Artificial Horizons - Attitude Indicators

The artificial horizon (gyro horizon) comprises of a vertical spin axis earth gyro having freedom of movement in all three planes, and indicates the aeroplane attitude relative to its pitch (lateral) and roll (longitudinal) axes, which is essential when a natural horizon is unavailable, e.g. When flying in cloud.

The instrument is either air or electrically driven, although the principal of operation is identical. The gyro spin axis is maintained vertical with reference to the centre of the earth, and a bar positioned at 90° to the spin axis represents the local horizon.

A symbol representing a miniature model aeroplane is fixed to the instrument case, and represents the rear view of the true aeroplane, which on some instruments is adjustable to suit the pilots own eye level, and the particular aeroplane pitch trim setting. A typical artificial horizon display is shown on the next page.

In flight, the aeroplane's movement about its pitch or roll axis is indicated instantaneously by movement of the case relative to a horizon (natural horizon) bar, which is held in the local horizontal by gyro rigidity. The position of the model aeroplane relative to the bar represents the attitude of the aeroplane to the natural horizon, whilst the position of a pointer relative to a fixed scale represents the aeroplane's angle of bank.

Dive and climb are indicated by the model aeroplane moving up and down with respect to the horizon bar, whilst the angle of bank is indicated by the model aeroplane appearing to bank in relation to the horizon bar. The indications expected during various flight attitudes are shown below.

The exact angle of bank is indicated by a pointer at the bottom of the instrument, and provides a direct indication of any change of attitude, without any lag being involved.

Air driven (Classic) Artificial Horizon

A schematic view of an air driven artificial horizon is shown on the next page. This type is commonly used in light aeroplane, and as a standby instrument in commercial aeroplanes. It is operated by a vacuum pump, which evacuates the air from the instrument case and gyro housing (inner gimbal). This creates a depression within the instrument, and the surrounding atmosphere enters the instrument through a filtered inlet.

The air then passes through channels to jets mounted within the inner gimbal, which direct air onto buckets cut into the periphery of the rotor, and cause the rotor to rotate at approximately 13,000 rpm, in an anti-clockwise direction when viewed from above.

The air is then evacuated through a pendulous unit, mounted below the rotor casing, via four ports that are controlled by two pairs of linked pendulous vanes, and provide a mechanism for maintaining the gyro spin axis in its vertical plane.

The rotor spins about a vertical axis (Z-Z1) and is mounted in bearings within a sealed case, which forms the inner gimbal.

The inner gimbal is mounted in bearings within a rectangular shaped outer gimbal and is free to rotate 55° either side of its horizontal position, about the lateral axis (Y-Y1). This enables the aeroplane's pitch attitude to be determined, and is directly indicated by movement of the horizon bar. The horizon bar arm is actuated by a guide pin, which protrudes from the gyro stabilised rotor housing (inner gimbal), and moves in a curved guide slot in the outer gimbal.

The outer gimbal is mounted in an air tight instrument case, with its pivots along the fore and aft axis (X-X1), and is free to rotate through 110° either side of its central position, in order to determine the roll attitude of the aeroplane. A background plate representing the sky is fixed to the front end of the outer gimbal and carries a bank pointer, which registers against a bank-angle scale. Movement in both cases is limited by resilient stops, which prevent any internal damage to the instrument.

The instrument is gyro stabilised, and arranged so that when the gyro is erect, the horizon bar is horizontal with reference to the earth's surface, and the angle of bank pointer is in its centre position, showing the gyro to be vertically erect with reference to the earth's surface.

Bank indication is given by an index on the sky plate, which reads against a scale printed on the glass face of the instrument. When the aircraft banks, the rotor, inner gimbal and outer gimbal remain rigid in their level position, whilst the instrument case, and hence printed scale, moves with the aircraft; thus the position of the sky plate index indicates the aircraft's bank angle against the scale.

Operation of an Air driven Artificial Horizon

During level flight the aeroplane's vertical axis is parallel to the rotor spin axis, with the guide pin in the centre of the slot in the outer gimbal, and the horizon bar centralised. During a climb or descent, the rotor, and hence inner gimbal will remains rigid with reference to the local vertical, whilst the outer gimbal and instrument case will move with the aeroplane, and turn about the Y-Y1 axis.

When the aeroplane starts to climb, the rear of the instrument case and outer gimbal will follow the nose of the aeroplane and will rise up. This will cause the guide pin to move, relative to the inner gimbal, thus displacing in the slot in the outer gimbal, and placing the horizon bar below the model aeroplane, giving a relative indication of a climb.

Conversely when the aeroplane starts to descend the rear of the instrument case and outer gimbal will be depressed with the nose of the aeroplane. The movement of the guide pin will cause the horizon bar to move above the model aeroplane, thus providing a relative indication of a dive.

During a roll manoeuvre the instrument case and model aeroplane will rotate about the fore and aft axis (X-X1), but the gyro assembly, including the inner gimbal, outer gimbal and horizon bar will remain level. The model aeroplane will thus turn in relation to the horizon bar, and will provide an indication of bank.

Air Driven Artificial Horizon Erection System

The air driven instrument incorporates a mechanical pendulous vane unit, which erects the gyroscope into its vertical position, and also maintains its spin axis in that position during its operation.

The unit is fastened to the underside of the rotor housing and consists of four knife-edged, pendulously suspended vanes, which are fixed in diametrically opposed pairs, on two shafts supported in the unit body. One shaft is parallel to the pitch axis (Y-Y,), whilst the other is parallel to the roll axis (X-X,) of the gyroscope.

In the sides of the unit body are four small, elongated ports, one located under each vane. Suction air, after spinning the gyro rotor, is exhausted through the ports, and the reaction of these diametrically opposed streams of air applies a force to the unit body.

The vanes, under the influence of gravity, always hang in the vertical position, and govern the amount of airflow from the ports. They also control the forces applied to the gyroscope by the exhaust air reaction forces. When the gyroscope is in its vertical position the knife-edge of each vanes will equally bisect each port, thus making all four port openings of equal dimension, as shown on the next page.

The air reactions will similarly be equal, and the resultant forces about each axis will be in balance. If the spin axis is however displaced from its vertical position, as shown below, the pair of vanes positioned on the Y-Y, axis will remain vertical, thus fully opening one port whilst the diametrically opposing port will be fully closed.

The increased reaction force produced by the air being expelled from the fully open port will result in a torque being applied to the gyro body in the direction of the arrow, and thus according to the law of precession, the unit will rotate about the pitch axis (Y-Y1),. The spin axis will therefore be returned to its local vertical or erect position, when the vanes will again equally bisect the ports, and will result in equal reaction forces again.

Errors Associated with the Air Driven Artificial Horizon

The air driven artificial horizon suffers from both acceleration and turning errors, and for the purpose of explanation it is assumed that the gyro rotor rotates in an anti-clockwise direction when viewed from above.

Acceleration Errors

This error is also known as the ‘Take-off error', since is most noticeable during the take-off phase of flight, and is caused by the pendulous unit and its associated vanes. The pendulous unit makes the rotor housing (inner gimbal) bottom-heavy, so that when the aeroplane accelerates, a force due to the unit's inertia, which is effective at the bottom of the rotor system, will act in the direction of the flight crew. The resulting force will be precessed through 90° in an anti-clockwise direction, and will lift up the right-hand side of the outer gimbal.

This will cause the sky-plate, which is attached to the outer gimbal to rotate anti-clockwise, and will indicate a false turn to the right against the bank angle index. Additionally during the acceleration both of the laterally (left and right) mounted side vanes will additionally be thrown aft, with the result that the right-hand side port will fully open and the left-hand side port will fully closed.

This will in turn produce a reaction force on the right-hand side, which when precessed through 90°, will lift the inner gimbal, and will indicate a false climb. A classic artificial horizon will thus indicate a false climbing turn to right during the take-off phase of flight.

Turning Errors

During a turn the longitudinally (fore and aft) mounted vanes on the air-driven artificial horizon will be displaced due to the centrifugal force acting on the pendulous unit. This will cause one port to open, whilst the opposing port will close, and a reaction force will be set up along the fore and aft axis of the aeroplane (X-X1).

After precessing the force through 90°, it will tend to lift the outer gimbal on the left or right hand side depending on the direction of the turn. This will result in a false bank indication, or 'Erection Error'. It follows that during a left turn the instrument will indicate a reduced left bank indication, whilst during a right turn the instrument will indicate a reduced right bank indication.

The centrifugal force will additionally cause the pendulous unit to swing outwards in the opposite direction to that of the turn, which will cause the inner gimbal to give a false indication of climb or descent. This is alternatively known as a ‘Pendulosity Error'. During a left turn the classic artificial horizon will indicate a false climb, and during a right turn will indicate a false descent.

These two forces will act together, and during a 360° turn will reach a maximum value at 180°, and return to zero when the turn is complete. In modern gyroscopes however the axis of rotation is slightly offset from its true vertical to counter these errors, although this is only valid for one particular rate of turn, and airspeed. The scales are similarly offset so that the indications are not affected during straight and level flight.

Construction of an Electrically Driven Artificial Horizon

An electrically driven artificial horizon is shown.

It is made up with the same basic components as the vacuum-driven type, except that the vertical spin axis gyroscope is a squirrel-cage induction motor. Unlike conventional induction motors, where the rotor normally revolves inside the stator, in order to make the motor small enough to be accommodated within the space available in a modern miniaturised instrument, the rotor is designed so that it rotates in bearings outside the stator. This ensures that the mass of the rotor is concentrated as near to the periphery as possible, thus ensuring maximum inertia, and adequate rigidity.

The "squirrel-cage motor" design is not only used in the artificial horizon, but is also used in other instruments that employ electrical gyroscopes. The motor assembly is carried in a housing that forms the inner gimbal, and is supported in bearings in the outer gimbal, which in turn is supported in bearings in the front, and rear casing of the instrument.

The horizon bar assembly is in two parts, and like the air driven version is similarly pivoted at the rear of the outer gimbal. The instrument is fitted with a torque motor erection system, which maintains the gyro in its vertical axis. The electrical motor rotates the rotor at approximately 22,500 rpm, and if the power supply fails it is indicated by a solenoid-actuated ‘OFF' flag, which appears in the face of the indicator.

Errors Associated with the Electrically Driven Artificial Horizon

The electrically driven artificial horizon like the air driven derivative similarly suffers from both acceleration and turning errors.

In the case of the electrically driven gyro horizon, the inner gimbal does not have a pendulous erection unit hanging below it as in the case of the air driven or classic version, and is therefore not subject to the apparent turn component of acceleration error.

However, the mercury in the longitudinally mounted switch will hang back and complete the circuit to the pitch torque motor, and will cause the instrument to indicate only a false climb, and not an apparent climbing turn to the right, as in the case of the air driven variant.

Turning Errors

The sole effect on an electrically driven gyro is to displace the mercury in the lateral mercury switch, to complete the circuit via one or other of the outer electrodes to the roll torque motor, causing the instrument to indicate only a false bank, and not a false indication of turn and climb or descent, as in the case of the air driven variant.

Remote Vertical Gyro

On many modern aeroplanes, the attitude indicator is fed from a remote vertical gyro unit, which is normally sited in the avionics bay. This gyro works in the same way as the electrical gyro just described except that it is not linked directly to a presentation. Pitch and bank data is fed to the remote (panel mounted) indicator by means of an electrical synchro transmission system.

The same attitude information can also be fed to the autopilot so that it can use the same data as the flight crew are viewing. The biggest advantage of using the remote vertical gyro is that it can provide greater degrees of freedom, and also the indicator can be constructed to present all attitudes with virtually unlimited freedom.

Standby Attitude Indicator

Many modern aeroplane employ integrated flight systems, which include indicators that can display not only pitch and roll attitude data from a remotely located vertical axis gyroscope, but also associated guidance data from radio navigation systems.

In these systems there is no longer a need for a separate artificial horizon to be fitted, but in order to satisfy the airworthiness requirements one has to be fitted, as a standby attitude indicator. This provides the necessary indication should the circuits controlling the aeroplane attitude display fail.

An example of the face of a typical standby indicator is shown above. This instrument uses an internal gyroscope, which is electrically operated and is powered during normal operation by the aeroplane's II5V 3-phase supply. If the normal power supplies fail a static inverter, will provide 28V DC from the battery busbar, and will automatically supply the standby artificial horizon. Power from this source is always available, so attitude indications are continually displayed.

In place of the conventional stabilised horizon bar method of displaying pitch and roll, a stabilised spherical element is adopted as the reference against an aeroplane symbol. The upper half of the element is coloured blue (sky) to display climb attitudes, while the lower half is black, to display descending attitudes. Each half is graduated in l0° increments up to 80° climb and 60° descent.

A pointer and scale indicates the bank angle in the normal manner. The indicator also has a pitch-trim adjustment and a fast-erection facility. When the knob is rotated in its "IN" position, the aeroplane symbol may be positioned through ±5° variable pitch trim. Pulling the knob out, and holding it, will alternatively energise a fast-erection circuit.