Narrative:

While acting as PF in the mu-2B-60 in the right seat during a part 135.293 sic check, I descended below the clearance altitude of 3200 ft until established on the approach. I ws established on the 220 degree inbound course to the VOR outside of the 13 DME fix. The check pilot, serving as PIC in the left seat and not flying, had failed the DME and the #2 ADF. Without DME, ZZZ can be idented where the 127 degree bearing to the NDB crosses the 040 degree radial from the VOR. I misread the ADF believing we had passed ZZZ, when in fact, we had about 3 mi to go when I started the descent to the next stepdown altitude of 2100 ft. The PIC was simulating being incapacitated by heart failure and did not intervene. At approximately 2500 ft MSL and 14 DME, tower called with a low altitude alert, confirmed our altitude, and commanded an immediate climb. My lack of situational awareness was inexcusable, however, I'm not sure why the PIC/check pilot allowed my mistake to progress so far. He should have pointed out my error at a much higher altitude.

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Original NASA ASRS Text

Title: AN MU2B FO DSNDED FROM A MANDATORY ALT PRIOR TO THE XING FIX CONSTRAINT. THE TWR CTLR COMMANDED A CLB.

Narrative: WHILE ACTING AS PF IN THE MU-2B-60 IN THE R SEAT DURING A PART 135.293 SIC CHK, I DSNDED BELOW THE CLRNC ALT OF 3200 FT UNTIL ESTABLISHED ON THE APCH. I WS ESTABLISHED ON THE 220 DEG INBOUND COURSE TO THE VOR OUTSIDE OF THE 13 DME FIX. THE CHK PLT, SERVING AS PIC IN THE L SEAT AND NOT FLYING, HAD FAILED THE DME AND THE #2 ADF. WITHOUT DME, ZZZ CAN BE IDENTED WHERE THE 127 DEG BEARING TO THE NDB CROSSES THE 040 DEG RADIAL FROM THE VOR. I MISREAD THE ADF BELIEVING WE HAD PASSED ZZZ, WHEN IN FACT, WE HAD ABOUT 3 MI TO GO WHEN I STARTED THE DSCNT TO THE NEXT STEPDOWN ALT OF 2100 FT. THE PIC WAS SIMULATING BEING INCAPACITATED BY HEART FAILURE AND DID NOT INTERVENE. AT APPROX 2500 FT MSL AND 14 DME, TWR CALLED WITH A LOW ALT ALERT, CONFIRMED OUR ALT, AND COMMANDED AN IMMEDIATE CLB. MY LACK OF SITUATIONAL AWARENESS WAS INEXCUSABLE, HOWEVER, I'M NOT SURE WHY THE PIC/CHK PLT ALLOWED MY MISTAKE TO PROGRESS SO FAR. HE SHOULD HAVE POINTED OUT MY ERROR AT A MUCH HIGHER ALT.

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