11.9 Flight Controls

Primary Flight Controls

• Primary controls are vital for aerodynamic force, enabling aircraft rotation across three axes, which results in changes to the direction and attitude.
• These consist of ailerons, elevators, and rudder, usually managed by a flight stick, wheel, or rudder pedals.

Ailerons

• Ailerons control rolls along the longitudinal axis.
• Located on the wings' trailing edge to leverage longitudinal movement.
• They operate in opposite directions to control lift.

Elevators

• Elevators manage movement around the lateral axis, affecting pitch for climbing or descending.
• Aft movement of the control column directs elevators upwards.
• This creates downward aerodynamic force, causing the tail to descend and the nose to ascend.

Rudder

• The rudder controls directional movement around the vertical axis.
• Controlled by pedals, when deflected, a horizontal force is exerted against airflow.
• Pushing the left pedal shifts the tail right, directing the nose left.

Trim Controls

• Trim control tabs on manually operated primary flying controls reduce crew workload.
• Trim systems stabilize the aircraft in the desired attitude by adjusting tab angles relative to control surfaces.

Types of Trim Systems

• Trim Tabs: Statically balance the aircraft by independent linkage, enabling hands-off flight.
• Servo Tabs: Linked directly to the flight control input, aerodynamically position control surfaces needing too much force.
• Balance Tabs: Coupled to the control surface linkage, assist the flight crew in moving the control surface.
• Spring Tabs: Assist to move the control surface when forces are high only during high-speed flight
• Anti-Balance Tabs: Increases the force needed by the flight crew to change flight control position.

Electrical and Hydraulic Trim Systems

• Faster modern aircraft utilize electrical and hydraulic systems for trim, moving control surfaces, without trim tabs.
• Trim adjustments are made via rocker switches, rotary knobs, and flight control computers (FCUs).
• Control surface positions are displayed on systems like EICAS and ECAM.

Lateral Trim

• Governed by a switch on the center console, electrically managed through the FCU.
• Adjusts the control wheel to maintain the trim position.

Directional (Yaw) Trim

• Controlled by a rotary knob on the center console, purely electrically controlled via the FCU.
• The EICAS displays the trimmed position.

Longitudinal Horizontal Stabilizer Trim Control

• Flight crew uses pitch trim switches on the control wheel for manual movement.
• Alternate pitch trim levers on the captain’s side offer mechanical control.
• Stabilizer position is shown on indicators, with a green band indicating correct takeoff position.

Active Load Control

Active Load Alleviation

• Modern aircraft use active load alleviation for performance, reducing costs and environmental impact.
• Wings with high aspect ratios, designed to be lightweight, cause flexibility.
• Aerodynamic loads cause wing deformations that alter aerodynamic loads.

Two Active Aeroelastic Techniques

• Maneuver Load Control (MLC): Reduces loads when maneuvering.
• Gust Load Alleviation (GLA): Reduces loads during in-flight turbulence.

Manoeuvre Load Control (MLC)

• Uses wing control surfaces to shift the center of lift.
• Reduces structural loading (bending moment).
• Ailerons deflect to reduce lift generation on the wing's outboard during gusts.

Gust Load Alleviation (GLA)

• GLA reduces gust load effects on aircraft.
• Employs control surfaces like ailerons, rudder, and elevators.
• It measures upward acceleration and compares it to commanded acceleration.

High Lift Devices

• High lift devices include trailing edge flaps, slats, leading edge flaps, and slots.
• They can be used independently or together and are managed using a lever on the flight deck.

Trailing Edge Flaps

• Commonly found on most aircraft, improve control and lift at slower speeds.
• Enables slower landings and shorter takeoff.
• Typically made of aluminum, with composites increasingly used for weight reduction.

Types of Trailing Edge Flaps

• Plain flaps: Hinge downwards to increase drag.
• Split flaps: Lower from under the wing’s trailing edge.
• Slotted flaps: Feature a slot that opens when extended.
• Fowler flaps: Extends, changing camber and increasing wing area.
• Fowler slotted flaps: Used in large aircraft.

Leading Edge Flaps

• Used with trailing edge flaps to increase lift and camber.
• Retract into the wing's leading edge.
• Made from aluminum or composite.

Types of Leading Edge Flaps

• Fixed Slots: Allows higher maximum lift
• Movable Slots: Move forward as the AOA increases.
• Leading Edge Flaps: Increases coefficient of lift and the camber of the wings.
• Leading Edge Cuffs: Extends the leading edge down and forward.
• Krueger Flaps: Rotates out in front of the main wing’s leading edge.

Lift Dump and Speed Brakes

• Spoilers and speed brakes reduce speed, assist roll, and reduce lift.
• The number of spoilers deployed varies based on function.

Spoilers

• Ground spoilers: Assist in braking during landing.
• Flight/Roll Spoilers: Assist ailerons in roll control during flight.
• Speed brakes: Reduce speed and increase descent rate during flight.

Lift Dump

• Flight/roll spoilers help the ailerons to control during roll.
• Ground spoilers are used to reduce lift immediately after landing.
• Spoiler panels also produce aerodynamic drag to help slow the aircraft.

Speed Brakes

• Speed brakes are deployed manually.
• They increase drag and reduce aircraft speed when extended.

Manually Operated Systems

• Manually operated systems use chains, cables, turnbuckles, pulleys, and control tubes.
• Common in smaller aircraft.
• Mechanical control where aerodynamic forces are not excessive.

Cable System

• The cable system is widely used, strong, and lightweight for transmitting control over distances.
• It incorporates cable assemblies, guides, linkages, and locking devices.

Control Cables

• Control cables are flexible steel wires.
• They exert a pulling force.
• Pulleys change cable direction or give support.
• Fairleads guide cables.
• Seals prevent pressure loss.

Tensiometers

• Tensiometers measure control cable tension.
• Tension is measured by the amount of force needed to make an offset in the cable between two hardened steel blocks called anvils.
• Cable rigging compensates for temperature changes.

Tension Regulators

• Cable tension regulators maintain cable tension against temperature.
• They allow the cable to be taken in and let out equally under all temperature conditions.

Push-Pull Control Rods

• Transmit forces in either direction.
• Can be solid or hollow.

Bell Cranks

• Constructed so that a series of levers can receive an input signal and deliver an output.
• Frequently change the direction of movement.

Torque Tubes

• Transmit torsional force, often actuating ailerons, flaps, and slats.

Screw Jacks

• Convert rotary motion into linear travel.

Hydraulic – Power Flying Control Systems

• Assistance to overcome aerodynamic forces to control the aircraft.
• Tabs and powered (hydraulic) control systems were designed.
• Aerodynamic loads provide feedback to the flight crew.

Hydraulic System

• Conventional cable or push-pull tube systems link flight deck controls with the hydraulic system.
• Hydraulic system redundancy is achieved by multiple systems and multiple pressure sources.

Hydraulic Power Control Units (PCUs)

• Used to position flight control surfaces.
• Combine individual hydraulic components into one assembly.
• Position the flight control surface in response to mechanical or electrical commands.
• Withstand flight loads and providing protection against flutter.

Servo Control Unit

• Divided into a power and a distribution section.
• The power section changes hydraulic pressure into mechanical force.
• Within the power section are the cylinder, piston, and piston rod.
• The distribution section includes the control valve and bypass valve.

Control or Active Mode

• The servo control unit enters the control/active mode of operation when the hydraulic pressure supply is switched on.
• Bypass valve moves when the servo control unit is pressurized
• Once the servo control unit is pressurized the bypass valve automatically moves
• Distribution control valve is connected to the valve input lever

Damping Mode

• Servo control unit is depressurized.
• Damping mode is used when the aircraft is on the ground.

Power-assisted Control

• Hydraulic power assists the pilot.
• Mechanical link to the control surface.
• Manual and power-assisted flying controls are known as reversible flying controls.
• The surface begins to deflect
• Valve input lever moves towards the neutral position

Fully Powered Control

• No physical connection between the pilot's controls and the control surfaces.
• No feedback of control forces to the pilot.

Pneumatic

• Primarily used as a backup to essential services.

Electrical

• Electronics are widely adopted.
• Multiple computers are incorporated in the control network.
• Translates physical control inputs made by the flight crew into relevant control surface deflections.

Fly-by-Wire

• Replaces manual flight controls with an electronic interface.
• The movements of flight controls are converted to electronic signals.
• Flight control computers determine how much to move the actuators.

Fly-by-Wire Operation

• Flight crew inputs are transduced into electrical signals sent to independent computers.
• Computers process data on flight conditions, servo-valves, and actuator positions.
• The data used by the system depends on the aircraft category.

Three gyroscopes

• Three gyroscopes fitted with sensors are fitted in the aircraft to sense movement changes in the pitch, roll, and yaw axes.
• Any movement results in signals being sent to the computer.

Fly-by-Wire - Safety and Redundancy

• Pre-flight safety checks are often performed using Built In Test Equipment (BITE).

Fly-by-Wire Control Modes/Laws

• Transforms the movement of the yoke or joystick, made by an aircraft flight crew, into movements of the aircraft control surfaces.

Two Commercial Passenger Aircraft Manufacturers Modes

• Normal Law: Varies at different stages of flight.
• Alternate Law: It happens after reconfiguration, and has modes identical to Normal Law.

Fly-by-Wireless

• The next generation, and provide efficiencies by removing the wires needed in fly-by-wire aircraft.

Artificial Feel

• Aircraft that move control surfaces solely by hydromechanical and/or electromechanical means deprive the flight crew of the feel of a mechanical control system.
• A feel unit is installed to provide the flight crew with an artificial feel of the air-load.

Mechanisms Used to Produce Artificial Feel

• Spring box.
• Hydraulics.

‘Q’ Pot Feel System

• Utilizes pitot pressure via a piston linked to the control column.

Yaw Damper

• Counters rolling during flight.
• Works by providing inputs to the rudder in proportion to the yaw rate.

Typical Yaw Damper System Consists Of

• Yaw damper engagement switch
• Warning annunciator
• Yaw damper coupler
• Yaw damper engagement solenoid
• Transfer valve
• Position transducer on the rudder power control unit
• Yaw damper rudder position indicator

Mach Trim

• Mach tuck results with shock waves.
• Mach trim operates so that it trims the nose of the aircraft up as Mach tuck begins to act on the aircraft.

Rudder Limiter

• High-speed aircraft limit control surface deflection during high-speed operation. At cruise speeds the rudder limiter restricts the rudder deflection to 7° left and right with full pedal travel
• For example, an aircraft may have 30° of rudder deflection at low speeds, with full pedal travel.

Gust Lock Systems

• Locks the controls when the aircraft is parked.
• May be separate from the control system or an integral part.
• Flight control surfaces of aircraft fitted with Power Control Units (PCUs) are protected by gust dampers that are integral to the PCU.

Balancing and Rigging

• Instructions for balancing control surfaces are given in the manufacturer’s service and maintenance manuals and must be followed closely.

Aircraft Balancing

• Control surfaces are rebalanced when weight is added or repairs are made.
• Balance is also checked after repainting.
• Unbalanced surfaces do not remain streamlined during flight.
• Extreme cases can cause fluttering and buffeting which can result in the complete loss of the aircraft.

Static Balance

• The tendency of an object to remain stationary when supported by its Centre of Gravity (CG).
• Indicated as either underbalance or overbalance.
• Better flight operations are gained by nose-heavy static overbalance. Most manufacturers support the existence of nose-heavy control surfaces.

Dynamic Balance

• Dynamic balance in a rotating body is where all the rotating forces are balanced within themselves so that no vibration is produced.
• Most commonly static rebalancing is performed and not dynamic balancing

Rebalancing Procedures

• The control surface and trim tabs are removed from the aircraft and mounted on a balance jig
• Trim tabs secured in the neutral position

Rebalancing Methods

• There are several methods of balancing (or rebalancing) control surfaces in use by the various manufacturers of aircraft.
• The most common are the calculation method, scale method, and balance beam method.

Aircraft Rigging

• It involves the adjustment and travel of the movable flight control surfaces that are attached to major aircraft structures.
• Set cable tension, adjust travel limits, and set travel stops.

Rigging Process

• Establishing the correct position of each of the main components in the transmission system.
• Then, adjusting the linking rods and cables.

Range of Movement

• For a control system to function properly, it must be correctly adjusted.
• Correctly rigged control surfaces move through a prescribed arc.

Three Steps When Checking Range of Movement

• Lock the flight deck control, bell cranks, and control surfaces in the neutral position.
• Adjust the cable tension.
• Adjust the control stops.

Stall Protection/Warning System

• Stall warning systems have advanced to include multiple computers that monitor aircraft configuration and flight data.

Pitch Attitude Protection

• Pitch attitude is the angle between the longitudinal axis (where the aircraft is pointed) and the horizon.

High Angle of Attack (AOA) Protection

• AOA is the angle between oncoming air and a reference line on the aircraft.
• It protects against stalling and wind shear and has priority over other protection.
• Protection limits the angle of attack commanded by the flight crew’s control column.