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ATTD Flag Showing in ADI

January 2012

The “ATTD” flag is showing in my Altitude Director Indicator (ADI). The problem could be with the remote vertical gyro valid missing due to the gyro not being valid or it could be the ADI itself is not positioning to where the gyro is telling it to be.

One way to determine which unit is at fault is to measure the voltage for the valid at the ADI connector.  A quicker and easier test is to see if the auto pilot still engages with the "ATTD" flag showing. 

Since in most systems the auto pilot needs the valid from the vertical gyro to engage, we can say the vertical gyro is OK if the auto pilot engages so the problem must be the ADI. This would apply to most any system with mechanical indicators.