Narrative:

We had flown this aircraft on two previous flights with the left vertical gyro inoperative. The selector was in 'left on aux' for those flights as well as this international flight. The first officer flew the aircraft with autopilot #2. Once level at FL360 we observed repeated 'flinching' of the autopilot rapidly up and then recovering to level flight in less than one second. I noted that my ADI would rapidly pitch up 2 or more degrees and then recover to level flight also in less than one second. This ADI fluctuation coincided with the autopilot 'flinch.' once I pointed this out; the first officer noted the same with her ADI. At this point; I weighed our options. I didn't want to continue to our international destination at night in mountainous terrain with the possibility of losing one or both of the remaining vertical gyros. The autopilot and ADI's continued this 'flinching' motion in 5 to 60 second intervals. We selected #1 dfgc and re-engaged the autopilot; but this didn't help. The autothrottles were already placarded as well as the APU. The prospect of entering the high density traffic area of our destination with mountainous terrain; a difficult arrival path to landing; with no autothrottles; no autopilot and the possibility of no artificial horizon displays (past midnight after a full day of flying) was more than I was willing to accept. We would also have to turn off the autopilot to continue. That would require us to descend out of rvsm airspace. With the resulting higher fuel burn; we would not be able to retain our landing alternate. The first officer and I discussed landing at either of two nearby airports; but our departure airport is the nearest major maintenance base with the appropriate parts and trained personnel to return this aircraft into service. We also discussed that there may not be a ground crew at the two nearby airports at this late hour qualified to perform the required security inspection of a replacement aircraft for our follow-on flight to our international destination. A new general declaration (gendex) reflecting a departure from a different city would also be required; but might be difficult to attain. It would only take a few more minutes to return to our departure airport rather than spiraling down from FL360. If the vertical gyros failed before landing at at our departure airport we could still see the horizon out the windshield because the moon was almost full and the night was now clear. During the descent; the captain's FMA display went blank with the exception of the 'no autoland' light which illuminated. The flight director command bars also disappeared. I selected FD 'both on 2' and this restored the displays and command bars to landing. The vertical gyros continued to operate to landing. The landing was uneventful.

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Original NASA ASRS Text

Title: A MD80's #1 GYRO was MELed; after reaching FL360 the autopilot was pitching up and down slightly with #2 selected; so unwilling to chance a night landing in mountainous terrain the crew returned to the departure airport for maintenance.

Narrative: We had flown this aircraft on two previous flights with the left vertical gyro inoperative. The selector was in 'Left on Aux' for those flights as well as this international flight. The First Officer flew the aircraft with autopilot #2. Once level at FL360 we observed repeated 'flinching' of the autopilot rapidly up and then recovering to level flight in less than one second. I noted that my ADI would rapidly pitch up 2 or more degrees and then recover to level flight also in less than one second. This ADI fluctuation coincided with the autopilot 'flinch.' Once I pointed this out; the First Officer noted the same with her ADI. At this point; I weighed our options. I didn't want to continue to our international destination at night in mountainous terrain with the possibility of losing one or both of the remaining vertical gyros. The autopilot and ADI's continued this 'flinching' motion in 5 to 60 second intervals. We selected #1 DFGC and re-engaged the autopilot; but this didn't help. The autothrottles were already placarded as well as the APU. The prospect of entering the high density traffic area of our destination with mountainous terrain; a difficult arrival path to landing; with no autothrottles; no autopilot and the possibility of no artificial horizon displays (past midnight after a full day of flying) was more than I was willing to accept. We would also have to turn off the autopilot to continue. That would require us to descend out of RVSM airspace. With the resulting higher fuel burn; we would not be able to retain our landing alternate. The First Officer and I discussed landing at either of two nearby airports; but our departure airport is the nearest Major Maintenance Base with the appropriate parts and trained personnel to return this aircraft into service. We also discussed that there may not be a ground crew at the two nearby airports at this late hour qualified to perform the required security inspection of a replacement aircraft for our follow-on flight to our international destination. A new general declaration (GENDEX) reflecting a departure from a different city would also be required; but might be difficult to attain. It would only take a few more minutes to return to our departure airport rather than spiraling down from FL360. If the vertical gyros failed before landing at at our departure airport we could still see the horizon out the windshield because the moon was almost full and the night was now clear. During the descent; the Captain's FMA display went blank with the exception of the 'No Autoland' light which illuminated. The flight director command bars also disappeared. I selected FD 'Both on 2' and this restored the displays and command bars to landing. The vertical gyros continued to operate to landing. The Landing was uneventful.

Data retrieved from NASA's ASRS site as of April 2012 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.