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AP4ATCO - Cockpit Indicators and Instruments
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19. Gyroscopic flight instruments
- Description of gyroscopic flight instruments, errors and their effects, abnormalities.
b) source (IANS) 4.1.4
c) additional sources FAA Instrument Flying Handbook (FAA-H-8083-15A) – chapters 3, 7 FAAPilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25A) – chapter 7 FAA The Pilot's Encyclopedia of Aeronautical Knowledge - chapter 6 _____________________________________
The follwing flight instruments: turn and bank indicator, artificial horizon and directional gyro indicator are using a gyroscope as the source for their indication and therefore they are sometimes referred as gyroscopic flight instruments. In order to understand the basic principle of operation of these instruments, it is necessary to explain the basic properties of a gyroscope.
A gyroscope is a fairly massive rotor, usually a wheel, mounted in light supporting rings called gimbals which have nearly frictionless bearings. It can spin about an axis in any direction. When the wheel is rotating, the gyroscope has two special properties: rigidity and precession.
Rigidity is the ability of the gyroscope to maintain its axis pointing in a fixed direction in space, unless subjected to an external force (also known as gyroscopic inertia). The rigidity is proportional to the number of rotations per minute (gyro rpm), mass of the rotor and its radius. So, gyroscope rigidity can be increased by increasing one or more of these factors.
If an external force is applied to the gyroscope to change the direction of the rotor axis, the gyro resists the angular movement and moves in a plane at a right angle to that of the applied force, the resulting movement being called precession. The precession rate is proportional to the force applied and inversely proportional to the rigidity. So, the greater the rigidity the smaller the rate of precession produced by a given applied force.
The gyroscope can be either air-driven or electric.
The system of air-driven gyroscope consists of:
- engine-driven vacuum pump that reduces the pressure inside the instrument case, and a - system of filters and hoses to supply the replacement air for the case.
The replacement air is filtered and then sucked inside the instrument case “over bucket” cuts on the rotor to make it spin like a water wheel. The system is independent of electric power and so is not vulnerable to the risk of electric failure. However, moisture, dust, and oil reduce bearing life, unbalance the frames, and so impair accuracy over time. In addition, adequate suction is difficult to maintain at high altitude (low density) and such system is subject to the risk of icing.
With an electric gyroscope, the rotor is an integral part of an AC motor. The system is subject to the risk of electric failure, but the rotor can rotate with higher and constant speed at all altitudes in a sealed instrument case, for longer life and greater accuracy.
Usually, the “stand by” instruments in aircraft are powered by air-driven system if the main gyroscopic flight instruments are powered by electrical means to provide alternate reference in case of power failure.
TURN AND BANK INDICATOR
Turn and bank indicator
The turn and bank indicator is also known as a turn coordinator. This instrument consists of two elements. One, measures the rate of roll or yaw. It is important to remember that the instrument does not display the bank angle, but the rate of turn (or the rate of bank) i.e. the number of degrees turned per second. The second part of the instrument indicates whether or not the aircraft turns are coordinated i.e. the aircraft is not slipping inwards or skidding outwards from the turn.
Sometimes the turn and slip indicator is used instead the turn and bank indicator. It has the capability of sensing the rate of turn, but not the rate of bank. This instrument will have a needle instead the symbolic airplane.
Basic operating principle and construction
The turn and bank indicator will have a gyroscope mounted with its precession axis at a 30 degree angle to the aircraft’s longitudinal axis. This allows the gyro to sense both turning and banking motion. In both cases (turn or bank) it appears that force is applied to change the position in space of the gyroscope’s axis of rotation. This will cause the gyroscope to resist the change by precession. The rate of precession is proportional to the rate of the force applied (in this particular case that is the rate of bank or the rate of turn). The rate of precession is measured with calibrated spring that transfers the indication to the pointer on the face of the instrument.
The other element is constructed of a heavy ball in a slightly curved closed tube of liquid, in which the liquid acts as a damper. This ball reacts to gravity, centripetal, and centrifugal forces, and shows whether or not the aircraft is in balance. When the aircraft is balanced the ball is centred in the tube. In all other cases the ball will be forced on one of the sides of the tube indicating skid or slip.
Interpretation The instrument’s scale is calibrated to indicate rates of turn to either side of the center zero, so the first graduation corresponds to Rate 1 standard turn (3O/sec). There may be further graduations for Rate 2 (6O/sec) or Rate half (1.5O/sec).
A turn is normally initiated by banking the aircraft which moves the aircraft symbol on the instrument to indicate the direction of bank and to enable the pilot to anticipate the resulting turn. Further on, the pilot controls the turn at a required rate according to the scale. A typical aircraft "rate one" turn is about 3° per second, which means it takes 2 minutes to complete a full orbit (360°). The balance of the turn is indicated on the second part of the instrument.
Balanced left turn at Rate 1 Unbalanced left turn. The aircraft slips inside the turn. The pilot needs to apply left rudder and/or right roll to balance the turn Unbalanced left turn. The aircraft skids outside the turn. The pilot needs to apply right rudder and/or left roll to balance the turn
Balanced right turn at Rate 1 Unbalanced right turn. The aircraft slips inside the turn. The pilot needs to apply right rudder and/or left roll to balance the turn Unbalanced right turn. The aircraft skids outside the turn. The pilot needs to apply left rudder and/or right roll to balance the turn
Errors and abnormal indications The correct reading of the turn and bank indicator is closely connected to the correct operation of its gyroscope. Abnormal indication is to be expected in case when the rotational speed of the gyroscope is not as required. This is possible when the suction is inadequate (high altitude, choked filter, leak in the suction tube system) or when there is major electrical failure in case of an electrically driven gyroscope. Two abnormal indications are possible: - Lower speed of the gyroscope. If this is the case, the rigidity of the gyroscope will be lowered. The instrument will under-read the present rate of turn. The aircraft will turn faster than expected by the pilot;
- Higher speed of the gyroscope. Opposite happens in this situation. The gyroscope rigidity is increased; consequently the instrument will over-read the rate of turn. The aircraft will turn slower than expected by the pilot.
ARTIFICIAL HORIZON Artificial horizon
The artificial horizon provides the pilot with information in terms of the aircraft’s attitude both in pitch and roll. It is primary instrument, replacing the natural horizon in poor or no visibility.
This instrument is one of the main attitude awareness tools that enable pilots to fly without external visual reference.
Basic operating principle and construction
The artificial horizon will have a gyroscope mounted with its rotation axis parallel to the Earth’s vertical. This means that the plane of rotation is always horizontal, so the gyroscope’s rigidity provides the lateral and longitudinal reference required.
When the aircraft is rolling and/or pitching, the gyroscope of this instrument maintains its position in space. In fact it is the instrument casing (together with the aircraft symbol) that is rolling and/or pitching around the gyroscope providing the required indication.
A complex control system is required to maintain the rotor axis always vertical in flight.
The instrument displays the attitude by means of pictorial representation of the aircraft flying over Earth’s surface. Aircraft symbolic representation (tail view) is fixed or painted centrally inside the glass face of the instrument. Behind this the horizon bar (representing ground and air) indicates the relative attitude of the aircraft in reference to the ground.
Nose-down attitude with left 20 O bank Nose-down attitude Nose-up attitude
Right 30 O bank Level flight no bank Nose-up attitude with left 20 O bank
Errors and abnormal indications Normal operation of the artificial horizon depends on the operation of the gyroscope and the control system responsible to maintain its rotational axis vertical. Sophisticated and complex control systems are capable of correcting the erroneous indications in the following cases: - Acceleration errors. Due to relatively big mass, the gyroscope is affected by acceleration. When an aircraft accelerates in level flight (during take off) false nose-up and right hand turn indication occur while deceleration will cause false nose-down and left turn indication, in cases when the rotor is rotating anticlockwise when viewing from above. If the rotor spin is clockwise the artificial horizon will give opposite errors;
- Turning error. Whenever the aircraft turns there is acceleration to the center of the turn (centripetal force). Similar like in previous case this affects the artificial horizon to over-read or under-read the bank angle by approximately 2O during the turn.
Abnormal indications are possible in the following cases:
- Limitations. The amount that the instrument case can move around the gyroscope is limited and controlled by fixed stops. If these limits (approx. ±85O in pitch and ±110O in roll) are reached then the fixed stop is acting like a force to the gyroscope. Therefore the gyroscope precession moves the rotational axis from the vertical. The indication is violent and erratic until the gyroscope aligns again by itself (10 to 15 minutes). However, on modern aircraft there are sophisticated control systems that can significantly reduce the required period to realign the gyro;
- Lower speed of the gyroscope. This is possible when the suction is inadequate (high altitude, choked filter, leak in the suction tube system) or when there is major electrical failure in the case of an electrically driven gyroscope. When this is the case, the rigidity of the gyroscope will be lowered. The instrument reading will be erratic and unreliable. DIRECTIONAL GYRO INDICATOR
Directional gyro indicator
The directional gyro indicator provides more stable directional reference in azimuth for maintaining accurate headings and for executing precise turns than the magnetic compass, which gives fluctuating readings during turns, accelerations and turbulence. However, the directional gyro indicator has no magnetic element and must initially be synchronized with the magnetic compass. It can not replace the magnetic compass.
Basic operating principle and construction The directional gyro indicator will have a gyroscope mounted with its rotation axis in the aircraft’s yawing plane. This means that the rotation axis is horizontal in level flight, and because of gyroscopic rigidity it provides the datum from which heading can be measured. When the aircraft is turning, the gyroscope of this instrument maintains its position in space. As for the artificial horizon, it is the instrument casing that turns around the gyroscope providing the required indication. A complex control system is required to maintain the rotor axis always in the yawing plane of the aircraft.
Interpretation The compass card on the instrument face has letters for the cardinal headings N, E, S, and W. Each numbered interval is every 30 degrees. The graduations are further divided by the longer marks every 10 degrees, and intervening short marks at the 5 degree points. The aircraft symbol is fixed (usually painted on the glass).
Errors and abnormal indications The direction gyro indicator is the most sensitive to erroneous and abnormal indication in comparison to the other gyroscopic instruments. Although the control systems are very complex and sophisticated, the following errors and abnormal indications have to be considered: - Instrument error. Due to manufacturing imperfection and friction the gyroscopic precession will misalign the gyroscope over time;
- Apparent error due to Earth’s rotation. Because the rotor axis remains still in space while the Earth is rotating beneath it will appear to the observer on the ground that the gyroscope has changed its position. The error rate depends on the latitude. It is equal to 0 on the equator and to 15 O /hour on the poles;
- Apparent error due to transport. Similar happens when the aircraft is flying from one position to another when a change of latitude is involved. Even if the apparent error due to Earth’s rotation is corrected, over time as aircraft travels and changes latitude the indication will have to be corrected again;
- Limitations. Same as for the artificial horizon the amount that the instrument case can move around the gyroscope is limited and controlled by fixed stops. If these limits are reached the indication is violent and erratic until the gyroscope aligns again by itself or by the control system;
- Lower speed of the gyroscope. As for the other gyroscopic instruments, when this is the case, the rigidity of the gyroscope will be lowered. The instrument reading will be erratic and unreliable.
In order to ensure correct indication of the directional gyro indicator it is necessary to align it with the magnetic compass at regular intervals, approximately every 10 to 15 minutes.
On some modern aircraft this alignment is performed automatically by the control system. Good example is the gyrosyn compass, a version of the directional gyro which automatically aligns itself to the earth's magnetic field via sensors located in the tail or wing tip, away from any magnetic fields generated by the aircraft’s equipment.
1. [Question type: true or false]
Q: In case of lowered speed of the gyroscope of turn/bank indicator, the instrument will under-read the rate of turn. A1: True A2: False
Correct answers: A1
2. [Question type: true or false]
Q: The amount that the artificial horizon case can move around the gyroscope is limited approx. ±85O in pitch and ±110O in roll
A1: False A2: True
Correct answer: A2
3. [Question type: multiple choice]
Q: In most general aviation airplanes it is necessary to align the gyro indicator with magnetic compass at regular intervals. Those intervals are:
A1: 1-2 minutes A2: 10-15 minutes A3: 30-40 minutes
Correct answers: A2
20. Cockpit indicators and instruments