What was found
AIRCRAFT WAS IN CRUISE AT 6000' WHEN ALL 4 CYLINDERS EXPERIENCED ABNORMAL EGT RISE, SLIGHT REDUCTION IN CHT, AND ENGINE SURGING. DIVERTED TO NEAREST AIRPORT. REMOVED MAIN ENGINE AIR FILTER AND DISCOVERED 600118-015 ALTERNATE AIR DOOR WITH CRACKS EMANATING FROM AND COMPLETELY DETACHED FROM 600355-507 PLATE ASSEMBLY (PHOTOS AVAILABLE). AD95-26-16 R1 WAS COMPLIED WITH. PART FAILED 30 FLIGHT HOURS AFTER LAST ANNUAL. REGARDLESS OF PRIOR COMPLIANCE WITH AD95-26-16 R1, RECOMMENDATION IS MADE TO INCORPORATE AGGRESSIVE INSPECTION OF THE ALTERNATE AIR DOOR FOR CRACKS AT EVERY ANNUAL/100HR INSPECTION AND ANY TIME THE AIR FILTER IS REMOVED.