Sabreliner 60
Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 85; Flight Crew Total 4500; Flight Crew Type 2500
Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
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.
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.