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Using MDA


nmurray

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Hi,

I am very likely confused here but despite reading the manuals I cannot get the MDA to work as I would have expected.

This may be the problem that I have not understood correctly and if this is the case then I hope someone can help me out.

This is the scenario I used for testing:

I am 20nm out at 3500ft with NAV1 and NAV2 tuned to the localizer/ILS and altitude and NAV set on the autopilot.

I set the Altitude Alerter to 500 feet and click MDA button.

MDA indicator lights.

Just before I intercept the Glide Slope I click the Approach Mode on the Autopilot.

The MDA indicator is still active.

As soon as Glide Slope is captured which is indicated by the G/S Capture indicator the MDA indicator goes out.

Now I believe the MDA should still indicate active.

I proceed with the approach anyway just to check what will happen.

I descend well below 500 feet while autopilot is still engaged and there is no attempt to level off at 500 feet.

Now have I missed the concept here or is this a bug?

I was using the 300 normal wheels for this test.

The G/A functions as I would have expected.

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MDA stands for Minimum Descend Altitude.

Thats a term used for non-precision approaches, like VOR/DME or Localizer only approaches.

When flying a full ILS approach, You are flying a presicion approach where MDA becomes DH - decision height.

See the difference - Altitude / height ??

Altitude relates to Your altimeter or altitude above mean sea level, height relates to height over ground (altitude over ground) and is indicated on the radar altimeter.

hats why MDA mode will be canceled at glideslope capture, cause thats when You start Your precision approach.

Finn

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Finn, thank you for your explanation, that clears it up for me.

I was confused or mislead by the documentation volume 2 where it states:

MDA: Is used during a coupled ILS approach, where the Altitude Alerter, after having captured both localizer and glideslope, should be set to the minimum descend altitude and the MDA button pressed.

I think you will agree that this should be corrected.

I will checkout MDA on a 'Localizer Only' approach tomorrow.

Again thanks for your response.

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Finn, thank you for your explanation, that clears it up for me.

I was confused or mislead by the documentation volume 2 where it states:

MDA: Is used during a coupled ILS approach, where the Altitude Alerter, after having captured both localizer and glideslope, should be set to the minimum descend altitude and the MDA button pressed.

I think you will agree that this should be corrected.

I will checkout MDA on a 'Localizer Only' approach tomorrow.

Again thanks for your response.

Yes that seems to be a wrong description.

Thanks for Your feedback.

Best regards

Finn

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