B737 crew had an altitude excursion.

Attributes
ACN	469690
Time
Date	200004
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : cmh.airport
State Reference	OH
Altitude	msl bound lower : 27000 msl bound upper : 27300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob.artcc
Operator	common carrier : air carrier
Make Model Name	B737-200
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zob.artcc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 120 flight time total : 19000 flight time type : 4000
ASRS Report	459690
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude conflict : airborne critical non adherence : clearance non adherence : required legal separation
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude none taken : detected after the fact
Consequence	other
Miss Distance	horizontal : 18000 vertical : 1000
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Cruising FL270, 5p177 autoplt on 'B' channel in altitude hold mode. Aircraft suddenly pitched up 5 degrees to 8 degrees. Autoplt remained engaged. Both pilots pushed over on control wheel. Altitude gain was 300 ft. TCAS commanded descent which we complied with. Leveled off again at FL270. This problem of rare autoplt rapid deviations highlights the need to remain focused, attentive and near if not physically on the flight controls. We reacted within 2-3 seconds.

Original NASA ASRS Text

Title: B737 CREW HAD AN ALT EXCURSION.

Narrative: CRUISING FL270, 5P177 AUTOPLT ON 'B' CHANNEL IN ALT HOLD MODE. ACFT SUDDENLY PITCHED UP 5 DEGS TO 8 DEGS. AUTOPLT REMAINED ENGAGED. BOTH PLTS PUSHED OVER ON CTL WHEEL. ALT GAIN WAS 300 FT. TCAS COMMANDED DSCNT WHICH WE COMPLIED WITH. LEVELED OFF AGAIN AT FL270. THIS PROB OF RARE AUTOPLT RAPID DEVIATIONS HIGHLIGHTS THE NEED TO REMAIN FOCUSED, ATTENTIVE AND NEAR IF NOT PHYSICALLY ON THE FLT CTLS. WE REACTED WITHIN 2-3 SECONDS.

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.