Narrative:

During cruise I heard ZLA tell air carrier X (descending) to lgb 'plan on 35 DME north of sxc (santa catalina VOR) at 11000 ft.' I made note. And since we were on the same routing, I figured they would give us the same altitude restriction. 3 mins later they said 'small transport plan on 35 north of sxc at 11000 ft.' both the captain and I had removed our head sets for the 1.5 hour flight. We also turned off the 'side tone' to avoid loud squeals when transmitting. We had approximately 75 KTS tailwind during our descent. Realizing I'd better start down to meet the restriction, we asked center for lower. The controller said 'small transport descend and maintain 11000 ft.' at this point I began calculating what rate of descent would be required to meet the 35 north at 11000 ft restriction. (In hindsight, the controller never gave us the restriction, just plan on it.) since the controller said plan on the restriction, I was busy planning the descent. After we had started down I failed to hear a call from center and failed to hear the captain read back the clearance 'small transport descend and maintain 13000 ft for traffic at 12000 ft.' again we had our side tone turned off. Since the altitude alerter is closer to the first officer side of the cockpit, I usually set the clearance altitude. But the last clearance I heard was 11000 ft, that remained in the alert box. As we continued the descent, center called out traffic as an air carrier widebody transport at 12 O'clock, 'do we have him in sight?' we responded 'affirmative, widebody transport in sight.' descending through 11800 ft the controller asked 'small transport, what is your altitude?' we responded 11800 ft for 11000 ft.' she responded 'negative, you were assigned 13000 ft.' 'small transport you were involved in a potential pilot altitude violation. Contact center [with the telephone number] on the ground.' (on the tape, the captain read back 13000 ft, but we both failed to adjust the altitude alerter.)

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Original NASA ASRS Text

Title: COMMUTER ACFT DSNDS BELOW ASSIGNED ALT.

Narrative: DURING CRUISE I HEARD ZLA TELL ACR X (DSNDING) TO LGB 'PLAN ON 35 DME N OF SXC (SANTA CATALINA VOR) AT 11000 FT.' I MADE NOTE. AND SINCE WE WERE ON THE SAME ROUTING, I FIGURED THEY WOULD GIVE US THE SAME ALT RESTRICTION. 3 MINS LATER THEY SAID 'SMT PLAN ON 35 N OF SXC AT 11000 FT.' BOTH THE CAPT AND I HAD REMOVED OUR HEAD SETS FOR THE 1.5 HR FLT. WE ALSO TURNED OFF THE 'SIDE TONE' TO AVOID LOUD SQUEALS WHEN XMITTING. WE HAD APPROX 75 KTS TAILWIND DURING OUR DSCNT. REALIZING I'D BETTER START DOWN TO MEET THE RESTRICTION, WE ASKED CTR FOR LOWER. THE CTLR SAID 'SMT DSND AND MAINTAIN 11000 FT.' AT THIS POINT I BEGAN CALCULATING WHAT RATE OF DSCNT WOULD BE REQUIRED TO MEET THE 35 N AT 11000 FT RESTRICTION. (IN HINDSIGHT, THE CTLR NEVER GAVE US THE RESTRICTION, JUST PLAN ON IT.) SINCE THE CTLR SAID PLAN ON THE RESTRICTION, I WAS BUSY PLANNING THE DSCNT. AFTER WE HAD STARTED DOWN I FAILED TO HEAR A CALL FROM CTR AND FAILED TO HEAR THE CAPT READ BACK THE CLRNC 'SMT DSND AND MAINTAIN 13000 FT FOR TFC AT 12000 FT.' AGAIN WE HAD OUR SIDE TONE TURNED OFF. SINCE THE ALT ALERTER IS CLOSER TO THE FO SIDE OF THE COCKPIT, I USUALLY SET THE CLRNC ALT. BUT THE LAST CLRNC I HEARD WAS 11000 FT, THAT REMAINED IN THE ALERT BOX. AS WE CONTINUED THE DSCNT, CTR CALLED OUT TFC AS AN ACR WDB AT 12 O'CLOCK, 'DO WE HAVE HIM IN SIGHT?' WE RESPONDED 'AFFIRMATIVE, WDB IN SIGHT.' DSNDING THROUGH 11800 FT THE CTLR ASKED 'SMT, WHAT IS YOUR ALT?' WE RESPONDED 11800 FT FOR 11000 FT.' SHE RESPONDED 'NEGATIVE, YOU WERE ASSIGNED 13000 FT.' 'SMT YOU WERE INVOLVED IN A POTENTIAL PLT ALT VIOLATION. CONTACT CTR [WITH THE TELEPHONE NUMBER] ON THE GND.' (ON THE TAPE, THE CAPT READ BACK 13000 FT, BUT WE BOTH FAILED TO ADJUST THE ALT ALERTER.)

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.