According to the operator, the purpose of the short, 22 minute flight was to deliver a load of oversized, oil drilling tools to a remote oil production site. The flightcrew reported that shortly after takeoff the first officer noticed that the airplane's elevator control movements appeared to be "momentarily stiff." While en route to the destination, the flight engineer was asked to inspect the cargo, and he subsequently discovered that two of the four, 31-foot long oil drilling tools had shifted aft, damaging the aft pressure bulkhead. The captain did not declare an emergency, and the airplane landed at the destination without incident. A postflight inspection revealed substantial damage to the aft pressure bulkhead assembly, as well as various frames, stringers and structural longerons. In addition, several attachments for the elevator flight control pulleys and flight control cable guides were damaged. The operator noted that the airplane's pressurization system had been removed years ago, when the airplane was converted to a cargo configuration. The operator reported that after loading the airplane, just before departure, the crew inspected the five nylon strapping mechanisms that secured the four drilling tools to the floor of the airplane. The crew reported that all five of the nylon straps were tight before departure, but gathered that one or more had loosened slightly during taxi and takeoff, allowing two of the tools to shift aft during takeoff. The crew also noted that the drilling tools were covered with ice and snow, which likely aided in the tools sliding along the aluminum, diamond plate-covered floor of the airplane.