I don't know whether these pilots were trying to instigate a spontaneous chevron formation like the geese that passed over Brookville Lake at sunset or if the pilot of the prop driven smaller plane just wanted a closer look at this jet (i think, I'm no expert it could be a turbo-prop. I don't know it it was some sort of airborne inspection, flight training or just somebody who isn't familiar with FAA regs. It turns out the FAA regulations for commercial aircraft is 1,000 feet vertically and 3-5 miles laterally depending on speed and altitude (the higher a jet flies the faster it goes, so above 10,000 feet it's 5 miles.) The rule for general aviation aircraft is comically vague: Reg 91.111: (a) No person may operate an aircraft so close to another aircraft as to create a collision hazard. (b) No person may operate an aircraft in formation flight except by arrangement with the pilot in command of each aircraft in the formation. And just for fun Reg. 91.13: No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
Pilots flying under IFR are subject to the same kinds of rules, and are still responsible for maintaining separation from other traffic when in visual conditions. The rules are the same: the pilot is given the discretion of deciding what a "safe distance" is. (I used to ride motorcycles in formation with as many as 20 guys or more. We all had tons of experience and trusted one another with our lives running two abreast at 80 or 90 mph on two lane roads with somebody five feet in front of you, another guy five feet behind you and another a foot or two to your left or right. It was not smart (and that was given with many of these characters) but it felt like the earth shook and the incredible roar of the engines through straight pipes rattled your internal organs. It was fun. Now when I see even two people riding abreast I just shake my head. And, of course, we never wore helmets) Frankly I was hoping the pilot of the prop plane would open up with a .30 caliber nose-mounted machine gun (plane looks to small for a .50 cal.) and I'd get some photos (or video) of a dog fight. Unfortunately for me they banked right and were soon out of sight. I'd be curious if anybody out there knows what was going on here. And what type aircraft these are.