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How to get Twin Otter Autopilot MDA button working ?


ChuckOman

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Tried coupling the autopilot to the ILS in APPR mode, and once stabilized on approach and past the FAF, setting the ALT window to the MDA and then pushing the MDA button.  The button flashes red briefly when I push it, but does not engage.  The systems manual says if you enable the MDA switch, the autopilot will level off at the MDA.  Has anyone been able to get this to work ?  What's the trick ?

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21 hours ago, ChuckOman said:

Tried coupling the autopilot to the ILS in APPR mode, and once stabilized on approach and past the FAF, setting the ALT window to the MDA and then pushing the MDA button.  The button flashes red briefly when I push it, but does not engage.  The systems manual says if you enable the MDA switch, the autopilot will level off at the MDA.  Has anyone been able to get this to work ?  What's the trick ?

 

I have found the MDA function works differently according to whether you have GS capture. If you have the GS, the MDA doesn't do anything (except flash like you say). If you don't have GS capture, it behaves as follows. If you are below the target height (even slightly) you will switch immediately to ALT Hold and hold at the target altitude (this also cancels APPR mode). If you are significantly below the target altitude this initiates a climb to that altitude (which makes it a more useful version of the ALT Alert button!) Either way, the MDA light doesn't stay on. If the target altitude is below your current altitude the MDA light stays on and you will switch ALT Hold when you have descended to within 300ft of the target altitude.

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Thanks Mark.

BTW I subsequently found an earlier post from several years back, saying that the Twin Otter systems manual description is wrong, and would be corrected.  (Hasn't happened yet, obviously.)  Apparently the MDA function is intended for use on non-precision LOC approaches flown with "dive an drive" strategy.  If it cancels LOC tracking at the MDA, however, that's not good if it occurs prior to the MAP !

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