An MD80 autoplt made an uncommanded transition from altitude hld to vertical speed at FL360.

Attributes
ACN	688949
Time
Date	200603
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zme.artcc
State Reference	TN
Altitude	agl single value : 36000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zme.artcc
Operator	common carrier : air carrier
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	688949
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	689111
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude
Independent Detector	other flight crewa other flight crewb
Resolutory Action	aircraft : equipment problem dissipated flight crew : regained aircraft control flight crew : returned to assigned airspace
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

Level at FL360 in clear; smooth conditions; autoplt engaged in altitude hld. No wave; no turbulence. I just happened to be doing an instrument scan when I saw altitude hld suddenly disappear in the FMA; followed by vertical speed. Throttles retarded and aircraft began an uncommanded descent. 1400 FPM down displayed in pitch window. I dialed pitch anu to counter the descent; but there was no response from the autoplt and the window was frozen at 1400 FPM down; and aircraft was in an uncommanded descent. I called the captain's attention to the problem; disconnected autoplt; and hand flew back to FL360. Autothrottles were engaged in mach sel and they responded properly with a thrust increase to maintain mach as I corrected the descent and returned the aircraft to altitude. Neither of us had our hands near the controls; and there was no way to account for the uncommanded descent. There were no annunciations; warnings; or other abnormal aircraft/DFGS indications. It is the strangest anomaly I have ever seen in this airplane. I think I was able to arrest the descent at about FL357; and the altitude alert never sounded. ATC did not question the excursion and there was no traffic conflict. We monitored autoplt operation very closely and discussed the possibility of using electronic interference procedures; but there was no reoccurrence -- the autoplt functioned normally the remainder of the flight. I have seen autoplt disconnects in wave; and I have seen reversions to vertical speed when perf disengaged -- but nothing like this. The autoplt was driving the airplane down at 1400 FPM for no apparent reason. If I hadn't have been staring at the FMA when it happened we might have lost a thousand feet or more before we caught the problem -- it happened that quickly. We made a discrepancy report. I'd be interested to find out if this has happened before; and what discrepancies were uncovered with the aircraft; if any.

Original NASA ASRS Text

Title: AN MD80 AUTOPLT MADE AN UNCOMMANDED TRANSITION FROM ALT HLD TO VERT SPD AT FL360.

Narrative: LEVEL AT FL360 IN CLR; SMOOTH CONDITIONS; AUTOPLT ENGAGED IN ALT HLD. NO WAVE; NO TURB. I JUST HAPPENED TO BE DOING AN INSTRUMENT SCAN WHEN I SAW ALT HLD SUDDENLY DISAPPEAR IN THE FMA; FOLLOWED BY VERT SPD. THROTTLES RETARDED AND ACFT BEGAN AN UNCOMMANDED DSCNT. 1400 FPM DOWN DISPLAYED IN PITCH WINDOW. I DIALED PITCH ANU TO COUNTER THE DSCNT; BUT THERE WAS NO RESPONSE FROM THE AUTOPLT AND THE WINDOW WAS FROZEN AT 1400 FPM DOWN; AND ACFT WAS IN AN UNCOMMANDED DSCNT. I CALLED THE CAPT'S ATTENTION TO THE PROB; DISCONNECTED AUTOPLT; AND HAND FLEW BACK TO FL360. AUTOTHROTTLES WERE ENGAGED IN MACH SEL AND THEY RESPONDED PROPERLY WITH A THRUST INCREASE TO MAINTAIN MACH AS I CORRECTED THE DSCNT AND RETURNED THE ACFT TO ALT. NEITHER OF US HAD OUR HANDS NEAR THE CTLS; AND THERE WAS NO WAY TO ACCOUNT FOR THE UNCOMMANDED DSCNT. THERE WERE NO ANNUNCIATIONS; WARNINGS; OR OTHER ABNORMAL ACFT/DFGS INDICATIONS. IT IS THE STRANGEST ANOMALY I HAVE EVER SEEN IN THIS AIRPLANE. I THINK I WAS ABLE TO ARREST THE DSCNT AT ABOUT FL357; AND THE ALT ALERT NEVER SOUNDED. ATC DID NOT QUESTION THE EXCURSION AND THERE WAS NO TFC CONFLICT. WE MONITORED AUTOPLT OPERATION VERY CLOSELY AND DISCUSSED THE POSSIBILITY OF USING ELECTRONIC INTERFERENCE PROCS; BUT THERE WAS NO REOCCURRENCE -- THE AUTOPLT FUNCTIONED NORMALLY THE REMAINDER OF THE FLT. I HAVE SEEN AUTOPLT DISCONNECTS IN WAVE; AND I HAVE SEEN REVERSIONS TO VERT SPD WHEN PERF DISENGAGED -- BUT NOTHING LIKE THIS. THE AUTOPLT WAS DRIVING THE AIRPLANE DOWN AT 1400 FPM FOR NO APPARENT REASON. IF I HADN'T HAVE BEEN STARING AT THE FMA WHEN IT HAPPENED WE MIGHT HAVE LOST A THOUSAND FEET OR MORE BEFORE WE CAUGHT THE PROB -- IT HAPPENED THAT QUICKLY. WE MADE A DISCREPANCY RPT. I'D BE INTERESTED TO FIND OUT IF THIS HAS HAPPENED BEFORE; AND WHAT DISCREPANCIES WERE UNCOVERED WITH THE ACFT; IF ANY.

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.