Autoflt system altitude set changed from FL330 to FL340 by itself. First officer noticed deviation at FL334. Excursion corrected by FL337 using manual control.

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Attributes
ACN	399607
Time
Date	199804
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : drk
State Reference	AZ
Altitude	msl bound lower : 33000 msl bound upper : 33700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla
Operator	common carrier : air carrier
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
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 : atp
Experience	flight time last 90 days : 220 flight time total : 11000 flight time type : 5200
ASRS Report	399607
Person 2
Affiliation	company : air carrier
Function	flight crew : captain
Qualification	pilot : atp
Experience	flight time last 90 days : 240 flight time total : 17000 flight time type : 12000
ASRS Report	399910
Events
Anomaly	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 : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Captain just finished an engine health check (autoplt in basic altitude hold mode) and had reengaged the automated pms altitude hold mode at FL330. Soon thereafter, I noticed aircraft was climbing through FL334 and alerted the captain. Using basic autoplt, he stopped the climb by FL337 and descended promptly after totally disconnecting the autoplt. I also noticed that the digital altitude window readout read FL340. As the pms was in the cruise mode, an inadvertent changing of the digital altitude level would not normally cause any change in altitude, a climb or descent mode must first be engaged. Likewise, an inadvertent activation of the vertical speed wheel (basic autoplt) would not cause the digital altitude window to read FL340. Therefore two separate inadvertent acts would have been necessary to achieve this effect without a malfunction of the pms. Therefore it is my belief that a momentary pms malfunction caused the digital discrepancy and resulted in the short duration altitude deviation described.

Original NASA ASRS Text

Title: AUTOFLT SYSTEM ALT SET CHANGED FROM FL330 TO FL340 BY ITSELF. FO NOTICED DEVIATION AT FL334. EXCURSION CORRECTED BY FL337 USING MANUAL CTL.

Narrative: CAPT JUST FINISHED AN ENGINE HEALTH CHECK (AUTOPLT IN BASIC ALT HOLD MODE) AND HAD REENGAGED THE AUTOMATED PMS ALT HOLD MODE AT FL330. SOON THEREAFTER, I NOTICED ACFT WAS CLBING THROUGH FL334 AND ALERTED THE CAPT. USING BASIC AUTOPLT, HE STOPPED THE CLB BY FL337 AND DSNDED PROMPTLY AFTER TOTALLY DISCONNECTING THE AUTOPLT. I ALSO NOTICED THAT THE DIGITAL ALT WINDOW READOUT READ FL340. AS THE PMS WAS IN THE CRUISE MODE, AN INADVERTENT CHANGING OF THE DIGITAL ALT LEVEL WOULD NOT NORMALLY CAUSE ANY CHANGE IN ALT, A CLB OR DSCNT MODE MUST FIRST BE ENGAGED. LIKEWISE, AN INADVERTENT ACTIVATION OF THE VERTICAL SPEED WHEEL (BASIC AUTOPLT) WOULD NOT CAUSE THE DIGITAL ALT WINDOW TO READ FL340. THEREFORE TWO SEPARATE INADVERTENT ACTS WOULD HAVE BEEN NECESSARY TO ACHIEVE THIS EFFECT WITHOUT A MALFUNCTION OF THE PMS. THEREFORE IT IS MY BELIEF THAT A MOMENTARY PMS MALFUNCTION CAUSED THE DIGITAL DISCREPANCY AND RESULTED IN THE SHORT DURATION ALT DEV DESCRIBED.

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.