Instrument Flight Rules (IFR) require that you file flight plans. Visual Flight Rules (VFR) are allowed in some areas (certainly NOT downtown). In order to use VFR, there needs to be a certain level of visibility. They STILL recommend filing flight plans with VFR in case there are problems so that rescue can be sent. VFR heavily uses a Sectional Chart. These look a bit like an inverted layer cake where airports control the airspace. There are some areas in the Seattle Metroplex where you can legally fly VFR without flight plans when the weather permits. Some of those areas are very narrow bands.
The big issue is take off and landing. If you're leaving from an airport or returning to an airport, you've got to get permission to enter that airspace.
The whole thing is very complex. If you've got a player willing to go through all those rules and plot out routes and such, then I'd yield to his legwork and let it work. If he doesn't, then KE is gonna send interceptors. I'm actually using a character that is designed around the whole airspace legality issues and loopholes. As a final note, KE needs clearance through controlled airspace just like runners. An inbound suborbital doesn't care if you're chasing a rogue helicopter, you have to stay out of the flight path.
