FAR 91.111 — Operating Near Other Aircraft

FAR 91.111 sets the basic rules for flying close to other aircraft. It exists to prevent mid-air collisions and to make sure that any close-proximity flying — like formation flight — happens only when every pilot involved has agreed to it.

The rule has three parts:

• No collision hazard. You may not operate your aircraft so close to another that you create a collision hazard. This applies any time you're near another airplane, regardless of airspace or flight rules.
• Formation flight requires arrangement. You may not fly in formation unless you have a prior arrangement with the pilot in command of each aircraft in the formation. Surprise join-ups or unannounced close-in maneuvering aren't allowed.
• No passengers for hire in formation. If you are carrying passengers for hire, formation flight is prohibited entirely — no exceptions, even with prior arrangement.

Operationally, this means you should keep safe separation from other traffic, brief any formation work in advance with all PICs, and never bring fare-paying passengers along on a formation flight.