A modified "zero-$g$" research aircraft is configured to simulate lunar gravity ($g_M = g/6$) inside its cabin during a parabolic flight segment. During this segment, an object released from rest in the cabin accelerates downward at $g/6$ relative to the cabin floor. What is the magnitude and direction of the aircraft's acceleration $\mathbf{a}$ in the ground frame during this segment? Take $g$ to be the gravitational acceleration at the aircraft's altitude, and ignore air drag on the released object.