Narrative:

While level in cruise flight southwest bound at FL400. I was filling out the flight log and engine trend data for the flight. Nothing seemed unusual we were flying almost into the setting sun; it was at about the 1-1:30 position; we were in smooth air and all seemed normal as I filled out the log sheet. I caught a glimpse of the sunset in my window (it was only visible out the co-pilot's window) I looked at the attitude indicator and we were rolling past 30 degrees bank and increasing and accelerating in roll along with an increasing nose down attitude and rapidly falling altimeter going through FL396. I grabbed the yoke rolled wings level squeezing the autopilot disconnect at the same time. There was no 'click' of the autopilot disconnecting affirming to me it already had disconnected. I over-corrected for the decent and ballooned up to FL404 then leveled at FL400; during this time the pilot not flying reported to center our altitude deviation and that we were correcting. No further anomalies were detected or noted with the autopilot for the rest of the flight. There is no chime when the autopilot disconnects on the plane so I have become very tuned in to the 'click' of the autopilot disconnecting. On occasion I miss it and this was one of those times. Coupled with it the plane very slowly went into an unusual attitude to the point that I did not notice or 'feel it in the seat'. It was a classic case of everything feels right but your senses are wrong. I am reminded again that even while carrying out the peripheral duties of operating the aircraft the primary job is still to fly the plane.

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Original NASA ASRS Text

Title: Sabreliner Captain experiences an undetected autopilot disconnect at FL400 while entering engine data in the logbook. The suns movement alerts him to a 30 degree bank angle and increasing descent rate which is quickly arrested.

Narrative: While level in cruise flight southwest bound at FL400. I was filling out the flight log and engine trend data for the flight. Nothing seemed unusual we were flying almost into the setting sun; it was at about the 1-1:30 position; we were in smooth air and all seemed normal as I filled out the log sheet. I caught a glimpse of the sunset in my window (it was only visible out the co-pilot's window) I looked at the attitude indicator and we were rolling past 30 degrees bank and increasing and accelerating in roll along with an increasing nose down attitude and rapidly falling altimeter going through FL396. I grabbed the yoke rolled wings level squeezing the autopilot disconnect at the same time. There was no 'click' of the autopilot disconnecting affirming to me it already had disconnected. I over-corrected for the decent and ballooned up to FL404 then leveled at FL400; during this time the pilot not flying reported to Center our altitude deviation and that we were correcting. No further anomalies were detected or noted with the autopilot for the rest of the flight. There is no chime when the autopilot disconnects on the plane so I have become very tuned in to the 'Click' of the autopilot disconnecting. On occasion I miss it and this was one of those times. Coupled with it the plane very slowly went into an unusual attitude to the point that I did not notice or 'feel it in the seat'. It was a classic case of everything feels right but your senses are wrong. I am reminded again that even while carrying out the peripheral duties of operating the aircraft the primary job is still to fly the plane.

Data retrieved from NASA's ASRS site 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.