Narrative:

The following situation happened twice in one week, and although it confused me the first time, at least I was somewhat ready the second. This is what happened the first time: it was a flight from lax to ord on dec/xa/99, flight xyz aircraft #abcd. We were flight planned and received a clearance for the angel 2 departure out of lax, this was my first time with this departure. I looked closely and discussed the departure with my copilot, we noted that the first turn seemed to be before the smo 160 degree radial. I thought this was odd as I have always done the first turn at the smo 160 degree radial. However both the map mode and, it seemed to us, that the commercial page showed it as an earlier turn. When we flew it (my leg) I asked for 'gear up, LNAV' and followed the flight director. We were then advised by the lax tower that I was turning too early (he was not happy). This caused a lot of consternation in the cockpit as we were now not sure of our navigation, and we lost some situational awareness as we were busy checking our position, LNAV, and telling ATC what we were doing. As a result we went about 300 ft high on level off. We were then given radar vectors by lax departure and resumed the angel 2 without further problems. We were concerned with the accuracy of our IRU's and checked them on the way to, and over trm. One showed a little off, but the other 2 and the average was right on, for the rest of the trip we were right on course. The second time this occurred was a couple days later. It was flight abc on dec/xb/99 lax to ord aircraft #abce. Exact same thing happened again however I was somewhat prepared and did not lose any situational awareness. On both occasions the FMC's show the turn before the smo 160 degree radial, both times the IRU's were accurate, both times ATC expected the turn at the smo 160 degree radial. So what's going on? The departure does not say where smoog waypoint is, just that it is depicted off the end of the runway, but how far? Either there is an error in the programming of the angle 2 or ATC is not familiar with what we are doing. Either of these are bad sits as things happen fast on the departure out of lax and we are at a very low altitude to be getting into navigation discussions with ATC. For one thing I would suggest that a cross-reference, and/or an indication of where the waypoints are physically located be included in the departure description on the commercial SID page. Second, tell ATC. Callback conversation with reporter revealed the following information: the problem described occurred on two separate aircraft. The captain indicated that waypoint smoog might not be depicted correctly in the FMC database. This would account for the early turn. The captain states that normally other sids call for turns at the smo 160 degree radial. He thinks the controllers expect this. The captain has notified his company and union air safety structure, but has not had a reply as yet.

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Original NASA ASRS Text

Title: B757 CREW HAD THE ACFT FMC LNAV INITIATE AN EARLY TURN, WHILE FLYING THE COMPANY TAILORED SPECIAL FMS SID 'ANGEL 2' DEP AT LAX.

Narrative: THE FOLLOWING SIT HAPPENED TWICE IN ONE WEEK, AND ALTHOUGH IT CONFUSED ME THE FIRST TIME, AT LEAST I WAS SOMEWHAT READY THE SECOND. THIS IS WHAT HAPPENED THE FIRST TIME: IT WAS A FLT FROM LAX TO ORD ON DEC/XA/99, FLT XYZ ACFT #ABCD. WE WERE FLT PLANNED AND RECEIVED A CLRNC FOR THE ANGEL 2 DEP OUT OF LAX, THIS WAS MY FIRST TIME WITH THIS DEP. I LOOKED CLOSELY AND DISCUSSED THE DEP WITH MY COPLT, WE NOTED THAT THE FIRST TURN SEEMED TO BE BEFORE THE SMO 160 DEG RADIAL. I THOUGHT THIS WAS ODD AS I HAVE ALWAYS DONE THE FIRST TURN AT THE SMO 160 DEG RADIAL. HOWEVER BOTH THE MAP MODE AND, IT SEEMED TO US, THAT THE COMMERCIAL PAGE SHOWED IT AS AN EARLIER TURN. WHEN WE FLEW IT (MY LEG) I ASKED FOR 'GEAR UP, LNAV' AND FOLLOWED THE FLIGHT DIRECTOR. WE WERE THEN ADVISED BY THE LAX TWR THAT I WAS TURNING TOO EARLY (HE WAS NOT HAPPY). THIS CAUSED A LOT OF CONSTERNATION IN THE COCKPIT AS WE WERE NOW NOT SURE OF OUR NAV, AND WE LOST SOME SITUATIONAL AWARENESS AS WE WERE BUSY CHECKING OUR POS, LNAV, AND TELLING ATC WHAT WE WERE DOING. AS A RESULT WE WENT ABOUT 300 FT HIGH ON LEVEL OFF. WE WERE THEN GIVEN RADAR VECTORS BY LAX DEP AND RESUMED THE ANGEL 2 WITHOUT FURTHER PROBS. WE WERE CONCERNED WITH THE ACCURACY OF OUR IRU'S AND CHECKED THEM ON THE WAY TO, AND OVER TRM. ONE SHOWED A LITTLE OFF, BUT THE OTHER 2 AND THE AVERAGE WAS RIGHT ON, FOR THE REST OF THE TRIP WE WERE RIGHT ON COURSE. THE SECOND TIME THIS OCCURRED WAS A COUPLE DAYS LATER. IT WAS FLT ABC ON DEC/XB/99 LAX TO ORD ACFT #ABCE. EXACT SAME THING HAPPENED AGAIN HOWEVER I WAS SOMEWHAT PREPARED AND DID NOT LOSE ANY SITUATIONAL AWARENESS. ON BOTH OCCASIONS THE FMC'S SHOW THE TURN BEFORE THE SMO 160 DEG RADIAL, BOTH TIMES THE IRU'S WERE ACCURATE, BOTH TIMES ATC EXPECTED THE TURN AT THE SMO 160 DEG RADIAL. SO WHAT'S GOING ON? THE DEP DOES NOT SAY WHERE SMOOG WAYPOINT IS, JUST THAT IT IS DEPICTED OFF THE END OF THE RWY, BUT HOW FAR? EITHER THERE IS AN ERROR IN THE PROGRAMMING OF THE ANGLE 2 OR ATC IS NOT FAMILIAR WITH WHAT WE ARE DOING. EITHER OF THESE ARE BAD SITS AS THINGS HAPPEN FAST ON THE DEP OUT OF LAX AND WE ARE AT A VERY LOW ALT TO BE GETTING INTO NAV DISCUSSIONS WITH ATC. FOR ONE THING I WOULD SUGGEST THAT A CROSS-REFERENCE, AND/OR AN INDICATION OF WHERE THE WAYPOINTS ARE PHYSICALLY LOCATED BE INCLUDED IN THE DEP DESCRIPTION ON THE COMMERCIAL SID PAGE. SECOND, TELL ATC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PROB DESCRIBED OCCURRED ON TWO SEPARATE ACFT. THE CAPT INDICATED THAT WAYPOINT SMOOG MIGHT NOT BE DEPICTED CORRECTLY IN THE FMC DATABASE. THIS WOULD ACCOUNT FOR THE EARLY TURN. THE CAPT STATES THAT NORMALLY OTHER SIDS CALL FOR TURNS AT THE SMO 160 DEG RADIAL. HE THINKS THE CTLRS EXPECT THIS. THE CAPT HAS NOTIFIED HIS COMPANY AND UNION AIR SAFETY STRUCTURE, BUT HAS NOT HAD A REPLY AS YET.

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.