C750 CREW HAD AN UNCOMMANDED ACFT PITCH UP AND CLIMB WHILE USING THE ACFT FLT GUIDANCE SYSTEM.

Narrative

ACFT IN STABLE FLT AT FL290 AFTER DESCENT FROM FL 410. AUTOPLT ENGAGE WITH NAV AND ALT HOLD MODES CAPTURED; SPEED APPROX MACH 0.89/340 KTS. ATC ISSUED DESCENT TO FL270. DESCENT INITIATED USING AUTOPLT WITH VERTICAL SPEED MODE ENGAGED AND DOWN WHEEL SELECTING 1000 FPM DESCENT; ACFT PITCHED UP AND CLIMBED BEFORE COMPLETE CTL WAS REGAINED AT APPROX FL293. DESCENT TO FL270 COMPLETED. THERE WAS NO COM WITH ATC REGARDING ALT; NOR ANY WARNINGS OR CONFLICTS. MAYBE AND SHOULD BE END OF STORY. BUT I RPTED THIS OCCURRENCE TO MY MANAGER. HIS RESPONSE WAS INTERESTING AT BEST. IN THAT THIS HAS 'HAPPENED REPEATEDLY' WITH THIS TYPE ACFT. 'THAT INSTALLED SOFTWARE (REVISION 7?) THAT CAUSES PITCH UP UNPREDICTABLY!' 'THAT NEW SOFTWARE (REVISION 8?) IS FORTHCOMING AND IN THE MEAN TIME NOTHING TO DO BUT BE CAREFUL.' CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED AT FL290 SPEED .89 MACH. 340 IAS WITH AUTOPLT ENGAGED TO ALT HOLD; ATC THEN ISSUED DSCNT TO FL270. THE RPTR SAID VERTICAL SPEED MODE WAS ENGAGED AND 1000 FT PER MINUTE WAS SELECTED AND STARTED TO DSND WHEN THE AIRPLANE PITCHED UP AND CONTROL WAS REGAINED AT FL293. THE RPTR STATED WHEN THE INCIDENT WAS RPTED TO THE CHIEF PLT THE RESPONSE WAS 'THIS HAPPENS REPEATEDLY' EXPLAINING THE PROB WAS IN THE SOFTWARE. THE RPTR SAID NEITHER THE COMPANY NOR THE MANUFACTURER HAS ISSUED ANY BULLETINS OR INFO TO THE CREWS OPERATING THIS ACFT. THE RPTR SAID NO SOFTWARE MODIFICATIONS HAD BEEN MADE TO DATE.