B737-800 CREW WAS CLRED FOR THE ILS RWY 26R APCH AT ATL. INSTEAD THEY SET THE ACFT UP FOR THE RNAV RWY 26R APCH. THE 2 APCHS DO NOT HAVE COMMON INTXNS OR ALTS. AN ALTDEV RESULTED.

Narrative

THE ATIS RPTED VISUAL APCHS BEING CONDUCTED TO RWYS 26R/27L. CAPT SAID HE WOULD FLY A VISUAL APCH USING INTERNAL RNAV/VNAV PROCS. SELECTED THE RNAV (GPS) RWY 26R APCH IN THE FMS DATABASE. WE RECENTLY HAVE BEEN REQUIRED BY THE COMPANY TO FLY 3 'RNAV' APCHS IN VMC CONDITIONS. THE ILS FREQ WAS TUNED UP AS A BACK UP ALSO. WE WERE ON BASE LEG AT 5000 FT AT ABOUT 20 DME. THE ACFT IN FRONT OF US WAS CLRED FOR THE ILS RWY 26R APCH (ATL). WE WERE VECTORED TO DOGLEG THEN GIVEN AN INTERCEPT HEADING. OUR CLRNC WAS; 'CLRED FOR ILS RWY 26R; MAINTAIN 5000 FT UNTIL FREAL INTXN.' FREAL IS 13.6 DME OFF THE LOC. WE SET UP THE ACFT TO FLY THE RNAV APCH. THE FIRST FIX IN THAT APCH IS 'CALES;' APPROX 10 MI FROM RWY; ALT AT CALES OF 3500 FT. AT ABOUT 18 DME; THE ACFT DSNDED OUT OF 5000 FT; AS IT HAD CAPTURED THE FMC GENERATED VNAV GLIDEPATH. WE WERE STILL ABOUT 4 MI FROM FREAL. AT APPROX 4200 FT; WE NOTICED WE HAD NOT REACHED FREAL YET; LEVELED OFF; THEN PRETTY MUCH PARALLELED THE ILS GS TO LNDG. EXPECTING TO BE CLRED FOR A VISUAL APCH; WE WEREN'T SET UP FOR THE ILS. WHEN THE ILS APCH CLRNC WAS GIVEN; WE SHOULD'VE FLOWN THAT CLRNC. ATC DID NOT SAY ANYTHING TO US.