A DC8-63 ON TKOF AT ROTATION THE #1 ENG L AND R SIDE COWLING DEPARTED THE ACFT.

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ON PREFLT WALKAROUND NOTED ALL COWLINGS CLOSED AND LOCKED; VERIFIED VISUALLY AND WITH PULL ON EACH COWLING HALF. UPON ARR; WE NOTED #1 ENG COWLINGS MISSING FROM ACFT. ENTIRE FLT WAS NORMAL; ALL ENG PARAMETERS NORMAL. WE HAD NO IDEA COWLINGS HAD FALLEN OFF. I SUSPECT THIS COULD BE AN AGING ACFT ISSUE; OR POSSIBLY RELATED TO STC SA 5455NM (STAGE III NOISE REDUCTION) WHICH INCLUDES MODIFICATIONS TO COWLINGS AND DUCT WORK. COWLING WAS FOUND AT PHL STILL LATCHED WITH FAILURE NOTED AT ATTACHMENT PINS (PRELIMINARY COMPANY FINDING). SUGGEST POSSIBLE ONE TIME FLEET DIRECTIVE OR AIRWORTHINESS DIRECTIVE. MAYBE COMPANY SHOULD MAKE LATCHING COWLINGS A REQUIRED INSPECTION ITEM PROC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A DC8-63 WITH P&W JTD3-7 ENGS. THE RPTR SAID THE COWLING WAS CHKED ON THE PREFLT WALKAROUND INSPECTION AND ALL ENGS WERE OK. THE RPTR STATED THE ACFT WAS RECENTLY MODIFIED TO MEET STAGE 3 NOISE REQUIREMENTS AND SOME COWLING WORK WAS ACCOMPLISHED. THE RPTR SAID THIS WORK WAS DONE SOMETIME PRIOR TO THIS FLT. THE RPTR SAID THE MAINT PRELIMINARY RPT WAS EITHER FALSE LATCHING OF THE UPPER PYLON TO COWLING HINGE PINS OR OUTRIGHT FAILURE OF THE PINS DUE TO AGING. THE ONLY DAMAGE TO THE AIRPLANE OTHER THAN THE COWLING WAS #1 ENG L SIDE BIFURCATED DUCT DAMAGED AND MINOR SCRATCHES TO THE LOWER WING SURFACE.