Autopilot Autothrust Flight Guidance
ATPL Instrumentation / Flight Guidance Systems
🧭 1️⃣ The Big Picture
Modern airliners have two major automatic control systems working together:
| System | Controls | Example |
|---|---|---|
| Autopilot (A/P) | Flight path (attitude, pitch, roll) | Keeps the aircraft flying the correct path |
| Autothrust (A/THR or A/T) | Engine thrust | Adjusts engine power to maintain speed or performance |
These two systems work in coordination to control the energy state of the aircraft:
- Pitch controls potential energy (altitude).
- Thrust controls kinetic energy (speed).
⚙️ 2️⃣ Energy Management Relationship
In simple terms:
- Autopilot pitch modes manage the aircraft’s vertical path (attitude or altitude).
- Autothrust modes manage the airspeed.
However, in different modes, their relationship changes — sometimes pitch controls speed and sometimes thrust controls speed.
✈️ 3️⃣ Typical Mode Pairings and Interactions
| Situation / Mode | Autopilot (Pitch Mode) | Autothrust Mode | Interaction / Control Logic |
|---|---|---|---|
| Climb (e.g., CLB) | Flight Path Angle / Vertical Speed / VNAV PATH | THR CLB / MCP SPD / MAN THR | Thrust sets climb power; pitch adjusts for target speed or climb rate. |
| Cruise (ALT HOLD) | Holds altitude | Speed (A/THR) | Pitch maintains altitude; autothrust maintains selected speed. |
| Descent (VNAV / FPA) | Follows descent path | Speed (A/THR) | Pitch controls path; thrust adjusts to maintain speed (typically idle descent). |
| Approach (G/S mode) | Follows glideslope | Speed (A/THR) | Autopilot controls vertical path; autothrust maintains approach speed. |
| Go-Around | Pitch to go-around attitude (TO/GA) | TO/GA thrust | Both systems engage TO/GA logic — thrust increases, pitch follows GA profile. |
🔄 4️⃣ How They Interact When You Select a Mode
When you select a new pitch mode, the autothrust may automatically change to complement it, and vice versa. They always work as a coordinated pair.
| You select… | System response | Result |
|---|---|---|
| A/P Pitch Mode → ALT HOLD | A/THR switches to SPEED mode | Pitch now holds altitude; thrust maintains speed. |
| A/P Pitch Mode → VS (Vertical Speed) | A/THR remains in SPEED | Pitch varies to achieve VS; A/THR keeps target speed. |
| A/THR Mode → THR CLB (climb thrust) | A/P switches to speed-based pitch mode (e.g., SPD or VNAV SPD) | Thrust set to climb; pitch controls speed. |
| You disengage A/THR (manual thrust) | A/P still controls pitch (altitude/path) | Pilot manually maintains speed using thrust levers. |
🧠 5️⃣ Key Rule — “One Controls Speed, the Other Controls Path”
- When autothrust is active (SPEED mode) → ➜ Pitch controls the vertical path (altitude, descent, glide path).
- When autothrust is fixed (THR or MAN) → ➜ Pitch now controls speed (since thrust is constant).
✅ This concept is sometimes called the Pitch–Power Couple or Energy Share Principle.
🧩 6️⃣ Example: Airbus A320 (for context)
| Flight Phase | Autopilot Vertical Mode | Autothrust Mode | Interaction |
|---|---|---|---|
| Climb | SRS / CLB | THR CLB | Pitch maintains speed, thrust provides climb power |
| Cruise | ALT / ALT* | SPEED | Pitch holds altitude, autothrust maintains Mach/IAS |
| Descent | DES / OP DES | IDLE / SPEED | Pitch maintains speed/path, thrust reduces to idle |
| Approach | G/S | SPEED | Pitch follows glideslope, autothrust maintains VAPP |
✈️ 7️⃣ In Summary
| Control Axis | Primary Function | Controlled By |
|---|---|---|
| Pitch | Flight path / vertical profile | Autopilot (A/P) |
| Thrust | Airspeed / energy | Autothrust (A/THR) |
| When thrust fixed | Pitch maintains speed | (Pilot or A/P adjusts pitch) |
| When speed fixed | Thrust maintains speed | (A/THR adjusts power) |
💬 8️⃣ Quick Analogy
Think of flying as balancing energy:
- The autopilot tilts the nose to trade altitude for speed.
- The autothrust adds or removes total energy. Both systems continuously coordinate to keep the aircraft on target — one handling where you point, the other how fast you go.