A320 flight crew thought autopilot was engaged; it wasn't, they overshot assigned altitude.

Attributes
ACN	497600
Time
Date	200101
Day	Tue
Place
Locale Reference	navaid : hbu.vortac
State Reference	CO
Altitude	msl bound lower : 21000 msl bound upper : 21600
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	artcc : zdv.artcc
Operator	common carrier : air carrier
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	climbout : vacating altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time type : 1545
ASRS Report	497600
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 10000 flight time type : 2293
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to assigned altitude
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Flight cleared to FL210, crew anticipating furthur climb and thought automatic pilot was engaged. We both noticed the aircraft wasn't leveleng off and so the PF (first officer) called the deviation and returned the aircraft to FL210 and reengaged the automatic pilot. Altitude deviation was 500 to 600 ft high. Neither of us could be absolutly sure the AP2 FMA was illuminated even though we both remember pushing the AP2 button and announcing AP2 on.

Original NASA ASRS Text

Title: A320 FLT CREW THOUGHT AUTOPILOT WAS ENGAGED; IT WASN'T, THEY OVERSHOT ASSIGNED ALTITUDE.

Narrative: FLT CLEARED TO FL210, CREW ANTICIPATING FURTHUR CLIMB AND THOUGHT AUTO PILOT WAS ENGAGED. WE BOTH NOTICED THE AIRCRAFT WASN'T LEVELENG OFF AND SO THE PF (FO) CALLED THE DEVIATION AND RETURNED THE AIRCRAFT TO FL210 AND REENGAGED THE AUTO PILOT. ALTITUDE DEVIATION WAS 500 TO 600 FT HIGH. NEITHER OF US COULD BE ABSOLUTLY SURE THE AP2 FMA WAS ILLUMINATED EVEN THOUGH WE BOTH REMEMBER PUSHING THE AP2 BUTTON AND ANNOUNCING AP2 ON.

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.