CAPT OF CARJ RETURNS FROM PHYSIOLOGICAL BREAK TO DISCOVER INCIPIENT STALL DUE TO UNMONITORED AIRSPD FOLLOWING MINOR THRUST REDUCTION BY FO. AUTOPLT DISCONNECTED AND RECOVERY OF AIRSPD AND ACFT CTL RESULTS IN LOSS OF 4000 FT FROM ASSIGNED ALT OF FL390.

Narrative

DURING ESTABLISHED CRUISE FLT; MACH .75; FL390; I EXITED THE COCKPIT TO USE THE LAVATORY. AFTER APPROX 2-4 MINS; I RETURNED TO THE COCKPIT AND WHILE IN THE PROCESS OF CLOSING AND SECURING THE COCKPIT DOOR; THE AUTOPLT DISCONNECTED AND THE ACFT ENTERED A SLIGHT ROLL (APPROX 5-10 DEGS MAX). MY INITIAL THOUGHT WAS THAT THE FO DISCONNECTED THE AUTOPLT; YET I IMMEDIATELY TURNED TO HIM AND A PFD (THE FO HAD CANCELED THE AUTOPLT DISCONNECT AUDIBLE WARNING). HE LOOKED BEWILDERED AND THE PFD INDICATED THE LOW SPD AND AN INDICATED AIRSPD APPROX 5 KTS ABOVE STALL. I IMMEDIATELY SET MAX THRUST AND SIMULTANEOUSLY INSTRUCTED THE FO TO PUSH THE NOSE OVER. STILL STANDING; MY OBSERVANCE WAS THE PITCH ATTITUDE REDUCTION. INDICATED AIRSPD INCREASE AND RATE OF DSCNT WERE NOT PROGRESSING AS I DESIRED AND I ISSUED SEVERAL ADDITIONAL INSTRUCTIONS TO PUSH THE NOSE DOWN AND THEN KNELT ON MY SEAT AND PUSHED NOSE DOWN ON MY YOKE; TELLING THE FO I WAS ASSISTING THE RECOVERY PROC. I DECLARED AN EMER DSCNT WITH ATC TO FL290 WHEN CONDITIONS PERMITTED. DURING THE RECOVERY PROCESS; THE FO INDICATED HE WAS HAVING DIFFICULTY WITH THE ACFT TRIM AND WE RECEIVED A 'TRIM IN MOTION' AURAL. I INSTRUCTED HIM TO STOP TRIMMING TO ASCERTAIN IF THE ISSUE WAS AUTOMATION OR MANUAL BY THE FO. THE PITCH TRIM INDICATOR WAS SHOWING SOME ERRATIC MOVEMENT; IE; +4 UNITS DOWN TO +1 UNITS QUICKLY. I FLEW THE ACFT TO ASCERTAIN ITS CTL ABILITY (APPROX FL350) AND IT APPEARED TO HANDLE WELL; YET MY PERCEPTION AT THE TIME WAS IT WAS 'SLUGGISH' IN PITCH. THE FO TOLD ME THAT HE HAD NO IDEA AS TO THE CAUSE OF THE LOW SPD CONDITION; THAT HE HADN'T TOUCHED THE THRUST OR INTENTIONALLY SLOWED THE ACFT. BASED UPON THESE FACTORS; MY INITIAL IMPRESSION WAS SOME TYPE OF PITCH ANOMALY; AND I ELECTED TO DIVERT TO ZZZ (NEAREST SUITABLE). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: AT THE TIME THE RPTR LEFT THE COCKPIT; A CABIN ATTENDANT WAS IN THE COCKPIT PER COMPANY SECURITY SOP. THE FLT ATTENDANT SHOULD HAVE BEEN SEATED AND STRAPPED IN PER SOP BUT WAS NOT. IN ADDITION; THE FO HAD NOT DONNED HIS OXYGEN MASK AS REQUIRED BY THE FARS ALTHOUGH THE RPTR FELT HE WAS IN THE PROCESS OF DOING SO WHEN HE LEFT THE COCKPIT. THE RPTR BELIEVES THE FO HAD REDUCED THE THRUST SLIGHTLY; AND APPARENTLY HAD NOT MONITORED THE EFFECTS OF THE THRUST REDUCTION. THE FO INTENDED TO REDUCE AIRSPD SLIGHTLY TO THE CRUISE MACH OF .75. INDICATED AIRSPD AT THAT TIME WAS APPROX 227 KIAS. WHEN THE CAPT RETURNED; THE FO WAS TURNED PHYSICALLY AWAY FROM THE INST PANEL AND ENGAGED IN CONVERSATION WITH THE FLT ATTENDANT. THE AIRSPD HAD DROPPED OFF TO JUST UNDER 180 KIAS. IN QUICK SUCCESSION; THE AUTOPLT DISCONNECTED AND SIMULTANEOUSLY THE STICK SHAKER AND AUTOPLT WARNINGS SOUNDED. THE RECOVERY EVENTS DETAILED IN THE ORIGINAL NARRATIVE THEN OCCURRED. RPTR NOTED THAT THIS ACR'S CARJ'S ARE EQUIPPED WITH BOTH LATERALLY AND VERTICALLY COMPLIANT FMCS SYS BUT THAT AUTOTHROTTLES ARE NOT PART OF THEIR INSTALLATION. HE FELT THAT AUTOTHROTTLES WOULD HAVE PREVENTED THE INCIDENT; NOTWITHSTANDING THE FO'S UNPROFESSIONAL LACK OF ATTN TO THE ACFT. HE EMPHASIZED THAT NO LOSS OF SEPARATION OCCURRED NOTWITHSTANDING THEIR DSCNT THROUGH AT LEAST 4 POTENTIALLY OCCUPIED RVSM ALTS. ALSO; THAT HE XMITTED THE EMER DSCNT AND WAS CLRED THROUGH FL290 IF NECESSARY BY ZZZ. CALLBACK CONVERSATION WITH RPTR ACN 651597 REVEALED THE FOLLOWING INFO: RPTR STATED MAINT CHECKED THE ACFT AND REPLACED THE STABILIZER CTL UNIT.