A LEARJET 25 IN CRUISE AT FL410 DEVIATED FROM ASSIGNED ALT DUE TO THE AUTOPLT INABILITY TO MAINTAIN ALT IN ALT HOLD MODE.

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WHILE IN CRUISE FLT; I EXITED THE COCKPIT FOR PHYSIOLOGICAL REASONS. THE FLYING CO-CAPT TRIED TO MAKE A SMALL CORRECTION FOR AUTOPLT HOLD DEVS. THE ALT HOLD CREEPS UP. EXCESSIVE AUTOPLT PITCH INPUTS AND FAILURE TO DISCONNECT THE AUTOPLT ALLOWED THE ACFT TO CLB 800 FT BEFORE STOPPING THE CLB. HE (OTHER PLT) CALLED ATC AND NOTIFIED THEM. I OVERHEARD THE XMISSION. HE SAID ATC WAS NOT CONCERNED OR UPSET. THIS IS A CHRONIC PROB IN THIS MAKE AND SERIES OF LEARJETS. THEY ARE OLD AND THE AUTOPLTS ARE ANTIQUATED. THE STATIC SOURCE FOR ALT HOLD IS SEPARATE AND INACCURATE. THERE IS NO AIR DATA COMPUTER FOR AUTOPLT INPUT. THE MANUFACTURER OR EVEN THE FAA SHOULD MANDATE A SOLUTION TO THIS PROB EITHER BY SVC BULLETIN OR AIRWORTHINESS DIRECTIVE NOTE. NO ONE IN THE INDUSTRY HAS A FIX FOR THIS PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A LEARJET 25 AND THE PROB IS THE ANTIQUATED AUTOPLT WHICH CANNOT MAINTAIN A CONSTANT ALT IN ALT HOLD MODE. THE RPTR SAID THE AUTOPLT WHEN IN ALT HOLD MODE MUST BE CONSTANTLY CORRECTED BY DISCONNECTING OR BY PITCH TRIM. THE RPTR STATED THE ACFT HAS NO AIR DATA COMPUTER FOR INPUT TO THE AUTOPLT JUST MAIN PITOT AND STATIC PORT INPUT. THE RPTR SAID THE FAA OR THE MANUFACTURER SHOULD MANDATE A SOLUTION TO THIS PROB BY A SVC BULLETIN OR AIRWORTHINESS DIRECTIVE. THE RPTR STATED WITH AN AIR DATA COMPUTER INPUT A NUMBER OF SAFETY FEATURES COULD BE BROUGHT UP-TO-DATE. THE RPTR SAID PRESENTLY THERE IS NO EQUIP ON THE MARKET THAT WOULD BE COMPATIBLE WITH THE AUTOPLT THE MANUFACTURER INSTALLED.