Going below DA(DH)

[Duchess_Driver]

On a precision approach the DA(DH) is the height at which the decision is made whether to continue the approach or not - and that is based having certain visual references at that altitude (height).

If at that altitude (height) those references are not acquired and the decision to go missed is taken, the natural flight path of the aeroplane would then take the aeroplane below DA(DH) before spool-up-pitch-up takes effect.

During a discussion the other day it was advocated that the decision should be made slightly earlier than DA(DH) to compensate for the time taken to get the aeroplane going up again, thus never descending below DA. However, I'm not convinced that this is correct - as by the name chosen, DECISION Altitude is where you decide if you can continue or not. Surely at this point on a precision approach being well within OCA, allowances are made for a transient descent below.

I understand that on a non-precision approach, MINIMUM DESCENT Altitude, MDA(MDH) is exactly that, the altitude below which the aeroplane should never go unless those references are attained, and if we're now saying that is also true of DA(DH), where's the difference?

Comments please...

[5150]

That thread was referring to a Non-Precision Approach:

You're correct in saying that MDA(H) cannot be busted unless you are visual so someone mentioned they actually call their minimums earlier to compensate for the inertia while initiating a missed approach and not bust their minimum.

DA(H) can be busted whilst in the process of initiating a missed approach

[keithl]

The discussion is as old as DA(H) itself, but certainly DA(H) can be bust during the go-round manoeuvre. Duchess Driver's reported discussion relates to the fact that, over the years, the question has been "What constitutes the making of the decision?" Is it when you decide in your head, when you announce it in words, or when you apply power or raise the nose?" Currently, it seems to be "when you say the words", but there are probably variations beween operators and regulators even now.
Another confusion factor is that many schools teach people to make the decision early, as a way of avoiding the whole issue for the purpose of passing the IRT. I know several pilots who didn't realise that this was just a training "fudge", not intended for real life.

[Duchess_Driver]

Keithl: Indeed this discussion was with an instructor, who was advocating the decision is made within tolerances for the IRT i.e. -0 +50, and therefore at say 240' you initiate the missed approach.

The examiner, therefore, would be quite within his rights to say that you've not completed the approach to minimums and the candidate fails.....catch 22?

[Intruder]

It is not unusual to go 50' or more below the DH when making a go-around at minimums. I know of no Check Airman or FAA inspector that would bust a Pilot on a check ride for going below DH, as long as the announcement "minimums; no runway" is made AT DH, and the Pilot Flying responds in a timely manner. In fact, with a 100' DH and only slightly longer than normal reaction times, an airplane may touch down during the go-around. That also is NOT in itself grounds for a bust.

[keithl]

DD: Don't worry about "Catch 22", because DA(H) is the alt(ht) by which the decision must be made. So they can't touch you for making it within that -0/+50ft zone.

[chevvron]

DA/DH is always calculated with an allowance for drift down/spool up and obstructions in the missed approach area so that you should maintain the necessary clearance above the ground if you commence go-around AT DA/DH.
Typically height loss for Cat C aircraft using radalt is given in Table 111-21-4 of ICAO Doc 8168 Vol 2 as 71ft (3 deg GP)
On non-precision approaches, there is no allowance because you're not necessarily descending at MDA/MDH when you reach MAPt