Narrative:

At FL370 en route from lax to tus we checked in with ZLA (hi) and he informed us (air carrier X abc) to use caution because there was a similar sounding callsign (air carrier X back course) on the frequency. After a few routine traffic calls from center, the controller issued a clearance to cross mohak intersection at FL250. The captain and I both heard 'air carrier X abc.' the captain acknowledged and clearly read back the clearance. Since mohak was on our route of flight and within reasonable range of our destination, the clearance made perfect sense. The captain rolled FL250 into our MCP altitude window and when we intercepted the descent profile to make the crossing restr, I descended us out of FL370. Passing approximately FL362 (1200-1500 FPM) the controller told us to maintain FL370. I immediately stopped the descent and began a climb back to FL370 while the captain queried the controller on our crossing restr at mohak. The controller informed us that the crossing restr had been given to 'air carrier X back course' and that he had told us at the time the clearance was given (to air carrier X back course) to maintain FL370. He also informed us that there was traffic below us. Our TCASII displayed traffic approximately 7 NM behind us at FL330. We continued to tus without further incident. Since both the captain and I were very aware that air carrier X back course was on the frequency, we were both highly vigilant to the potential callsign mix-up. There were no distrs in the cockpit (ie, flight attendants, deep conversation, etc), we were both listening closely to the radio. It became obviously that both air carrier X back course and we read back the clearance at the same time. We never acknowledged or read back ZLA's instructions to maintain FL370 after we acknowledged the mohak crossing restr. 2 things that could have prevented this altitude deviation come to mind: 1) if we had reported leaving FL370 (to make the mohak crossing restr, and 2) if the controller had pressed us for an acknowledgement after he told us to maintain FL370. Also, there must be some standard way for a controling agency to temporarily change a call sign for one of the aircraft when callsigns this similar are on the same frequency and/or route. Supplemental information from acn 518167: only the tapes will tell whether we misheard the original clearance (we are sure we did not, of course). What is clear is that both air carrier X abc and air carrier X back course flts both acknowledged the clearance. Due to the nature of the radios, the only one to notice if both aircraft acknowledge the clearance is the receiver, in this case ATC. Once again, with similar call signs all parties need to use caution.

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Original NASA ASRS Text

Title: A B737-300 FLC ACCEPTS A CLRNC MEANT FOR ANOTHER FLT WITH A SIMILAR SOUNDING CALL SIGN WITH CTLR INTERVENING TO STOP THEIR DSCNT W OF MOHAK INTXN NEAR TUS, AZ.

Narrative: AT FL370 ENRTE FROM LAX TO TUS WE CHKED IN WITH ZLA (HI) AND HE INFORMED US (ACR X ABC) TO USE CAUTION BECAUSE THERE WAS A SIMILAR SOUNDING CALLSIGN (ACR X BC) ON THE FREQ. AFTER A FEW ROUTINE TFC CALLS FROM CTR, THE CTLR ISSUED A CLRNC TO CROSS MOHAK INTXN AT FL250. THE CAPT AND I BOTH HEARD 'ACR X ABC.' THE CAPT ACKNOWLEDGED AND CLRLY READ BACK THE CLRNC. SINCE MOHAK WAS ON OUR RTE OF FLT AND WITHIN REASONABLE RANGE OF OUR DEST, THE CLRNC MADE PERFECT SENSE. THE CAPT ROLLED FL250 INTO OUR MCP ALT WINDOW AND WHEN WE INTERCEPTED THE DSCNT PROFILE TO MAKE THE XING RESTR, I DSNDED US OUT OF FL370. PASSING APPROX FL362 (1200-1500 FPM) THE CTLR TOLD US TO MAINTAIN FL370. I IMMEDIATELY STOPPED THE DSCNT AND BEGAN A CLB BACK TO FL370 WHILE THE CAPT QUERIED THE CTLR ON OUR XING RESTR AT MOHAK. THE CTLR INFORMED US THAT THE XING RESTR HAD BEEN GIVEN TO 'ACR X BC' AND THAT HE HAD TOLD US AT THE TIME THE CLRNC WAS GIVEN (TO ACR X BC) TO MAINTAIN FL370. HE ALSO INFORMED US THAT THERE WAS TFC BELOW US. OUR TCASII DISPLAYED TFC APPROX 7 NM BEHIND US AT FL330. WE CONTINUED TO TUS WITHOUT FURTHER INCIDENT. SINCE BOTH THE CAPT AND I WERE VERY AWARE THAT ACR X BC WAS ON THE FREQ, WE WERE BOTH HIGHLY VIGILANT TO THE POTENTIAL CALLSIGN MIX-UP. THERE WERE NO DISTRS IN THE COCKPIT (IE, FLT ATTENDANTS, DEEP CONVERSATION, ETC), WE WERE BOTH LISTENING CLOSELY TO THE RADIO. IT BECAME OBVIOUSLY THAT BOTH ACR X BC AND WE READ BACK THE CLRNC AT THE SAME TIME. WE NEVER ACKNOWLEDGED OR READ BACK ZLA'S INSTRUCTIONS TO MAINTAIN FL370 AFTER WE ACKNOWLEDGED THE MOHAK XING RESTR. 2 THINGS THAT COULD HAVE PREVENTED THIS ALTDEV COME TO MIND: 1) IF WE HAD RPTED LEAVING FL370 (TO MAKE THE MOHAK XING RESTR, AND 2) IF THE CTLR HAD PRESSED US FOR AN ACKNOWLEDGEMENT AFTER HE TOLD US TO MAINTAIN FL370. ALSO, THERE MUST BE SOME STANDARD WAY FOR A CTLING AGENCY TO TEMPORARILY CHANGE A CALL SIGN FOR ONE OF THE ACFT WHEN CALLSIGNS THIS SIMILAR ARE ON THE SAME FREQ AND/OR RTE. SUPPLEMENTAL INFO FROM ACN 518167: ONLY THE TAPES WILL TELL WHETHER WE MISHEARD THE ORIGINAL CLRNC (WE ARE SURE WE DID NOT, OF COURSE). WHAT IS CLR IS THAT BOTH ACR X ABC AND ACR X BC FLTS BOTH ACKNOWLEDGED THE CLRNC. DUE TO THE NATURE OF THE RADIOS, THE ONLY ONE TO NOTICE IF BOTH ACFT ACKNOWLEDGE THE CLRNC IS THE RECEIVER, IN THIS CASE ATC. ONCE AGAIN, WITH SIMILAR CALL SIGNS ALL PARTIES NEED TO USE CAUTION.

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.