What was found
PARTIAL LOSS OF ENG PWR OCCURRED JUST AFTER T/O. MANIFOLD PRESS DROPPED TO 15 INCHES HG AND RPM DROPPED TO 1900. PRE T/O ENG RUN UP, ALTERN AIR CHECK WAS REPORTED AS (NORMAL). POST FLT INSP REVEALED THAT ALTERN AIR (HEAT BOX) DOOR HAD SEPARATED FROM ALTERNATE AIR BOX ASSY, WAS INGESTED INTO FUEL SERVO INTAKE, PARTIALLY BLOCKED ENG INDUCT AIR FLOW, RESULTED IN PARTIAL LOSS OF PWR. MS20470AD3 RIVETS (3EA) THAT SECURE DOOR TO AIR BOX HAD FAILED. IMPROPERLY BUCKED RIVETS DUE TO EXCESS SPRING TENSION DURING ASSY AND/OR TOO MUCH SPRING TENSION FOR RIVETS TO BEAR. HEAT BOX DOOR SHOULD BE TETHERED IN SUCH WAY, IF HINGE ATTACHMENTS SHOULD FAIL DOOR WON`T BE INGESTED AND/OR SECURED WITH STRONGER FAIL SAFE HDWR.