CPR SMT HAS ALTDEV THEN HDG DEV ON DSCNT DUE PAX CONVERSATION.

Narrative

INBOUND TO FLL TO CLR CUSTOMS. ZMA CLRED ME TO DSND TO 7000 AND INTERCEPT 042 RADIAL OF FLL. NEXT LEG OF FLT HOME WAS FLL TO LA. THE WX IN FLL WAS GOOD; HOWEVER 50 NM W THROUGH N THERE WERE NUMEROUS TSTMS. PAX NOTICED THE WX AND ASKED ABOUT THE NEXT LEG OF FLT. I USED THE STORMSCOPE AND RADAR TO TRY TO SHOW HIM MY IDEA OF BEST RTE FOR DEP FROM FLL. DURING MY EXPLANATION; THE ALT ALERTER WENT OFF AND; THINKING IT WAS THE 1000 FT PRIOR TONE; I ENGAGED THE ALT CAPTURE MODE. A FEW SECONDS LATER; I SCANNED THE ALTIMETER AND SAW ALT PASSING 6400 FT. IMMEDIATELY INITIATED CLB BACK TO 7000 FT. THE FACT THAT THE ALTDEV HAD HAPPENED OCCUPIED MY MIND FOR A COUPLE OF MINS. ATC CALLED AND ASKED MY HDG BECAUSE I HAD OVERSHOT THE INBOUND RADIAL. THEY ISSUED ME A NEW VECTOR BUT DID NOT MENTION ANYTHING ABOUT THE ALT. LESSONS LEARNED: IN THE TKOF; CLB; DSCNT; AND APCH PHASES OF FLT; A STERILE COCKPIT IS THE BEST POLICY. I SHOULD HAVE TOLD THE PAX I WOULD ANSWER HIS QUESTIONS LATER; ESPECIALLY SINCE I HAD BEEN WATCHING THE WX FOR SOME TIME AND HAD DECIDED WHAT MY PLAN OF ACTION WOULD BE ON THE NEXT LEG. ONCE THE ALTDEV WAS CORRECTED; SHOULD HAVE PUT IT INTO THE BACK OF MY MIND TO BE STUDIED LATER. IF I HAD DONE THIS; I FEEL I WOULD NOT HAVE OVERSHOT THE RADIAL. IN ALL MY YRS OF ACCIDENT AND VIOLATION FREE FLYING I HAVE TRIED TO LEARN FROM MY EXPERIENCES. THIS ONE HAS BEEN ESPECIALLY INSTRUCTIVE.