Narrative:

Eastside 3 arrival into sna requires 190 KTS at kayoh, and we were cleared to cross at 7000 ft. In the B737 with VNAV capability we are recommended to set the next altitude on the arrival in the MCP -- not the lowest altitude -- then program the FMC. However, it is acceptable on a complicated profile descent to set the lowest altitude in the MCP and program the FMC for the profile descent. In that way it should hit every altitude and airspeed automatically, in path descent. The problem this day was that on the eastside arrival we are required to cross kayoh at 190 KTS, so we must use flaps 5 degrees. What I didn't know was that as soon as flaps are extended, the FMC automatically drops into VNAV speed, and ignores all path programming. Although I had 7000 ft at kayoh programmed, we descended right through 7000 ft to about 6600 ft. We caught it, but not before descending through the required altitude. Boeing, and air carrier training departments, should ensure that every pilot understands that when flaps are extended, the FMC drops out of VNAV path and into VNAV speed. If the altitude in the MCP is set to the bottom of the profile descent, the autoplt will ignore all FMC programming and descend in VNAV speed all the way down to the altitude in the MCP. Since this occurrence, I have discussed this with many other pilots and nobody knows about this. It is very dangerous.

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Original NASA ASRS Text

Title: B737 ALTDEV SNA. RPTR CITES LACK OF TRAINING IN FMC OPS AND PROCS TO USE AUTOFLT IN STAR ARRS.

Narrative: EASTSIDE 3 ARR INTO SNA REQUIRES 190 KTS AT KAYOH, AND WE WERE CLRED TO CROSS AT 7000 FT. IN THE B737 WITH VNAV CAPABILITY WE ARE RECOMMENDED TO SET THE NEXT ALT ON THE ARR IN THE MCP -- NOT THE LOWEST ALT -- THEN PROGRAM THE FMC. HOWEVER, IT IS ACCEPTABLE ON A COMPLICATED PROFILE DSCNT TO SET THE LOWEST ALT IN THE MCP AND PROGRAM THE FMC FOR THE PROFILE DSCNT. IN THAT WAY IT SHOULD HIT EVERY ALT AND AIRSPD AUTOMATICALLY, IN PATH DSCNT. THE PROB THIS DAY WAS THAT ON THE EASTSIDE ARR WE ARE REQUIRED TO CROSS KAYOH AT 190 KTS, SO WE MUST USE FLAPS 5 DEGS. WHAT I DIDN'T KNOW WAS THAT AS SOON AS FLAPS ARE EXTENDED, THE FMC AUTOMATICALLY DROPS INTO VNAV SPD, AND IGNORES ALL PATH PROGRAMMING. ALTHOUGH I HAD 7000 FT AT KAYOH PROGRAMMED, WE DSNDED RIGHT THROUGH 7000 FT TO ABOUT 6600 FT. WE CAUGHT IT, BUT NOT BEFORE DSNDING THROUGH THE REQUIRED ALT. BOEING, AND ACR TRAINING DEPTS, SHOULD ENSURE THAT EVERY PLT UNDERSTANDS THAT WHEN FLAPS ARE EXTENDED, THE FMC DROPS OUT OF VNAV PATH AND INTO VNAV SPD. IF THE ALT IN THE MCP IS SET TO THE BOTTOM OF THE PROFILE DSCNT, THE AUTOPLT WILL IGNORE ALL FMC PROGRAMMING AND DSND IN VNAV SPD ALL THE WAY DOWN TO THE ALT IN THE MCP. SINCE THIS OCCURRENCE, I HAVE DISCUSSED THIS WITH MANY OTHER PLTS AND NOBODY KNOWS ABOUT THIS. IT IS VERY DANGEROUS.

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.