Narrative:

We were involved in a possible altitude deviation. We departed lgb and were issued a left turn to 200 degrees (off runway 30) and 3000'. Coast approach handed us off to ZLA and soon after issued us a left turn to 360 degrees and a climb to 11000'. I dialed my altitude alerter to 11000' and my first officer read back 'one-one-thousand.' we were issued traffic as air carrier Y at 10 O'clock and 11000'. After reaching approximately 10800' the controller came back with 'maintain one-zero-thousand,' which we thought was strange, but immediately read back 'one-zero-thousand' and descended. We were then informed of a 'possible altitude deviation' and and a phone number for ZLA. After landing at smf I called ZLA and talked with the area supervisor. He said that his controller issued us 10000' and we read back 'one-zero-thousand.' however, he didn't say whether we were issued this altitude the first time along with the turn to 360 degrees, or if it was when air carrier Y was nearing our position. He said that air carrier Y came within 4.6 mi (out of a 5 mi standard sep) of our position at the same altitude. We had air carrier Y in sight at all times and there was no danger of a near miss. Air carrier Y crew also reported having us in sight as well. The error was either on the part of the controller in which his voice was not clear on the altitude (sometimes when the controllers speak fast, 'one-zero' and 'one-one' sound similar), or in fact he did issue us the wrong altitude. Or, the error could have been with us? Maybe my first officer did not communicate to me what he repeated back to ATC, however, I thought I heard him correctly repeat back what I dialed into the altitude alerter the first time. When 2 aircraft are converging, controllers should be particularly careful about how they speak to pilots. Altitudes should be issued slowly and always be listened to and verified when pilots repeat them back. Very often controllers issue 4 to 5 instructions in the same breath, such as 'turn left 330 degrees, maintain 2000' till established, cleared for ILS 30 approach, contact tower 119.4 at the OM, and maintain 160 KTS until 5 mi final.' it is sometimes very hard to get all these instructions clear to repeat back. Many pilots simply say 'roger.' how do the controllers really know that all instructions were clearly understood? Callback conversation with reporter revealed the following: reporter has received a letter from FAA saying they were investigating the incident because less than standard separation had resulted from altitude deviation. Says they were already above 10000' when controller issued traffic at 11000'. They saw the traffic and kept climbing. I explained to reporter that terminal controllers could apply visual sep, but that center controllers could not use visual sep. This was news to reporter. Explained immunity provisions of ASRS program briefly.

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Original NASA ASRS Text

Title: ALT DEVIATION BY COMMUTER FLT CREW CAUSED LESS THAN STANDARD SEPARATION WITH ACR. PLT DEVIATION.

Narrative: WE WERE INVOLVED IN A POSSIBLE ALT DEVIATION. WE DEPARTED LGB AND WERE ISSUED A LEFT TURN TO 200 DEGS (OFF RWY 30) AND 3000'. COAST APCH HANDED US OFF TO ZLA AND SOON AFTER ISSUED US A LEFT TURN TO 360 DEGS AND A CLB TO 11000'. I DIALED MY ALT ALERTER TO 11000' AND MY F/O READ BACK 'ONE-ONE-THOUSAND.' WE WERE ISSUED TFC AS ACR Y AT 10 O'CLOCK AND 11000'. AFTER REACHING APPROX 10800' THE CTLR CAME BACK WITH 'MAINTAIN ONE-ZERO-THOUSAND,' WHICH WE THOUGHT WAS STRANGE, BUT IMMEDIATELY READ BACK 'ONE-ZERO-THOUSAND' AND DSNDED. WE WERE THEN INFORMED OF A 'POSSIBLE ALT DEVIATION' AND AND A PHONE NUMBER FOR ZLA. AFTER LNDG AT SMF I CALLED ZLA AND TALKED WITH THE AREA SUPVR. HE SAID THAT HIS CTLR ISSUED US 10000' AND WE READ BACK 'ONE-ZERO-THOUSAND.' HOWEVER, HE DIDN'T SAY WHETHER WE WERE ISSUED THIS ALT THE FIRST TIME ALONG WITH THE TURN TO 360 DEGS, OR IF IT WAS WHEN ACR Y WAS NEARING OUR POS. HE SAID THAT ACR Y CAME WITHIN 4.6 MI (OUT OF A 5 MI STANDARD SEP) OF OUR POS AT THE SAME ALT. WE HAD ACR Y IN SIGHT AT ALL TIMES AND THERE WAS NO DANGER OF A NEAR MISS. ACR Y CREW ALSO RPTED HAVING US IN SIGHT AS WELL. THE ERROR WAS EITHER ON THE PART OF THE CTLR IN WHICH HIS VOICE WAS NOT CLEAR ON THE ALT (SOMETIMES WHEN THE CTLRS SPEAK FAST, 'ONE-ZERO' AND 'ONE-ONE' SOUND SIMILAR), OR IN FACT HE DID ISSUE US THE WRONG ALT. OR, THE ERROR COULD HAVE BEEN WITH US? MAYBE MY F/O DID NOT COMMUNICATE TO ME WHAT HE REPEATED BACK TO ATC, HOWEVER, I THOUGHT I HEARD HIM CORRECTLY REPEAT BACK WHAT I DIALED INTO THE ALT ALERTER THE FIRST TIME. WHEN 2 ACFT ARE CONVERGING, CTLRS SHOULD BE PARTICULARLY CAREFUL ABOUT HOW THEY SPEAK TO PLTS. ALTS SHOULD BE ISSUED SLOWLY AND ALWAYS BE LISTENED TO AND VERIFIED WHEN PLTS REPEAT THEM BACK. VERY OFTEN CTLRS ISSUE 4 TO 5 INSTRUCTIONS IN THE SAME BREATH, SUCH AS 'TURN LEFT 330 DEGS, MAINTAIN 2000' TILL ESTABLISHED, CLRED FOR ILS 30 APCH, CONTACT TWR 119.4 AT THE OM, AND MAINTAIN 160 KTS UNTIL 5 MI FINAL.' IT IS SOMETIMES VERY HARD TO GET ALL THESE INSTRUCTIONS CLEAR TO REPEAT BACK. MANY PLTS SIMPLY SAY 'ROGER.' HOW DO THE CTLRS REALLY KNOW THAT ALL INSTRUCTIONS WERE CLEARLY UNDERSTOOD? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR HAS RECEIVED A LETTER FROM FAA SAYING THEY WERE INVESTIGATING THE INCIDENT BECAUSE LTSS HAD RESULTED FROM ALT DEVIATION. SAYS THEY WERE ALREADY ABOVE 10000' WHEN CTLR ISSUED TFC AT 11000'. THEY SAW THE TFC AND KEPT CLBING. I EXPLAINED TO RPTR THAT TERMINAL CTLRS COULD APPLY VISUAL SEP, BUT THAT CENTER CTLRS COULD NOT USE VISUAL SEP. THIS WAS NEWS TO RPTR. EXPLAINED IMMUNITY PROVISIONS OF ASRS PROGRAM BRIEFLY.

Data retrieved from NASA's ASRS site as of August 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.