A B737-700 DEPARTING LAS ON THE BOACH2 DEP OVERSHOOTS THEIR ASSIGNED ALT OF 19000 FT WHEN THE AUTOPLT FAILS TO LEVEL OFF EVEN THOUGH ALT CAPTURE IS ANNUNCIATED.

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DURING THE LAS BOACH TWO RNAV DEP WE WERE GIVEN AN UNRESTR CLB TO FL190. AT APPROX 16500 FT LAS DEP GAVE US A SPD REDUCTION TO 270 KTS. MY FO KEPT CLB PWR SET AND INCREASED THE PITCH TO SLOW THE ACFT AND THEN MAINTAIN THE ASSIGNED AIRSPD. AT FL180 WE BOTH CALLED OUT FL180 FOR FL190. AT APPROX 17500 FT THE AMBER ALT CAPTURE ILLUMINATED BUT AT FL185 WE BOTH NOTICED THAT THE AUTOPLT WASN'T DECREASING THE IVSI OR PITCH AT A NORMAL RATE TO CAPTURE FL190. MY FO THEN PULLED THE PWR BACK AND BEGAN A PUSH OVER IN AN EFFORT TO LEVEL OFF AT FL190. (AT APPROX FL185 MY FO NOTICED THAT WE STILL HAD ABOUT 2500 FPM ON THE IVSI AND THAT THE AUTOPLT WAS STILL IN CTL WHEEL STEERING.) AT FL188 IT WAS APPARENT THAT WE COULD NOT LEVEL OFF WITHOUT AN EXCESSIVE PUSHOVER MANEUVER. IN AN EFFORT TO MAINTAIN 1 G AND NOT MAKE IT UNCOMFORTABLE OR DANGEROUS FOR THE PAX/FLT ATTENDANTS MY FO WAS ABLE TO ARREST OUR ASCENT AT ABOUT FL195. HE IMMEDIATELY CORRECTED BACK TO FL190. LAS DEP ASKED IF WE WERE LEVELING AT FL190 AND WE ASSURED HIM WE WERE CORRECTING. (THE AUTOPLT NEVER MADE AN ATTEMPT TO CAPTURE THE ASSIGNED ALT.) I DON'T THINK THE ALT CAPTURE FEATURE OF THE AUTOPLT IS VERY SMOOTH. CONSEQUENTLY; I DON'T USE THE ALT CAPTURE FEATURE. AT 1000 FT BEFORE LEVELOFF I SET 1000 FPM ON THE VERT SPD AND DECREASE THE PWR SETTING TO MAINTAIN PROPER SPD. THEN AT 500 FT BEFORE LEVELOFF I SET 500 FPM AND REDUCE PWR AGAIN. THIS ALWAYS KEEPS ME IN THE LOOP AND MAKES FOR A SMOOTH LEVELOFF. (I PASSED THIS ON TO MY FO AS A TECHNIQUE TO PRECLUDE A REPEAT INCIDENT.) SUPPLEMENTAL INFO FROM ACN 711644: WHILE ON THE BOACH DEP WE WERE FOLLOWING AN MD80 ACFT AND ASSIGNED FL190 WHILE CLBING. INITIALLY; WE WERE ASKED TO SLOW TO 290 KTS FOR SPACING; BUT LATER ASKED TO SLOW AGAIN TO 270 KTS AS THE MD80 HAD NOT ACCELERATED BEYOND THAT. THIS OCCURRED AT ABOUT 16000 FT. I PITCHED THE ACFT UP SLIGHTLY TO SLOW THE ADDITIONAL 20 KTS. AT AROUND 17500 FT I NOTED THAT 'ALT CAPTURE' WAS ILLUMINATED. WE CHANGED ALTIMETER SETTINGS AT FL180 AND WHEN I LOOKED BACK AT THE VV1; I NOTED THAT WE STILL HAD A HIGH RATE OF CLB AND VERIFIED THAT 'ALT CAPTURE' WAS STILL ILLUMINATED. AT FL188 IT BECAME APPARENT THAT THE ACFT WOULD NOT LEVEL AT FL190 EVEN THOUGH THE VV1 WAS BEGINNING TO DECREASE. I APPLIED FORWARD PRESSURE ON THE YOKE AND WAS SURPRISED THAT THE ACFT WAS RESPONSIVE; INDICATING THAT CTL WHEEL STEERING IN PITCH WAS STILL AVAILABLE (NOT NORMAL IN ALT CAPTURE). WE BOTH ACKNOWLEDGED THAT THE ACFT WAS GOING TO LEVEL OFF ABOVE FL190 AND WERE ABOUT TO NOTIFY ATC WHEN THEY CALLED REMINDING US THAT FL190 WAS OUR ASSIGNED ALT. I WAS ABLE TO LEVEL THE ACFT AT ABOUT FL196 AND BEGAN A DSCNT BACK DOWN TO FL190 BUT ATC REASSIGNED US FL200. WE CALLED ATC UPON LNDG AT SNA AND SPOKE TO THEM ABOUT THE INCIDENT. UNSURE. I'LL NEED TO RESEARCH THE FRM TO SEE IF THERE ARE ENVELOPES WHERE THE AUTOPLT IS UNABLE TO CAPTURE A GIVEN ALT. WE MAY HAVE INADVERTENTLY PLACED THE ACFT IN SUCH AN ENVELOPE ATTEMPTING TO SLOW THE ACFT WHILE CLBING.