Aileron feel and centering unit

[Driver 170]

Ok a brief description

The aileron movement can be accomplished by three different systems. The most important aileron movement is done by the control wheels on the flight deck. These control wheels are connected to each other, by means of cables. This means, that if one control wheel deflects, the other will do exactly the same. Via cables, the control wheels are connected to the aileron Feel and Centring Unit, which is located in the main wheel well. This unit changes the neutral position of the control wheel after trimming and adds ‘feel’ to the control wheel. This is done so that the pilots have a feel of what kind of forces are applied to the ailerons. The Feel and Centring Unit moves a cable to the aileron PCU, which in turn converts ca-ble movements to hydraulic movements. The PCU deflects the ailerons accordingly to the control wheel input. These PCU’s are powered by hydraulic systems A and B (we'll talk about these systems later). The hydraulic systems are powered by bleed air that is generated by the engines.


So for the actuall pilots who actually fly the NG what exactly is the centring unit and what does it do to the control wheel during takeoff and in flight? I just can't understand this and how it works.

Cheers

[Exup]

AILERONS - NORMAL OPS
• when you move the aileron control, the aileron input shaft rotates and applies input to the aileron feel and centering unit (AFCU)
- the AFCU contains 2 springs which provide artificial feel
- the AFCU applies input to the 2 PCUs which move the aileron quadrants
- the aileron quadrants move the aileron wing cables
- the aileron body quadrant is connected to the aileron spring cartridge
- the spring cartridge is connected to the spoiler input shaft by a crank and 4 shear rivets
• flight spoilers are hydraulically actuated by the spoiler mixer in response to inputs from the aileron PCUs and aileron spring cartridge
• a transfer mechanism with a torsion spring is at the base of the right control column
- it supplies a mechanical link between the control wheels and lets either pilot make the roll control commands
- it incorporates both the transfer mechanism and the lost motion device

AILERON AUTOPILOT ACTUATORS
• when an A/P is engaged an FCC controls the aileron autopilot actuators
• an A/P actuator gives mechanical input to the AFCU and from there the ailerons are controlled the same as during normal ops



AILERONS - ABNORMAL OPS
• if the ailerons or spoilers are jammed, force applied to the left and the right control wheels will identify which system, ailerons or spoilers, is usable and which control wheel, left or right, can provide roll control by overpowering the torsion spring in the transfer mechanism
- the aileron spring cartridge and the transfer mechanism isolate the aileron system from the flight spoilers
• if the left control wheel cannot move, the right control wheel can be used to override the force of the torsion spring and the spring cartridge
- the lost motion device, attached to the right control wheel, engages at 12° of control wheel movement and moves the flight spoilers only
• if the right control wheel cannot move, the left wheel can be used to override the force of the torsion spring and the force of the feel and centering mechanism
- this allows control of the ailerons only
• if one aileron cannot move, the shear rivets at that quadrant will shear and isolate that aileron
- the other aileron operates normally

Hope this helps & for info the hydraulic systems are powered by engine driven pumps in normal use not bleed air.

[Yeelep]

Think of a steering column that isn't connected to anything. It will rotate either way without resistance. Now attach a V shaped ramp (cam) to the steering shaft and a spring loaded bearing to the column that rides in the valley of the V. Now when you turn the wheel the bearing rides up one side of the V which progressively increases the force to turn the wheel (feel). Let go of the wheel and it will rotate until the bearing has returned to the bottom of the V (centering). That in very basic terms is how it works. The other thing the AFCU does is input aileron trim by shifting the neutral (centered) point. This is done by using an electric actuator to rotate the bearing around the shaft, moving the neutral point.

Normal hydraulics are by two electric and two engine driven hydraulic pumps.

[Driver 170]

Great description that helps alot to visualise it :)

And the feel system for the aileron and elevator is basically for high speed so you can still move the control surfaces at high speed. Because of friction and aerodynamic forces?

[MarkerInbound]

Don't know about the NG but earlier 737s had a bit of bleed air feed to the hydraulic reservoirs to ensure positive fluid flow to the pumps. Probably where the confusion arose.