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ELECTRICALLY DRIVEN ARTIFICIAL HORIZON.
These gyros contain rotors which are electrically driven with the gyro rotor acting as the armature of an induction motor. Brush assemblies are not used since the friction would result in abnormal precession. The rotor/armature is inductively excited. The electric artificial horizon is subject to the same operational requirements of the air driven artificial horizon, except for the method of obtaining rotor rotation and the design of the erection mechanism.
OPERATIONAL LIMITATIONS.
Unlike the directional gyro, the artificial horizon has no attitude limits. If, however, pitch reaches 90°, the horizon bar display will "tumble" 180° to an inverted position and will again "tumble" 180° when the aircraft returns to a less extreme flight attitude. The modern-day artificial horizon will not be damaged by such an attitude excursion and will correct itself in a short time. There are no roll limitations to the present artificial horizon. Another important but not widely understood operating limitation of air driven instruments is erection of the horizon bar from a full stop and from a residual running condition. This can lead to wasted service time and invalid operating complaints. When the gyro rotor is at rest and power is applied, the erection mechanism exerts maximum authority and rapid, noticeable erection results. However, if power is removed from the spinning rotor (such as when an engine is shut down while briefly discharging passengers) the gyro rotor continues to rotate at high speed but the erection mechanism is not functional. When power is again applied to
34-20-00 Page 34-13 Issued: April 1, 1997
the air driven artificial horizon the erection mechanism again begins to function. However, due to gyro rigidity because of high rotor speed, erection of the instrument takes considerably longer than normal. In flight, the air-driven artificial horizon exhibits small errors at roll out after a coordinated turn, skids and small pitch changes after acceleration and deceleration. The electric artificial horizon exhibits small errors in pitch and roll out from a coordinated turn and also small pitch changes after acceleration or deceleration. In both cases, the erecting mechanisms quickly return the gyro to its proper position. The electric artificial horizon is considered generally more efficient in operation and less subject to error than the air driven artificial horizon.
TROUBLESHOOTING.
Unless an obvious malfunction, such as inability to erect, spinning, or great horizon bar displacement, none of which can be corrected by manually caging the instrument, requires repair or replacement of the instrument, service is restricted to the instrument installation and power source. Typical installation examples of artificial horizon malfunctions are due to such problems as: restricted air flow due to air line kinks or leaks, contaminated air filters, deteriorating electrical grounds, sagging instrument panel shock mounts, systems regulators, faulty vacuum/ pressure gauges.
— Note —
Air pressure must be 4.5 - 5.2 hg.
Only after the system has proven to be good should the instrument be “pulled” for replacement or repair.
DIRECTIONAL GYRO.
Both air and electrical directional gyros are used, depending on the option package installed. Both types are displacement type gyros with "free" rotors mounted in gimbal assemblies. It is important that if a magnetic slaving system, flight director or autopilot is coupled to the DG, the A. F. C. S. manufacturer's Service Manual should be consulted.
AIR DRIVEN DIRECTIONAL GYRO.
The air driven directional gyro is driven by the pneumatic system which is supplied by engine driven dry-pneumatic pumps either on pressure or vacuum. It is of prime importance to realize that air VOLUME, and not air pressure, spins the gyro rotor. The air filter can become contaminated and restrict airflow, reducing gyro rotor speed, while the pressure regulator will automatically adjust air pressure within proper limits.
— Note —
The gyro air filter must be clean or replaced before adjusting gyro air pressure.
Airflow directed at the gyro rotor vanes, causes the rotor to spin approximately 17,000 to 22,000 RPM, thus providing the gyroscopic ability to remain rigid in space. The instrument case moves freely about the spinning gyro rotor in three dimensions by the use of a Gimbal Assembly and the displacement or Azimuth readings are presented on the instrument face. This results in a positive and stable presentation.
Since the directional gyro has no reference to magnetic north, it must be set from the magnetic compass. The directional gyro will agree only with the magnetic heading from which it was set, since an other subsequent magnetic compass headings are subject to deviation, northerly turning, acceleration, deceleration
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