airbus: MCDU pages (FMGS-centric)

CORE PHILOSOPHY (Pages)

• Boeing: Pages are task-specific and more distributed
• Airbus: Pages are structured and sequential (INIT → F-PLN → PERF)

IDENTIFICATION & STATUS

Boeing

• IDENT – Aircraft model, engines, nav database

Airbus

• IDENT – Same purpose (aircraft & database)

👉 Same function, same exam meaning

POSITION INITIALISATION

Boeing

• POS INIT – IRS alignment and present position

Airbus

• INIT A – IRS alignment and initial position

👉 Different names, identical purpose

ROUTE CREATION

Boeing

• RTE – Origin, destination, en-route waypoints
• DEP/ARR – Runway, SID, STAR, approach

Airbus

• F-PLN – Full route displayed as one continuous plan
• DEP/ARR – Runway, SID, STAR, approach

👉 Boeing = route split across pages 👉 Airbus = route shown as a single flow

PERFORMANCE INITIALISATION

Boeing

• PERF INIT – Weights, reserves, CI, cruise level
• TAKEOFF / APPROACH REF – Speeds, flap settings

Airbus

• INIT B – Weights, fuel, reserves
• PERF – TO, CLB, CRZ, DES, APPR pages

👉 Airbus performance is phase-based 👉 Boeing performance is page-based

LEGS / WAYPOINT MANAGEMENT

Boeing

• LEGS – Waypoint-by-waypoint sequencing
• Manual intervention common (direct-to, deletes)

Airbus

• F-PLN – Same page used for:

  • Waypoints
  • Constraints
  • Direct-to

👉 Boeing: LEGS page = heart of navigation 👉 Airbus: F-PLN page = heart of navigation

PROGRESS & MONITORING

Boeing

• PROG – Position, ETA, fuel, waypoint progress

Airbus

• PROG – Same function

👉 Identical role

SECONDARY / SUPPORT PAGES

Boeing

• VNAV – Vertical profile details
• FIX – Radials, distance rings
• HOLD – Holding patterns

Airbus

• RAD NAV – Manual radio tuning
• FIX INFO – Fix rings, radials
• HOLD – Holding patterns

KEY EXAM COMPARISON TABLE

ONE-LINE MEMORY HOOKS (EXAM GOLD)

• Boeing: “Many pages, pilot builds the route step by step”
• Airbus: “Few core pages, system manages the flight plan flow”

✈️ Boeing vs Airbus — Modes Overview

BIG PHILOSOPHY REMINDER

• Boeing: Pilot sets targets → aircraft follows
• Airbus: Pilot chooses managed or selected → aircraft decides

LATERAL MODES (Direction)

Boeing

• HDG SEL – Fly selected heading
• LNAV – Follow FMC lateral route
• LOC / VOR – Radio navigation
• ROL – Basic roll hold (fallback)

Airbus

• HDG (Selected) – Fly selected heading (knob pull)
• NAV (Managed) – Follow FMGS route (knob push)
• LOC – Localizer
• RWY / RWY TRK – Initial guidance after takeoff

VERTICAL MODES (Path)

Boeing

• VNAV – FMC vertical profile (climb/cruise/descent)
• FLCH – Pitch for speed, thrust for climb/descent
• V/S – Selected vertical speed
• ALT HOLD / ALT ACQ – Level-off modes

Airbus

• CLB / DES (Managed) – FMGS vertical profile
• OP CLB / OP DES (Selected) – Pitch for speed, ignore constraints
• V/S – FPA – Vertical speed or flight path angle
• ALT / ALT* – Altitude hold/acquire

SPEED / ENERGY MODES

Boeing

• Speed set by:

  • MCP window or
  • FMC (VNAV)

• Autothrottle moves thrust levers

• Thrust modes shown on FMA

Airbus

• Managed speed (FMGS) or Selected speed
• Autothrust active in detents
• Thrust levers do not move

APPROACH MODES

Boeing

• APP – Arms LOC + GS
• VNAV PATH – For non-precision approaches
• FLARE / ROLLOUT – Autoland (if equipped)

Airbus

• APPR – Arms LOC + GS
• FINAL APP – Managed non-precision
• FLARE / ROLLOUT – Autoland

MODE SELECTION INTERFACE

Boeing (MCP)

• Windows always active

• Pilot selects:

  • Heading
  • Speed
  • Altitude

• Then selects the mode

Airbus (FCU)

• Push = Managed
• Pull = Selected
• Windows may be dashed (managed)

PROTECTION PHILOSOPHY

Boeing

• No hard limits
• System warns
• Pilot can override everything

Airbus

• Envelope protections (Normal Law)
• Inputs are limited for safety
• Protections degrade with failures

QUICK EXAM COMPARISON TABLE

ONE-LINE MEMORY HOOKS (EXAM GOLD)

• Boeing: “Select a target, then choose a mode”
• Airbus: “Push to manage, pull to command”

✈️ Boeing vs Airbus — FMS Components Overview

Core Philosophy Reminder

• Boeing: Pilot commands the system
• Airbus: Pilot manages the system

CENTRAL COMPUTERS (Brains)

Boeing

• FMC – Flight Management Computer

• Calculates:

  • lateral route (LNAV)
  • vertical profile (VNAV)
  • fuel & performance

• Usually dual FMCs with cross-talk

Airbus

• FMGC – Flight Management & Guidance Computer

• Combines:

  • flight management
  • guidance laws

• Usually dual FMGCs

👉 Same role, Airbus integrates guidance more tightly

PILOT INTERFACE (Input)

Boeing

• CDU – Control Display Unit

• Used to:

  • enter route
  • enter performance
  • modify waypoints

• Works with FMC logic

Airbus

• MCDU – Multi-function Control Display Unit

• Used to:

  • initialise FMGS
  • manage F-PLN and PERF

• Strongly structured workflow

👉 CDU = pilot-driven 👉 MCDU = system-driven

MODE SELECTION (Autoflight Control)

Boeing

• MCP – Mode Control Panel

• Pilot sets:

  • speed
  • heading
  • altitude

• Then selects mode (LNAV, VNAV, FLCH)

Airbus

• FCU – Flight Control Unit

• Pilot:

  • push = managed
  • pull = selected

• Windows may be dashed (managed)

👉 Boeing: windows always command 👉 Airbus: windows may be advisory

AUTOPILOT & GUIDANCE

Boeing

• AFDS (Autopilot + Flight Director)
• Executes FMC commands if selected
• Pilot can override instantly

Airbus

• AP / FD integrated with FMGC
• Guidance laws apply protections
• Degrades through flight control laws

SPEED & THRUST CONTROL

Boeing

• Autothrottle (A/T)
• Moves thrust levers
• Speed controlled via MCP or FMC

Airbus

• Autothrust (A/THR)
• Thrust levers fixed in detents
• Speed controlled electronically

DISPLAY SYSTEMS

Both

• PFD – attitude, speed, altitude, FMA
• ND – route, waypoints, constraints, weather

👉 FMA is critical on both — more so on Airbus

SUPPORTING INPUT SYSTEMS (Both)

• IRS / ADIRS – position & attitude
• ADCs – airspeed & altitude
• GNSS
• Radio NAVAIDs
• Fuel & configuration sensors
• Aircraft clock (time reference)

EXAM COMPARISON TABLE (Components)

ONE-LINE MEMORY HOOKS (EXAM GOLD)

• Boeing: “Pilot commands computers”
• Airbus: “Pilot manages computers”