Widebody transport flight crew selects VNAV to climb and aircraft descends instead.

Attributes
ACN	332150
Time
Date	199603
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : zjx
State Reference	FL
Altitude	msl bound lower : 30400 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zjx
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 200 flight time total : 8000 flight time type : 70
ASRS Report	332150
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 non adherence : clearance non adherence : far
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

The captain was flying. He was cleared from FL310 to FL350. He selected VNAV on the autoflt panel and instead of climbing, the aircraft abruptly nosed down and descended approximately 600 ft before the captain deselected the autoplt and hand flew it back up to FL310 and then to FL350. Nothing was said and it happened very quickly. The turbulence could have created the problem with the autoflt computer. Careful monitoring of the autoplt is essential.

Original NASA ASRS Text

Title: WDB FLC SELECTS VNAV TO CLB AND ACFT DSNDS INSTEAD.

Narrative: THE CAPT WAS FLYING. HE WAS CLRED FROM FL310 TO FL350. HE SELECTED VNAV ON THE AUTOFLT PANEL AND INSTEAD OF CLBING, THE ACFT ABRUPTLY NOSED DOWN AND DSNDED APPROX 600 FT BEFORE THE CAPT DESELECTED THE AUTOPLT AND HAND FLEW IT BACK UP TO FL310 AND THEN TO FL350. NOTHING WAS SAID AND IT HAPPENED VERY QUICKLY. THE TURB COULD HAVE CREATED THE PROB WITH THE AUTOFLT COMPUTER. CAREFUL MONITORING OF THE AUTOPLT IS ESSENTIAL.

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.