*** Do not use this for flying refer to your AFM and company policy OM ****

Let’s start with a littles background. An instrument approach is the final phase of connecting the airways to a runway. Airways – STAR, IAP. (Sometimes there are transitions as well).

The basic theory is that the goal of the IAP is to align the airplane with the runway centerline while securing obstacle clearance.

Though usually the minimums are expressed in terms of visibility, each type of approach has a legal minimum descend altitude MDA or decision altitude DA, based on statistics of navigation accuracy, system accuracy etc. ILS CAT I has a 200 foot above threshold elevation, VOR approach 250 feet, RNP approach (no SBAS or GBAS) 250 feet etc.

Every now and then you will see two minimums one at 200 and the other one higher. If you look carefully, you will see that the lower minimums require a higher than 2.5% or 200 feet/mile climb gradient. This implies that there are obstacles in the vicinity that require a higher than the standard (as mentioned above) climb gradient.

But sometimes you see a much higher decision altitude than the minimum legal attitude.

The decision altitude has one and only one use. The pilot(s) must establish good enough visual contact with the runway or approach lights (leaving aside other non-straight-in approaches) that’s it. At the decision height we are required to decide a weather we see the runway and can continue to a safe landing.

Coming back to a high decision altitude. When you see a really high decision altitude it usually means that the IAP planner could not guaranty obstacle clearance below the specified altitude based on the planning criteria either on approach or as usually is the case during missed approach.  If you look at the ILS plate to Batumi Georgia you can see a pretty big mountain just past the runway.

Do not use for flight


So, what do you do if you have to go around below the decision height? (Remember it is only for visibility we are not committed)

Well, the simple answer is that the published missed approach procedure does not guaranty obstacle clearance, You can not just fly it.  it is the crews responsibility and you must brief and or maintain visual obstacle clearance. As simple as that.

It is a serious threat to proper decision making below the decision altitude this kind of an operation (we might find ourselves less inclined to go around when necessery) and should be briefed accordingly.

Stay safe.


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