Narrative:

Airbus standard rnp value for non-precision approach navigation approachs is 0.37. Our procedures require us to verify the 0.37 prior to crossing the FAF. Our procedures also specify to use the default value unless the chart specifically tells us to use a different value. However; the manual also says that the default value is only 0.37 when the FMGC has DME-DME updating available. If it only has VOR-DME updating; the default value is 0.61. This is often the case in pvr. In fact; I have seen the default value as high as 0.78 on the final approach segment. If we are supposed to be using 0.37 for the approach; the chart in pvr needs to tell us to hard-tune it in the FMGC to prevent an automatic increase in rnp value. The current situation sets the crew up for a doubling of rnp inside the FAF with absolutely no notification whatsoever; leading to a xtrack error that is potentially double before a navigation accuracy downgrade. This qualifies as very bad when close to the ground. As an aside; the rnp value automatically increases at pvr even in the GPS-equipped airplanes; which shouldn't happen. Obviously; if the FMGC is receiving GPS updates; that's far more accurate and either DME-DME or VOR-DME updating is irrelevant. Please give us some instructions on the chart to hard-tune an rnp of 0.37 for non-precision approachs at pvr. As another aside; in non-GPS airplanes this will also cause us to only do constant descent approach procedures because we'll get a navigation accuracy downgrade every time.

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Original NASA ASRS Text

Title: A320 CAPT REPORTS ANP VALUES HIGHER THAN APPROACH LIMITS DURING VOR DME 2 RWY 22 APPROACH AT MMPR.

Narrative: AIRBUS STANDARD RNP VALUE FOR NON-PRECISION APCH NAV APCHS IS 0.37. OUR PROCS REQUIRE US TO VERIFY THE 0.37 PRIOR TO XING THE FAF. OUR PROCS ALSO SPECIFY TO USE THE DEFAULT VALUE UNLESS THE CHART SPECIFICALLY TELLS US TO USE A DIFFERENT VALUE. HOWEVER; THE MANUAL ALSO SAYS THAT THE DEFAULT VALUE IS ONLY 0.37 WHEN THE FMGC HAS DME-DME UPDATING AVAILABLE. IF IT ONLY HAS VOR-DME UPDATING; THE DEFAULT VALUE IS 0.61. THIS IS OFTEN THE CASE IN PVR. IN FACT; I HAVE SEEN THE DEFAULT VALUE AS HIGH AS 0.78 ON THE FINAL APCH SEGMENT. IF WE ARE SUPPOSED TO BE USING 0.37 FOR THE APCH; THE CHART IN PVR NEEDS TO TELL US TO HARD-TUNE IT IN THE FMGC TO PREVENT AN AUTOMATIC INCREASE IN RNP VALUE. THE CURRENT SITUATION SETS THE CREW UP FOR A DOUBLING OF RNP INSIDE THE FAF WITH ABSOLUTELY NO NOTIFICATION WHATSOEVER; LEADING TO A XTRACK ERROR THAT IS POTENTIALLY DOUBLE BEFORE A NAV ACCURACY DOWNGRADE. THIS QUALIFIES AS VERY BAD WHEN CLOSE TO THE GND. AS AN ASIDE; THE RNP VALUE AUTOMATICALLY INCREASES AT PVR EVEN IN THE GPS-EQUIPPED AIRPLANES; WHICH SHOULDN'T HAPPEN. OBVIOUSLY; IF THE FMGC IS RECEIVING GPS UPDATES; THAT'S FAR MORE ACCURATE AND EITHER DME-DME OR VOR-DME UPDATING IS IRRELEVANT. PLEASE GIVE US SOME INSTRUCTIONS ON THE CHART TO HARD-TUNE AN RNP OF 0.37 FOR NON-PRECISION APCHS AT PVR. AS ANOTHER ASIDE; IN NON-GPS AIRPLANES THIS WILL ALSO CAUSE US TO ONLY DO CONSTANT DSCNT APCH PROCS BECAUSE WE'LL GET A NAV ACCURACY DOWNGRADE EVERY TIME.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.