Wednesday, January 28, 2009

Aproaching Minimums - what is the decision altitude/height

There is a point in the approach to an airport when the copilot or the aircraft systems call Minimums or Decision height. If you ever wondered what that means read below.

Decision altitude (DA) or Decision Height (DH) is a specific altitude or height in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been acquired. This allows the pilot sufficient time to safely re-configure the aircraft to climb and execute the missed approach procedures while avoiding terrain and obstacles.

If you already do not know what is the difference between altitude and height in aviation a simple (not entirely "legal") explanation is that the altitude of an aircraft is measured from sea-level (local air pressure is a factor), while its height is measured from ground level(e.g. with a radar altimeter).

A minimum descent height (MDH) or minimum descent altitude (MDA) is the equivalent of the DA for non-precision approaches, however there are some significant differences. It is the level below which a pilot making such an approach must not allow his or her aircraft to descend unless the required visual reference to continue the approach has been established.

So how can I see what is the MDA for the airport I will land on? Very simple: in the approach chart for that runway. Most of the charts have it in the top part (as the one at the top of the post). It will list both DA and DH.

And where it is setup so my aircraft calls it correctly?
This differs from the aircraft but Airbus has it in the FMGC (approach page) and at Boeing you can set it up in the EFIS panel. G/A planes have mostly a knob on the radar altimeter.

What should I do when I hear the "magic word" minimums?
If you do not see the runway: Go around!
If you see the runway: land.

You can read more about Instrument approach procedures from wikipedia.

Disclaimer: This post is a simple explanation for beginners in flight simming, so the explanation here may not be complete. DO NOT USE FOR REAL NAVIGATION!

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