Aircraft Inner Loop Control System
The components of an inner (closed) loop control system in an aircraft shown in Figure are:

Attitude Sensor
A rate gyro senses disturbance of the aircraft in one axis only.

Transducer
Converts mechanical movement of the gyro into an electrical signal.

Signal Processor
The error detector. Compares the signals from the transducer with the input signals, determines
the required corrective action (the error) and transmits a signal to the servomotor. Receives
and compares position and rate of movement feedback signals from the servomotor.

Servomotor
Converts processed signal into movement of the aircraft flight controls proportional to rate
and direction of signal. Uses hydraulic, electrical or pneumatic power.

Aerodynamic Feedback
The attitude reached by the aircraft is sensed by the rate gyro which gives a measure of the
output.
A disturbance to the selected flight path produces an error signal; the autopilot operates to
move the aircraft back towards its stabilized condition. This causes the error signal from the
transducer to be progressively reduced and therefore removes the control surface deflection
after the disturbance has been corrected.

Feedback control systems used for positional control e.g. aircraft flying controls, are usually
referred to as servo systems or servomechanisms. A servomechanism can be defined as a
closed loop control system where a small input is converted into a larger output in a strictly
proportionate manner.

To increase the damping forces at altitude could mean an increase in the overall size of the stabilizing surfaces but this would also increase drag. Another option is to produce an aircraft that is dynamically stable at lower and middle altitudes and have a system to automatically counter Dutch roll. This system detects the yaw then applies rapid, small and effective rudder deflections stopping the Dutch roll before it starts. This system we call a Yaw Damper.

The yaw damper will either be the third axis of an autopilot system or will be an addition to the
third axis of autopilot control

In automatic flight systems, controlling the aircraft’s flight typically involves managing Azimuth, Elevation, and Velocity, which correspond to the following:

1. Azimuth

• Definition: The horizontal angle or direction of the aircraft relative to a reference, usually north or the aircraft’s intended track.
• Control: This is managed by the autopilot’s yaw control (rudder) and roll control (ailerons) to maintain the correct course or heading.

2. Elevation

• Definition: This refers to the aircraft’s vertical position, usually measured in altitude.
• Control: Managed by pitch control (elevators) and also the autothrottle system to maintain a desired altitude or climb/descent profile.

3. Velocity

• Definition: The aircraft’s speed, typically indicated as ground speed or airspeed.
• Control: Managed by the autothrottle system and the pitch and roll controls to maintain a desired speed during various phases of flight.

An Instrument Landing System (ILS) is a short-range navigational aid which provides azimuth
and vertical guidance during the approach to an airport runway. The system comprises groundbased transmitting elements and also receiving elements carried on board the aircraft.

With CWS engaged, the A/P manoeuvres the aeroplane in response to control pressures
applied by either pilot. The control pressure is similar to that required for manual flight. When
control pressure is released, the A/P holds existing attitude.

At 1500 feet Radio Altitude (RA)
Shortly after capturing both LOC and G/S and descending below 1500 ft RA, the second A/P
couples with the flight controls, FLARE mode armed is annunciated and the A/P go-around
mode arms but is not annunciated. ROLL OUT mode if available will also now arm.

The autoland status will also now be annunciated as either “LAND 2” (or “LAND 3” for fail-operational
aircraft).
The pitch and roll axes cannot be manually overridden into CWS. Attempts to do so will result
in A/P disengagement

Fail-operational (Fail-active)

This status is defined as the ability of a system to withstand a failure without affecting the overall functioning of the system and without causing degradation of performance beyond the limits required for automatic landing. The system requires a minimum of three autopilots. However, it is possible for an aircraft to have a fail-operational category with only two autopilots provided that there is suitable duplicate
monitoring for each channel.

The roll and pitch channels are used as the primary control channels. It is these two channels to which outer loop signals are fed to control the various modes.

The rudder channel is basically a stability channel.
It is common to have interaction between the roll channel and rudder channel to assist in coordinated turns and to give faster stability response.

Compass gyro and turn and slip gyro provides inputs to the rudder channel

Three axis control is required for autoland.

Fail-operational (Fail-active)

This status is defined as the ability of a system to withstand a failure without affecting the overall functioning of the system and without causing degradation of performance beyond the limits required for automatic landing. The system requires a minimum of three autopilots. However, it is possible for an aircraft to have a fail-operational category with only two autopilots provided that there is suitable duplicate
monitoring for each channel.

Manometric (or Air) Data
Raw data inputs which come under this heading are those associated with altitude, airspeed/
Mach No. and vertical speed. Each of these provides current aircraft status for outer loop
control in the pitch channel of the autopilot

Sensing may be carried out either by independent sensor units, or by a Central Air Data
Computer (CADC). The sensors operate on the same fundamental principles as the basic pitotstatic instruments, the measuring elements being coupled to appropriate types of electrical
transducers instead of instruments.

Examples of Outer Loop Inputs in Pitch

Altitude Hold (ALT HLD)

The altitude hold mode gives pitch commands to hold either:

1. The MCP-selected altitude, or
2. The uncorrected barometric altitude at which the ALT HOLD switch was pressed.

ALT HOLD engages in two conditions:

• At the MCP-selected altitude:
  • Indicated by the annunciation of ALT HOLD.
  • The ALT HOLD switch light is extinguished.
• Not at the MCP-selected altitude:
  • Indicated by the annunciation of ALT HOLD.
  • The ALT HOLD switch light is illuminated.

ALT HOLD not at the MCP-selected altitude occurs in these cases:

• Pushing the ALT HOLD switch while not at the MCP-selected altitude.
• Selecting a new MCP altitude while in ALT HOLD at the currently selected altitude.

Additional notes:

• ALT HOLD is inhibited after glide slope capture.
• When in ALT HOLD at the selected altitude, LVL CHG, V/S, and V NAV climb and descend functions are inhibited until a new altitude is selected.
• The altitude selected on the MCP is referenced to the Captain’s barometric altimeter setting for the “A” autopilot and FDS, and to the First Officer’s barometric setting for the “B” autopilot and FDS.
• After ALT HOLD engages, changes in the altimeter barometric settings do not affect the selected altitude reference

Altitude Acquire (ALT ACQ)

The altitude acquire mode is a transition maneuver entered automatically from:

• V/S, LVL CHG, or V NAV climb or descent to an MCP-selected altitude.

ALT ACQ mode is also armed during:

• Climbing or descending in CWS with an autopilot engaged.

ALT ACQ engagement:

• Annunciated as ALT ACQ in pitch when leveling off in either V/S or LVL CHG.
• V NAV remains annunciated throughout the altitude acquire mode when leveling in V NAV.

ALT ACQ engagement is inhibited in these cases:

• When the ALT HOLD switch is pressed.
• After glide slope capture.

Fail-Passive (Fail-Soft)

Fail-passive is defined as the system’s ability to:

• Withstand a failure without endangering passenger safety.
• Avoid producing excessive deviations in the flight path.
• However, it removes the system’s capability to complete an automatic landing.

Key requirement:
The minimum number of autopilots required for a fail-passive capability is two.

Status Annunciator

The system performs autoland in either a ‘LAND 2’ status or a ‘LAND 3’ status, depending on the number of channels that are armed and engaged.

• LAND 2:
  • Indicates dual redundancy of engaged flight control computers, sensors, and servos.
  • Operates in a fail-passive mode.
• LAND 3:
  • Indicates triple redundancy of power sources, engaged flight control computers, sensors, and servos.
  • Operates in a fail-operational mode.

Each status is displayed on an autoland status annunciator

Fail-Passive (Fail-Soft)

Fail-passive is defined as the system’s ability to:

• Withstand a failure without endangering passenger safety.
• Avoid producing excessive deviations in the flight path.
• However, it removes the system’s capability to complete an automatic landing.

Key requirement:
The minimum number of autopilots required for a fail-passive capability is two.

At About 5 Feet Gear Altitude (GA)

1. Flare Mode Disengagement:
   • The flare mode is disengaged, transitioning to touchdown.
2. LOC Disengagement:
   • The LOC (localizer) mode disengages.
3. Roll-Out Mode Engagement:
   • If available, the roll-out mode engages.

At About 1 Foot Gear Altitude (GA)

1. Pitch Attitude Adjustment:
   • The pitch attitude of the aircraft is decreased to 2°.
2. Nose Gear Contact at Touchdown:
   • At touchdown, a command signal is sent to the elevators to:
     • Lower the aircraft’s nose.
     • Bring the nose landing gear wheels into contact with the runway.
     • Hold the nose gear wheels on the runway during roll-out

With CWS engaged, the A/P manoeuvres the aeroplane in response to control pressures
applied by either pilot. The control pressure is similar to that required for manual flight. When
control pressure is released, the A/P holds existing attitude

Three Axis System

• Control Axes: Roll (Ailerons), Pitch (Elevators), and Yaw (Rudder)
• Function: Controls stability across all three axes.
• Characteristics: Provides full attitude control, including coordinated turns and autoland capability.
• Control Loops: Three inner loops, one for each axis of control (roll, pitch, and yaw).
• Yaw Control: Often interacts with the roll channel to maintain coordinated flight and faster stability response.

A three-axis autopilot is required for autoland functionality, providing complete control over the aircraft’s attitude in all axes.

Air Data Computer may internally be split into the following modules for
assessment and onward transmission of data obtained through the temperature, static and
pitot pressure gathering devices – Altitude, Computed Airspeed, Mach Speed, True Airspeed
and using data from the altitude module via a Rate of Climb module will give vertical speed.

Inner loop stability is obtained by:

1. Rate Gyro Feedback (Attitude Sensor) – Detects any unintended aircraft motion and provides real-time feedback.
2. Error Detection & Correction (Signal Processor) – Compares actual attitude with the desired state and generates corrective signals.
3. Smooth Control Surface Adjustments (Servomotor) – Moves control surfaces proportionally to minimize oscillations and avoid overcorrection.
4. Aerodynamic Feedback (Rate of Change Monitoring) – Ensures progressive reduction of control inputs as the aircraft stabilizes.
5. Damping of Oscillations – Prevents excessive movements by adjusting the control response rate.
6. Closed-Loop Feedback Mechanism – Continuously refines control inputs based on aircraft response to maintain a stable attitude.

An autothrottle system is a computer controlled, electromechanical system which can control
the thrust of an aircraft’s engines within specific design parameters. The throttle position of
each engine is controlled to maintain a specific value of thrust in terms of:
• Fan Speed (N1)
• Engine Pressure Ratio (EPR) or
• Target Airspeed (set by SPD on mode control panel)

The modes of operation of the autopilot during the various flight phases can be seen from the
following table:

as you can see in any phase of the flight it either controls ,THRUST REFERENCE, Mach or Speed.

THRUST REFERENCE = EPR

• CWS Mode: When CWS (Control Wheel Steering) is engaged, the autopilot responds to control pressures applied by the pilot. The pilot controls the aircraft by applying pressure to the controls, and the autopilot holds the current attitude when control pressure is released.
• Aileron Control: If the aileron pressure is released and the bank angle is 6 degrees or less, the autopilot will automatically level the wings and maintain the current heading.
• Inhibition of Heading Hold: The heading hold feature is inhibited under certain conditions:
  • Below 1500 ft RA with the landing gear down.
  • After VOR or LOC capture with TAS (True Airspeed) of 250 knots or less.
  • During the Approach (APP) mode after LOC capture.

Touch Control Steering (TCS):

• TCS vs. CWS: While both CWS (Control Wheel Steering) and TCS allow the pilot to manually control the aircraft, TCS works differently. Instead of merely commanding the autopilot to hold a certain attitude when control pressures are released (like in CWS), TCS disengages the automatic control channels and servomotors entirely when the pilot holds the TCS button depressed.

How TCS Works:

• Manual Control: When the pilot presses and holds the TCS button, they take full manual control of the aircraft’s pitch or roll. The autopilot servos and automatic control systems are temporarily disconnected, so the pilot is in complete control of the aircraft without any automated assistance.
• Desired Attitude: The pilot can then manually maneuver the aircraft to the desired attitude using the aircraft’s controls (typically the yoke or control wheel).
• Automatic Control Re-engagement: Once the TCS button is released, the autopilot system can be re-engaged, and it will take over again, holding the aircraft at the attitude the pilot left it at.

 Differences between TCS and CWS:

• TCS disengages the autopilot’s control servos entirely, allowing the pilot to control the aircraft manually by holding the TCS button.
• CWS, on the other hand, maintains autopilot control but responds to the pilot’s inputs, allowing the pilot to override and adjust the aircraft’s attitude temporarily while the autopilot continues to manage the overall flight.

Fail-operational (Fail-active)

This status is defined as the ability of a system to withstand a failure without affecting the overall functioning of the system and without causing degradation of performance beyond the limits required for automatic landing. The system requires a minimum of three autopilots. However, it is possible for an aircraft to have a fail-operational category with only two autopilots provided that there is suitable duplicate
monitoring for each channel.

In  Automatic Landing Altitude information essential for vertical guidance to touchdown is always provided by signals from a radio altimeter which becomes effective as soon as the aircraft’s altitude is within the
altimeter’s operating range (typically 2500 feet).

In a modern transport aircraft equipped with an advanced flight guidance system that includes automatic landing capability, the system typically employs an integrated flight management system (FMS) and autopilot that can handle both the outer loop (navigation and flight path guidance) and inner loop (autopilot control of aircraft attitude and flight dynamics). The outer loop inputs you’ve listed relate to the modes used for managing navigation and vertical guidance during various phases of flight, including approaches and landings.

Roll Channel:

The roll channel is responsible for managing the aircraft’s lateral guidance (roll control) and navigation along the flight path, including heading and course corrections.

1. Heading Hold:
   • Maintains the aircraft’s current heading. This mode keeps the aircraft flying in the direction it is already heading, without automatic course changes unless the pilot intervenes.
2. Heading Select:
   • Allows the pilot to select a specific heading, and the autopilot will steer the aircraft to that heading. It’s commonly used during enroute navigation or when executing specific turns.
3. VOR Intercept and Track:
   • Uses a VOR (Very High Frequency Omnidirectional Range) navigation aid to intercept and track a specific VOR signal. The aircraft adjusts its heading to fly to the VOR station or along a course defined by the VOR.
4. LOC Intercept and Track:
   • Tracks the localizer (LOC) signal, which is part of the Instrument Landing System (ILS). This mode ensures the aircraft intercepts and follows the localizer course during the approach to the runway, maintaining alignment with the runway centerline.
5. Inertial Navigation (Inertial Nav) or L NAV:
   • Inertial Navigation (Inertial Nav) uses inertial reference systems to provide highly accurate position and heading information, independent of external signals. This mode is used for enroute navigation.
   • L NAV (Lateral Navigation) is a system that provides lateral flight path guidance based on GPS, waypoints, or other navigation aids. It’s part of the overall FMS and ensures the aircraft follows the pre-programmed route.

Pitch Channel:

The pitch channel manages vertical guidance, controlling the aircraft’s altitude, speed, vertical speed, or Mach number.

1. Altitude Hold:
   • Maintains the aircraft at a constant altitude. The autopilot will make minor adjustments to keep the aircraft at the pre-selected altitude.
2. Speed Hold:
   • Keeps the aircraft at a constant airspeed by adjusting the throttle or pitch attitude. This is typically used during level flight where the goal is to maintain a set speed.
3. Mach Hold:
   • Maintains a constant Mach number (the ratio of the aircraft’s speed to the speed of sound). This is often used for high-speed cruise flight at higher altitudes where maintaining a Mach number is more important than a specific airspeed.
4. Vertical Speed:
   • Controls the aircraft’s rate of climb or descent. The pilot sets a desired vertical speed (in feet per minute or meters per second), and the autopilot adjusts pitch to achieve and maintain this rate of climb or descent.
5. V NAV (Vertical Navigation):
   • V NAV is part of the Flight Management System (FMS) that controls the aircraft’s vertical flight path. It calculates and manages the aircraft’s altitude and vertical speed during the flight based on waypoints, airways, and flight plan. During an approach, V NAV may manage the descent profile according to the route and constraints provided in the FMS.

• CWS Mode: When CWS (Control Wheel Steering) is engaged, the autopilot responds to control pressures applied by the pilot. The pilot controls the aircraft by applying pressure to the controls, and the autopilot holds the current attitude when control pressure is released.
• Aileron Control: If the aileron pressure is released and the bank angle is 6 degrees or less, the autopilot will automatically level the wings and maintain the current heading.
• Inhibition of Heading Hold: The heading hold feature is inhibited under certain conditions:
  • Below 1500 ft RA with the landing gear down.
  • After VOR or LOC capture with TAS (True Airspeed) of 250 knots or less.
  • During the Approach (APP) mode after LOC capture.

Fail-passive (Fail-soft)
This is defined as the ability of the system to withstand a failure without endangering passenger
safety, and without producing excessive deviations in the flight path but removing its capability
to complete an automatic landing

WHAT IS REDUNDANCY?

Redundancy is like having a backup or extra copies of something important, just in case the first one doesn’t work.

Imagine you have two light bulbs in your room. If one light bulb burns out, you still have the other one to keep your room bright. So, you don’t have to sit in the dark!

In an airplane, redundancy is when there are extra parts or systems, like having two engines or two computers. If one stops working, the extra one keeps everything running smoothly. It helps to keep things safe and working well.

Redundancy is accomplished by providing two or more systems of each type so a failure of one system will
not affect the operation of the complete system.

Automatic approach, flare and landing sequence is based on a system that utilizes triple digital flight control computer channels, allowing for redundancy to operate in the fail-operational and fail passive conditions .

At 1500 feet RA:

• Second A/P couples: After the aircraft has captured both the localizer (LOC) and glideslope (G/S), and the aircraft descends below 1500 feet RA, the second autopilot (A/P) engages and begins working alongside the first autopilot. This is done for redundancy and increased safety during the critical phase of the approach and landing.
• FLARE Mode Armed: As the aircraft nears the runway, the FLARE mode is armed. This mode prepares the autopilot to automatically initiate a gentle nose-up attitude to soften the landing just before touchdown.
• Go-Around Mode Armed (Not Annunciated): In case the aircraft needs to abandon the landing (for example, if a problem arises or the runway is blocked), the go-around mode is armed. However, this mode is not annunciated (i.e., it doesn’t show up on the Flight Mode Annunciator – FMA) unless activated.
• ROLL OUT Mode (if available) Armed: If the aircraft is equipped with ROLL OUT mode, it will also be armed. This mode automatically steers the aircraft along the runway after landing, helping the pilot keep the aircraft on the centerline during rollout. If the system is available, this mode becomes part of the automatic landing process.
• Autoland Status: The aircraft’s autoland status will be displayed, and it will show either “LAND 2” or “LAND 3” on the FMA, depending on the aircraft’s fail-operational capability.
  • LAND 2: This indicates that the autoland system is fail-passive (the second autopilot is monitoring and can take over if there’s an issue, but the system isn’t fully redundant).
  • LAND 3: This is for fail-operational aircraft, which means the autoland system has full redundancy (two autopilots are actively controlling the aircraft, so if one fails, the other can still perform the landing).

Off-Line Channels Engaged:

• Off-Line Channels typically refer to redundant systems or autopilot channels that are disengaged during normal operation but can be engaged to provide backup and ensure system reliability.

The inner loop is a classic example of a closed loop control system

. The basic
elements of a closed loop control system are

• Input
• Error detector / Signal Processor
• Output
• Control element
• Feedback

Outer Loop Inputs in Pitch: Altitude Hold (ALT HLD)

The Altitude Hold (ALT HLD) mode is a fundamental feature in the pitch axis of an autopilot system. It ensures that the aircraft maintains a selected altitude automatically. Below is an explanation of how the ALT HLD mode functions and the different conditions under which it operates:

ALT HOLD Functionality:

• Purpose: ALT HLD commands the autopilot to hold the aircraft at a specific altitude. The aircraft either holds the altitude selected on the Mode Control Panel (MCP) or the altitude at which the ALT HOLD switch was pressed (uncorrected barometric altitude).
• How It Engages:
  • ALT HOLD at the MCP Selected Altitude: This occurs when the aircraft is at the altitude previously selected on the MCP. When this happens, ALT HOLD engages and the ALT HOLD switch light extinguishes.
  • ALT HOLD Not at the MCP Selected Altitude: If the aircraft is not at the MCP selected altitude when ALT HOLD is activated, ALT HOLD still engages, but the ALT HOLD switch light illuminates to indicate that the altitude being held is not the one set on the MCP.

• A rate gyro provides the flight control computer with rate of rotation information (how fast the aircraft is turning).
• This information can be integrated to provide displacement or attitude data (the aircraft’s actual orientation or attitude), allowing the autopilot or navigation systems to make precise adjustments to keep the aircraft on course or maintain stable flight.

When the autopilot is engaged automatic trim is available only in pitch. This is called Automatic
Pitch Trim or simply Auto-trim. Auto-trim is active only when the autopilot is engaged

There are mechanical trim backups in pitch and yaw which will override the automatic flight control computer trim. The pitch trim is operated by moving the pitch trim wheel which will cause the horizontal stabilizer to move, the autopilot to disconnect and override the flight control computers pitch attitude.

L.NAV is an outer loop lateral input to the roll channel using data from the FMC (Flight Management Computer).

• Outer Loop: The outer loop refers to the higher-level flight guidance systems that handle navigation, such as following a flight plan or route. It includes inputs like L.NAV that provide guidance for the aircraft’s overall path.
• Lateral Input: L.NAV specifically guides the aircraft along the horizontal or lateral path, which means it controls the aircraft’s heading or course.
• Roll Channel: The roll channel is responsible for controlling the aircraft’s roll, typically through the ailerons, ensuring the aircraft stays on the desired course.
• FMC (Flight Management Computer): The FMC provides navigation data to the autopilot, such as waypoints, flight routes, and current position, which the L.NAV system uses to follow the desired lateral flight path.

So, L.NAV takes data from the FMC and sends it as an input to the roll channel to guide the aircraft on its lateral course.

The Mach trim system automatically corrects for Mach tuck, a nose-down pitch caused by the movement of the center of pressure at high subsonic speeds. It operates independently of the autopilot and is armed throughout flight but only activates during high-speed conditions when the aircraft nears its critical Mach number. This ensures the aircraft maintains the desired attitude and prevents an uncontrollable pitch-down at high speeds

The altitude selected on the MCP is referenced to the Captain’s barometric altimeter setting for
the “A” autopilot and FDS, and to the First Officer’s barometric setting for the “B” autopilot
and FDS.

After ALT HOLD engages, changes in the altimeter barometric settings do not change
the selected altitude reference

• CWS Mode: When CWS (Control Wheel Steering) is engaged, the autopilot responds to control pressures applied by the pilot. The pilot controls the aircraft by applying pressure to the controls, and the autopilot holds the current attitude when control pressure is released.
• Aileron Control: If the aileron pressure is released and the bank angle is 6 degrees or less, the autopilot will automatically level the wings and maintain the current heading.
• Inhibition of Heading Hold: The heading hold feature is inhibited under certain conditions:
  • Below 1500 ft RA with the landing gear down.
  • After VOR or LOC capture with TAS (True Airspeed) of 250 knots or less.
  • During the Approach (APP) mode after LOC capture.

Automatic Synchronization in Autopilot Systems

Automatic synchronization is a crucial process in autopilot systems, ensuring that the transition from manual to automatic control is smooth and without snatching (abrupt or jerky movements). Here’s a breakdown of how it works:

1. Pre-Engage Requirements:
   • Before the autopilot system can engage, certain pre-engage requirements must be met, such as ensuring the autopilot circuits are electrically complete and operational.
   • It’s important that the aircraft is trimmed for the desired flight attitude before autopilot engagement. “Trimming” means that the aircraft is in a neutral or balanced condition where no excessive control inputs are needed to maintain the current flight attitude.
2. Smooth Take-Over:
   • When the autopilot engages, the take-over (the transfer of control from manual to automatic) must happen smoothly, with no jerky or abrupt movements. This prevents the aircraft from experiencing sudden changes in attitude that might be uncomfortable or even dangerous.

When the autopilot is engaged automatic trim is available only in pitch. This is called Automatic
Pitch Trim or simply Auto-trim. Auto-trim is active only when the autopilot is engaged

There are mechanical trim backups in pitch and yaw which will override the automatic flight control computer trim. The pitch trim is operated by moving the pitch trim wheel which will cause the horizontal stabilizer to move, the autopilot to disconnect and override the flight control computers pitch attitude.

EU-OPS Requirements

Single pilot operation under IFR or at night.
An operator shall not conduct single pilot IFR operations unless the aero plane is equipped with
an autopilot with at least ALTITUDE HOLD and HEADING MODE.

This means that the aircraft
must have at least a two-axis autopilot.

Installation of automatic pilot system
Each automatic pilot system must be approved and must be designed so that the autopilot
can be quickly and positively disengaged to prevent it from interfering with the control of the
aeroplane.

Unless there is automatic synchronizing, each system must have a means to readily indicate to
the pilot the alignment of the actuating device in relation to the control system it operates

EU-OPS Requirements

Single pilot operation under IFR or at night.
An operator shall not conduct single pilot IFR operations unless the aero plane is equipped with
an autopilot with at least ALTITUDE HOLD and HEADING MODE.

This means that the aircraft
must have at least a two-axis autopilot.

Installation of automatic pilot system
Each automatic pilot system must be approved and must be designed so that the autopilot
can be quickly and positively disengaged to prevent it from interfering with the control of the
aeroplane.

Unless there is automatic synchronizing, each system must have a means to readily indicate to
the pilot the alignment of the actuating device in relation to the control system it operates

• High-Speed Protection:
  • The system actively prevents the aircraft from exceeding its design dive speed (Vd/Md), not just the operating limits (VMO/MMO). It does so by adding a nose-up demand to the pilot’s input if the aircraft exceeds VMO/MMO, thereby reducing the potential for overspeed.
• Pitch Attitude Protection:
  • This protection feature limits the aircraft’s pitch attitude to 30° nose-up and 15° nose-down to avoid excessive climb or descent attitudes. In fly-by-wire aircraft, this system is combined with high-angle-of-attack and high-speed protections to maintain safe flight profiles, using inputs from the attitude gyros and air data computers

Fail-operational (Fail-active)

This status is defined as the ability of a system to withstand a failure without affecting the overall functioning of the system and without causing degradation of performance beyond the limits required for automatic landing. The system requires a minimum of three autopilots. However, it is possible for an aircraft to have a fail-operational category with only two autopilots provided that there is suitable duplicate
monitoring for each channel.

The Autoland sequence is considered complete when the following events have occurred:

1. Touchdown: The aircraft has made contact with the runway. The elevators are controlled to bring the nose landing gear in contact with the runway.
2. Autothrottle Disengagement: After touchdown, the autothrottle system automatically disengages approximately 2 seconds after the aircraft makes contact with the runway, and reverse thrust is applied.
3. Autopilot Disengagement: The autopilot system remains engaged until manually disengaged by the pilot. However, if the autopilot remains engaged throughout the rollout, the autoland sequence is only complete once the system is manually disengaged by the pilot after the aircraft has safely landed.
4. Rollout: The aircraft continues to roll out along the runway after touchdown, typically with the rollout mode engaged (if available), and the aircraft comes to a complete stop

The high angle of attack protection is an aerodynamic protection but
thrust is required to maintain the flight path and the auto-thrust function would automatically
provide TOGA thrust when the aircraft reaches a certain value (called floor ) before it gets to
max. The input to the circuit is the angle of attack and the output is applied to the elevators
and the auto-thrust.

Tracking Through VOR “Cone of Confusion”
The ‘cone of confusion’ is an area overhead a VOR navigation beacon where the signals
are unusable. Thus an aircraft transiting the VOR will receive no usable signals for a period
depending upon its ground speed and altitude.

As the aircraft approaches the VOR the radials are converging and the course deviation indicator
becomes more sensitive. At some point, before it enters the cone of confusion, the information
from the selected inbound radial becomes unusable due to the convergence.

At this point the
VOR signals are ‘cut off’ by the ‘over station sensing’ circuits i.e. the roll channel automatically
de-couples from the radio beam and controls the aircraft through the cone of confusion on
the drift-corrected heading existing when the radio signals are de-coupled. In other words the
autopilot goes into Heading Hold for a set period after which it reverts to the VOR Mode.

To avoid ‘snatching’ in a non-synchronized autopilot system, the pilot should ensure the aircraft is properly trimmed before engaging or disengaging the autopilot. This helps avoid abrupt movements and ensures a smooth transition to and from autopilot control.

With CWS engaged, the A/P manoeuvres the aeroplane in response to control pressures applied by either pilot. The control pressure is similar to that required for manual flight.

When control pressure is released, the A/P holds existing attitude.

autopilot corrections affecting pitch are indeed carried out by a combination of autotrim and elevators. Here’s how these two systems work together in the context of autopilot operations:

1. Autopilot and Autotrim:

• Autotrim is an automated system that adjusts the trim of the aircraft without pilot input. It automatically compensates for any changes in the aircraft’s configuration (like fuel burn, speed changes, or flap deployment) that could affect the aircraft’s pitch attitude.
• The autopilot, when engaged, monitors the aircraft’s flight condition and, if necessary, adjusts the trim to keep the aircraft flying in a stable attitude. Autotrim ensures that the aircraft remains in balance, reducing the need for continuous control input from the autopilot.

2. Role of Elevators:

• Elevators are the primary control surfaces that manage the aircraft’s pitch (up and down attitude). The pilot or autopilot adjusts the elevators to maintain or change the aircraft’s pitch.
• During autopilot operation, the autopilot uses elevator inputs (through electrical actuators) to make corrections to the aircraft’s pitch. However, for continuous stable flight, the autopilot may also rely on the autotrim system to adjust the elevator trim and ensure smooth, controlled flight without requiring excessive input or force from the autopilot

The EU-OPS (European Union – Operational Standards) regulations outline the requirements for autopilot systems in aircraft, specifically regarding single-pilot IFR (Instrument Flight Rules) operations and the installation and operation of autopilot systems

Autopilot Requirements for Single Pilot IFR or Night Operations:

• An aircraft operating single-pilot under IFR (Instrument Flight Rules) or at night must be equipped with an autopilot capable of at least:
  • Altitude Hold (to maintain a selected altitude) and
  • Heading Mode (to maintain a set heading).
• This means that a two-axis autopilot is required. These two axes generally refer to the pitch axis (altitude control) and the roll axis (heading control).