Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Deviation - Altitude Undershoot; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Other / Unknown
Flight Crew Became Reoriented
Human Factors; Procedure
We were assigned a crossing restriction to cross ZZZ VOR at FL180. I watched my copilot adjust the altitude selector and program the FMS. I reprogrammed the angle of descent and was certain I selected VNAV myself on the mode select panel. We were the only aircraft in the sky last night so everything was silent. We were just cruising along when I started wondering about our next ATC handoff. That's when I noticed we were crossing ZZZ VOR still at FL310. VNAV had apparently not been selected. I immediately initiated a descent and as soon as the descent was establish the controller; sounding a little confused; gave us that frequency change I had been waiting for.
[Report narrative contained no additional information.]
BE40 flight crew reported a failure to begin a descent when cleared for a crossing restriction on the route of flight.