Hello all,
my question refers mainly to a CAT I ILS with a DH of 200'.
my question is the following:
is DA the altitude at which you make the decision to either continue visually or execute the missed approach (thus implying that if you make the decision to go missed, you WILL momentarily descend somewhat below DA if you account the reaction time, engine acceleration time and aircraft vertical acceleration time) or is it a HARD STOP altitude which you should never go below unless you have the required visual reference?
I have always been of the opinion that you cannot go not even 1 foot below DA, and therefore you should take whatever action necessary to initiate the go-around in order to maintain the aircraft at or above DA (in other words start adding power and rotating to GA pitch some X ft above DA).
However after reading several discussions on other forums (
here's one for example ) and talking to other instructors I am noticing that many pilots have a different view on this.
They believe you can reach DA, make your decision, and that it is "understood" that you will descend somewhat below DA.
One of the arguments I heard was that if its called "decision altitude" it means thats when you make the decision. However, I don't think this argument holds, because if we compare this to a V1 scenario, "decision speed", thats not really the time you start making decisions. Thats the point where the decision has already been made for you.
I was wondering what was your input on this, especially when it comes to airline SOPs. If the power advanced prior to, or at DA?
What about pitch rotation?
Whould the answer be different if the autopilot was shooting the approach? (I've never flown with an advanced AP/FD/AT so I don't know if the autopilot would automatically execute a GA at DA or if the pilot needs to "manually" tell it to execute TOGA thrust and pitch attitude).
In most airline SOPs, is it "understood" that you will descend somewhat below DH?
If not, how much above DA do you generally start advancing the throttles?
I understand the amount of "driftdown" from the time the throttles are advanced to the time the aircarft starts climbing will be different from say, an older 747 with older engines and slower spool-up times having a momentum of over half-million pounds descending at 700/800fpm flying at Vref, to a C-172 at 450fpm shooting the approach at over twice Vs1 that can afford to stop the descent rather abruptly even without immediately adding power due to the higher stall margin. However, its also true that the airlines have the advantage of a 2 person crew, where the PNF is looking outside when approaching DA. In a single-pilot operation, if the pilot waits until he hits DA before looking out, then has to re-transition to his intruments and execute the go-around, there will also be some degree of altitude loss.
All inputs are appreciated.
palgia
PS. By the way, when flying in the US with a sensitive altimeter, an error of +/-75ft is allowed, therefore when your altimeter reads 200ft yo may well be only 125ft above TDZE...which is not necessarily AGL. Not the time I'd like to risk getting any lower.