Altitude excursion undetected by SF340 crew for 1700 ft and apparently never noticed by ARTCC.

Attributes
ACN	490690
Time
Date	200011
Day	Fri
Local Time Of Day	1201 To 1800
Place
State Reference	WI
Altitude	msl bound lower : 17700 msl bound upper : 19700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau.artcc
Operator	common carrier : air carrier
Make Model Name	SF 340A
Operating Under FAR Part	Part 121
Flight Phase	descent : vacating altitude
Route In Use	arrival : on vectors enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 86 flight time total : 3500 flight time type : 2300
ASRS Report	490690
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : cfi pilot : commercial
Experience	flight time total : 3110 flight time type : 1810
ASRS Report	490966
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : published procedure non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to original clearance
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While cruising at FL180 ZAU gave us a descent at pilot's discretion to 12000 ft. I was the PF and commenced the descent. I then realized we were still 105 DME from the wausau VOR and decided to delay the descent. By this time the aircraft had descended to 17700 ft. I pressed the 'altitude' button on the mode select panel (msp) of the autoplt. This mode of the autoplt captures the altitude of the aircraft at the time the button is pushed. I decided to put the autoplt into a 200 FPM climb to get us back to FL180. I did not realize that 12000 ft had already been put into the altitude pre-selector. I had expected the aircraft to start a 200 FPM climb and capture 18000 ft but because 12000 ft was in the pre-select, the autoplt never captured FL180 and continued to climb to FL197 before the first officer noticed the deviation and we started down to 12000 ft. The controller never had us verify our altitude nor did he mention the deviation. During the climb we were distracted from flying due to paperwork that we both were completing. Improper monitoring of the autoplt and the paperwork distraction led to this deviation. Having 1 pilot monitor the aircraft will prevent a recurrence of this incident.

Original NASA ASRS Text

Title: ALT EXCURSION UNDETECTED BY SF340 CREW FOR 1700 FT AND APPARENTLY NEVER NOTICED BY ARTCC.

Narrative: WHILE CRUISING AT FL180 ZAU GAVE US A DSCNT AT PLT'S DISCRETION TO 12000 FT. I WAS THE PF AND COMMENCED THE DSCNT. I THEN REALIZED WE WERE STILL 105 DME FROM THE WAUSAU VOR AND DECIDED TO DELAY THE DSCNT. BY THIS TIME THE ACFT HAD DSNDED TO 17700 FT. I PRESSED THE 'ALT' BUTTON ON THE MODE SELECT PANEL (MSP) OF THE AUTOPLT. THIS MODE OF THE AUTOPLT CAPTURES THE ALT OF THE ACFT AT THE TIME THE BUTTON IS PUSHED. I DECIDED TO PUT THE AUTOPLT INTO A 200 FPM CLB TO GET US BACK TO FL180. I DID NOT REALIZE THAT 12000 FT HAD ALREADY BEEN PUT INTO THE ALT PRE-SELECTOR. I HAD EXPECTED THE ACFT TO START A 200 FPM CLB AND CAPTURE 18000 FT BUT BECAUSE 12000 FT WAS IN THE PRE-SELECT, THE AUTOPLT NEVER CAPTURED FL180 AND CONTINUED TO CLB TO FL197 BEFORE THE FO NOTICED THE DEV AND WE STARTED DOWN TO 12000 FT. THE CTLR NEVER HAD US VERIFY OUR ALT NOR DID HE MENTION THE DEV. DURING THE CLB WE WERE DISTRACTED FROM FLYING DUE TO PAPERWORK THAT WE BOTH WERE COMPLETING. IMPROPER MONITORING OF THE AUTOPLT AND THE PAPERWORK DISTR LED TO THIS DEV. HAVING 1 PLT MONITOR THE ACFT WILL PREVENT A RECURRENCE OF THIS INCIDENT.

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.