Narrative:

Flying level at 9000 ft MSL, during descent phase of approach to chico, ca, we were being radar vectored to the ILS runway 13L at controller in charge. Copilot was flying aircraft. I asked controller for a lower altitude and he replied 'turn 20 degrees right, lower in 5 mi.' I read back clearance and copilot initiated turn. At this point I began looking at left leading edge to estimate ice buildup, as we were picking up light to moderate rime ice. Copilot then asked 'what altitude were we down to?' I looked forward and observed us descending through 8500 ft MSL. I replied 'no descent, we are maintaining 9000 ft' at which time he realized his mistake and returned to 9000 ft MSL immediately. Controller asked if we were having trouble maintaining altitude, and I replied that we had hit a downdraft. We were following proper sterile cockpit procedures and company procedures of calling 1000 ft and 300 ft prior to all altitudes. Had the jetstream been equipped with an altitude alert, I believe I would have heard the deviation chime at 200 ft off altitude and could have corrected copilot's mistake. I fail to see why the FAA requires GPWS and soon TCASII in 19 seat aircraft, yet not a simple altitude alerter. It should be a required item. My company certainly won't spend the money unless it is required. We can't even get approach chart yoke clips installed because it costs too much.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 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.