Narrative:

We were cleared to intercept 061 degrees right from tys and to maintain 3600' MSL until crossing leabs intersection (T45 15.1 DME) and then go down to 3000' MSL and we were also cleared for the sdf runway 5 mor approach. Normally 3000' MSL is maintained to jxt radio beacon and then down to 1860' MSL. We were established outbnd on tys 061 degrees right and our DME was selected to tys. When we passed 15.1 DME from tys we started down to 3000' and when we passed 24.9 DME from tys we left 3000' MSL going to 1860'. At 2300' MSL approximately, knoxville approach contacted us and asked us to verify our position and altitude and we said we were past 24.9 DME from tys at 2300' MSL. Approach control said she showed us at a different place and that she had a low altitude warning on us. I double checked with morristown DME and realized that we had descended too early. I told approach we were in VMC, but that we would climb back up to 3600' MSL, which we did. The rest of the approach went normally. We later found out that the DME equipment in the airplane showed erratic distances, and we got it changed the next day. At 3600' we were IMC and on the way down we broke out in VMC at 2500-2600' MSL. The DME reading in the airplane also went on and off during this approach.

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Original NASA ASRS Text

Title: ATX SMT ON APCH GETS MSAW FROM CTLR. ALT CROSSING RESTRICTION NOT MAINTAINED PER PROC.

Narrative: WE WERE CLRED TO INTERCEPT 061 DEGS R FROM TYS AND TO MAINTAIN 3600' MSL UNTIL XING LEABS INTXN (T45 15.1 DME) AND THEN GO DOWN TO 3000' MSL AND WE WERE ALSO CLRED FOR THE SDF RWY 5 MOR APCH. NORMALLY 3000' MSL IS MAINTAINED TO JXT RADIO BEACON AND THEN DOWN TO 1860' MSL. WE WERE ESTABLISHED OUTBND ON TYS 061 DEGS R AND OUR DME WAS SELECTED TO TYS. WHEN WE PASSED 15.1 DME FROM TYS WE STARTED DOWN TO 3000' AND WHEN WE PASSED 24.9 DME FROM TYS WE LEFT 3000' MSL GOING TO 1860'. AT 2300' MSL APPROX, KNOXVILLE APCH CONTACTED US AND ASKED US TO VERIFY OUR POS AND ALT AND WE SAID WE WERE PAST 24.9 DME FROM TYS AT 2300' MSL. APCH CTL SAID SHE SHOWED US AT A DIFFERENT PLACE AND THAT SHE HAD A LOW ALT WARNING ON US. I DOUBLE CHKED WITH MORRISTOWN DME AND REALIZED THAT WE HAD DSNDED TOO EARLY. I TOLD APCH WE WERE IN VMC, BUT THAT WE WOULD CLB BACK UP TO 3600' MSL, WHICH WE DID. THE REST OF THE APCH WENT NORMALLY. WE LATER FOUND OUT THAT THE DME EQUIP IN THE AIRPLANE SHOWED ERRATIC DISTANCES, AND WE GOT IT CHANGED THE NEXT DAY. AT 3600' WE WERE IMC AND ON THE WAY DOWN WE BROKE OUT IN VMC AT 2500-2600' MSL. THE DME READING IN THE AIRPLANE ALSO WENT ON AND OFF DURING THIS APCH.

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.