Fms Autoflight

✈️ FMS Cooperation with Autoflight Systems

Big Picture

The FMS computes the flight path. The autoflight systems execute it.

FMS = brains (planning & guidance) Autoflight = muscles (flying the aircraft)

Systems Involved

Autoflight Systems

• Autopilot (AP) – flies pitch/roll
• Flight Director (FD) – gives guidance bars
• Autothrottle / Autothrust – controls thrust for speed or path

Data Link

• Continuous exchange between:

  • FMS computers
  • Autoflight computers
  • Displays (FMA on PFD)

How They Cooperate (Generic Flow)

1. Crew programs FMS

   • Route, SID/STAR, constraints
   • Weights, speeds, CI

2. FMS computes

   • Lateral path (LNAV)
   • Vertical path (VNAV)
   • Speed targets
   • Thrust requirements

3. Pilot selects modes

   • Via MCP (Boeing) or FCU (Airbus)

4. Autoflight executes

   • AP follows pitch/roll commands
   • A/T or A/THR sets thrust
   • FD shows guidance if AP off

5. Crew monitors

   • FMA confirms who is controlling what
   • Pilot intervenes if needed

Boeing Cooperation Logic

Boeing philosophy: Pilot commands

• FMS provides targets and guidance

• Pilot must:

  • Set altitude, speed, heading in MCP
  • Select LNAV / VNAV / FLCH

• Autoflight follows MCP + FMS logic

Key point: 👉 The aircraft obeys the windows first, FMS second.

Example:

• VNAV active → FMC controls profile
• FLCH selected → FMC vertical path ignored

Airbus Cooperation Logic

Airbus philosophy: Pilot manages

• FMS directly commands guidance

• Pilot chooses:

  • Managed (push) → FMGS controls
  • Selected (pull) → pilot overrides

• Autoflight closely integrated with FMGS

Key point: 👉 In managed modes, FMGS drives AP and A/THR directly.

Speed & Thrust Cooperation

Boeing

• Autothrottle moves thrust levers

• Speed from:

  • MCP window or
  • FMC (VNAV)

Airbus

• Autothrust active in detents
• Speed controlled electronically
• Thrust levers do not move

FMA — The Contract with the Pilot

On both aircraft, the Flight Mode Annunciator tells you:

• Who controls lateral
• Who controls vertical
• Who controls speed/thrust

👉 If it’s not on the FMA, it’s not happening.

Failure & Degradation Cooperation

Boeing

• Loss of FMS → revert to basic modes
• Pilot authority unchanged

Airbus

• Loss of FMGS or sensors → law degradation
• Managed modes lost, selected modes remain

Exam Summary Table

The FMS computes the flight path, and the autoflight systems execute it; Boeing requires pilot-commanded targets, while Airbus allows the FMS to manage guidance directly in managed modes.