MDA or DA

28.10.2004

Submission:

I am an Airbus A320 captain and we fly all our non-precision approaches to an MDA using Baro VNAV for our vertical guidance for NDB and VOR approaches. We treat the MDA as a DA by going slightly below MDA during the go around procedure. Only one of our fleet of fifty aircraft has GPS. I personally think what we are doing is incorrect and illegal. But I am having a difficult time getting my point across to our company flight standards department. US Airways has GPS primary feed to their A320 FMS and they are allowed to use that to fly their own special "precision" approaches wherever they fly. NWA and United I believe fly to an MDA and level off till reaching the VDP or MAP and then commence missed approach. They have their autopilot go into altitude capture at the MDA (rounded up to the nearest hundred as displayed on the FCU or Mode Control Panel. What are the criteria to allow aircraft to fly using a DA technique on a non-precision approach? GPS could be one but also DME/DME FMS are allowed if it meets certain criteria or tolerances. I think its RNP.3. Do you know which publication I could refer to find out.

Steve Erickson

United States

Answer or Commentary:

(I.W.

I tend to agree that going below the MDA is incorrect. There is an altitude height loss associated with the transition from descent to climb and is taken into account in the procedure design process. In an ICAO precision approach it is an aircraft category related value and in a non-precision approach it is covered by the applied minimum obstacle clearance. However this altitude height loss is inherent in the transition from descent to climb and does not allow a lower minimum if the pilot thinks his plane doesn't lose height in this transition phase. The documents to check are ICAO Pans-Ops (doc 8168 ) Volume I and II and the FAA TERPS manual.)