This thesis describes a longitudinal dynamic analysis of a large, twin-fuselage aircraft that is connected solely by the main wing with two tails unattached by a horizontal stabilizer. The goal of the analysis is to predict the aircraft’s behavior in various flight conditions. Starting with simple force diagrams of the longitudinal directions, six equations of motion are derived: three equations defining the left fuselage’s motion and three equations defining the right fuselage’s motion. The derivation uses a state-vector approach. Linearization of the system utilizes a Taylor series expansion about different trim points to analyze the aircraft for small disturbances about the equilibrium. The state transition matrix shows that there is a coupling effect from the reactionary moments caused by the two empennages through the connection of the main wing. By analyzing the system in multiple flight conditions: take-off, climb, cruise, and post-separation of payload, a general flight envelope can be developed which will give insight as to how the aircraft will behave and the overall controllability of the aircraft. The four flight conditions are tested with published Boeing 747 data confirmed from multiple sources. All four flight conditions contain unstable phugoid modes that imply instability increases with decreasing torsional spring stiffness of the wing or as the structural damping drops below 4%.