Autoflight Practice

1. Automatic Flight Control Systems (AFCS): General Idea

An Automatic Flight Control System (AFCS) is designed to control the aircraft automatically while still behaving in a way that feels predictable and safe to the pilot.

At the heart of AFCS design is a fundamental trade-off between two opposing characteristics:

Stability

The tendency of the aircraft to remain on a given flight path
Minimises oscillations and sudden movements
Provides a smooth, comfortable ride

Controllability

How quickly and accurately the aircraft responds to control inputs
Allows the system to track commands such as altitude, heading, or vertical speed

Because:

High stability → slower response
High responsiveness → reduced stability

…the AFCS must continuously manage this balance. This balance defines the man–machine interface of autoflight: the system must feel stable and capable of accurately following the desired flight path.

2. Stability vs Controllability and the Effect of Speed

Aircraft operate over a very wide speed range, which strongly affects control behaviour:

Low speed
- Control surfaces are less effective
- Aircraft response is sluggish
High speed
- Control surfaces are very powerful
- Small deflections can cause large responses and structural loads

Variable Gain in AFCS Feedback Loops

To handle this, modern AFCS use feedback loops with variable gain.

Increased Gain

Faster response to deviations
Reduced stability margin
Useful at low speed to improve control effectiveness

Decreased Gain

Increased stability
Slower, smoother responses
Useful at high speed to:
- Avoid overly sharp manoeuvres
- Protect the airframe from overstress

In practice, AFCS computers continuously adjust gain based on:

Flight phase
Aircraft configuration
Density altitude
Speed

The system also compensates for changes in longitudinal stability caused by:

Flap extension
Slat deployment

3. Control Laws

The rules governing how the autopilot commands the aircraft are known as control laws.

In AFCS, control laws define:

Maximum control surface deflections
How the system responds to deviations
Pitch and roll limits the autopilot will not exceed

Control laws ensure that:

The aircraft responds appropriately for its current speed
Structural and aerodynamic limits are respected
The system remains predictable and safe

The term control laws is also used in manual fly-by-wire systems (for example, Airbus aircraft). Although applied differently, the purpose is the same:

To provide correct control inputs for the current airspeed, configuration, mass, and centre of gravity.

4. Closed-Loop Control System Elements (AFCS)

According to the EASA learning objectives, an AFCS is a closed-loop control system.

It contains the following elements:

Input signal
- Desired state (e.g. selected altitude or heading)
Error detector
- Compares desired state with actual state
Signal processor
- Applies control laws and gain
Control element
- Actuator or servo driving the control surface
Feedback signal
- Aircraft response fed back to the error detector

This continuous loop allows the AFCS to:

Detect deviations
Correct them automatically
Maintain the desired flight path

5. Key Points to Remember (Exam-Focused)

AFCS design is always a compromise between stability and controllability
Feedback loop gain is adjusted continuously:
- High gain at low speed → better control authority, less stability
- Low gain at high speed → more stability, structural protection
Control laws define:
- System response
- Control limits
- Pitch and roll boundaries
A closed-loop control system uses:
- Input
- Error detection
- Signal processing
- Actuator
- Feedback

1. Purpose of Autothrust

Autothrust (A/THR) or Autothrottle (A/T) is designed to automatically control engine thrust in order to maintain a selected flight parameter.

Its main purposes are to:

Maintain speed or thrust as required by the active flight mode
Reduce pilot workload, especially during high-workload phases and when hand-flying
Provide consistent, predictable thrust management

Terminology differs by manufacturer:

Boeing → Autothrottle (A/T)
Airbus (and others) → Autothrust (A/THR)

Functionally, the systems achieve the same goal.

2. What Autothrust Controls

Autothrust can operate in two basic control philosophies, depending on which system (pitch or thrust) is controlling speed.

Thrust Mode

Autothrust sets and holds a fixed thrust level
Typical thrust references:
- TO/GA
- CLB
- MCT / CON
- IDLE
Used when the vertical mode controls speed

Examples

LVL CHG
VNAV SPD
Airbus OP CLB / OP DES

Here:

Pitch → controls speed
Autothrust → controls thrust

Speed Mode

Autothrust varies thrust to maintain a selected speed
Speed target comes from:
- MCP
- FMC / FMGS

Used when the vertical mode controls flight path

Examples

ALT HOLD
V/S
G/S
VNAV PTH

Here:

Pitch → controls flight path
Autothrust → controls speed

Engine Limits and Protection

Regardless of mode:

Autothrust commands N1 (or EPR on some engines)
Commands are limited by:
- FMC-computed thrust limits
- Engine control logic (FADEC, PMC, ECU)
Engine computers always prevent exceedance of certified limits (“low-wins logic”)

3. Key Autothrust Modes (EASA-Relevant)

The following thrust modes are commonly examined:

TO/GA – Take-off and go-around thrust
THR CLB / THR MCT / N1 / EPR / THR HOLD
- Fixed thrust modes
- Used for climb or maximum continuous thrust
SPEED / MCP SPD
- Variable thrust to maintain speed
THR IDLE / RETARD / ARM
- Idle thrust for descent and landing
Landing
- Typically RETARD or THR IDLE during the flare (type-dependent)

Mode Indications

Pre-EFIS aircraft → Thrust Mode Annunciator (TMA)
EFIS aircraft → Engine display + Flight Mode Annunciator (FMA)

➡️ The FMA is the primary reference for the active autothrust mode

4. Variants of Autothrust Systems

There are two main design philosophies.

Moving Thrust Levers (Boeing-style)

Thrust levers are motorised
Lever position always reflects commanded thrust
Mode selection via MCP
TOGA switches located on or under the thrust levers
Pressing TOGA physically drives levers to the TOGA position

Fixed Thrust Levers (Airbus-style)

Thrust levers are not motorised
Levers placed in detents:
- TOGA
- FLEX/MCT
- CLB
Lever position itself determines thrust mode
No TOGA switches:
- TOGA is commanded by moving levers to the TOGA detent

5. N1 Thrust Mode Operation

N1 mode commands and maintains the N1 limit calculated by the FMC.

Thrust Limit Management

Active thrust limit is:
- Selected on the CDU N1 LIMIT / REF page
- Displayed above the N1 gauges
Typical limits:
- TO
- CLB
- CON / MCT

Example Sequence

Before take-off
- FMC computes take-off N1 (possibly reduced via assumed temperature)
- Pressing TOGA → A/T enters N1 mode and sets this value
After acceleration altitude
- Selecting N1 on MCP → active limit changes to CLB (e.g. ~89% N1)
After engine failure
- CON/MCT selected to provide maximum continuous thrust for engine-out climb

Even if a higher value is selected:

FADEC / PMC logic prevents engine exceedance

6. Engine Control Unit / PMC (Non-FADEC Systems)

On older engines:

Fuel flow is primarily controlled by a hydro-mechanical ECU
A Power Management Controller (PMC) may be added

PMC Functions

Fine-tunes fuel flow electronically
Maintains accurate N1
Provides limited exceedance protection
Allows automatic climb thrust without lever movement

Failure Case

If PMC fails:
- ECU reverts to manual, lever-based control
- No automatic thrust limit protection
- Pilot must manually avoid exceedances

7. Autopilot–Autothrust Interaction (Critical Concept)

Autopilot and autothrust must always be interpreted together.

Vertical pitch mode determines:
- Whether pitch controls speed or flight path
Autothrust mode complements this:
- Thrust mode or speed mode

Key Rule to Remember

One system controls speed, the other controls flight path — never both at the same time

Understanding this interaction is essential for:

Mode awareness
Correct aircraft energy management
Avoiding unexpected pitch or thrust changes

8. Key Exam Takeaways

Autothrust automatically controls engine thrust to reduce workload
Two operating philosophies:
- Thrust mode → fixed thrust
- Speed mode → variable thrust
Active mode is always confirmed on the FMA
Two design philosophies:
- Moving levers (Boeing-style)
- Fixed detent levers (Airbus-style)
N1/EPR limits are calculated by the FMC and protected by engine logic
Autopilot pitch mode and autothrust mode must always be analysed together