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Harrier etc ADI

Old 10th Jan 2021, 12:13
  #1 (permalink)  
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Join Date: Apr 2015
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Question Harrier etc ADI

Hi all,

I've been trying to figure out the operation of the 3 axis ADI fitted as a standby instrument to the later Harriers (a Ferranti FSI100 I think). I see that there are 2 warning flags on it. The "Power" one seems pretty self explanatory to me but I can't figure out the "DG" (Directional Gyro) flag. I appreciate that on this type of instrument, the pilot must manually align the direction indicator with the magnetic compass (at worst - I realise more sophisticated data is normally available) but I don't really understand what the DG flag would be telling the pilot. Surely if the gyro is still running then it would provide as good a heading as it would pitch & bank angle, so does this mean that somehow directional indication is being maintained from some other source?

Many thanks in advance.
ZG862 is offline  
Old 10th Jan 2021, 13:04
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When you think about how gyroscopes work you will realise that there are two of them in this unit: one rotating around the vertical axis to drive the attitude indicator, and one rotating around a horizontal axis to provide directional information. The fact that this information is displayed by rotating the attitude indicator display ball around its vertical axis should not be taken to imply that the designers somehow came up with a 3-axis gyroscope: that is a physical impossibility.

The DG flag indicates that the directional gyro has failed, prompting the pilot to disregard the heading indication (which canít be made to go away due to the nature of the device).
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Old 11th Jan 2021, 17:59
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Originally Posted by Easy Street View Post
When you think about how gyroscopes work you will realise that there are two of them in this unit: one rotating around the vertical axis to drive the attitude indicator, and one rotating around a horizontal axis to provide directional information. The fact that this information is displayed by rotating the attitude indicator display ball around its vertical axis should not be taken to imply that the designers somehow came up with a 3-axis gyroscope: that is a physical impossibility.

The DG flag indicates that the directional gyro has failed, prompting the pilot to disregard the heading indication (which canít be made to go away due to the nature of the device).
Now you have encouraged me to think about it, the answer seems obvious. Without your prompt I'd still be scratching my sorry, balding head. Many thanks for your help!
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