Hawker 800 Captain had an autopilot pitch up while attempting to descend due to a 'max speed' flag with the speed 15 knots below maximum. An altitude deviation above the original cruise altitude occured. Queries to Honeywell; the avionics package manufacturer; indicate that this is normal operation.

Narrative

With the autopilot engaged at FL240; we were beginning our decent to 12;000. We were indicating about 15 KTS BELOW Red Line (320 KTS). I selected vertical speed and rolled in about 2;000 FPM DOWN; the nose began to lower and then we showed a 'Max Speed' flag; and the aircraft immediately began to pitch up to a 1;500 FPM CLIMB. By the time I recovered; we had climbed up to 24;400 FT. The speed was never within 10 KTS of max speed; and the climb took me by surprise. I have personally experienced this phenomenon in a decent before today; but never at our maximum clearance altitude; which directly related to our altitude deviation. ATC did inquire to say our altitude and we were already in the decent at that point passing back through 24;000. We have the Honeywell Primus Epic avionics package installed and we have had discussions with them regarding this bug where the aircraft autopilot pitches up steeply on a 'Max Speed' flag; when we are still at least 10-15 KTS SLOWER than Red Line. They say it is normal operation.