Flight Controls GENERAL definitions and Explanations.

021 Aircraft General Knowledge theory known as airframe and systems, electrics, powerplant & emergency equipment.
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Flight Controls GENERAL definitions and Explanations.

Postby CaptainMFT » 12 Jul 2017 02:57

:!: Power Assisted Flight Controls: 

In this system hydraulic power will assist the pilot in moving the controls. 

The pilot will still get feed back of the air loads imposed on the control. 

In the event of hydraulic failure the pilot can still operate the control. 

Manual and power assisted flying controls are known as reversible flying controls. 

Aircraft with reversible flight controls should be fitted with a gust lock (flying control lock). 

:!: Powered Flying Controls: 

In a fully powered system there is no physical connection between the pilot's controls and the control surfaces and therefore there is no feedback of control forces to the pilot. 

This type of powered control system is known as an irreversible control and requires some form of artificial feel. 

:!: Artificial Feel: 

The simplest artificial feel system is spring feel. 

Sophisticated systems creates an artificial feel that is proportional to EAS squared. Artificial feel systems are often known as "Q" (the dynamic force element) feel systems. 

All artificial feel systems operate in parallel to the flying controls. 

Some systems can modify this to give lower control forces at high speed (feel proportional only to EAS) or high control forces at high speed (control forces proportional to EAS^3). 

In the pitch control system 'g' feel is sometimes used which increases control forces as the load factor increases. 

Artificial feel may also be referred to as a 'force gradient unit'. 

:!: Gear Change: 

Gear changing or ratio changing in a control system is an attempt to match the need for large control deflections at low speed with the need to safeguard the aircraft from overstress at high speed. A full cockpit control deflection gives full control deflection at low speed. but restricted control deflection at high speed. e.g. rudder system. 

:!: Datum Shift: 

Datum shift is an adjustment of the relationship between the control column range of movement and the control range of movement to make the maximum deflection available in the desired sense. 

:!: Power Control Unit (PCU) or The Servo Control Unit: 

It is used to actuate powered flying controls. 

PCUs consist of a balanced double acting actuator. 

Body of the actuator is attached to the flying control surface. 

Input and the ram is attached to the structure of the aircraft. 

Moving the selector (by pilot) will move the spool valve, opening one side of the actuator to pressure and the other to return. 

As the piston is fixed to the aircraft structure, the body of the actuator will move (same direction as the spool valve). 

Since body of the actuator is attached to the flying control surface it will move the control surface. 

At the same time the spool valve will be centered, holding the control surface in its new position. 

Incase of a hydraulic failure the interconnecting valve drops to break the actuator hydraulic lock, allowing manual controls through the spool valve. 

:!: Fly-By–Wire Systems 

Fly-By-Wire Systems remove the direct linkage from the control wheel to the PCU. This is an irreversible control system. 

Control Runs 

e.g. The control wheel is turned to give a roll input. 

Force of the roll application and the displacement of the control wheel are sensed by transducers, which convert these inputs into analog electrical signals. 

The analog signals are sent to the Actuator Control Electronics (ACE). 

The ACE takes these inputs plus inputs from other aircraft systems and computes a required control deflection and sends digital electronic signals to the PCUs. 

The PCUs operate hydraulically to move the control surface and simultaneously send a position feedback signal to the ACE. 

When the position feedback signal equals the commanded position the ACE stops the PCU command. 

Autopilot Control 

The autopilot acts with the Primary Flight Computer (PFC) to send signals directly to the ACE. 

The ACE computes the commands, for the PCUs. 

The autopilot and PFC commands are also transmitted to backdrive actuators, DC electric motors, which move the control wheels to follow up the autopilot commands. 

Safety Features 

A breakout mechanism is provided to allow the other pilot to control the aircraft if a control wheel jams. 

All actuators are duplicated. 

To allow for redundancy and error monitoring there are three autopilot computers, three primary flight computers and four ACEs. 

The system will also incorporate degraded operation modes, to allow limited use with software failures. 

:!: Load Limiting Devices: 

These devices will prevent extension of a control surface above its limiting speeds. It will auto-retract the flaps and slats at speeds above the limit. 

:!: Alpha / Speed Locks: 

Retraction of the leading edge slats is inhibited if the aircraft is at high alpha or below a minimum airspeed. This system is known as Alpha or Speed Lock.


Good luck,

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