B737 crew experiences uncommanded climb while in cruise at FL280.

Attributes
ACN	723631
Time
Date	200701
Place
Locale Reference	airport : hou.airport
State Reference	TX
Altitude	msl single value : 28000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zhu.artcc artcc : lfff.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 270 flight time total : 6000 flight time type : 1500
ASRS Report	723631
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude 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	Ambiguous

Narrative:

We were in cruise flight at FL280 with a clearance to descend at pilot's discretion to FL240. LNAV/VNAV selected; FL240 in the altitude selection window. FL280 set in the cruise page of the FMS. While waiting for the VNAV to start the descent; the aircraft made an uncommanded climb. Upon noticing the deviation the captain took control and corrected and we made a normal descent and landing.

Original NASA ASRS Text

Title: B737 CREW EXPERIENCES UNCOMMANDED CLB WHILE IN CRUISE AT FL280.

Narrative: WE WERE IN CRUISE FLT AT FL280 WITH A CLRNC TO DSND AT PLT'S DISCRETION TO FL240. LNAV/VNAV SELECTED; FL240 IN THE ALT SELECTION WINDOW. FL280 SET IN THE CRUISE PAGE OF THE FMS. WHILE WAITING FOR THE VNAV TO START THE DSCNT; THE ACFT MADE AN UNCOMMANDED CLB. UPON NOTICING THE DEV THE CAPT TOOK CTL AND CORRECTED AND WE MADE A NORMAL DSCNT AND LNDG.

Data retrieved from NASA's ASRS site as of January 2009 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.