MDA or DA 2

28.10.2004

Submission:

Thank you for your response. Mr Massimini points out that FAR requirements are further refined by the airline operation manual. We call our manual a Flight Operations Manual, or FOM. Our Ops Specs which are part of the FOM specifically disallow "conducting IFR approach procedures using vertical navigation (VNAV) utilizing a published MDA as a DA (H). However, in what appears to be a complete contradiction, we fly all of our non precision approaches in VNAV and RNAV with monitoring of raw data so as to insure nav accuracy. Some people claim that we are accurate if the bearing of the RMI needles are five degrees or less. We overlay an RNAV/Baro VNAV approach on a published VOR/NDB, then have the auto pilot fly the approach in a managed mode (airbus lingo) to the MDA at a constant rate. When we get to the MDA we are descending about 500 to 700 fpm. If the environment is not in sight and we go missed we will penetrate the MDA by a substantial margin.

I believe that the reason we are not allowed to fly RNAV/VNAV approaches per ops specs is because the required nav accuracy is not there. To fly a Baro VNAV approach to an MDA utilizing the DA technique which allows for penetration on the missed requires a certain degree of nav accuracy. I think it is contained in RNP 0.3. When I examine the Prog page in the FMGS during an approach phase, I see the required amount is 0.37 and estimated position error is usually somewhat less. Of course, if the EPE grows larger than the required, we get a "Nav Accuracy Downgrade" message. Unfortunately, our training does not say to carry out a missed approach at this time. It just says to monitor raw data and make a Nav Accuracy Check (which is the 5 degree bearing displacement) One FCOM Vol says five degrees and or within three miles. .. Now seriously, would one want to even contemplate flying an approach in hard IFR to an MDA we are going to penetrate using the DA technique knowing that a Nav accuracy is good enough with a five degrees bearing error or a 3 mile distance error between raw data and computed data displayed on the NAV Display?

Steve Erickson

United States