Narrative:

I heard the controller say: 'maintain 4000 until intercepting localizer.' immediately after, the localizer came alive, and I centered the needle. Believing I was cleared for the approach, I began descent in accordance with the approach plate. Approach controller called us as we were passing 2800 ft and advised we were supposed to still be at 4000 ft. She gave us a turn off the localizer and we were vectored to the approach for the other runway. After landing, I realized the controller had never said 'cleared for the approach,' and I realized that I had flown what I had expected to hear, not what I actually heard. However, the word sequence I had heard is that which would lead into...'cleared for the approach,' I was told to call the approach supervisor, which I did, and he said he would have to submit paperwork to explain the less than standard separation which occurred with a light plane as a result of our descent. I did not know the controller was working this aircraft, as we were VMC above an undercast and did not see it, nor did we hear any radio calls to it. She was also working military aircraft in the radar pattern and was fairly busy. Cockpit workload was high at this point, and my copilot was involved in changing aircraft confign and operating the pressurization system. My copilot was new to the company and the aircraft, and was slightly behind during critical phases of flight. He had some doubt about the descent I initiated, but I feel he was not effective in communicating that he thought we were not cleared for the approach. Perhaps he was reluctant to question a more senior pilot. Perhaps he was unsure or confused by the wording of the controller's instructions. I dismissed his query about the altitude assignment by showing that I was established on the localizer course. If he had been more experienced or if I had flown with him before, maybe I would have listened closer and paid more attention to what he was trying to tell me. You can bet I'll listen better next time, no matter how green the copilot, and I will verify such clrncs with the controller if either of us has the slightest doubt or question!

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Original NASA ASRS Text

Title: COMMUTER ACFT DSNDS BELOW ASSIGNED ALT ON ILS APCH.

Narrative: I HEARD THE CTLR SAY: 'MAINTAIN 4000 UNTIL INTERCEPTING LOC.' IMMEDIATELY AFTER, THE LOC CAME ALIVE, AND I CENTERED THE NEEDLE. BELIEVING I WAS CLRED FOR THE APCH, I BEGAN DSCNT IAW THE APCH PLATE. APCH CTLR CALLED US AS WE WERE PASSING 2800 FT AND ADVISED WE WERE SUPPOSED TO STILL BE AT 4000 FT. SHE GAVE US A TURN OFF THE LOC AND WE WERE VECTORED TO THE APCH FOR THE OTHER RWY. AFTER LNDG, I REALIZED THE CTLR HAD NEVER SAID 'CLRED FOR THE APCH,' AND I REALIZED THAT I HAD FLOWN WHAT I HAD EXPECTED TO HEAR, NOT WHAT I ACTUALLY HEARD. HOWEVER, THE WORD SEQUENCE I HAD HEARD IS THAT WHICH WOULD LEAD INTO...'CLRED FOR THE APCH,' I WAS TOLD TO CALL THE APCH SUPVR, WHICH I DID, AND HE SAID HE WOULD HAVE TO SUBMIT PAPERWORK TO EXPLAIN THE LTSS WHICH OCCURRED WITH A LIGHT PLANE AS A RESULT OF OUR DSCNT. I DID NOT KNOW THE CTLR WAS WORKING THIS ACFT, AS WE WERE VMC ABOVE AN UNDERCAST AND DID NOT SEE IT, NOR DID WE HEAR ANY RADIO CALLS TO IT. SHE WAS ALSO WORKING MIL ACFT IN THE RADAR PATTERN AND WAS FAIRLY BUSY. COCKPIT WORKLOAD WAS HIGH AT THIS POINT, AND MY COPLT WAS INVOLVED IN CHANGING ACFT CONFIGN AND OPERATING THE PRESSURIZATION SYS. MY COPLT WAS NEW TO THE COMPANY AND THE ACFT, AND WAS SLIGHTLY BEHIND DURING CRITICAL PHASES OF FLT. HE HAD SOME DOUBT ABOUT THE DSCNT I INITIATED, BUT I FEEL HE WAS NOT EFFECTIVE IN COMMUNICATING THAT HE THOUGHT WE WERE NOT CLRED FOR THE APCH. PERHAPS HE WAS RELUCTANT TO QUESTION A MORE SENIOR PLT. PERHAPS HE WAS UNSURE OR CONFUSED BY THE WORDING OF THE CTLR'S INSTRUCTIONS. I DISMISSED HIS QUERY ABOUT THE ALT ASSIGNMENT BY SHOWING THAT I WAS ESTABLISHED ON THE LOC COURSE. IF HE HAD BEEN MORE EXPERIENCED OR IF I HAD FLOWN WITH HIM BEFORE, MAYBE I WOULD HAVE LISTENED CLOSER AND PAID MORE ATTN TO WHAT HE WAS TRYING TO TELL ME. YOU CAN BET I'LL LISTEN BETTER NEXT TIME, NO MATTER HOW GREEN THE COPLT, AND I WILL VERIFY SUCH CLRNCS WITH THE CTLR IF EITHER OF US HAS THE SLIGHTEST DOUBT OR QUESTION!

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.