LOC Disagree question
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Over the past couple of flights after having tuned in a VOR to both sides but on white needles ive been getting a LOC DISAGREE cas message. I can't find any details about it, but i can probably intuit that it's about a comparitor flagging either the CRS or CDI between 1 and 2. What are the triggers for this CAS that would cause it when both L and R are on FMS mode?
Thanks,
Crash -
You're correct. It is a comparator between the left and right NAV receivers. I use my signal degradation system to trigger the message if the derivatives of the CDI or GS (GS DISAGREE) get too high. Whether this should trigger when they are not selected as the active Nav Source, I'm not sure. Perhaps other Collins equipment manuals might have a clue.
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@Black-Square according to the Avionics manual on page 603 the LOC DISAGREE is a comparator between the L / R LOC deviation indicators, which indicates that the system does not respond to VOR signals and would therefore be based on the frequency of the RTU being between 108.1 and 111.95 & the tenths digit being odd. Additionally the comparator for other similar collins products indicates the flag gets set when the deviation greater than or equal to 0.2 CDI dots (specifically displayed dots not reception signal).
So the full logical tree is
LOC_DISAGREE_bool = If LOC_frequency && signal && displayed deviation >= 0.2 dots then true.
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Thanks for finding that. I have changed the logic to be exclusive to localizer signals. The strength, I use as a proxy for the differential deviation to simplify the math, but also to add some hysteresis, so the message is not flashing on and off constantly.
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Thanks for finding that. I have changed the logic to be exclusive to localizer signals. The strength, I use as a proxy for the differential deviation to simplify the math, but also to add some hysteresis, so the message is not flashing on and off constantly.
@Black-Square Cool, it appears the signal comes out of P3 (LH PFD) 23-26 and P6 23-26 (RH PFD), which passes through the input output concentrator. Each display has a comparator so either can trip the flag based on that side's associated signal. Had to really go diving into the wiring diagram for that and it's still not totally clear to me because of the block diagrams how the signal propagates.
