Hypothetically, can 2 airplanes (or rather 2 pilots) agree on a frequency that they know it's not used by any airport or service in the area and use it to communicate between each other, for whatever reason? Would it be legal to do so?
I don't know about other countries, but in the US, it's both possible and legal, as long as the frequency you pick happens to be one of the ones authorized for air-to-air communications. This AOPA article has more info, or you can check the official list at 47 CFR 87.187, but the basics are:
- 122.75 for regular fixed-wing aircraft.
- 123.025 for helicopters.
- 123.3 or 123.5 for gliders and balloons.
- 123.45 (a.k.a. "Fingers") was originally for flight testing, but has become a de facto air-to-air frequency just because it's so easy to remember. (So, technically not legal, but that doesn't stop anyone.)