As a general rule, an aircraft comprises a fuselage having a front portion in which a cockpit is arranged, a main portion that bears the wings and a rear fuselage that supports the horizontal stabilizer.
Normally, the rear fuselage ends in a modular tail cone end. In particular, the tail cone end houses the auxiliary power unit (APU) of the aircraft.
Thus, when assembling an aircraft, the interface between the tail cone end and the rest of the rear fuselage of the aircraft is normally designed with four fittings or lugs, two upper and two lower, for supporting the loads, plus a balancer fitting in order to align or centering the positioning of the sections during the assembly process. This type of construction derives from the fail safe operability standards requested by the air navigation laws. Hence, in case of losing one of the fittings or lugs the aircraft always has four more in order to carry the loads.
Each of the four lugs take two reactions and the balancer fitting takes one more reaction so in total, nine reactions are taken with this configuration in order to satisfy the failsafe requirements by redundancy. The result is an hyper static junction.
The four lugs work in a hyperstatic mode and the balancer fitting only acts when a failure in one of the lugs occurs.
In addition, since the tail cone end houses the auxiliary power unit (APU) of the aircraft, the joint between the tail cone and the rest of the rear fuselage depends on the location of the air intake of the auxiliary power unit. Thus, in cases where the auxiliary power unit air intake is in a lower location, the balancer is in an upper position whereas in cases where the auxiliary power unit air intake is in an upper location, the balancer is in a lower position.
The positions of the lugs are constrained by the fuselage shape. Those positions are also affected by the cutout that the tail cone end has at its end and are also related to trimmable horizontal stabilizer.
The main forces and moments to be supported by the attachments come from flexion, dynamic landing, crash-landing, etc, applied to the tail cone. The heavier is the APU, the higher the loads that the tail cone end supports will be.
The currently applied hyper static solutions, even with bearings and tolerances provided for the design, are not able to accomplish enough accuracy to assure the positions with the repair operations.
The method of assembly of the tail cone end to the rest of the rear fuselage of an aircraft traditionally used comprises the following steps:                a) To install the balancer which guides and aligns the sections of the tail cone and the rest of the rear fuselage;        b) To install the four fittings or lugs.        
As the four fittings are mounted at the same time, big stresses appear and usually the whole assembly has to be forced.
USPTO Backgrounds
USPTO Backgrounds