A DHC8-400 flight crew failed to note that maintenance had left the alternate gear door handle open during a required line check. As a result they were unable to retract the gear after takeoff.

Narrative

We were notified of required maintenance line check inspection on the aircraft prior to our return flight. After maintenance signed the aircraft off; I proceeded to conduct a walk-around. We were trying to get an already significantly delayed flight out as quickly as possible. We briefed the flight per SOP's and conducted a Before Start and Engine Start checklist before pushing back. After takeoff and 'gear up' command; I raised the gear handle and the landing gear would not retract. 3 green lights; as well as the 3 red unsafe lights and gear handle light remained illuminated. After safely navigating out of the terminal area; we ran the appropriate QRH checklists. I scanned the flight deck and discovered the alternate gear door hatch opened. We contacted company and maintenance by radio; and followed recommendations to close the hatch and raise the gear. This was successful. After another gear cycle to ensure proper operation; we continued to our destination without incident. We need to develop a standard aircraft configuration when control is transferred from flight crew to maintenance and vice versa. If switches and levers are always in the same position; we can minimize risk of overlooking these in the preflight.