Narrative:

After departure was given a heading of 030 degree to intercept the crp 045 degree right to the psx 218 degree right. A clearance was given to climb to FL210 as filed. The 030 degree heading was flown until it became apparent that, due to winds, the 045 degree crp would not be intercepted prior to the psx 218 degree. I began intercepting the psx 218 degree and transferred this information from the #2 HSI to the #1 HSI and the autoplt captured the navigation mode to track inbound to psx. At that time I began programming the vlfirst officermega for the gland intersection. This would be the next fix after psx and is part of the clearance to fly the gland five arrival route into the houston terminal area. I received a message from houston center advising me I had exceeded my assigned altitude. I verified that I was approaching 22500', disconnected the autoplt and made an immediate high rate descent to 21000' and established my cruise at 21000'. The autoplt/flight director had been engaged approximately 1500'agl when departing crp and had been continually engaged until the call from center. It had tracked the assigned heading until capturing the psx 218 degree right. It had been trimmed to the proper climb speed/attitude. The altitude preselect had been set for 21000' and the altitude preselect mode had been selected on the a/P controller. There was no indication of any malfunction in it on this flight or the previous leg of this flight which was from ft worth, tx to corpus christi. An audible signal normally precedes a selected altitude by 1000'. I do not recall hearing this alert signal at 20000', at which time the aircraft was climbing approximately 1500' per minute. It appears that when the autoplt shifted from heading mode to navigation mode it disconnected the altitude preselect mode. I do not know if this particular autoplt does this regularly since this is the first time I have flown this aircraft. I regularly fly the same make of aircraft and autoplt and I have not experienced this difficulty with the other aircraft I normally fly. Maintenance will be advised of the problem and maintenance flight checks made to verify problems in the autoplt/flight director if able to duplicate the same conditions. WX at the time/place of occurrence was good VMC at 21000' with tops of lower clouds around 10000'. There was no other traffic in the area and no conflicts were noted or called out by the center controller.

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Original NASA ASRS Text

Title: SMT CLIMBING ON AUTOPILOT OVERSHOT ASSIGNED ALT.

Narrative: AFTER DEP WAS GIVEN A HDG OF 030 DEG TO INTERCEPT THE CRP 045 DEG R TO THE PSX 218 DEG R. A CLRNC WAS GIVEN TO CLB TO FL210 AS FILED. THE 030 DEG HDG WAS FLOWN UNTIL IT BECAME APPARENT THAT, DUE TO WINDS, THE 045 DEG CRP WOULD NOT BE INTERCEPTED PRIOR TO THE PSX 218 DEG. I BEGAN INTERCEPTING THE PSX 218 DEG AND TRANSFERRED THIS INFO FROM THE #2 HSI TO THE #1 HSI AND THE AUTOPLT CAPTURED THE NAV MODE TO TRACK INBND TO PSX. AT THAT TIME I BEGAN PROGRAMMING THE VLF/OMEGA FOR THE GLAND INTXN. THIS WOULD BE THE NEXT FIX AFTER PSX AND IS PART OF THE CLRNC TO FLY THE GLAND FIVE ARR RTE INTO THE HOUSTON TERMINAL AREA. I RECEIVED A MESSAGE FROM HOUSTON CENTER ADVISING ME I HAD EXCEEDED MY ASSIGNED ALT. I VERIFIED THAT I WAS APCHING 22500', DISCONNECTED THE AUTOPLT AND MADE AN IMMEDIATE HIGH RATE DSCNT TO 21000' AND ESTABLISHED MY CRUISE AT 21000'. THE AUTOPLT/FLT DIRECTOR HAD BEEN ENGAGED APPROX 1500'AGL WHEN DEPARTING CRP AND HAD BEEN CONTINUALLY ENGAGED UNTIL THE CALL FROM CENTER. IT HAD TRACKED THE ASSIGNED HDG UNTIL CAPTURING THE PSX 218 DEG R. IT HAD BEEN TRIMMED TO THE PROPER CLB SPD/ATTITUDE. THE ALT PRESELECT HAD BEEN SET FOR 21000' AND THE ALT PRESELECT MODE HAD BEEN SELECTED ON THE A/P CTLR. THERE WAS NO INDICATION OF ANY MALFUNCTION IN IT ON THIS FLT OR THE PREVIOUS LEG OF THIS FLT WHICH WAS FROM FT WORTH, TX TO CORPUS CHRISTI. AN AUDIBLE SIGNAL NORMALLY PRECEDES A SELECTED ALT BY 1000'. I DO NOT RECALL HEARING THIS ALERT SIGNAL AT 20000', AT WHICH TIME THE ACFT WAS CLBING APPROX 1500' PER MINUTE. IT APPEARS THAT WHEN THE AUTOPLT SHIFTED FROM HDG MODE TO NAV MODE IT DISCONNECTED THE ALT PRESELECT MODE. I DO NOT KNOW IF THIS PARTICULAR AUTOPLT DOES THIS REGULARLY SINCE THIS IS THE FIRST TIME I HAVE FLOWN THIS ACFT. I REGULARLY FLY THE SAME MAKE OF ACFT AND AUTOPLT AND I HAVE NOT EXPERIENCED THIS DIFFICULTY WITH THE OTHER ACFT I NORMALLY FLY. MAINT WILL BE ADVISED OF THE PROB AND MAINT FLT CHKS MADE TO VERIFY PROBS IN THE AUTOPLT/FLT DIRECTOR IF ABLE TO DUPLICATE THE SAME CONDITIONS. WX AT THE TIME/PLACE OF OCCURRENCE WAS GOOD VMC AT 21000' WITH TOPS OF LOWER CLOUDS AROUND 10000'. THERE WAS NO OTHER TFC IN THE AREA AND NO CONFLICTS WERE NOTED OR CALLED OUT BY THE CENTER CTLR.

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.