A quasisteady analysis of the aeroelastic stability of the lateral (antisymmetric) modes of the 747/Orbiter vehicle demonstrates that the interference effect of the Orbiter wake on the 747 tail furnishes an aerodynamic undamping contribution to the low-frequency elastic modes. Likewise, the upstream influence of the 747 tail and aft fuselage on the Orbiter beavertail tail fairing also is undamping. Fortunately, these undamping effects cannot overpower the large damping contribution of the 747 tail, and the yaw modes are damped for the configurations analyzed.