AN ATP RATED PLT A CHEYENNE TURBOPROP NEAR BYRAN; TX; DEVIATED FROM HIS ASSIGNED ALT.

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I HAVE JUST INSTALLED 2 GARMIN 430 UNITS AND 1 SANDEL EFIS SYS CONNECTED TO A300 BENDIX 300 AUTOPLT. WHILE CRUISING AT 15000 FT; ENTERED AN APCH TO CFD (COULTER FIELD; BRYAN; TX). THIS IS A VOR/DME APCH WITH A DME ARC. ENTERED PROC; SELECTED LOAD -- HELD ACTIVE RTE. APPROX 1 MIN LATER; ALT HOLD ON AUTOPLT TRIPPED OFF. AUTOPLT TRIPPED OFF. ACFT WENT INTO A STEEP BUT EVEN DSCNT. MANUAL COMMAND OF ACFT WAS TAKEN TO STOP DSCNT. CTR CALLED AS I CALLED THEM; ASKED WHY DEV OF ALT. I RESPONDED WITH SHORT DESCRIPTION OF AUTOPLT DISCONNECT. NO FURTHER COMMENT FROM CTR. I CANCELED IFR AT THAT TIME. WOULD RECOMMEND THAT ALL PLTS UPGRADING TO DUAL/FM GPS'S CONNECTED TO EACH OTHER WITH SANDEL EQUIP USE EXTRA CARE IN INITIAL FLTS AS; AFTER TALKING TO OUR INSTALLER; SOME OF THE FEATURES DON'T ALWAYS REACT. ALL THE NEW RADIO EQUIP IS 1999. AUTOPLT IS 1979. SYS APPEARS TO WORK WELL TOGETHER. RECOMMEND VERY CAREFUL FLT WATCH UNTIL FAMILIAR WITH USE OF SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ON CALLBACK RPTR STATES THAT THIS WAS THE FIRST FLT AFTER INSTALLING THIS NEW EQUIP ON HIS ACFT. THAT READING THE BOOKS AND ACTUALLY FLYING THE ACFT ARE TWO DIFFERENT THINGS. HE EXPECTS THAT AFTER A FEW HOURS WITH THE NEW EQUIP WILL GREATLY REDUCE THE LEARNING CURVE.