A B737-700 at FL280 with LNAV engaged and without VNAV engaged aircraft started an uncommanded climb...

Attributes
ACN	465850
Time
Date	200003
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : ric.airport
State Reference	VA
Altitude	msl single value : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc.artcc
Operator	common carrier : air carrier
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Route In Use	enroute airway : j55.airway
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : atp pilot : commercial
Experience	flight time last 90 days : 200 flight time total : 6000 flight time type : 700
ASRS Report	465850
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : altimeter other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude
Consequence	other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

Level at FL280 with clearance to cross ridgy at FL240 LNAV engaged and before engaging VNAV the aircraft while on autoplt started a climb by itself. We were approximately FL285 when the correction was completed. Checked all the entries in the CPU, found no errors. The only way the aircraft could have flown away from the FL280 would have been vertical speed. This was never touched. The altitude of FL240 showed in the select window. This was truly a strange event. The rest of the flight was uneventful. Callback conversation with reporter revealed the following information: the reporter said the aircraft was a boeing 737-700. The reporter stated the autoplt performance was reported in the logbook but no feedback has been received from maintenance on the fix.

Original NASA ASRS Text

Title: A B737-700 AT FL280 WITH LNAV ENGAGED AND WITHOUT VNAV ENGAGED ACFT STARTED AN UNCOMMANDED CLB.

Narrative: LEVEL AT FL280 WITH CLRNC TO CROSS RIDGY AT FL240 LNAV ENGAGED AND BEFORE ENGAGING VNAV THE ACFT WHILE ON AUTOPLT STARTED A CLB BY ITSELF. WE WERE APPROX FL285 WHEN THE CORRECTION WAS COMPLETED. CHKED ALL THE ENTRIES IN THE CPU, FOUND NO ERRORS. THE ONLY WAY THE ACFT COULD HAVE FLOWN AWAY FROM THE FL280 WOULD HAVE BEEN VERT SPD. THIS WAS NEVER TOUCHED. THE ALT OF FL240 SHOWED IN THE SELECT WINDOW. THIS WAS TRULY A STRANGE EVENT. THE REST OF THE FLT WAS UNEVENTFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THE ACFT WAS A BOEING 737-700. THE RPTR STATED THE AUTOPLT PERFORMANCE WAS RPTED IN THE LOGBOOK BUT NO FEEDBACK HAS BEEN RECEIVED FROM MAINT ON THE FIX.

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.