HSI and altitude selected changed without any flight crew input.

Attributes
ACN	97987
Time
Date	198810
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : gve
State Reference	VA
Altitude	msl bound lower : 16000 msl bound upper : 41000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 210 flight time total : 16000
ASRS Report	97987
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During our descent from FL410 to 11000' to sabbi intersection. Near dca while looking at my HSI I saw the heading bug slew from about 017 degrees to 000 degrees west/O input from the crew. While discussing this with the first officer we also noticed the altitude. Select had changed from the 11M assigned altitude to 10M. This normally only happens during a complete power loss. I have never seen this in flight before. My point is that while on autoplt and FMC 'left' navigation. We might not have noticed the slight change in the heading bug or the altitude select setting and we would have leveled off at 10000' instead of 11000'. I believe we were at 16000 or 17000' when we became aware of the altitude select change so there was no problem this time. I have been very impressed with the large transport and the FMC and what it can do for you, also this time I can see what it can do to you. If this incident had resulted in a violation I myself would find this report hard to believe. This report was also made to the company.

Original NASA ASRS Text

Title: HSI AND ALT SELECTED CHANGED WITHOUT ANY FLT CREW INPUT.

Narrative: DURING OUR DSCNT FROM FL410 TO 11000' TO SABBI INTXN. NEAR DCA WHILE LOOKING AT MY HSI I SAW THE HDG BUG SLEW FROM ABOUT 017 DEGS TO 000 DEGS W/O INPUT FROM THE CREW. WHILE DISCUSSING THIS WITH THE F/O WE ALSO NOTICED THE ALT. SELECT HAD CHANGED FROM THE 11M ASSIGNED ALT TO 10M. THIS NORMALLY ONLY HAPPENS DURING A COMPLETE PWR LOSS. I HAVE NEVER SEEN THIS IN FLT BEFORE. MY POINT IS THAT WHILE ON AUTOPLT AND FMC 'L' NAV. WE MIGHT NOT HAVE NOTICED THE SLIGHT CHANGE IN THE HDG BUG OR THE ALT SELECT SETTING AND WE WOULD HAVE LEVELED OFF AT 10000' INSTEAD OF 11000'. I BELIEVE WE WERE AT 16000 OR 17000' WHEN WE BECAME AWARE OF THE ALT SELECT CHANGE SO THERE WAS NO PROB THIS TIME. I HAVE BEEN VERY IMPRESSED WITH THE LGT AND THE FMC AND WHAT IT CAN DO FOR YOU, ALSO THIS TIME I CAN SEE WHAT IT CAN DO TO YOU. IF THIS INCIDENT HAD RESULTED IN A VIOLATION I MYSELF WOULD FIND THIS RPT HARD TO BELIEVE. THIS RPT WAS ALSO MADE TO THE COMPANY.

Data retrieved from NASA's ASRS site as of August 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.