FO OF A GULFSTREAM II G2 MOMENTARILY LOST CTL OF THE ACFT DURING DSCNT DUE TO AN AUTOPLT INCORRECT MODE SETTING CAUSING THE ACFT TO OVERSHOOT XING ALT. ARTCC CTLR SAID NOTHING ABOUT OVERSHOOT AND SUBSEQUENTLY CLRED RPTR TO A LOWER ALT.

Narrative

WHILE ON AN IFR FLT PLAN FROM PHX TO SLC (VMC; SMOOTH AIR); ZLC CLRED US FROM FL390 TO FL330. AT THIS TIME; THE AUTOPLT WAS ENGAGED AND FLYING THE ACFT VIA OUR FMS'S FOR AREA NAV. THE PNF SELECTED FL330 INTO OUR ALT ENCODER (USED FOR AUTOPLT CAPTURE AND ALERTING) AND REARMED THE SYS TO AUTO (IT CONSISTS OF AUTO/OFF/MANUAL). I CONFIRMED THE NEW ALT WITH HIM AND PROCEEDED TO DSND BY MANUALLY SELECTING THE VERT DSCNT WHEEL ON THE AUTOPLT SYS TO A 1500 FPM DSCNT; WHILE SIMULTANEOUSLY REDUCING PWR. DURING OUR DSCNT; ZLC RECLRED US OFF OUR ORIGINAL FLT PLAN AND STAR AND TO CROSS 45 MI S OF SLC VOR AT 17000 FT. I USED THE VERT NAV GUIDANCE ON THE FMS TO SELECT THOSE PARAMETERS TO DETERMINE MY DSCNT RATE. WE NEEDED ABOUT 2000 FPM TO MAKE THAT RESTR. AGAIN; THE PNF SELECTED THE ALT ENCODER TO 17000 FT; AND ALSO ADJUSTED ACFT ALTIMETER TO LCL SETTING AND ARMED SYS; WHICH I CONFIRMED. I INCREASED THE RATE OF DSCNT TO 2000 FPM IN WHICH THE AUTOPLT RESPONDED. WHILE PASSING FL220; THE ACFT WENT INTO AN UNCOMMANDED DSCNT OF 6000+ FPM. I IMMEDIATELY RESPONDED BY DISCONNECTING THE AUTOPLT (WHICH ALSO CLICKED OFF ALT ENCODER -- WENT FROM AUTO TO OFF) AND REDUCING PWR. DURING THIS PERIOD THE PNF WAS OFF COMMUNICATING WITH THE FBO AT OUR DEST (SLC) AND WAS NOT AWARE OF WHAT HAD HAPPENED. THERE WAS ALSO A PAX IN THE JUMP SEAT -- NON PLT. AS I REGAINED CTL OF THE ACFT I INADVERTENTLY DSNDED THROUGH MY ALT OF 17000 FT TO 14000 FT. THE CTLR NEVER SAID A WORD. I WAS SO DISTR BY THE UNCOMMANDED DSCNT OF THE APCH AND REGAINING CTL; I NEVER RESET THE ALT ENCODER TO MANUAL; SO I WOULD RECEIVE A WARNING (VISUAL ONLY; NO AURAL ALERT). I WAS DISTR BY THE FACT OF THE PAX IN THE JUMP SEAT; TO WHOM I EXPLAINED WHAT WAS GOING ON WITH THE ACFT. THE PNF DID NOT REALIZE IT EITHER DUE TO COMMUNICATING WITH OTHER FREQ. WHEN WE DID NOTICE OUR ALT; ZLC HAD HANDED US OFF TO SLC APCH WHO CLRED US TO 13000 FT. THEY NEVER SAID ANYTHING ABOUT OUR ASSIGNED ALT. THE FLT PROCEEDED AND WE COMPLETED THE REST OF OUR MISSION SAFELY AND NORMALLY. I ATTRIBUTE THIS INCIDENT TO THE FACT OF BEING DISTR BY THE UNCOMMANDED DSCNT OF THE AUTOPLT; IN WHICH THE ALT ENCODER WAS INADVERTENTLY TURNED OFF. I WAS ALSO DISTR BY THE PAX IN THE JUMP SEAT. AS ONE CORRECTION TO THIS INCIDENT; I FEEL THAT WHEN I DISCONNECTED THE AUTOPLT; THE ALT ENCODER SHOULD GO TO THE MANUAL MODE; NOT THE OFF MODE; THEREBY I WOULD HAVE HAD SOME FORM OF A WARNING SYS. SELECTING IT TO OFF SHOULD BE AN INPUT WE DO OURSELVES. AFTER THE FLT; I DISCUSSED THE INCIDENT WITH THE OTHER PLT SO SITS LIKE THIS IN THE FUTURE DON'T ARISE.