A LEARJET 25 IN CRUISE AT FL230 EXPERIENCED A 700 FT ALT EXCURSION DIRECTLY CAUSED BY THE MAIN GEAR DOORS OPENING AND POSSIBLY THE MAIN GEAR EXTENDING AT 350 KTS INDICATED. CAUSE UNDER INVESTIGATION.

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ON DEC/MON/03; I WAS ASSIGNED AN FO POS IN A LEARJET 25. THE RTE OF FLT WAS FROM ZZZ1 TO ZZZ. THE FLT WAS UNEVENTFUL UNTIL THE MECHANICAL IRREGULARITY. WE WERE CRUISING AT FL230; ASSIGNED AN UNUSUALLY HIGH NUMBER OF VECTORS ENRTE TO ZZZ. (I FLY THIS SPECIFIC RTE 4 NIGHTS A WK.) THE ACFT WAS W OF OUR NORMAL RTE; APPROX 45 MI NW OF THE ZZZ VOR. ATC ASSIGNED A HEADING (SOUTHEASTERLY) AND GAVE A TA AT FL240. SHORTLY THEREAFTER THE ACFT WAS EXCESSIVELY NOISY AND MADE AN ENORMOUS PITCHING MOMENT. THE ACFT CLBED 700 FT. AS THE PNF; I WAS TRYING TO EVALUATE THE SIT; ASSIST THE CAPT/PF; AND ADDRESS ATC. WE ASCERTAINED THAT THE GEAR DOORS OPENED INFLT; WITH THE GEAR (WE THINK) DOWN; WE WERE INDICATING 350 KTS AT THE TIME. (NORMAL GEAR SPD IS 200 KTS IN THE LEARJET 25.) AFTER THE ACFT UPSET OCCURRED; AND ACFT CTL WAS OBTAINED; I DECLARED AN EMER. (WHILE WE WERE TRYING TO EVALUATE AND FLY THE ACFT; ATC CONTACTED US SEVERAL TIMES REGARDING THE ALTDEV.) THEREAFTER; WE DIVERTED TO ZZZ2 AND HAD NO FURTHER ACFT ABNORMALITIES. NOTE: THE ALTDEV WAS CAUSED SOLELY BY THE MECHANICAL IRREGULARITY OF THE GEAR DOORS OPENING AT FL230 AND 350 KTS. DUE TO THIS; THERE WAS NO OTHER ACTION WE COULD HAVE TAKEN TO AVOID THE CONFLICT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS WORKED ON SEVERAL DAYS BEFORE BEING JACKED. THE RPTR SAID THE AIRPLANE WAS JACKED AND THE RPT WAS DUPLICATED. THE DOORS OPENED AND THE GEAR DROPPED OUT. THE RPTR STATED THAT THE COMPONENT OR COMPONENTS CAUSING THE FAILURE IS UNKNOWN AS THE INCIDENT IS STILL UNDER INVESTIGATION.