The design is the visual features of the FOLDABLE TILTROTOR AIRCRAFT as shown in solid lines in the drawings, whether those visual features are features of shape, configuration, pattern or ornament, or any combination of those features. The portions shown in dotted lines do not form part of the design.FIG. 1 is a tridimensional view of the FOLDABLE TILTROTOR AIRCRAFT of the present design, with the rotor nacelles facing forward;FIG. 2 is a front elevational view of FIG. 1;FIG. 3 is a right side elevational view of FIG. 1;FIG. 4 is a left side elevational view of FIG. 1;FIG. 5 is a top plan view of FIG. 1;FIG. 6 is a bottom plan view of FIG. 1;FIG. 7 is a tridimensional view of the FOLDABLE TILTROTOR AIRCRAFT of the present design, with the rotor nacelles facing upward;FIG. 8 is a front elevational view of FIG. 7;FIG. 9 is a right side elevational view of FIG. 7;FIG. 10 is a left side elevational view of FIG. 7;FIG. 11 is a top plan view of FIG. 7;FIG. 12 is a bottom plan view of FIG. 7;FIG. 13 is a tridimensional view of the FOLDABLE TILTROTOR AIRCRAFT of the present design in a folded position;FIG. 14 is a front elevational view of FIG. 13;FIG. 15 is a right side elevational view of FIG. 13;FIG. 16 is a left side elevational view of FIG. 13;FIG. 17 is a top plan view of FIG. 13; andFIG. 18 is a bottom plan view of FIG. 13.