Autoflight Concepts

✈️ Autoflight Concepts — Clear Review

1. AFCS / AFDS — Overall Concept

The Autoflight Control System (AFCS) uses guidance from the FMS/FMC and/or pilot-selected targets to fly the aircraft.

It controls:

• Pitch & roll → Autopilot (AP) / Flight Director (FD)

• Thrust → Autothrust / Autothrottle

• 3D flight path → from:

  • FMS (managed guidance), or
  • MCP / FCU (selected values)

👉 FMS plans the path, AFCS flies it.

2. Principle of Operation

• FMS/FMC:

  • Stores route and vertical profile

• AFCS:

  • Converts this into:

    • control surface movement
    • thrust changes

Autothrust logic:

• Cruise: adjusts thrust to maintain speed
• Climb: sets climb thrust
• Descent: sets idle or reduced thrust

Control laws:

• Use feedback loops and variable gain
• Maintain stability across speed, mass, and configuration changes

3. Autopilot & Autothrust Engagement

Interface

• MCP (Boeing) / FCU (Airbus)

Typical Boeing example (B737):

• Two APs (CMD A / CMD B)
• Engagement confirmed on FMA
• A/T armed with one switch
• A/T becomes active when a speed or vertical mode requires it

Normal rules:

• Only one AP used in normal flight
• Exception: dual AP during ILS for fail-operational autoland

4. Flight Director (FD) vs Autopilot (AP)

Flight Director:

• Displays command bars
• Receives exactly the same commands as the autopilot

Relationship:

• AP ON → aircraft follows FD commands
• AP OFF → pilot follows FD commands

👉 FD = visual cross-check of what AP is commanding

Operating protocols:

• PF selects modes as required
• PM monitors FMA and confirms correctness
• Both pilots call out mode changes
• AP should not be used with FD off (except basic systems)

5. Airbus Flight Path Concept — Managed vs Selected

Core rule

• Managed = FMGS controls
• Selected = pilot controls

FCU logic:

• Push = Managed (“you fly it”)
• Pull = Selected (“I fly it”)

Examples:

• Speed

  • Pull → selected IAS/Mach
  • Push → managed speed

• Heading

  • Pull → selected HDG
  • Push → NAV (FMGS lateral)

• Altitude

  • Pull → OP CLB / OP DES (ignore constraints)
  • Push → managed CLB / DES

• Vertical Speed

  • Pull → set V/S
  • Push → zero V/S (level off)

⚠️ Airbus V/S zero ≠ Boeing ALT HOLD

6. Human Factors & Automation Use

Key safety concepts:

• Always know:

  • Active modes
  • Armed modes
  • What the aircraft will do next

• Clear PF / PM roles

• PM performs most mode monitoring

• Use automation appropriate to workload

• Guard against:

  • mode confusion
  • complacency
  • loss of situational awareness

The AFCS converts FMS guidance or pilot-selected targets into pitch, roll, and thrust commands, with pilots responsible for mode awareness and correct automation use.

✈️ Boeing vs Airbus — Autoflight Comparison

Core Philosophy

• Boeing: Pilot commands → automation executes The aircraft flies what the pilot selects.

• Airbus: Pilot manages → automation decides The aircraft flies what the system manages, within protections.

Autoflight Architecture

Boeing

• AFDS (Autopilot + Flight Director)
• FMC computes path
• MCP = command panel
• Autoflight follows MCP windows first, FMC second

Airbus

• FMGS tightly integrated with AP/FD
• FCU = management panel
• Autoflight follows FMGS in managed modes

Mode Selection Logic

Boeing (MCP)

• Pilot:

  1. Sets speed / heading / altitude
  2. Selects a mode (LNAV, VNAV, FLCH)

• Windows are always active

• Aircraft will not change altitude or speed unless set

Airbus (FCU)

• Pilot chooses how much authority to give
• Push = Managed
• Pull = Selected
• Windows may be dashed (managed)

👉 Boeing = target driven 👉 Airbus = authority driven

Lateral Modes

Vertical Modes

Speed & Thrust Control

Boeing

• Autothrottle moves thrust levers

• Speed from:

  • MCP window, or
  • FMC (VNAV)

Airbus

• Autothrust does not move levers
• Levers fixed in detents
• Speed controlled electronically

Flight Envelope Protection

Boeing

• No hard protections
• System warns only
• Pilot can override everything

Airbus

• Hard protections (Normal Law):

  • Stall
  • Overspeed
  • Excessive bank / pitch

• Inputs are limited for safety

Flight Director Relationship

Both

• FD and AP receive identical commands
• FD = visual check of AP behaviour

Key difference

• Airbus relies more heavily on FMA discipline
• Boeing relies more on pilot-set targets

Failure & Degradation

Boeing

• Failures → simpler modes
• Pilot authority unchanged

Airbus

• Failures → law degradation
• Protections gradually removed

Human Factors Emphasis

Boeing

• Risk: forgetting to update MCP targets
• Strength: intuitive manual override

Airbus

• Risk: mode confusion (managed vs selected)
• Strength: workload reduction & protections

Exam Comparison Table (Quick)

One-Line Exam Memory Hooks

• Boeing: “Set the window, then choose the mode”
• Airbus: “Push to manage, pull to command”

✈️ Typical Climb / Descent Trap Scenarios

(Boeing vs Airbus)

1. Altitude Set but No Climb/Descent

Boeing Trap

What happens

• New clearance: “Climb FL350”
• Pilot selects VNAV
• Altitude window not changed
• Aircraft does not climb

Why

• Boeing always obeys the MCP altitude window

Avoid 👉 Set altitude first, then select the mode

Airbus Trap

What happens

• New clearance: “Climb FL350”
• Pilot sets altitude but forgets to PUSH
• Aircraft stays level

Why

• Altitude knob left in selected/armed state
• Managed CLB not engaged

Avoid 👉 Set altitude → PUSH for managed climb 👉 Confirm CLB on FMA

2. Constraints Ignored Unintentionally

Boeing Trap

What happens

• Pilot selects FLCH in climb or descent
• FMC vertical constraints ignored

Why

• FLCH = pitch for speed, not path

Avoid 👉 Use VNAV if constraints must be respected

Airbus Trap

What happens

• Pilot PULLS altitude instead of pushing
• OP CLB / OP DES
• Vertical constraints ignored

Why

• Selected mode overrides FMGS path

Avoid 👉 Push = managed 👉 Check for CLB/DES, not OP CLB/DES

3. Early Descent Below Cleared Level

Boeing Trap

What happens

• Pilot sets lower altitude early
• VNAV PATH captures TOD later
• FLCH selected early
• Aircraft starts descending immediately

Why

• FLCH obeys window, not FMC TOD

Avoid 👉 Leave VNAV engaged until TOD

Airbus Trap

What happens

• Pilot sets lower altitude and pulls
• Aircraft enters OP DES
• Starts descending before TOD

Why

• Selected descent ignores managed profile

Avoid 👉 Set altitude → PUSH, not pull 👉 Verify DES (managed) on FMA

4. Speed Control Confusion

Boeing Trap

What happens

• Pilot changes MCP speed
• VNAV SPD drops to MCP SPD
• Unexpected pitch/thrust change

Why

• Speed window overrides FMC speed

Avoid 👉 Only change MCP speed if intended 👉 Check speed mode on FMA

Airbus Trap

What happens

• Pilot pulls speed knob unintentionally
• Leaves managed speed
• Aircraft accelerates/decelerates unexpectedly

Why

• Selected speed overrides FMGS

Avoid 👉 Watch dashed vs solid speed window 👉 Confirm SPD mode on FMA

5. Vertical Speed “Chasing” the Profile

Boeing Trap

What happens

• Pilot uses V/S in descent
• High rate selected
• Energy management lost

Why

• V/S ignores path and energy

Avoid 👉 Use VNAV or FLCH unless necessary

Airbus Trap

What happens

• Pilot uses V/S to “help” managed DES
• Aircraft drifts off profile

Why

• V/S replaces managed vertical path

Avoid 👉 Use V/S sparingly 👉 Return to managed DES when possible

6. Mode Misinterpretation (FMA Not Checked)

Common Trap (Both)

What happens

• Pilot assumes aircraft is in the “right” mode
• FMA shows something else
• Aircraft does something unexpected

Avoid 👉 FMA = truth 👉 Verbal callouts after every change

EXAM GOLD — QUICK COMPARISON

One-Line Exam Memory Hooks

• Boeing: “Windows rule the airplane”
• Airbus: “Push vs Pull decides everything”
• Both: “If it’s not on the FMA, it’s not happening”

✈️ Autoland Logic (Boeing vs Airbus)

What Autoland Is

Autoland is a fully automatic landing using:

• Autopilot(s)
• Autothrottle / Autothrust
• ILS (LOC + GS)

It is designed to allow landing in low visibility and to remain safe after failures.

Basic Autoland Requirements (Both)

To perform an autoland, the aircraft must have:

• Serviceable ILS (CAT II / CAT III)

• Multiple autopilots

• Redundant:

  • radios
  • computers
  • sensors

• Correct aircraft status (no disqualifying failures)

Autoland Phases (Common Logic)

1. APP mode armed

   • LOC and GS captured

2. Dual autopilot engagement

3. LAND mode

   • Precise lateral & vertical tracking

4. FLARE

   • Automatic pitch-up

5. ROLLOUT

   • Centerline tracking after touchdown

6. Autothrust retard

   • Idle thrust at touchdown

Boeing Autoland Logic

Boeing philosophy: Pilot commands, system executes

Key Features

• Two (or three) autopilots:

  • CMD A + CMD B

• Autoland status shown on PFD

• Fail status annunciated clearly

Fail Categories

• Fail-Passive

  • One failure
  • Aircraft remains controllable
  • Manual landing required

• Fail-Operational

  • One failure tolerated
  • Autoland continues automatically

Typical Indications

• LAND 3 → Fail-operational
• LAND 2 → Fail-passive
• NO AUTOLAND → Not permitted

👉 Pilot must monitor LAND status before decision height.

Airbus Autoland Logic

Airbus philosophy: System manages within protections

Key Features

• AP1 + AP2 automatically engage
• Autoland is normal and expected
• Deep integration with flight control laws

Fail Categories

• CAT 3 DUAL

  • Fail-operational

• CAT 3 SINGLE

  • Fail-passive

• CAT 2

  • Limited redundancy

Indications

• Shown on FMA
• Protections remain active in Normal Law

👉 Airbus prioritizes automatic protection and continuity.

Boeing vs Airbus — Key Differences

Decision Height Logic (Exam Favorite)

• Fail-Operational

  • Can continue below DH

• Fail-Passive

  • Failure below alert height → go-around

• Failure above alert height

  • Autoland not permitted

Common Exam Traps

❌ Forgetting to engage both autopilots ❌ Misreading LAND status / CAT status ❌ Assuming autoland = no monitoring ❌ Ignoring alert height logic

One-Line Exam Summary

Autoland uses multiple autopilots and ILS guidance to land automatically, with Boeing emphasizing pilot-commanded redundancy and Airbus emphasizing managed protection and continuity.

✈️ Fail-Passive vs Fail-Operational (Autoland)

Big Idea (remember this first)

• Fail-passive: If something fails, the aircraft stays safe but you must take over.
• Fail-operational: If something fails, the aircraft keeps landing automatically.

Fail-Passive

What it means

• A single failure causes the automatic landing to stop.

• Aircraft remains:

  • stable
  • controllable

• Pilot must take control and land manually or go around.

Key characteristics

• No hazardous deviation after failure
• Autopilot disengages or downgrades
• Safe outcome depends on pilot action

Typical indication

• Boeing: LAND 2
• Airbus: CAT 3 SINGLE

Example

• One autopilot fails during approach
• System disconnects autoland
• Aircraft remains on a safe flight path
• Pilot continues manually or goes around

Fail-Operational

What it means

• A single failure is tolerated.
• Autoland continues to touchdown and rollout.
• No pilot intervention required.

Key characteristics

• Redundant systems automatically reconfigure
• Aircraft remains fully automatic
• Required for CAT III operations

Typical indication

• Boeing: LAND 3
• Airbus: CAT 3 DUAL

Example

• One autopilot channel fails after alert height
• Remaining systems take over
• Aircraft continues autoland normally

Alert Height Logic (Exam Favourite)

• Failure ABOVE alert height

  • Autoland not permitted
  • Pilot must intervene

• Failure BELOW alert height

  • Fail-operational: landing continues
  • Fail-passive: go-around or manual takeover

Fail-Passive vs Fail-Operational (Table)

One-Line Exam Answer

Fail-passive systems remain safe after a failure but require pilot intervention, while fail-operational systems tolerate a failure and continue the automatic landing.

✈️ Alert Height vs Decision Height

Big Picture (remember first)

• Decision Height (DH): “Do I land or go around?”
• Alert Height (AH): “Can the system still land by itself?”

They serve different purposes.

Decision Height (DH)

What it is

• A height above runway elevation

• Where the pilot must decide:

  • Continue landing, or
  • Go around

Based on

• Visual reference (runway environment)
• Aircraft & crew capability

Applies to

• CAT I
• CAT II
• CAT III

At DH

• If required visual cues are not acquired → GO-AROUND
• Pilot decision is mandatory

Alert Height (AH)

What it is

• A system monitoring height
• Used only to assess autoland system integrity

Based on

• System failures, not visual cues
• Monitors autopilots, sensors, computers

Applies to

• CAT III only

At AH

• If a critical failure occurs ABOVE AH:

  • Autoland not permitted

• If a failure occurs BELOW AH:

  • Fail-operational: landing continues
  • Fail-passive: pilot must take over / go around

⚠️ No pilot decision at AH unless a failure occurs.

Key Difference (Exam Gold)

Simple Examples

CAT II approach

• DH = 100 ft

• No alert height

• At DH:

  • No visual → go-around

CAT III fail-operational approach

• AH = 200 ft

• No DH (or DH = 0)

• Failure at:

  • 300 ft: go-around
  • 100 ft: autoland continues

Boeing & Airbus Context

• Boeing: AH tied to LAND 2 / LAND 3 logic
• Airbus: AH tied to CAT 3 SINGLE / DUAL logic

👉 Same ICAO definitions, different cockpit indications.

Common Exam Traps ❌

• ❌ “Alert height is a pilot decision height”
• ❌ “AH replaces DH”
• ❌ “Visual reference required at AH”

✔️ AH = system check ✔️ DH = pilot decision

One-Line Exam Answer

Decision Height is the height at which the pilot decides to land or go around based on visual reference, while Alert Height is a CAT III system-monitoring height used to determine whether autoland may continue after failures.