Narrative:

Flight from san to lax at 10000' I had to leave ATC frequency to get ATIS information. I advised first officer to monitor ATC (he was flying) and while I was off frequency he acknowledged a descent clearance from coast approach. When I returned to frequency I saw he had 6000' set in the altitude alert and that he had programmed the RNAV to cross seal beach VOR at 6000'. I asked him to confirm that we had been cleared to cross seal beach at 6000' and he did confirm it. After changing to lax approach control, approach asked us to say our altitude. We were leaving 6300' for 6000' about 1 mi from seal beach. I said 'we are leaving 6300'; we'll cross seal beach at 6000'', I assumed he was concerned that we would make our crossing restriction. Lax approach did not mention that he expected us at a different altitude and simply said 'roger, maintain 6000'.' I thought his query was odd, however, and then remembered the normal seal beach crossing altitude was 7000'. I then pressed the first officer on his certainty that we were cleared to 6000'. He then seemed a lot less certain than he had been. I asked approach for a land-line number and called them after landing. I was told there was no problem but they had expected us at 7000' instead of 6000'. The question remains, did coast approach mis-speak and say 6000' instead of 7000' or did the controller mis-hear the readback? There are many loose threads in this situation, first, ideally both pilots should monitor both ATC and ATIS frequencys. Easy to say but usually utterly impossible. With ATIS messages which are entirely too lengthy and non-stop xmissions on ATC, it is normally necessary for one pilot to leave ATC in order to receive the ATIS. This incident has moved me to implement a procedure which I have considered for some time. Regardless of who is flying, I, the captain, will never again leave the ATC frequency. If it is necessary for some one to leave the frequency, it will be the first officer, oven if he is flying the leg. As for the first officer's procedures, he, like many pilots, is a very 'take-charge' type personality, not prone to realize his own human fallibility. He may or may not have taken the clearance as given, but his fault lies in not being sure. I, of course counseled him about this but it will likely not have any lasting effect. Human behavior modification is a challenging task. The last thing I would like to say on the subject is that this and probably 90% of other ATC problems would be prevented if we had data-link transmission of ATC and ATIS information. I will probably be retired before it becomes a reality. It's a crying shame, since the technology has been available for many yrs. As you can see, this is more than a cya report, a chance to say a few things I've been storing up. I have a great deal of respect for the ASRS system. Let's hope the folks in washington keep listening to what you have to say. Supplemental information from acn 147766. We were in the 12TH hour of a 14 hour duty day. We were at 290 KTS, and the FMC is supposed to first slow you to 250 KTS before descending below 10000', but, as I found out, if you are at 10000' already, it won't do that. I have flown that leg about 500 times and we are always cleared to cross sli at 7000', not 6000'. I was so tired at the time I couldn't remember if we were cleared to 6000' or 7000'.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT INTO LAX.

Narrative: FLT FROM SAN TO LAX AT 10000' I HAD TO LEAVE ATC FREQ TO GET ATIS INFO. I ADVISED F/O TO MONITOR ATC (HE WAS FLYING) AND WHILE I WAS OFF FREQ HE ACKNOWLEDGED A DSNT CLRNC FROM COAST APCH. WHEN I RETURNED TO FREQ I SAW HE HAD 6000' SET IN THE ALT ALERT AND THAT HE HAD PROGRAMMED THE RNAV TO CROSS SEAL BEACH VOR AT 6000'. I ASKED HIM TO CONFIRM THAT WE HAD BEEN CLRED TO CROSS SEAL BEACH AT 6000' AND HE DID CONFIRM IT. AFTER CHANGING TO LAX APCH CTL, APCH ASKED US TO SAY OUR ALT. WE WERE LEAVING 6300' FOR 6000' ABOUT 1 MI FROM SEAL BEACH. I SAID 'WE ARE LEAVING 6300'; WE'LL CROSS SEAL BEACH AT 6000'', I ASSUMED HE WAS CONCERNED THAT WE WOULD MAKE OUR XING RESTRICTION. LAX APCH DID NOT MENTION THAT HE EXPECTED US AT A DIFFERENT ALT AND SIMPLY SAID 'ROGER, MAINTAIN 6000'.' I THOUGHT HIS QUERY WAS ODD, HOWEVER, AND THEN REMEMBERED THE NORMAL SEAL BEACH XING ALT WAS 7000'. I THEN PRESSED THE F/O ON HIS CERTAINTY THAT WE WERE CLRED TO 6000'. HE THEN SEEMED A LOT LESS CERTAIN THAN HE HAD BEEN. I ASKED APCH FOR A LAND-LINE NUMBER AND CALLED THEM AFTER LNDG. I WAS TOLD THERE WAS NO PROB BUT THEY HAD EXPECTED US AT 7000' INSTEAD OF 6000'. THE QUESTION REMAINS, DID COAST APCH MIS-SPEAK AND SAY 6000' INSTEAD OF 7000' OR DID THE CTLR MIS-HEAR THE READBACK? THERE ARE MANY LOOSE THREADS IN THIS SITUATION, FIRST, IDEALLY BOTH PLTS SHOULD MONITOR BOTH ATC AND ATIS FREQS. EASY TO SAY BUT USUALLY UTTERLY IMPOSSIBLE. WITH ATIS MESSAGES WHICH ARE ENTIRELY TOO LENGTHY AND NON-STOP XMISSIONS ON ATC, IT IS NORMALLY NECESSARY FOR ONE PLT TO LEAVE ATC IN ORDER TO RECEIVE THE ATIS. THIS INCIDENT HAS MOVED ME TO IMPLEMENT A PROC WHICH I HAVE CONSIDERED FOR SOME TIME. REGARDLESS OF WHO IS FLYING, I, THE CAPT, WILL NEVER AGAIN LEAVE THE ATC FREQ. IF IT IS NECESSARY FOR SOME ONE TO LEAVE THE FREQ, IT WILL BE THE F/O, OVEN IF HE IS FLYING THE LEG. AS FOR THE F/O'S PROCS, HE, LIKE MANY PLTS, IS A VERY 'TAKE-CHARGE' TYPE PERSONALITY, NOT PRONE TO REALIZE HIS OWN HUMAN FALLIBILITY. HE MAY OR MAY NOT HAVE TAKEN THE CLRNC AS GIVEN, BUT HIS FAULT LIES IN NOT BEING SURE. I, OF COURSE COUNSELED HIM ABOUT THIS BUT IT WILL LIKELY NOT HAVE ANY LASTING EFFECT. HUMAN BEHAVIOR MODIFICATION IS A CHALLENGING TASK. THE LAST THING I WOULD LIKE TO SAY ON THE SUBJECT IS THAT THIS AND PROBABLY 90% OF OTHER ATC PROBS WOULD BE PREVENTED IF WE HAD DATA-LINK XMISSION OF ATC AND ATIS INFO. I WILL PROBABLY BE RETIRED BEFORE IT BECOMES A REALITY. IT'S A CRYING SHAME, SINCE THE TECHNOLOGY HAS BEEN AVAILABLE FOR MANY YRS. AS YOU CAN SEE, THIS IS MORE THAN A CYA RPT, A CHANCE TO SAY A FEW THINGS I'VE BEEN STORING UP. I HAVE A GREAT DEAL OF RESPECT FOR THE ASRS SYS. LET'S HOPE THE FOLKS IN WASHINGTON KEEP LISTENING TO WHAT YOU HAVE TO SAY. SUPPLEMENTAL INFO FROM ACN 147766. WE WERE IN THE 12TH HR OF A 14 HR DUTY DAY. WE WERE AT 290 KTS, AND THE FMC IS SUPPOSED TO FIRST SLOW YOU TO 250 KTS BEFORE DSNDING BELOW 10000', BUT, AS I FOUND OUT, IF YOU ARE AT 10000' ALREADY, IT WON'T DO THAT. I HAVE FLOWN THAT LEG ABOUT 500 TIMES AND WE ARE ALWAYS CLRED TO CROSS SLI AT 7000', NOT 6000'. I WAS SO TIRED AT THE TIME I COULDN'T REMEMBER IF WE WERE CLRED TO 6000' OR 7000'.

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.