Aircraft Flight Control Axes

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Questions and Answers

What is the primary function of the Flight Management Computer (FMC) in an aircraft?

To provide real-time updates on weather conditions

To control engine power settings independently of the autopilot

To integrate various aircraft systems and assist in flight planning and navigation (correct)

To manage fuel efficiency during takeoff and landing

Which of the following systems does NOT directly interface with the Flight Management Computer (FMC)?

Weather radar systems (correct)

Air data systems

Central maintenance computer

Inertial reference systems

When the autopilot and autothrottle systems are engaged, what does the aircraft rely on to follow the flight plan?

Control Display Unit inputs only

Weather conditions and terrain analysis

Pilot manual adjustments and confirmations

Flight Management Computer outputs for guidance (correct)

What capability allows the FMC to navigate and avoid restricted areas during a flight?

Automatic navigation guidance Signup and view all the answers

Which aspect of flight does the FMC NOT directly manage?

Direct control of the flight control surfaces Signup and view all the answers

What happens when the control column is moved to the left?

The aircraft banks to the left. Signup and view all the answers

How do spoilers function compared to ailerons?

They extend upwards and cause a wing to drop. Signup and view all the answers

What is the consequence of pulling the control column back?

The elevators rise, causing the aircraft to climb. Signup and view all the answers

What is primarily responsible for moving the control surfaces in small light aircraft?

The pilot's arm and leg muscles. Signup and view all the answers

What effect does pushing the right rudder pedal have on the aircraft?

The aircraft yaws to the right. Signup and view all the answers

What is the primary purpose of trim tabs?

To relieve the pilot from maintaining control input. Signup and view all the answers

Where are elevators located on an aircraft?

On the trailing edge of the horizontal tail plane. Signup and view all the answers

Which control surface primarily affects the pitch of an aircraft?

Elevators. Signup and view all the answers

What is the primary purpose of damping systems in aircraft control?

To correct non-pilot induced attitude changes Signup and view all the answers

What phenomenon is countered by a rudder damping system?

Dutch roll motion of the aircraft Signup and view all the answers

How does a yaw damper control system operate?

By minimizing yaw motion to prevent roll actions Signup and view all the answers

What type of movement does a swept wing airplane experience if disturbed in the yaw axis?

Low frequency oscillation known as Dutch roll Signup and view all the answers

What is the role of artificial feel in aircraft flight controls?

To replicate mechanical control surface opposition forces Signup and view all the answers

What forces are simulated by the artificial feel system to aid pilot control?

Both spring and hydraulic forces Signup and view all the answers

What happens to aircraft controls in mechanically operated systems as airspeed increases?

Difficulty in exerting force increases Signup and view all the answers

During operation, what requirement is there for a pilot to move the control column in a hydraulic system?

Compressing the spring and overcoming hydraulic force Signup and view all the answers

What must be done with the turn knob after acquiring the selected roll attitude?

It must be returned to its detent position. Signup and view all the answers

Which mode allows the aircraft to maintain its altitude at the time of selection?

Altitude Hold Signup and view all the answers

In what situation will the Vertical Speed Hold mode automatically disengage?

When the aircraft reaches the selected altitude while altitude select is in use. Signup and view all the answers

What is the primary function of the Heading Hold mode?

To keep the aircraft wings level after autopilot engagement. Signup and view all the answers

How does the Autothrottle feature assist the pilot?

By maintaining the aircraft's speed through thrust adjustments. Signup and view all the answers

What does the pilot use to input commands in an aircraft with Control Wheel Steering (CWS)?

Pressure applied to the controls. Signup and view all the answers

Which statement correctly describes the relationship between Turbulence mode and autopilot operation?

Turbulence mode returns the autopilot to MAN mode during severe turbulence. Signup and view all the answers

What distinguishes the autopilot engage switch with CWS from one with MAN mode?

CWS is not fitted with a flight controller. Signup and view all the answers

What does the term 'fly-by-wire' (FBW) specifically indicate in a control system?

A purely electrically signalled control system without mechanical links. Signup and view all the answers

How does a fly-by-wire (FBW) system enhance the safety and stability of an aircraft?

By interposing a computer system that modifies pilot inputs based on pre-set parameters. Signup and view all the answers

What is a primary benefit of using a fly-by-wire system as compared to conventional systems?

Lighter overall system weight and the ability to make smaller flight control surfaces. Signup and view all the answers

What role do sensors like gyros and accelerometers play in a fly-by-wire system?

They detect uncommanded attitude changes and generate outputs for flight control. Signup and view all the answers

What is the significance of having multiple independent channels in a fly-by-wire system?

It provides redundancy to maintain full capability despite possible system failures. Signup and view all the answers

What is the main purpose of the Control Wheel Steering (CWS) mode in an autopilot system?

To allow the pilot to manually fly the aircraft while the autopilot is engaged Signup and view all the answers

Which issue can an Automatic Flight Control System (AFCS) specifically address in an aircraft's performance?

Stability or control deficiencies such as Dutch roll. Signup and view all the answers

What happens to the pitch attitude of the aircraft when the pitch wheel is moved during autopilot engagement?

The aircraft’s pitch attitude changes Signup and view all the answers

Which device converts the input force from the Control Wheel Steering into an electrical signal?

LVDT Signup and view all the answers

Which of the following statements accurately describes an aspect of electric actuator operation in a fly-by-wire system?

Electrical signals modify pilot commands and directly control surface deflection. Signup and view all the answers

What characteristic allows FBW aircraft to achieve lighter designs compared to conventional aircraft?

A reduction in the aggregate weight of system components and relaxed stability requirements. Signup and view all the answers

What is a typical force that a pilot would apply to override the autopilot system?

25 to 35 pounds on the control wheel Signup and view all the answers

What does the trim indicator display in an aircraft system?

The signal magnitude and polarity to the flight control actuator Signup and view all the answers

Which of the following statements about the autopilot system is false?

The autopilot cannot be overridden by the pilot at any time. Signup and view all the answers

In the command mode of autopilot, what role does the pilot play?

The pilot provides no control input but monitors the system. Signup and view all the answers

What is indicated when a pointer on the trim indicator remains continuously deflected?

The servo amplifier needs to provide a continuous correction signal. Signup and view all the answers

Study Notes

Flight Control Axes

Aircraft movement in flight is controlled within three stabilized planes, around axes passing through the Centre of Gravity (CG).
The three principal axes are:
- Longitudinal (roll or X-axis): runs from nose to tail through the CG.
- Lateral (pitch or Y-axis): runs parallel to a line between wingtips, intersecting the X-axis at the CG.
- Normal/vertical (yaw or Z-axis): runs perpendicular to the other two axes, intersecting them at the CG.

Roll

Movement around the longitudinal axis is called rolling.
Lateral stability is controlled by ailerons.

Pitch

Movement around the lateral axis is called pitching.
Longitudinal stability is controlled by elevators.

Yaw

Movement around the vertical axis is called yawing.
Directional stability is controlled by the rudder.

Flight Control Surfaces

In straight and level flight, a control surface acts as an extended airfoil.
Deflecting a control surface changes the amount of lift it produces.
- Deflecting downwards increases lift.
- Deflecting upwards decreases lift.
Primary control surfaces include ailerons, elevators, and rudder.

Ailerons/Spoilers

Control the roll attitude of an aircraft.
Located on the trailing edge of the wings near the tips.
Connected in opposition for coordinated roll.
Raising one aileron lowers the opposite one, increasing lift on one wing and decreasing lift on the other, causing the aircraft to roll.
Spoilers can also be used as substitutes for ailerons. -They operate on the same principle, but only extend upwards, causing lift loss.

Elevators

Control the pitch of the aircraft.
Located on the horizontal tail plane.
Pulling the control column back raises the elevators, decreasing lift on the tail and causing the nose to rise (climb).
Pushing the control column forward lowers the elevators, increasing lift on the tail and causing the nose to drop (dive).

Rudder

Control the yaw of the aircraft.
Located on the vertical fin.
Pushing the right rudder pedal forward deflects the rudder to the right, causing the aircraft to yaw to the right.

Manually Operated Flight Controls

Small aircraft use pilot muscle to move control surfaces connected by cables.
The pilot moves control surfaces by repositioning the control column or rudder pedals until the desired attitude and heading is achieved.
Trim tabs maintain a straight and level flight or heading.

Powered Flight Controls

In larger aircraft, actuators and hydraulics are used for more powerful control.

Power-Assisted Control

The pilot's control stick connects to the control surface via a control lever.
When the pilot moves the stick, the control lever pivots, and the control surface moves.
Hydraulic fluid flows to the actuator, causing it to move the surface even further.

Power-Operated Control

The pilot's stick only moves the control lever, with the actuator directly connected to the control surface.
The hydraulic power from the actuator moves the control surface without direct pilot pressure.

Fly-by-Wire System

The system replaces mechanical links with electrical signals.
A computer system interposes between the pilot and the control surfaces to maintain stability and structural integrity.
The aircraft can be lighter and have more redundancy using electrical signals, with smaller control surfaces using electric actuators or servomotors that process and receive electrical signals for control surface movements.

Damping Systems

Automatically correct non-pilot-induced attitude changes.
- Example: elevator damping counteracts porpoising and rudder damping counteracts Dutch roll.
Swept-wing aircraft oscillate in yaw axis and develop Dutch roll, with improved yawing using yaw damper systems.

Artificial Feel

A system that replicates the aerodynamic forces on control surfaces without hydraulic assistance.
This allows the pilot to feel the opposing load and safely operate controls.
The system is used for elevator control in most systems and is used for greater force at high speeds, with greater resistance to movements.

Autopilot

Used to maintain stable flight conditions and make precise maneuvers, like landing.
Classified by the number of axes for control.
Single-axis – controls roll only. Multi-axis – controls pitch, roll, and yaw.
The autopilot responds to uncommanded attitude changes and maintains the initial attitude.

Basic Capability of Autopilot

Holds attitude from engagement to return to wings-level (if a bank exists).
Maintains altitude/attitude, even if the aircraft is climbing, descending or in level flight.

Closed Loop Servo System

Autopilot and AFCS systems use closed-loop servo systems with two loops:
- Inner loop (Attitude loop): compares desired and actual attitude, and the error in attitude generates a control surface movement signal.
- Outer loop (external systems): employs sensors, such as from heading hold or autoland, to generate attitude changes.

Autopilot Elements

Sensing elements: detect uncommanded aircraft movement (attitude gyros, rate gyros, accelerometers, pitot-static).
Command elements: select the autopilot mode, and control wheel or steering, manage system inputs, and process signals.
Output elements: move control surfaces (actuators); electric servomotors, pneumatic actuators, or electromagnetic transfer valves.

Flight Control Computer

An aircraft's autopilot system may have one, two, or three independent autopilot systems, depending on operator requirements.
The flight control computer receives inputs and provides outputs.
May include multiple units.

Autopilot Flight Control Computers

An aircraft capable of Autoland requires a minimum of two totally independent autopilots for safe and reliable operations in any conditions.
Fail-Passive: A warning system for a single input failure to prevent the loss of the whole autopilot system.
Fail-Operational: Redundancy in case of dual autopilot or disengagement.

Command Elements – Autopilot Mode Select Panel

The primary interface between the flight crew and the autopilot.
Switches and circuits for on/off control and mode selection of the autopilot channels and autothrottle system.

Flight Controller

Allows the pilot to change the aircraft's attitude without disengaging the autopilot.
When in the manual mode, the turn knob commands the roll channel to perform a roll maneuver.
The pitch wheel controls the pitch attitude of the aircraft.

Control Wheel Steering

The pilot's force on the control wheel acts as an input signal to the flight control computer.
The computer moves control surfaces, with the pilot maintaining active control during the entire procedure.
The pilot can override the autopilot using a reasonable amount of force.

Trim Indicator

Visual display of the commanded signal applied to the flight actuator or servomotor.
Each pointer represents a flight control and moves to reflect the commanded signal being applied.
Return to zero-trim when the command is satisfied.

Autopilot Disengage Switches

Located on each control yoke.
Pressing either disengages the autopilot from operating the flight controls and releases the autopilot.

Flight Director Display

Generates information about the aircraft’s attitude, used to monitor the autopilot activity.

Flight Mode Annunciator

Displays the status of the selected autopilot and autothrottle modes visually.

Output Elements – Autopilot Actuators

Move control surfaces; servomotors, servo actuators, and other similar names according to the control system, for example, rudder servo or pitch actuator.
Convert electrical signals to mechanical motion using AC- or DC-powered motors, torque motors, or solenoid-controlled valves.

Electro-Pneumatic Actuator

Used in smaller aircraft.
Use vacuum from the engine compressor to move the diaphragm to a designated position.
Two actuators (push and pull) are needed for each control surface.

Electro-Mechanical Actuator

Uses an electric (DC) servomotor with a gearbox and an electromechanical clutch.
Servomotor operation depends on whether the autopilot is engaged or disengaged.

Continuous DC Motor Actuator

Uses a servomotor with a dual set of contra-rotating magnetic clutches.
Torque is transferred to the flight control mechanism when the autopilot is engaged.
Used for precise and timely response.

AC Servomotor Actuator

Used in AC-powered autopilot systems.
Uses a reversible squirrel cage motor, a reduction gearbox, and a DC-powered clutch assembly to provide torque via electrical signals.

Electro-Hydraulic Actuator

Modified power-assisted or power-operated hydraulic actuator.

Torque Limiting

Prevents overstressing the aircraft by limiting the autopilot servo amplifier output and introducing a means for mechanical force limits.
Uses torque limiting resistors in output amplifier circuits or mechanical clutches to achieve this.
A computer servo amp signal to the actuator limits the attitude, airspeed, and other factor commands, preventing inputs beyond aircraft capabilities.

Autopilot Operation - Engagement

Before the system is engaged, power supplies must be validated, and systems must be operational.
Signal and engaged circuits need confirming.
Systems stay in standby mode.

Autopilot Modes

Heading Hold.
Heading Select.
Altitude Hold.
Altitude Select.
Vertical Speed Hold.
Airspeed or Mach Hold.
Turbulence.
Autothrottle.
Radio Navigation (e.g., LOC and glideslope in autoland).
Land.

Flight Management System (FMS)

Automatically controls the aircraft from takeoff to landing at the destination.
The system consists of a flight management computer (FMC), control display unit (CDU), and a data or bus system.
It has integrated flight planning, performance management, and automatic navigation.
Uses radio navigational systems, inertial reference systems, air data systems, thrust management computers, and other flight control computers.

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Description

This quiz covers the three principal axes of aircraft movement: longitudinal, lateral, and normal axes. Learn how each axis affects roll, pitch, and yaw, along with the corresponding control surfaces such as ailerons, elevators, and rudders. Test your understanding of aircraft stability and maneuverability.