A student and I were practicing ground reference maneuvers near our local airport, minding our own business when I caught sight of some traffic approaching our area. I advised the student to turn to the South to get clear of the area so we could get back to maneuvers.
The other aircraft got closer and I recognized it as a Yak trainer out of one of the T-hangars from our airport.
As we turned South the Yak followed. Assuming this was coincidence I advised the student to make a 180 that would put us out of the way of the other plane.
Looking back I saw him working his way into formation with us. After a few more turns it was clear that he wanted to so some unarranged formation flying.
Flight instruction is not all fun and games, and we had work to do, so I called him on the radio, assuming he would still be on the CTAF of our local airport. I asked him to not follow us as we planned to do some maneuvers. He politely accepted and flew away. It would have been fun to fly in formation with this other plane, but was it legal? See below.
Formation Flying Regulations
§91.111 Operating near other aircraft.
(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.
(c) No person may operate an aircraft, carrying passengers for hire, in formation flight.
The short answer is, yes, formation flying is legal. However, there are several things to consider here. First, look at 91.111(b) above. In order to begin formation flying, you must have an “arrangement” in advance with each pilot.
Note that planes flying in formation with a lead aircraft are also flying in formation with each other, so really all of the pilots involved need to meet together at the same time to ensure that everybody is on the same page and has an “arrangement” with every other pilot.
91.111(a) is another interesting clause because it essentially washes the FAAs hands of any formation flying mishaps or problems. Just because you have made an arrangement to fly formation doesn’t mean you can do whatever you want. You have a responsibility to stay far enough away to avoid a collision, but you also must be far enough away to avoid a collision hazard!
By now you can see that obviously, it was not legal for the Yak to enter formation flight with my plane. We did not arrange it in advance and he did not call me on the radio to let me know he wanted to fly formation, even though that might be a sufficient arrangement in advance.
If someone ever gets in formation with you, do what you can to get away from them until you have time to speak to them and work out the details.