Light Transport; Low Wing; 2 Turbojet Eng
Flight Crew Instrument; Flight Crew Multiengine
Confusion; Fatigue; Time Pressure; Situational Awareness
Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Human Factors; Procedure
[We] took wrong aircraft in the dark at DAL. [We were] assigned Aircraft Y; [I] noted a different aircraft [parked] where Aircraft Y [was] left the night before.  [I] asked where Aircraft Y was; and ground crew pointed to an airplane on the dark west ramp. We [did our] preflight and loaded without noticing it was not Aircraft Y. We left inadvertently in Aircraft X; [and] therefore we had an invalid dispatch; since we were in [the] wrong tail number. We did not notice any other light transport aircraft in the dark ramp area. Approached aircraft from nose and did not notice wrong tail number. Suggestions: Better awareness on preflight. Even looked at oil with my flashlight and but did not see the wrong side number.
A Captain reported taking the wrong aircraft from the ramp on a dark morning; consequently; their dispatch release was invalid.