Let's talk about genre. In video games, genre is, interestingly, usually used to talk about the interface rather than the content. That is to say, a sci-fi based space flight sim and a World War II flight sim are considered to have more in common than a World War II flight sim and a World War II first person shooter.
One of the great white hopes of video games is the successful genre fusion. The game that melds together two different types of game into a satisfying, crunchy whole. There have been some worthy contenders here and there - the Mario RPG line has steadily evolved into a satisfying action/RPG fusion. Actraiser is a mildly successful SimCity/Sidescroller fusion.
Air Fortress, a largely forgotten title by HAL Laboratory, splits its focus between the shooter an the Metroid genre, the Metroid genre already, it should be noted, being a fusion of the side-scroller and Zelda. The player begins as, so far as I can tell, a man strapped to a rocket flying towards what the player is helpfully informed is an Air Fortress. The game does not give much more plot than this, but it quickly becomes clear that one is to destroy the Air Fortresses, and really, in the world of NES games, this is an adequate substitute for actual motivation. The player goes through a fairly standard "shoot enemy spaceships" Gradius-style game before getting to the entrance to the fortress and switching to a side-scroller infiltration game where you shoot your way through.
The side-scroller has one of those classically quirky video game concepts, namely the self-depleting life bar. In this case, moving slowly depletes your life force, making it one of the rare games where you can walk yourself to death. Thankfully standing still increases it, and enemies do sufficient damage that this is basically never actually a concern. So you maneuver through generic if maze-like space stations to destroy them.
The problem with these fusion games is that, ultimately, they do neither of the things they set out to do particularly well. Mix and match is not, in the end, creativity, and these hybrid games end up being unsatisfying because they lack any clear and interesting play mechanic.
Which is often the case. Rarely does mixing two things lead to a third better thing. This is why musical supergroups usually blow, and why you should not dip a steak in chocolate.
Speaking of play mechanics, it turns out that you can't do a 3-D Flight Simulator on the NES. Pity nobody told the developers of Airwolf.