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Let's say you continue below the DA/DH on an IAP because you have the APL insight; and of course no lower then 100 ft above TDZE.

Now you arrive and don't see any required visual cues to continue to land.

  1. When would you decide to go missed on an ILS? since you already descended below your DA
  2. If you were to execute a missed procedure below the DA/DH on the same scenario above are you assured obstacle clearance? (my thoughts are no, obstacle clearance is only assured if its executed at the MAP, at or above MDA or DA/DH with a 200ft/nm climb)
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When would you go missed if below MDA/DA(H)?

You're only allowed to operate below the MDA/DA(H) if all of the following are true (14 CFR 91.175(c)):

  1. You can land using normal descent rate and maneuvers
  2. You have at least the flight visibility required for the approach
  3. You have at least one visual reference from the list (not just the approach lighting system)

91.175(e) says that you must go missed if those conditions are not met and you're below the MDA, at the MAP, at the DA/DH, or "at any time after that until touchdown", i.e. below MAP/DA(H). Essentially, if any of the conditions in 91.175(c) are no longer true, then you have to go missed immediately. Practically, the most likely issue is seeing the runway environment, but you should also go missed if your approach is unstable, for example.

On an ILS approach specifically, that would most likely mean that you saw the approach lights at DA/DH (or above), followed the glideslope down to 100' above TDZE, but then didn't see any more visual references. That should be very unlikely if you really do have the flight visibility required for the approach but I'm sure it could happen somehow, e.g. there's ground fog in the wrong place at the wrong time. Or something else could go wrong: the runway lighting completely fails because of a power cut, or there's an obstruction on the runway.

Is obstacle clearance assured if going missed below MDA/DA(H)?

No. The Instrument Procedures Handbook (chapter 4) says several times that if you go missed below MDA/DA(H) then you have no guarantees and it gives some general advice on what to do:

If the aircraft initiates a missed approach at a point other than the missed approach point, from below MDA or DA (H), or on a circling approach, obstacle clearance is not provided by following the published missed approach procedure, nor is separation assured from other air traffic in the vicinity.

[...]

In the event a balked (rejected) landing occurs at a position other than the published missed approach point, the pilot should contact ATC as soon as possible to obtain an amended clearance. If unable to contact ATC for any reason, the pilot should attempt to re−intercept a published segment of the missed approach and comply with route and altitude instructions. If unable to contact ATC, and in the pilot’s judgment it is no longer appropriate to fly the published missed approach procedure, then consider either maintaining visual conditions (if possible) and reattempt a landing, or a circle−climb over the airport.

This part is directed at pilots using night vision systems, but it looks like good advice for any pilot considering going below MDA/DA(H):

If a pilot initiates a go-around at a point below DA/DH or after the MAP, obstacle clearance is not necessarily provided by following the published missed approach procedure. Prior planning is recommended and should include contingencies between the published MAP and touchdown with reference to obstacle clearance, aircraft performance, and alternate escape plans.

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I don’t really know the answer to 1), but you’re correct on 2) - obstacle clearance is not assured by regulations for a go-around below minima. However, due to all-engine aircraft performance most of the time far exceeding the procedure design assumptions, obstacle clearance is probably still fine operationally (at least in virtually all large commercial aircraft - can’t tell about small craft and GA), as long as they are flown properly and don’t lose an engine between descending through DA and re-gaining a height corresponding to the original obstacle clearance surface.

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