Altitude knob set to 11000' altitude but aircraft leveled at 10000'.

Attributes
ACN	159454
Time
Date	199010
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zzz
State Reference	US
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zzz
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : cfi pilot : instrument pilot : atp pilot : commercial
Experience	flight time last 90 days : 225 flight time total : 14000 flight time type : 5000
ASRS Report	159454
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : atp pilot : instrument
Events
Anomaly	altitude deviation : overshoot
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : regained aircraft control other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During descent we were cleared to 11000' and the first officer dialed it into the autoplt. We both saw that it said 11000'. This aircraft has an autoplt which captures and levels automatically. However, we descended through 11000' be a few hundred ft before I caught it. We both looked and the altitude had changed on its own to 10000'. I correct immediately and ATC (by phone) said, 'no problem.' no action necessary. I have seen this before on this autoplt where altitude is not in altitude detent (it's between altitudes) and sometimes switches altitudes. Altitudes need to physically be checked to ascertain that altitude in window is correct and knob is not in between altitudes. If the altitude reset knob is not in its correct groove it can FLIP over to another altitude. I've seen it before and we feel certain this happened to us.

Original NASA ASRS Text

Title: ALT KNOB SET TO 11000' ALT BUT ACFT LEVELED AT 10000'.

Narrative: DURING DSNT WE WERE CLRED TO 11000' AND THE F/O DIALED IT INTO THE AUTOPLT. WE BOTH SAW THAT IT SAID 11000'. THIS ACFT HAS AN AUTOPLT WHICH CAPTURES AND LEVELS AUTOMATICALLY. HOWEVER, WE DSNDED THROUGH 11000' BE A FEW HUNDRED FT BEFORE I CAUGHT IT. WE BOTH LOOKED AND THE ALT HAD CHANGED ON ITS OWN TO 10000'. I CORRECT IMMEDIATELY AND ATC (BY PHONE) SAID, 'NO PROB.' NO ACTION NECESSARY. I HAVE SEEN THIS BEFORE ON THIS AUTOPLT WHERE ALT IS NOT IN ALT DETENT (IT'S BTWN ALTS) AND SOMETIMES SWITCHES ALTS. ALTS NEED TO PHYSICALLY BE CHKED TO ASCERTAIN THAT ALT IN WINDOW IS CORRECT AND KNOB IS NOT IN BTWN ALTS. IF THE ALT RESET KNOB IS NOT IN ITS CORRECT GROOVE IT CAN FLIP OVER TO ANOTHER ALT. I'VE SEEN IT BEFORE AND WE FEEL CERTAIN THIS HAPPENED TO US.

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.