Autopilot System Modes: Synchronization, TO/GA, CWS, and FMA Awareness

Autopilot Engagement & Synchronization

For a smooth transition from manual to automatic flight, the autopilot system uses a synchronization function.

• Purpose: Prevents "snatching" or jerking of controls upon engagement.
• Mechanism: Before engagement, the system checks that the aircraft is in trim and matches the autopilot's internal commands with the current position of the control surfaces/servos.
• Failure: If synchronization fails, the autopilot will typically refuse to engage.

Mixed Modes: TO/GA (Take-Off / Go-Around)

The Take-Off / Go-Around (TO/GA) mode is a combined mode involving both Autopilot and Autothrust.

• Activation: Typically activated by a switch on the throttle levers.
• Go-Around Function:
  • Disconnects Approach modes (like G/S and LOC).
  • Commands a pitch-up attitude (e.g., for 1000-2000 fpm climb).
  • Autothrust advances to Go-Around thrust (reduced or full limit).
  • Logic: The go-around mode will command a climb regardless of the missed approach altitude selected on the MCP (it demands a positive climb gradient away from the ground).

Vertical Modes

Vertical modes control the aircraft's pitch/thrust to manage altitude or vertical path.

• Vertical Speed (V/S): Maintains a specific rate of climb/descent (e.g., 1000 ft/min). Airspeed is controlled by pitch/thrust balance (potential risk of stalling if thrust is insufficient).
• Flight Path Angle (FPA): Maintains a specific geometric angle of climb/descent relative to the horizon (e.g., -3.0°). Adjusts V/S automatically if groundspeed changes.
• Level Change (LVL CHG) / Open Climb/Descent: Pitch controls airspeed, Thrust is set to Climb/Idle. Used for efficient altitude changes.
• Altitude Hold (ALT): Captures and maintains a specific barometric altitude.

Lateral Modes

Lateral modes control roll to manage heading or track.

• Heading (HDG) / Track (TRK): Follows a pilot-selected Vector.
• LNAV (Lateral Navigation): Follows the FMS flight plan (great circle route between waypoints).
• VOC/LOC: Intercepts and tracks radio navigation signals.

Control Wheel Steering (CWS)

Control Wheel Steering (CWS) is a hybrid mode available on some aircraft.

• Operation: The pilot maneuvers the aircraft manually using the yoke. When the pilot releases the pressure, the autopilot holds the existing attitude (pitch and roll).
• Annunciation: Displays "CWS P" (Pitch) and "CWS R" (Roll) on the FMA.
• Distinction: The autopilot does NOT follow internal commands in this mode; it responds to pilot pressure.

Flight Mode Annunciator (FMA) awareness

It is critical for pilots to monitor the Flight Mode Annunciator (FMA) (top of PFD).

• Undesired States: Failing to notice mode changes can lead to danger. Example: Attempting to capture a Glideslope (G/S) before capturing the Localizer (LOC) can result in a descent into unprotected airspace.
• Normal Law: In Fly-By-Wire aircraft, "Normal Law" provides full flight envelope protection. Reversion to "Direct Law" or "Alternate Law" degrades these protections.

Power Supply & Backup

• ADC Dependency: The Autopilot relies on the Air Data Computer (ADC). An ADC failure affects all associated systems (AP, FD, Autothrust).
• Disconnection: Disconnecting the AP (e.g., via instinctive cutout) triggers a temporary warning.