Narrative:

The captain who was the PF briefed to fly the ILS runway 21L. We even discussed at length the lack of an associated DME with the ILS and how we both felt all ILS's should have an associated DME. The step-down fixes on the approach were predicated on a VOR DME. We did not brief which VOR. In setting up for the approach the captain had his radio set up on the ILS frequency. My radio was tuned to the detroit VOR for situational awareness purposes in IMC conditions as it is located on the field. We were being vectored at 5000 ft and were cleared for the approach. The DME showed approximately 22 mi out. The captain; believing we were inside of robbi (25.2 DME) started down to 4000 ft instead of waiting for the GS to capture. At 4100 ft ATC queried our altitude and informed us we should be at 5000 ft. I responded that we were inside of robbi. The controller said we were still 2 mi outside of robbi. Now certain something was wrong; the captain started a climb back up to 5000 ft. I looked at the approach plate and realized that the step-down fixes were predicated on the carleton VOR; not on the detroit VOR. I dialed in the correct VOR frequency and the approach was continued without further event. Special attention needs to be paid to briefing and agreeing on exactly which NAVAID the DME is predicated on. Additionally; for some reason the pilot group of this airline is very fond of using the 'dive and drive' method and stepping down prior to the FAF even on precision approachs (versus just capturing the GS from the altitude they were cleared for the approach on). While certainly sometimes appropriate; doing so is often de-stabilizing; unnecessary; adds workload; and obviously can be problematic as with this situation. Supplemental information from acn 764028: we completed the arrival and landing with no further problems. ATC did not appear to be concerned with the deviation. We thanked them prior to switching to tower. Use on-field DME for arrival fixes; not off-field VOR. If for terps reasons or other minutia; make it more noticeable that DME is to off-field VOR (like 250 KT restr at lax on departure plates). I take the hit for this. We briefed it well and thought we had a good plan; however; it was based on bad information and the good information was available -- we just missed it.

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Original NASA ASRS Text

Title: A B737-300 CREW REPORTS DESCENDING EARLY ON THE DTW ILS 21L AFTER USING THE INCORRECT VOR DME TO IDENTIFY ROBBI.

Narrative: THE CAPT WHO WAS THE PF BRIEFED TO FLY THE ILS RWY 21L. WE EVEN DISCUSSED AT LENGTH THE LACK OF AN ASSOCIATED DME WITH THE ILS AND HOW WE BOTH FELT ALL ILS'S SHOULD HAVE AN ASSOCIATED DME. THE STEP-DOWN FIXES ON THE APCH WERE PREDICATED ON A VOR DME. WE DID NOT BRIEF WHICH VOR. IN SETTING UP FOR THE APCH THE CAPT HAD HIS RADIO SET UP ON THE ILS FREQ. MY RADIO WAS TUNED TO THE DETROIT VOR FOR SITUATIONAL AWARENESS PURPOSES IN IMC CONDITIONS AS IT IS LOCATED ON THE FIELD. WE WERE BEING VECTORED AT 5000 FT AND WERE CLRED FOR THE APCH. THE DME SHOWED APPROX 22 MI OUT. THE CAPT; BELIEVING WE WERE INSIDE OF ROBBI (25.2 DME) STARTED DOWN TO 4000 FT INSTEAD OF WAITING FOR THE GS TO CAPTURE. AT 4100 FT ATC QUERIED OUR ALT AND INFORMED US WE SHOULD BE AT 5000 FT. I RESPONDED THAT WE WERE INSIDE OF ROBBI. THE CTLR SAID WE WERE STILL 2 MI OUTSIDE OF ROBBI. NOW CERTAIN SOMETHING WAS WRONG; THE CAPT STARTED A CLB BACK UP TO 5000 FT. I LOOKED AT THE APCH PLATE AND REALIZED THAT THE STEP-DOWN FIXES WERE PREDICATED ON THE CARLETON VOR; NOT ON THE DETROIT VOR. I DIALED IN THE CORRECT VOR FREQ AND THE APCH WAS CONTINUED WITHOUT FURTHER EVENT. SPECIAL ATTN NEEDS TO BE PAID TO BRIEFING AND AGREEING ON EXACTLY WHICH NAVAID THE DME IS PREDICATED ON. ADDITIONALLY; FOR SOME REASON THE PLT GROUP OF THIS AIRLINE IS VERY FOND OF USING THE 'DIVE AND DRIVE' METHOD AND STEPPING DOWN PRIOR TO THE FAF EVEN ON PRECISION APCHS (VERSUS JUST CAPTURING THE GS FROM THE ALT THEY WERE CLRED FOR THE APCH ON). WHILE CERTAINLY SOMETIMES APPROPRIATE; DOING SO IS OFTEN DE-STABILIZING; UNNECESSARY; ADDS WORKLOAD; AND OBVIOUSLY CAN BE PROBLEMATIC AS WITH THIS SITUATION. SUPPLEMENTAL INFO FROM ACN 764028: WE COMPLETED THE ARR AND LNDG WITH NO FURTHER PROBS. ATC DID NOT APPEAR TO BE CONCERNED WITH THE DEV. WE THANKED THEM PRIOR TO SWITCHING TO TWR. USE ON-FIELD DME FOR ARR FIXES; NOT OFF-FIELD VOR. IF FOR TERPS REASONS OR OTHER MINUTIA; MAKE IT MORE NOTICEABLE THAT DME IS TO OFF-FIELD VOR (LIKE 250 KT RESTR AT LAX ON DEP PLATES). I TAKE THE HIT FOR THIS. WE BRIEFED IT WELL AND THOUGHT WE HAD A GOOD PLAN; HOWEVER; IT WAS BASED ON BAD INFO AND THE GOOD INFO WAS AVAILABLE -- WE JUST MISSED IT.

Data retrieved from NASA's ASRS site as of January 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.