Narrative:

Event occurred at cruise; non-radar environment; HF position reports with ATC. Both I (right seat) and the relief pilot (left seat) were at the controls when both yokes started to vibrate to the degree of a light buzz. I was the pilot not flying and autopilot number 1 was engaged. I immediately checked altitude; airspeed and vsi. All parameter were normal. Aircraft was holding altitude; airspeed and had a zero vsi. Next we checked all circuit breakers to include autopilot and stall warning systems; and I did a quick test of the warning lights. All circuit breakers were in. The dull continuous vibration remained making it difficult to hold the yoke for any length of time. We tried swamping to autopilot number 2; this did not alleviate the situation. I consulted the QRH and aircraft operating manual (aom) for any reference to flight control malfunction or inappropriate stall warning or stall warning system malfunction. There was not applicable reference in the QRH or aom for our situation. At this point the captain relieved the relief pilot from the left seat and informed me that I should take my rest and that he and the relief pilot would work the situation. I let the relief pilot into the right seat and headed into crew rest. Before I retired I briefed the captain that we checked all our lights and circuit breakers and found no anomalies. We consulted the aom and QRH and found no reference for our situation. Upon returning to the cockpit after my two and a half hours of rest; the vibration has ceased in both yokes.

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Original NASA ASRS Text

Title: A MD11's control columns were mildly vibrating at cruise with no other indications. Both stall warning cannon plugs were found loose and when the left was connected; with the right disconnected; the vibration ceased.

Narrative: Event occurred at cruise; non-radar environment; HF position reports with ATC. Both I (right seat) and the Relief Pilot (left seat) were at the controls when both yokes started to vibrate to the degree of a light buzz. I was the pilot not flying and autopilot number 1 was engaged. I immediately checked altitude; airspeed and VSI. All parameter were normal. Aircraft was holding altitude; airspeed and had a zero VSI. Next we checked all circuit breakers to include autopilot and stall warning systems; and I did a quick test of the warning lights. All circuit breakers were in. The dull continuous vibration remained making it difficult to hold the yoke for any length of time. We tried swamping to autopilot number 2; this did not alleviate the situation. I consulted the QRH and Aircraft Operating Manual (AOM) for any reference to flight control malfunction or inappropriate stall warning or stall warning system malfunction. There was not applicable reference in the QRH or AOM for our situation. At this point the Captain relieved the Relief Pilot from the left seat and informed me that I should take my rest and that he and the Relief Pilot would work the situation. I let the Relief Pilot into the right seat and headed into crew rest. Before I retired I briefed the Captain that we checked all our lights and circuit breakers and found no anomalies. We consulted the AOM and QRH and found no reference for our situation. Upon returning to the cockpit after my two and a half hours of rest; the vibration has ceased in both yokes.

Data retrieved from NASA's ASRS site as of July 2013 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.