An A320 Captain described an FMGC coding anomaly on the SLC QWENN 3 RNAV which required the crew to program the arrival to Runways 34L; 34R and 35 in a non-standard method. A track deviation resulted when the FMGC was programmed using the standard methodology.

Narrative

Cleared via the QWENN 3 RNAV arrival into SLC; expecting a visual approach to RW34L. The First Officer (pilot flying) setup the arrival and approach in the FMGC; first selecting the ILS 34L; then the QWENN STAR. The FMGC course matched the QWENN arrival until FFU VOR; then diverged from the arrival and followed the FFU transition to the ILS34L. Although no 'via' was selected; the FMGC defaulted to the ILS FFU transition; which diverges from the QWENN arrival prematurely. The course should have followed the QWENN routing past FFU to PLAGE; then via heading 341 degrees for vectors to the ILS. After passing FFU; SLC Approach asked if we were proceeding toward PLAGE; and we answered in the affirmative while quickly correcting ten degrees to the right to track the arrival rather than the ILS transition. No apparent loss of separation took place; and no TCAS advisories or warnings were received. With the airport in sight a visual approach clearance was issued and flown to Runway 35.In Airbus aircraft it is a normal procedure to select the ILS then the STAR when setting up an approach in the FMGC. At KSLC; this leads to confusion and a potential violation when the QWENN arrival is in use. Using standard procedures it is nearly impossible to program the FMGC to fly the QWENN arrival as published. After the event occurred; we conducted an experiment to see whether and how the arrival and approach could be programmed correctly; and we discovered when the ILS 34L and the QWENN procedure are programmed; the (incorrect) FFU transition appears whether no 'VIA' or 'NO VIA' are selected. The only consistent way to build the complete QWENN arrival is by NOT selecting an ILS approach to the runway; and this is (1) NOT standard procedure; and (2) leads to a last minute scramble to program the ILS on final approach after flying the QWENN. An alternative would be to manually delete the FFU ILS transition fixes; then manually enter the missing QWENN fixes; but this is also NOT standard procedure as the QWENN is an RNAV arrival and not to be manually modified. I would suggest that the QWENN arrival and ILS 34L / 34R / and 35 approaches at KSLC be re-coded in the FMGC navigation database so that the complete QWENN routing is the default program; and the FFU transition on the ILS approaches appears only when 'VIA FFU' is selected. Until this is accomplished; a note should be placed in the Arrival section of the KSLC-1 airport info page warning crews not to diverge from the QWENN routing after passing FFU; and to ensure that the QWENN routing is flown ALL THE WAY TO PLAGE INTERSECTION.