In the second part of the Triple Seven flight control series, we will understand the elevator mechanism. Before looking at the elevator, it's important to understand the control column function of the aircraft. The control column's backward and forward motion is used to control the elevator. The columns pivot on their respective torque tubes. When the pilot flying the aircraft moves the control column to change the pitch attitude, the other pilot should get an indication of the column movement. Therefore, the two columns are connected using a breakout mechanism.

The breakout mechanism ensures when one column is moved, the other moves along with it. However, if one of the control columns gets jammed, the pitch control of the aircraft will be compromised. In this situation, applying sufficient force on the other column will cause the breakout mechanism to disengage, releasing the column for free movement. After a pitch command, when released, the column must return to its neutral position. This is done by the elevator field units connected to the column torque tubes.

When the column was moved, work was done against the spring in the field units. Releasing the column causes the spring to return it to neutral. Now, let's see how the column controls the elevator. Column movement results in movement of the connecting rods of the elevator fuel unit. Position transducers measure the displacement and send an analog signal to the actuator control electronics (ACE). The ACE converts it into a digital signal and sends it to the primary flight computer (PFC).

The PFC calculates the final elevator position and informs the ACE. The ACE sends a signal to the elevator power control units (PCUs). There are four PCUs, two for each elevator, mounted on the rear spar of the horizontal stabilizer. The PCU is an electrically controlled hydraulic actuator. PCUs use different hydraulic systems of the aircraft for redundancy. The ACE signal causes the PCU actuators to move the elevator in the commanded direction. Position transducers on the PCU give the elevator position feedback. The computers, with the help of feedback signals, ensure the elevator has deflected exactly to the desired position, allowing precise control of the elevator.

When the autopilot system is engaged, it controls the pitch of the aircraft. The autopilot computer, to give a pitch command, has to send a signal to the primary flight computer (PFC). The PFC will calculate the elevator deflection for the pitch change requested by the autopilot computer and send the signal to the ACE. The rest of the function is similar to a manual command. The ACE will move the elevator PCUs as per PFC instructions. However, the PFC does one more important calculation when there is an autopilot-requested pitch change. It informs the autopilot computer to move the control columns to match the elevator deflection.

The autopilot computer controls the backdrive actuators connected to the column torque tubes. First, the computer engages the clutch on the actuator and then runs the motor, moving the control column. Moving the control column with the backdrive actuator gives an indication to the pilots whenever the autopilot system is changing the pitch of the aircraft. Once the column reaches the position to match the elevator deflection, the motors are stopped. After the pitch command is over, the autopilot disengages the actuator and the elevator fuel unit returns the column back to neutral.