The ornamental design for an instrumentpanel for an aircraft, as shown and described.
FIG. 1 is a front perspective view of an instrument panel for an aircraft showing our new design;
FIG. 2 is a front view thereof, the back view forming no part of the claimed design;
FIG. 3 is a bottom view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a first side view thereof; and,
FIG. 6 is a second side view thereof.
The repeating short dash broken lines illustrate structure and environment that form no part of the claimed design. The single long dash and single short dash broken lines illustrate boundaries of the claimed design that form no part thereof.