There is a team of CM's who work on scenarios. (CM's are player volunteers who help put on special events in Aces High.)
The scenario CM team picks a battle, generally based upon a historical battle, but sometimes based on a "what if" situation. We pick a topic based on what scenarios have been run previously, so that we have variety in the types of aircraft and objectives. We generally don't pick the same theater of operation for consecutive scenarios, for example.
Once we pick a battle, there is research into what forces were in the battle, which aircraft, in which proportion, what some of the goals were, what radar was or wasn't used, what the weather was like, what the terrain was like, etc. This can be something that the scenario team already is familiar with, or it can involve reading several books on the topic and doing a lot of Internet-based research.
When the research portion is complete, the team comes up with a scenario design. The goals of the design are historical realism, play balance, action, and fun. The goal of historical realism involves which aircraft were available, in what quantities, going after which objectives, physical locations (i.e., which terrain and bases), weather, etc. The goal of play balance is the attempt to make it such that, if each side had equal-capability players, the result would end up near even, which is sometimes hard to figure out in advance. The goal of action is to set things up so that each player is likely to encounter battle during the frame. The goal of fun covers things not yet covered, such as variety, goals that we hope will interest people, and so on.
These factors pull in different directions. Realism alone, for example, would often result in a setup where one side is completely overmatched or where players on each side never rarely encounter each other. So, judgement must be used on how to come up with a good mix of these various factors.
The design process results in a first draft of a rules writeup, which includes the aircraft available in what quantities, goals, targets, victory points, settings (of wind, ack, ship hardness, building hardness, downtime, etc.), how many lives, and so on. The design is then considered, debated, and adjusted until we have a final draft.
The process of producing a scenario (up to the point of running it) generally involves the spare time of several people for a couple of months.
