ALT EXCURSION.

Narrative

FLYING LEVEL AT 9000 FT MSL; DURING DSCNT PHASE OF APCH TO CHICO; CA; WE WERE BEING RADAR VECTORED TO THE ILS RWY 13L AT CIC. COPLT WAS FLYING ACFT. I ASKED CTLR FOR A LOWER ALT AND HE REPLIED 'TURN 20 DEGS R; LOWER IN 5 MI.' I READ BACK CLRNC AND COPLT INITIATED TURN. AT THIS POINT I BEGAN LOOKING AT L LEADING EDGE TO ESTIMATE ICE BUILDUP; AS WE WERE PICKING UP LIGHT TO MODERATE RIME ICE. COPLT THEN ASKED 'WHAT ALT WERE WE DOWN TO?' I LOOKED FORWARD AND OBSERVED US DSNDING THROUGH 8500 FT MSL. I REPLIED 'NO DSCNT; WE ARE MAINTAINING 9000 FT' AT WHICH TIME HE REALIZED HIS MISTAKE AND RETURNED TO 9000 FT MSL IMMEDIATELY. CTLR ASKED IF WE WERE HAVING TROUBLE MAINTAINING ALT; AND I REPLIED THAT WE HAD HIT A DOWNDRAFT. WE WERE FOLLOWING PROPER STERILE COCKPIT PROCS AND COMPANY PROCS OF CALLING 1000 FT AND 300 FT PRIOR TO ALL ALTS. HAD THE JETSTREAM BEEN EQUIPPED WITH AN ALT ALERT; I BELIEVE I WOULD HAVE HEARD THE DEV CHIME AT 200 FT OFF ALT AND COULD HAVE CORRECTED COPLT'S MISTAKE. I FAIL TO SEE WHY THE FAA REQUIRES GPWS AND SOON TCASII IN 19 SEAT ACFT; YET NOT A SIMPLE ALT ALERTER. IT SHOULD BE A REQUIRED ITEM. MY COMPANY CERTAINLY WON'T SPEND THE MONEY UNLESS IT IS REQUIRED. WE CAN'T EVEN GET APCH CHART YOKE CLIPS INSTALLED BECAUSE IT COSTS TOO MUCH.