How Flight Controls Work | Part 2 : Elevator

Main ideas / concepts covered

1. Role of the elevator and control column

   • The control column is the pilot’s interface for commanding the aircraft’s pitch attitude.
   • Backward/forward movement of the control column is used to control the elevator.
   • Each control column pivots on its torque tube.

2. Two control columns: coordination via a breakout mechanism

   • When the pilot flying moves a control column, the other pilot must see matching column movement.
   • The two columns are connected using a breakout mechanism so that:
     • If one column moves, the other follows.
   • Failure condition: if a control column gets jammed, pitch control could be compromised.
   • Recovery behavior:
     • Applying sufficient force to the other (non-jammed) column can disengage the breakout mechanism.
     • The released column then becomes free to move for control.
   • After a pitch command ends and the column is released, the column must return to neutral.

3. Returning the control column to neutral using elevator feel units

   • Column neutral restoration is handled by elevator feel (field) units connected to the column torque tubes.
   • When the pilot moves the column, work is done against a spring in the feel units.
   • When released, the spring returns the column to neutral.

4. Electrical/compute chain from column movement to elevator motion

   • Column movement causes the connecting rods to move the elevator feel unit / elevator actuator linkage.
   • Position transducers measure displacement and output analog signals.
   • The ACE (Actuator Control Electronics):
     • converts analog signals to digital
     • sends them to the PFC (Primary Flight Computer).
   • The PFC:
     • calculates the final commanded elevator position
     • informs the ACE of the required target.

5. Actuation hardware: PCUs and hydraulic redundancy

   • The ACE commands elevator PCUs (Power Control Units).
   • There are four PCUs total:
     • two per elevator
     • mounted on the rear spar of the horizontal stabilizer
   • A PCU is an electrically controlled hydraulic actuator.
   • Different hydraulic systems are used for redundancy.
   • PCU position transducers provide feedback of actual elevator position.
   • Computers use feedback to ensure the elevator deflects to the exact desired position (precise control).

6. Autopilot mode: how pitch commands are translated

   • When autopilot is engaged, autopilot controls aircraft pitch by requesting an elevator change.
   • The autopilot computer sends a pitch command to the PFC.
   • The PFC computes the required elevator deflection and sends it to the ACE.
   • The ACE then commands the elevator PCUs similarly to manual control.

7. Autopilot “backdriving” the control columns

   • Additionally, when autopilot requests a pitch change:
     • the PFC informs the autopilot computer to move the control columns to match elevator deflection.
   • The autopilot computer uses backdrive actuators connected to the column torque tubes:
     • engages a clutch
     • runs a motor to move the control column
     • provides pilot indication of the elevator change
   • Once the column reaches the matching position:
     • motors stop
     • after the pitch command ends, the autopilot disengages the actuator
     • the elevator feel unit returns the column to neutral

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Methodology / process flow (step-by-step)

Manual pitch control (pilot moves control column)

• Pilot flying moves the control column:
  • column pivots on its torque tube
  • causes elevator command linkage movement
• Breakout mechanism coordinates both columns:
  • other pilot receives matching column movement
  • if one column jams, sufficient force on the other:
    • disengages breakout
    • releases the functional column for movement
• Elevator feel units:
  • spring resists movement during command
  • returns column to neutral after release
• Signal/compute/actuation chain:
  • connecting rods move linkage
  • position transducers measure displacement → output analog signal
  • ACE converts analog → digital and sends to PFC
  • PFC computes final commanded elevator position → sends target to ACE
  • ACE commands the elevator PCUs (electrically controlled hydraulic actuators)
  • PCUs move elevator to commanded direction
  • feedback transducers let computers correct for exact positioning

Autopilot pitch control (autopilot commands elevator + moves columns)

• Autopilot computer sends pitch command to PFC
• PFC computes required elevator deflection
• PFC → ACE → PCUs move elevator to requested position (with feedback control)
• PFC informs autopilot computer to move control columns to match elevator deflection:
  • autopilot computer engages clutch on backdrive actuator
  • motor backdrives column via torque tube mechanism
  • pilots see column movement aligned with commanded elevator deflection
• When matching position is reached:
  • motors stop
• When pitch command ends:
  • autopilot disengages actuator
  • feel unit returns column to neutral

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Speakers / sources featured

• No individual speakers are identified in the subtitles.
• Sources mentioned as system components:
  • Control column
  • Breakout mechanism
  • Elevator feel (field) units
  • Position transducers
  • ACE (Actuator Control Electronics)
  • PFC (Primary Flight Computer)
  • PCUs (Power Control Units)
  • Autopilot computer
  • Backdrive actuators
  • Clutch, motors, springs
• The subtitle ends with [Music], indicating background music.