Narrative:

Thanks for TCASII. On approach to sna, on the kayoe arrival. The flight had been uneventful to this point. Level at 10000 ft we received what the captain and I both perceived was a clearance to 5000 ft. I, PNF, read back clearance as 'air carrier X cleared to 5000 ft.' we started down passing 8000 ft. I noticed another aircraft on TCASII about 1 O'clock position and 3 mi at 7000 ft. I point it out to the captain. At 7700 ft he slowed the descent and at 7500 ft we received an RA and leveled off at 7500 ft. I immediately called socal and told him that we were responding to an RA. He said our clearance was to 8000 ft. I told him we both heard 5 and I read back 5000 ft. He apologized for not catching it on the readback. I don't know if he originally said 5 or I heard him wrong. The rest of the flight was uneventful. Approach was very busy. A GA pilot was jamming the frequency trying to get an IFR clearance. The controller was very busy. He apologized twice before we left the frequency. In the future I will question clrncs that might cross other aircraft flight paths. The kayoe arrival -- we were on radar vectors. Both pilots heard 5000 ft on the radios and 5000 ft was in the altitude window as per SOP.

Related reports:

Google
 

Original NASA ASRS Text

Title: FLC OF AN LGT WERE RESPONDING TO A TCASII RA AND HAD LEVELOFF IN DSCNT FOR THAT REASON. WHEN THEY NOTIFIED ATC OF THEIR RESPONSE TO TCASII, ATC ADVISED THAT THEY OVERSHOT ASSIGNED ALT. BOTH CREW MEMBERS HAD HEARD AND SET INTO ALT ALERTER THE ALT READBACK TO ATC AFTER ASSIGNMENT. ATC APOLOGIZED FOR THE MISUNDERSTANDING.

Narrative: THANKS FOR TCASII. ON APCH TO SNA, ON THE KAYOE ARR. THE FLT HAD BEEN UNEVENTFUL TO THIS POINT. LEVEL AT 10000 FT WE RECEIVED WHAT THE CAPT AND I BOTH PERCEIVED WAS A CLRNC TO 5000 FT. I, PNF, READ BACK CLRNC AS 'ACR X CLRED TO 5000 FT.' WE STARTED DOWN PASSING 8000 FT. I NOTICED ANOTHER ACFT ON TCASII ABOUT 1 O'CLOCK POS AND 3 MI AT 7000 FT. I POINT IT OUT TO THE CAPT. AT 7700 FT HE SLOWED THE DSCNT AND AT 7500 FT WE RECEIVED AN RA AND LEVELED OFF AT 7500 FT. I IMMEDIATELY CALLED SOCAL AND TOLD HIM THAT WE WERE RESPONDING TO AN RA. HE SAID OUR CLRNC WAS TO 8000 FT. I TOLD HIM WE BOTH HEARD 5 AND I READ BACK 5000 FT. HE APOLOGIZED FOR NOT CATCHING IT ON THE READBACK. I DON'T KNOW IF HE ORIGINALLY SAID 5 OR I HEARD HIM WRONG. THE REST OF THE FLT WAS UNEVENTFUL. APCH WAS VERY BUSY. A GA PLT WAS JAMMING THE FREQ TRYING TO GET AN IFR CLRNC. THE CTLR WAS VERY BUSY. HE APOLOGIZED TWICE BEFORE WE LEFT THE FREQ. IN THE FUTURE I WILL QUESTION CLRNCS THAT MIGHT CROSS OTHER ACFT FLT PATHS. THE KAYOE ARR -- WE WERE ON RADAR VECTORS. BOTH PLTS HEARD 5000 FT ON THE RADIOS AND 5000 FT WAS IN THE ALT WINDOW AS PER SOP.

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.