Abstract: An aircraft has the space between the bulkheads on the interior of the aircraft filled with a polyisocyanurate solid closed cell foam material. The foam is applied so that it adheres to both the inside of the aluminum skin of the fuselage and the facing sides of the bulkheads. The foam may be applied by either spraying or by pouring the resin with appropriate catalyst materials to cause the resin to form the foam. The foam acts to significantly strengthen the aircraft structure and thereby increase the time of usage of aging aircraft before the catastrophic failure of the adherence of the skin to the bulkheads.