Autopilot Modes
🧭 1️⃣ What Autopilot Modes Do
An autopilot works through control loops — it can:
- Stabilize the aircraft (keep it steady), and
- Guide the aircraft (make it follow a desired path).
So we divide the functions into two groups:
| Group | Function | Controlled by |
|---|---|---|
| Basic (Stabilization) | Keeps the aircraft steady around its center of gravity | Inner loops |
| Guidance (Navigation / Flight Path Control) | Makes the aircraft follow a route or path | Outer loops |
⚙️ 2️⃣ Basic Modes — “Keep the Aircraft Stable”
These are the foundation modes that ensure the aircraft stays steady. They do not follow a specific route — they just maintain the current attitude or rate.
| Basic Mode | What It Does | Example Use |
|---|---|---|
| Pitch & Bank Hold (Attitude Hold) | Keeps the current pitch and bank angles | Level off after a climb or turn |
| Wings Level Hold | Keeps wings level (zero bank) | Used after turbulence or takeoff |
| Vertical Speed Hold | Keeps a selected climb or descent rate (ft/min) | Smooth climb or descent without changing pitch manually |
✅ These come from the inner control loops, which react quickly to keep the aircraft stable (similar to “short-term corrections”).
✈️ 3️⃣ Guidance Modes — “Make the Aircraft Go Somewhere”
These are the outer-loop functions — they tell the aircraft where to go and how to get there*. They control the flight path (altitude, heading, and speed).
A. Vertical Guidance Modes
Control motion up and down (vertical plane):
| Mode | What It Does |
|---|---|
| IAS / Mach Hold | Maintains selected speed by adjusting pitch |
| Altitude Hold (ALT HOLD) | Keeps the selected altitude |
| Vertical Speed (V/S) | Climb or descend at a fixed rate (e.g., 1000 ft/min) |
| FPA (Flight Path Angle) Hold | Keeps a selected angle of climb or descent |
| VNAV (Vertical Navigation) | Follows altitude/speed profile programmed in FMS |
| Glide Slope (GS) | Tracks ILS glide slope during approach |
B. Lateral Guidance Modes
Control motion left and right (horizontal plane):
| Mode | What It Does |
|---|---|
| Heading Hold (HDG HOLD) | Maintains current heading |
| HDG SEL / TRK SEL | Turns to and holds a selected heading or track |
| LNAV (Lateral Navigation) | Follows the lateral path from the FMS route |
| VOR / LOC Mode | Intercepts and tracks VOR radial or localizer |
| Approach Mode (APP) | Combines localizer + glide slope capture for ILS approach |
| Wings Level | Keeps the aircraft straight (no roll) |
🧩 4️⃣ Relationship Between Loops
Outer Loop → Guidance (Flight Path)
↑
│
Inner Loop → Stability (Pitch, Roll)
- Inner loop (basic modes) keeps the aircraft stable.
- Outer loop (guidance modes) commands the aircraft where to go.
The guidance loops send small attitude commands to the inner loops, which execute them precisely.
🧠 5️⃣ Simple Analogy
Think of the autopilot as a two-layer system:
| Layer | Analogy | Function |
|---|---|---|
| Inner loop (basic) | Like car suspension — keeps the ride smooth and stable | Controls pitch, roll, and bank |
| Outer loop (guidance) | Like GPS + steering — tells where to go | Controls altitude, heading, and navigation path |
✅ 6️⃣ Quick Summary Table
| Type | Axis Controlled | Typical Modes | Purpose |
|---|---|---|---|
| Basic (Inner loop) | Pitch / Roll stabilization | Pitch hold, Bank hold, V/S hold, Wings level | Keeps aircraft stable |
| Guidance (Outer loop) | Flight path control (Vertical & Lateral) | ALT, IAS, Mach, VNAV, HDG, LNAV, LOC, GS | Controls route and profile |
Would you like me to show a diagram of inner vs outer control loops (basic vs guidance modes), showing how commands flow from the pilot/FMS → autopilot → control surfaces?