Narrative:

We were being vectored for an approach at dtw and were told we were #8 for the approach, expect 21R ILS. I set up 21R ILS for the flying pilot and we continued being vectored. After about 10 min, they changed our runway to 21L. I got out the approach plate and set up 21L for the pilot. 21R localizer has ILS DME but 21L localizer does not, so I held the DME from the right side so we would know our distance from the airport. We intercepted the 21L localizer as instructed and flew it inbound. Approach said, 'maintain 5000 until robbi cleared for ILS 21L. Call the tower now.' I called the tower as instructed and the pilot flying intercepted the G/south and started to descend on it. While talking to the tower, I glanced at the approach plate and saw 24.7 DME on robbi intersection and did not say anything to the pilot flying because we were showing 18.1 DME. At this point, we were going through pre-landing checklists and the tower cleared us to land on 21L. After completing the checklist, I looked at the approach plate again (copy enclosed) and realized that by descending on the G/south, we had left 5000' too early. The 18.1 DME was from the airport and not from crl VOR which is how robbi is defined. Other than leaving altitude too soon, the approach and landing was normal and uneventful. Nothing was said to us by the tower or approach control about leaving 5000' before robbi. I believe this situation could have been avoided by a more complete review of the approach procedure when they changed runways on us.

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Original NASA ASRS Text

Title: GA LTT OVERSHOT ALT CROSSING RESTRICTION ON APCH.

Narrative: WE WERE BEING VECTORED FOR AN APCH AT DTW AND WERE TOLD WE WERE #8 FOR THE APCH, EXPECT 21R ILS. I SET UP 21R ILS FOR THE FLYING PLT AND WE CONTINUED BEING VECTORED. AFTER ABOUT 10 MIN, THEY CHANGED OUR RWY TO 21L. I GOT OUT THE APCH PLATE AND SET UP 21L FOR THE PLT. 21R LOC HAS ILS DME BUT 21L LOC DOES NOT, SO I HELD THE DME FROM THE RIGHT SIDE SO WE WOULD KNOW OUR DISTANCE FROM THE ARPT. WE INTERCEPTED THE 21L LOC AS INSTRUCTED AND FLEW IT INBND. APCH SAID, 'MAINTAIN 5000 UNTIL ROBBI CLRED FOR ILS 21L. CALL THE TWR NOW.' I CALLED THE TWR AS INSTRUCTED AND THE PLT FLYING INTERCEPTED THE G/S AND STARTED TO DSND ON IT. WHILE TALKING TO THE TWR, I GLANCED AT THE APCH PLATE AND SAW 24.7 DME ON ROBBI INTXN AND DID NOT SAY ANYTHING TO THE PLT FLYING BECAUSE WE WERE SHOWING 18.1 DME. AT THIS POINT, WE WERE GOING THROUGH PRE-LNDG CHKLISTS AND THE TWR CLRED US TO LAND ON 21L. AFTER COMPLETING THE CHKLIST, I LOOKED AT THE APCH PLATE AGAIN (COPY ENCLOSED) AND REALIZED THAT BY DSNDING ON THE G/S, WE HAD LEFT 5000' TOO EARLY. THE 18.1 DME WAS FROM THE ARPT AND NOT FROM CRL VOR WHICH IS HOW ROBBI IS DEFINED. OTHER THAN LEAVING ALT TOO SOON, THE APCH AND LNDG WAS NORMAL AND UNEVENTFUL. NOTHING WAS SAID TO US BY THE TWR OR APCH CTL ABOUT LEAVING 5000' BEFORE ROBBI. I BELIEVE THIS SITUATION COULD HAVE BEEN AVOIDED BY A MORE COMPLETE REVIEW OF THE APCH PROC WHEN THEY CHANGED RWYS ON US.

Data retrieved from NASA's ASRS site as of August 2007 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.