PF; FO; WAS OVERLY CONCERNED ABOUT POTENTIAL TURB IN AREA OF WX. REQUESTED LOWER ALT AND ATC DIDN'T IMMEDIATELY RESPOND; PROBABLY DUE TO COORDINATING; SO PLT DECIDED TO DSND RAPIDLY ANYWAY WITHOUT CLRNC. RPTR; CAPT; WAS DUMBFOUNDED BY ACTIONS OF PF; BUT FAILED TO TAKE CTL OF ACFT.

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DSNDING TO CROSS TYGER INTXN AT 12000 FT. PF REQUESTED 10000 FT MSL DUE TO TURB WHILE FLYING UNDER A BUILDUP WITH BASES AT 16000 FT. CONDITIONS VMC. RECEIVED CLRNC TO 10000 FT BUT PF WAS CONCERNED AND SAID TO REQUEST 6000 FT MSL DUE TO TURB. (NO PAX ON BOARD; JUST CREW). I ASKED FOR 6000 FT MSL BUT RECEIVED NO ANSWER AND I CONTINUED THE IN-RANGE CHKLIST WITH 10000 FT SELECTED ON THE ALT ALERT. PF IS VERY APPREHENSIVE FLYING IN TURB (AT THE TIME IT WAS LIGHT WITH JUST OCCASIONAL MODERATE TURB) AND SAID TO GET 6000 FT FROM CTR. THE DSCNT RATE WAS AROUND 3000-4000 FPM AND I HEARD THE ALT ALERT AND LOOKED UP FROM THE CHKLIST AND SAW WE WERE AT 8500 FT AND DSNDING. I WAS AWESTRUCK AND STARTED TO POINT OUT THE DEV WHEN THE CTLR SAID OUR ASSIGNED ALT WAS 10000 FT. THE PF THEN TOOK HIS MICROPHONE AND SAID TO CTR; 'ACFT IDENT; WE'RE GETTING THE HECK BEAT OUT OF US AND WE'RE GOING TO 6000 FT NOW!' THERE WAS NO RESPONSE FROM THE CTLR FOR ABOUT 20 SECONDS WITH US DSNDING ALL THE WHILE. HE THEN CLRED US TO 6000 FT WHICH I DIALED ON THE ALT SELECTOR. OUR DSCNT RATE WAS STILL HIGH AND THE PF OVERSHOT 6000 FT AND FINALLY LEVELED OFF AT 5000 FT. WE THEN CLBED BACK TO 6000 FT AND WE WERE HANDED OFF TO ZKC. THOUGH I WAS IN THE R SEAT; I WAS STILL PIC OF THE FLT (PF IS TYPE RATED IN THE ACFT). PF'S FLYING ABILITIES HAVE BEEN WEAK AND HE SEEMS TO BE VERY APPREHENSIVE FLYING IN ANY WX (ALTHOUGH HE IS 15 YRS MY SENIOR WITH LOTS OF FLT TIME). HE HAS NEVER MADE SUCH A DEV SUCH AS THIS AND I WAS NOT EXPECTING IT. I ASSUME RESPONSIBILITY AND SHOULD HAVE TAKEN OVER FLYING THE AIRPLANE AND CORRECTED THE ALT BUT I WAS DUMBFOUNDED THAT HE SHOULD DO SUCH A THING IN THE FIRST PLACE. OUR COMPANY IS TRYING TO UPGRADE HIM TO CAPT AND I WAS SEEING IF HE COULD MAKE THE PROPER DECISIONS; AND RPT THEM TO THE CHIEF PLT. I DO REALIZE THAT I SHOULD NOT HAVE BEEN DOING THE CHKLIST WHILE DSNDING TO AN ASSIGNED ALT BUT OUR '1000 FT ABOVE' CALLOUT HAD BEEN MADE AND I WENT BACK TO THE CHKLIST. I WAS PROBABLY ALSO COMPLACENT AS HE HAD FLOWN ANOTHER LEG EARLIER IN THE DAY AND DID IT VERY WELL. THE TURB CERTAINLY WAS NOT THAT BAD AND WE WEREN'T IN AN EMER SIT AND HE HAS BEEN FLYING THIS ACFT FOR 1 1/2 YRS NOW. I WILL BE MUCH MORE VIGILANT FLYING WITH HIM NOW (ESPECIALLY IN WX OR TURB) AND WHEN DSNDING TO AN ASSIGNED ALT WILL NOT DIVERT MY ATTN TO OTHER THINGS (IE; CHKLIST) UNTIL WE HAVE LEVELED OFF. ABOVE ALL; I SHALL EXERCISE MY RESPONSIBILITIES AS A PIC AND ASSUME COMMAND OF THE ACFT IMMEDIATELY UPON A DEV RATHER THAN SEEING HOW HE DECIDES TO CORRECT THE SIT. PLEASE NOTE: WHEN FLYING AS A CREW YOU DO HAVE TO HAVE SOME TRUST IN YOUR PARTNER SO I CAN PERFORM MY RESPONSIBILITIES IN THE COCKPIT. FLYING AND MAINTAINING AN ASSIGNED ALT IS BASIC FLYING AND WITH A PERSON OF HIS EXPERIENCE AND RATINGS I DID NOT FEEL AS IF I HAD TO MONITOR HIS FLYING ABILITIES AS WITH A NEW PLT. WE HAVE FLOWN MANY TIMES BEFORE WITH NO MAJOR PROBS. I NOW HAVE LOST MY CONFIDENCE IN HIM -- NOW WHAT?