Narrative:

With captain hand flying, we were cleared for VOR runway 13 approach to jfk. We were level at 3000 ft about 10 mi from the cri VOR on 043 degree radial. Copilot started descent to 1500 ft early and by the time I pointed it out to him and told him to climb back to 3000 ft we descended approximately 300 ft (2700 ft MSL). He somehow was confused about descent point thinking it was 10 mi instead of 6 DME. He knew he needed to be 1500 ft (mandatory altitude). At cri VOR so started fairly rapid descent. The approach was briefed and copilot said he had made the approach many times before but somehow was confused on DME position.

Related reports:

Google
 

Original NASA ASRS Text

Title: FLC DSNDS BELOW ASSIGNED ALT BEFORE DSCNT FIX.

Narrative: WITH CAPT HAND FLYING, WE WERE CLRED FOR VOR RWY 13 APCH TO JFK. WE WERE LEVEL AT 3000 FT ABOUT 10 MI FROM THE CRI VOR ON 043 DEG RADIAL. COPLT STARTED DSCNT TO 1500 FT EARLY AND BY THE TIME I POINTED IT OUT TO HIM AND TOLD HIM TO CLB BACK TO 3000 FT WE DSNDED APPROX 300 FT (2700 FT MSL). HE SOMEHOW WAS CONFUSED ABOUT DSCNT POINT THINKING IT WAS 10 MI INSTEAD OF 6 DME. HE KNEW HE NEEDED TO BE 1500 FT (MANDATORY ALT). AT CRI VOR SO STARTED FAIRLY RAPID DSCNT. THE APCH WAS BRIEFED AND COPLT SAID HE HAD MADE THE APCH MANY TIMES BEFORE BUT SOMEHOW WAS CONFUSED ON DME POS.

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.