The Decision Altitude (DA) is a specified altitude in the Precision Approach or approach with vertical guidance at which a Missed Approach must be initiated if the required visual reference to continue the approach has not been established.
This would only be used with ILS and LNAV/VNAV approaches.
At the DA altitude a pilot must state whether they want to continue with the landing and land or go around and try again.
There are many reasons for a go around at DA.
- The pilot could be too high above glide
- Too low below glide
- To fast
- To slow
You must also only make your decision whilst still on glide, you can not dive low and then make it or fly up and make it. Your decision needs to be final and done at that point to continue the flight safely.