Narrative:

The flight was being conducted in an medium large transport aircraft for a scheduled airline. We were operating a passenger charter between denver and cody, wy, for our commuter airline. I am a qualified check airman for both the medium large transport aircraft and the more advanced and automated medium large transport Y. However, I only have about 75 hours in the medium large transport X as compared to over 700 hours in the medium large transport Y. The first officer is also a qualified check airman on both models of the medium large transport with many thousands of hours in the medium large transport Y. I was sitting in the left seat and was flying the aircraft at the time of the altitude deviation. We were level at FL350 when we received a clearance to descend. We accepted FL230 and started down. During the descent the other pilot was on the #2 radio talking to the company and was having trouble getting the WX for cody. While he was waiting for them to call back I was explaining to him about the automated WX service and how to tune it up. Sometime during the descent we heard 1 chime. Since he is very familiar with the aircraft I asked him if that was the cabin crew trying to call us. We tried to raise them on the interphone but to no avail. Just after passing through 16000 ft I asked him what altitude we were cleared to. He said that he didn't know as he had been on and off the radio since shortly after we started our descent. We both agreed that the altitude indicator was set to FL230. I leveled off around 15500 ft and immediately asked center what altitude we were cleared to as we had just passed 16000 ft. The response was 'ah, ah, I'm checking' or something similar to that. Finally, the controller came back on and said that he had only cleared us down to FL230 but that we were now cleared to 12000 ft. I continued the descent and leveled off at 12000 ft. I asked him point blank if he had a problem with the altitude deviation and he said no. During the descent from FL350 until we reached 12000 ft we were clear of clouds and had excellent in-flight visibility. There were only 2 aircraft on the frequency and the other aircraft was not in our area sector. After we leveled at 12000 ft we had to keep requesting lower altitudes as we were in and out of the clouds and could not get low enough to sight cody visually. Finally, at 10 mi southeast of the airport we broke through the last cloud bank and made a visual approach into cody. Once on the ground I called the salt lake center and spoke to the supervisor. He indicated that he had not heard of any altitude problems but that he would check into it. I told him that I did not want to cause any problems and that I was satisfied with how the controller had handled the situation. Against my wishes, the supervisor indicated that he was going to talk to the controller on duty.

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Original NASA ASRS Text

Title: CAPT OF MLG ACR ACFT INADVERTENTLY DSNDED BELOW ASSIGNED ALT DUE TO DISTR.

Narrative: THE FLT WAS BEING CONDUCTED IN AN MLG ACFT FOR A SCHEDULED AIRLINE. WE WERE OPERATING A PAX CHARTER BTWN DENVER AND CODY, WY, FOR OUR COMMUTER AIRLINE. I AM A QUALIFIED CHK AIRMAN FOR BOTH THE MLG ACFT AND THE MORE ADVANCED AND AUTOMATED MLG Y. HOWEVER, I ONLY HAVE ABOUT 75 HRS IN THE MLG X AS COMPARED TO OVER 700 HRS IN THE MLG Y. THE FO IS ALSO A QUALIFIED CHK AIRMAN ON BOTH MODELS OF THE MLG WITH MANY THOUSANDS OF HRS IN THE MLG Y. I WAS SITTING IN THE L SEAT AND WAS FLYING THE ACFT AT THE TIME OF THE ALTDEV. WE WERE LEVEL AT FL350 WHEN WE RECEIVED A CLRNC TO DSND. WE ACCEPTED FL230 AND STARTED DOWN. DURING THE DSCNT THE OTHER PLT WAS ON THE #2 RADIO TALKING TO THE COMPANY AND WAS HAVING TROUBLE GETTING THE WX FOR CODY. WHILE HE WAS WAITING FOR THEM TO CALL BACK I WAS EXPLAINING TO HIM ABOUT THE AUTOMATED WX SVC AND HOW TO TUNE IT UP. SOMETIME DURING THE DSCNT WE HEARD 1 CHIME. SINCE HE IS VERY FAMILIAR WITH THE ACFT I ASKED HIM IF THAT WAS THE CABIN CREW TRYING TO CALL US. WE TRIED TO RAISE THEM ON THE INTERPHONE BUT TO NO AVAIL. JUST AFTER PASSING THROUGH 16000 FT I ASKED HIM WHAT ALT WE WERE CLRED TO. HE SAID THAT HE DIDN'T KNOW AS HE HAD BEEN ON AND OFF THE RADIO SINCE SHORTLY AFTER WE STARTED OUR DSCNT. WE BOTH AGREED THAT THE ALT INDICATOR WAS SET TO FL230. I LEVELED OFF AROUND 15500 FT AND IMMEDIATELY ASKED CTR WHAT ALT WE WERE CLRED TO AS WE HAD JUST PASSED 16000 FT. THE RESPONSE WAS 'AH, AH, I'M CHKING' OR SOMETHING SIMILAR TO THAT. FINALLY, THE CTLR CAME BACK ON AND SAID THAT HE HAD ONLY CLRED US DOWN TO FL230 BUT THAT WE WERE NOW CLRED TO 12000 FT. I CONTINUED THE DSCNT AND LEVELED OFF AT 12000 FT. I ASKED HIM POINT BLANK IF HE HAD A PROBLEM WITH THE ALTDEV AND HE SAID NO. DURING THE DSCNT FROM FL350 UNTIL WE REACHED 12000 FT WE WERE CLR OF CLOUDS AND HAD EXCELLENT INFLT VISIBILITY. THERE WERE ONLY 2 ACFT ON THE FREQ AND THE OTHER ACFT WAS NOT IN OUR AREA SECTOR. AFTER WE LEVELED AT 12000 FT WE HAD TO KEEP REQUESTING LOWER ALTS AS WE WERE IN AND OUT OF THE CLOUDS AND COULD NOT GET LOW ENOUGH TO SIGHT CODY VISUALLY. FINALLY, AT 10 MI SE OF THE ARPT WE BROKE THROUGH THE LAST CLOUD BANK AND MADE A VISUAL APCH INTO CODY. ONCE ON THE GND I CALLED THE SALT LAKE CTR AND SPOKE TO THE SUPVR. HE INDICATED THAT HE HAD NOT HEARD OF ANY ALT PROBLEMS BUT THAT HE WOULD CHK INTO IT. I TOLD HIM THAT I DID NOT WANT TO CAUSE ANY PROBLEMS AND THAT I WAS SATISFIED WITH HOW THE CTLR HAD HANDLED THE SITUATION. AGAINST MY WISHES, THE SUPVR INDICATED THAT HE WAS GOING TO TALK TO THE CTLR ON DUTY.

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.