Basic theory of flight

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In order to understand the whole functioning of an automatic landing system, the very basic idea of how a stable flight is maintained gets mandatory. This chapter will focus on all the basic forces acting on an aircraft and the stability pattern involved. After this the control axes will be studied along with stability around control axes.

It is essential to know how an aircraft maintains a stable flight and how an autopilot works to stabilize an aircraft in air. It is only after this that Automatic Landing systems can be studied and understood and only then the design area of this project can be studied.

Basic Forces On An Aircraft

Four basic forces act on an Airplane

  • Lift
  • Weight
  • Thrust
  • Drag

Lift and Weight:
Let us consider these forces, the aircraft gets its lift from the wings and this acts vertically upwards and the aircraft's weight acts vertically downwards, so to fly straight and level lift must be equal to weight.

Thrust and Drag:

The thrust comes from the engines and the drag is the resistance to the air from such things as engine mountings, fuselage, etc.

If any of these component forces are out of equilibrium, e.g. lift were greater than weight, the aircraft would go higher and conversely if weight was greater than lift then the aircraft would descend.

The other two components speed and drag, if out of equilibrium, will either increase or decrease speed.


The tendency of an aircraft to return to its original trimmed attitude, after having been disturbed is called stability.

If a sudden down current, of air causes the nose of an aircraft to lower, and if after the disturbance is over, the aircraft automatically (i.e. no help from the pilot) returns to its original attitude, it is said to be stable.

If after being disturbed the aircraft does not return to its original attitude but stays exactly in the attitude to which it was moved by the disturbance, then it is said to be neutrally stable.

If an aircraft has been disturbed from its original attitude and, once disturbed, moves progressively further and further away from its original attitude, it is said to be unstable.

Control Axes

An aircraft can move or deviate from its attitude in various ways. It can move up, down or sideways. These movements are realized around any or all of the three axes which are known as the control axes.

These control axes are:

  • Longitudinal Axis
  • Lateral Axis
  • Directional Axis

The aircraft,

  • Pitches about Axis A - Lateral axis.
  • Rolls around Axis B - Longitudinal axis.
  • Yaws about axis C - Normal axis.

The point where all these axes intersect each other the Center of Gravity ( C of G) of the aircraft.

Stability Around Control Axes

Longitudinal Stability

In this figure the lateral axis is shown going from wing tip to wing tip. Aircraft movement about that axis is known as pitching, but in stability terms the movement of an aircraft about the lateral axis is called longitudinal stability. This is because it is the longitudinal axis which actually does the moving although the moment is about the lateral axis.

Lateral Stability

The axis running fore and aft through the C of G is called the longitudinal axis, and movement about this axis is called rolling. If a stable aircraft has a disturbance whereby the wing tips move up or down and the aircraft returns to its original attitude, then the aircraft is said to have lateral stability.

Directional Stability

The normal axis on an aircraft is vertical and passes through the C of G, and the movement about it is known as Yawing. The stability of an aircraft about the normal axis is called directional stability. It can also be noticed that the movement about the normal axis involves movement of both the longitudinal and the lateral axes. If a stable aircraft has a disturbance because of which it moves about its normal axis and returns to its original attitude, then the aircraft is said to have directional stability.

Until now, we have made all the previous observations based on an aircraft in a straight and level flight. Now we will make some adjustments to our conclusion when the scenario changes.

Here, Lift resultant should be equal to Weight resultant based on simple vector.

As it can be seen in Fig 1.8, when the aircraft rolls to one side, the lift resultant is slightly inclined in the direction on the lower wing whereas the weight resultant remains vertical. While this happens it leads to a phenomenon called side slipping.

When the lift resultant moves to the left, (i.e. when the aircraft rolls to the left) the aircraft will also move to the left, because there will be a sideways force produced. This sideways movement, which occurs when rolling, is called side slipping.

Automatic Pilot System Principles

Autopilot Control Axes

Now let's look at the control axes with reference to automatic control. The purpose of an automatic control system is to keep an aircraft on a correctly stabilized flight path by sensing and correcting any departures from the flight path. Around the three axes of movement three systems can exist.

  • Single axis system
  • Dual axis system
  • Triple axis system
  • Single axis system - This relates to a basic form of control, and it is usually performed in the roll attitude by the ailerons. In an aircraft with this type of equipment, the pilot would input a command to turn it automatically, other inputs might be compass or radio navigation signals.
  • Dual axis system - The attitude control in the dual configuration will normally be via the pitch and roll axis, thus controlling the elevators and the ailerons. An additional mode of operation in dual axis systems is altitude hold.
  • Triple axis system - This is a system which uses the automatic control of the aircraft in the pitch, roll and yaw planes, and it is more usually associated with modern high performance aircraft. Additional inputs might be air data and inertial navigation.

To better understand the concept mentioned above, let's consider the following example and let's assume a pitch function described in Fig2.1. If there is a change in attitude because of some turbulence, the attitude reference will note the change in pitch and it will send a signal to be processed by a pitch computer, and then by some servomechanism the elevators will reposition. The servomotor then sends a feedback signal to reduce the input error signal and limit the control. This signal is equal and opposite to the input error signal and the resultant will stop the servomotor and the elevator in a position which will return the aircraft back to its correct attitude. When the aircraft is returned to its correct position, the error signal will reduce and the elevator will NULL out to its original position.

This is also a simple example of a Single axis system.

Basic Control System Logic Used

There are two basic control system logic used to sense a change in attitude and respond to the change accordingly.

These are

  • Rate
  • Proportional


In a rate system, the aircraft cannot respond and communicate with the system informing it about the correction already made. This leads to a lot of oscillation.


The proportional system will produce a control surface movement proportional to the value of the input signal

2.2 - Rate and Proportional system

Automatic Landing System.

This chapter will describe the principles and other necessary condition related to automatic landing system.

Principles Of Auto-Landing System.

Automatic landing system is not a very new development in the aircraft industry, in fact in the very early stages of aircraft development a very simple and basic form of auto-landing existed.

The aircraft then would have two wires attached to it with different weight and set-up at different lengths and when the weight touched the ground it would release a spring controlled switch which would light up the corresponding display in the cockpit for the pilot's assistance.Another utility that was available was a lever type of object that was arranged at a particular angle around the undercarriage, this would be attached to the control apparatus and when activated just before touchdown, it would move the control apparatus to the rear and then at this point the engines would be turned off.

The idea used for auto-land was improved, and for this it was required to input signals from the ground instrument landing system (ILS) into the automatic flight control which would control the required Pitch/Roll function of the aircraft during its descent.

The signals are called localizer and glide slope. The localizer is the signal beam which is in line with the runway and the glide slope gives the landing glide path. A simple radio altimeter would control other maneuvers such as flare, kick off drift and touchdown.


While using an automatic landing system it is essential that any error must be rectified well in advance because of the fact that the aircraft is at a very low altitude and chances of accidents thus are very high. The system will thus monitor all parameters at all times and allow the pilot to take control whenever he/she wants at their will in case of critical faults.

Below mentioned are some of the very basic requirements which have to hold on for a successful auto-land system.

  • Any failure of the systems redundancy must be alerted to the pilot by any visual or audio warning.
  • A dangerous condition should not be evident itself as the result of a single fault.
  • The loading of the aircraft must be perfect.
  • In case of any failure, no unusual movements must be demanded.
  • Roll must be within tolerable limits
  • Modes of operation must be visible in cockpit at all times.

Approach Categories.

The decision height - This is the height below which the pilot must never go below unless required visual reference has been made. This height is monitored by a radio altimeter.

Runway Visual Range - This is the maximum distance along the runway where the landing lights are visible to naked eye. This information is relayed to the pilot by the air traffic controller and this provides the pilot with the latest information on runway visibility.

CAT 1 - For a successful category 1 landing the decision height must be 200 ft and runway visual range must be 800 meters.

CAT2 - For a successful category 2 landing, the decision height must be between 200-100 ft and the runway visual range must be between 800-400 meters.

CAT3a - Here the decision height could be anywhere between ground level up to 100 ft and the runway visual range must be at least 200 meters for a successful land.

CAT3b - In this category the decision height could be very well along the runway with external visual reference and the runway visual range should be at a minimum of only 50 meters.

CAT3c - This category can work along the runway and it needs no external reference at all.

System Reliability And Redundancy.

There are two types of failures what can occur in an automatic landing system; they are Fail-soft and Fail-operational.

  • Fail-soft - this describes the ability of the system to handle a failure without risking passenger safety and without making very large deviation from flight path.
  • Fail-operational - This is a system in which one or more failures can happen, but this failure would not jeopardize the landing procedure.

In order to maintain a safe landing procedure and to reduce the errors in the capture of events, sometimes more than one autopilot systems are made to work together.

Simplex - This defines a single autopilot system and its appropriate number of channels. A single failure in this will shut down the system.

Multiplex - This is a system with more than one sensibly independent system and sub-channels used collectively so that in case of any failure to one component the remaining components are alone capable of performing controlling procedures.

Duplicate-monitored - This arrangement has two systems connected in parallel and with separate power supplies. These systems keep checking themselves against each other with the help of comparator circuits. In this only one system is working at a time and incase of requirement the other system enables itself.

Automatic Landing Sequence.

The profile of an automatic approach, flare and landing sequence is illus­trated in the figure and 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 already defined. Depending upon the number of channels that are available, the system performs a 'LAND 2' status or 'LAND 3' status auto-land. Thus, 'LAND 2' signifies there is dual redundancy of engaged flight control computers, sensors and servos (fail passive operation) while 'LAND 3' signifies triple redundancy of power sources, engaged flight control com­puters, sensors and servos (fail operational). Each status is displayed on an auto-land status display.

During cruise and initial stages of approach to land, the control system operates as a single channel system, controlling the aircraft about its pitch and roll axes and providing the appropriate flight director commands. Since multichannel operation is required for an automatic landing, at a certain stage of the approach, the remaining two channels are armed by pressing a switch on the flight control panel. The operation of this switch also arms the localizer and glide slope modes. Both of the 'off-line' channels are continually supplied with the relevant outer loop control signals and operate on a comparative basis the entire time.

Altitude information essential for vertical guidance to touchdown is always provided by signals from a radio altimeter.

When the aircraft has descended to 1,500 feet radio altitude, the localizer and glide slope beams are captured, and the armed 'off-line' control channels are then automatically engaged. The localizer and glide slope beam signals control the aircraft about the roll and pitch axes so that any deviations are automatically corrected to maintain alignment with the runway. At the same time, the auto-land status displays 'LAND 2' or 'LAND 3', depending upon the number of channels 'voted into operation' for landing the aircraft, and flare is also armed.

At a radio altitude of 330 feet, the aircraft's horizontal stabilizer is automatically repositioned to begin trimming the aircraft to a nose-up attitude. The elevators are also deflected to counter the trim and to provide subsequent pitch control in the trimmed attitude.

When an altitude is reached at which the landing gear is 45 feet above the ground (referred to as gear altitude) the flare mode gets engaged automatically. The gear altitude calculation, is based upon radio altitude, pitch attitude, distance between the landing gear, fuselage and the radio altimeter antenna. The flare mode takes over pitch attitude control from the glide slope, and generates a pitch command to bring the aircraft onto a 2 feet/second descent path. At the same time, a throttle retard command signal is supplied to the automatic throttle control system to reduce engine thrust to the limits compatible with the flare path. Prior to touchdown, and about 5 feet gear altitude, the flare mode is disengaged and there is transition to the touchdown and roll-out mode. At about 1 foot gear altitude, the pitch attitude of the aircraft is decreased to 2°, and at touchdown, a command signal is supplied to the elevators to lower the aircraft nose so that the landing gear can make contact with the ground all the way till the roll out. Then the reverse thrust is applied and the auto throttle system is disengaged.

Go- Around Mode.

This mode can be activated when the altitude of the aircraft is less than 2000 feet. If the pilot decides not to land or is not satisfied with the performance of the automatic landing procedure, he can simply press the Go-around button which will automatically increase the thrust to full power and this will also pitch the aircraft to a safe nose up angle to start altitude ascend.

Aircraft Landing Performance Enhancement.

Now that we know the efficiency of the automatic-landing system, if we optimize the landings at a busy airport we can then boost an airport's efficiency. If this optimized data is somehow incorporated into the automatic flight control system then we can have a totally independent solution which would automatically lead to better airport efficiency.

A sample is shown below with graphical simulation.

Current Issues in Automatic Landing System.

Even though the automatic landing system being used now is perfect for standard conditions of weather, but there is still some concern when it comes to implementing it in snowy conditions where the surface of the runway gets altered because of different level of snow at various points. This happens because of how the glide slope information is relayed to the aircraft. Usual snowfall does not really create any hassle for the working of the current automatic landing system, but in some rare and unique conditions it can adversely reduce the performance of the automatic landing system.

The glide slope which an instruments landing system projects uses a pair of horizontally paired antennas on the side of the runway and then a very high frequency (VHF) pattern is created because of the ground reflection which makes a null at chosen elevation angle. It is this that forms an inverted cone which determines the glide slope angle.

In case where the ground surface is not even, then the glide slope formed will be altered and this could cause the auto-land system to fail. Although this is a very extreme situation but this is possible, and this is why there are very strict regulations regarding placement of obstructions in this region and it is a rule to remove accumulation of more than 15-20 inches.


1. E.H.J. Palett and Shawn Coyle, Automatic Flight Control 4th ed., Blackwell Science, Great Britain, 1995.

2. Avionics Fundamental training guide published, prepared and developed by United Airlines.

3. Boeing 747-400 Maintenance Manual, 2003.

4. Walton Eric, Effect of Wet Snow on the Null-Reference ILS System, IEEE Transactions on Aerospace and Electronic Systems, Vol. 29, No. 3, July 1993.

5. Byung J. Kim, Computer Simulation Model for Airplane Landing-Performance Prediction, 1996.

6. V. Ungvichiangtd, Terrain Reflection Model Applied to ILS Glide Scope, January 1982.

7. Airbus 320 Maintenance Manual, 2003.