Artificial Horizon (AH)

Artificial Horizon

Display of an Artificial Horizon

• Artificial horizon provides attitude of aircraft using a vertical axis gyro
• The gyro spin axis is maintained vertical to earth surface by gravity
• Gull wing etched on the outer glass moves with aircraft in pitch and roll
• The horizon image attached to an earth gyro maintains its orientation
• Nose up indication shows gull wing above the horizon
• Gull wing attached to outer gimbal moves down by a guide pin system
• Horizon bar attached to the inner gimbal retains its vertical position
• Artificial horizon indications are limited to 60 and 110 degrees in pitch and roll

Principle of Artificial Horizon

• Pitch-up indication is shown by gull wing above the horizon
• Nose-up movement rotates the case and attached outer gimbal up
• Guide pin protruding from the inner gimbal maintains the horizon bar down
• Horizon bar is now below the gull-wing producing the nose-up indication
• Pitch-down indication is shown by gull wing below the horizon
• Nose-down movement rotates the case and attached outer gimbal down
• Guide pin protruding from the inner gimbal maintains the horizon bar up
• Horizon bar is now above the gull-wing producing the nose-down indication

Self Correction in Air Driven Artificial Horizon

• Vertical gyro in artificial horizon cannot drift but can topple
• Two pairs of pendulous vanes are designed to auto correct gyro topple
• Topple of spin axis opens one wane and closes its opposite one
• Imbalanced pressure of exiting air creates an external force
• This force is precesses through 90 degrees to self correct topple

Acceleration Errors Due to Self Correction

• Acceleration in level attitude induces errors due to self correct mechanism
• Artificial horizon will indicate a false nose up attitude
• Pitch error is due to the effect of acceleration on the pendulous vanes
• Right wing will appear to be down on the instrument
• Roll error is due to the inertia of the bottom-heavy rotor housing
• Acceleration and deceleration will therefore result in false information
• Pitch up with right bank during acceleration
• Pitch down with left bank during deceleration

Turning Errors Due to Self Correction

• Turns induce errors due to centripetal force on the artificial horizon
• Turn through 90 deg – under-reads bank-angle – pitch up indication
• Turns through 180 deg – correct bank-angle – pitch up indication
• Turns through 270 deg – over-reads bank-angle – pitch up indication
• Turns through 360 deg – correct bank-angle – correct indication

Electrical Artificial Horizon

• Electrically driven artificial horizon uses electrical motor
• The rotor rotates clockwise and is tied to gravity by two mercury switches
• The switches correct for pitch and roll axis on base of rotor
• Spin axis is retained vertical to the earth surface
• Mercury switches are connected to torque motors
• Switch for rolling plane is paired with torque motor on inner gimbal
• Switch for pitching plane is paired with torque motor on outer gimbal

Advantages of Electrical Artificial Horizon

• Electrically driven artificial horizon has some advantages
• Powerful electric motor provides high spin rate
• Artificial horizon has high rigidity and low precession
• Instrument is more accurate and sticks more to earth reference
• Fast erect system erects gyro at 120 deg per minute
• Reduced acceleration and turn errors as compared to air driven gyro
• Does not have a heavy erection chamber or pendulous vanes

Errors of Electrical Artificial Horizon

• Errors are a result of inertia effect on mercury switch
• Mercury switch falsely initiates precession of pitch torque motor
• The error is corrected by use of a cut-off switch
• Pitch cut-off activates on acceleration of 0.18 g
• Roll cut-off activates at bank angle of 10 deg

Remote Vertical Gyro

• Remote vertical gyro is also called as vertical axis data generation unit
• Vertical gyro unit is located far away from flight deck in remote location
• Remote gyro operates a synchro mechanism for multiple purposes
• Vertical gyro unit is large and heavy with high spin rate due to powerful motor
• Remote gyros have higher accuracy due to high rigidity and less precession

Advantage of Remote Vertical Gyro

• Vertical gyro unit is large and heavy with high spin rate due to powerful motor
• Remote gyros have higher accuracy due to high rigidity and less precession
• One vertical gyro synchro system sends attitude signals to multiple systems
• Steering computer and amplifier unit
• Servo units within Attitude director indicator
• Auto flight control system

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