In a part 91 formation flight within a mode C veil, are all the aircraft required to have their transponders on? Two aircraft are planning on conducting a standard formation flight (aircraft remaining within 1 mile of each other) as follows in the U.S.:

Maintaining VFR below 10,000 MSL;
Remaining outside/clear of Class B/C/D airspace;
The route will be totally within the 30 mile mode-C veil of an airport listed in appendix D to Part 91, for example - Houston or Atlanta; (FAR 91.215)
FAR 91.111 will be complied with: see below:


§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.


No flight plan will be filed;
No communication with ATC will be established. (takeoff/landing will be at non-towered, uncontrolled airports in Class G airspace);
Both  aircraft will be equipped with a properly certified operable (4096 3/A, w/Mode C) transponder in accordance with FAR 91.215;

Question: Is it legal for only the agreed upon formation leader to have his/her transponder on with the other aircraft's
tansponder set to &quot;standby?&quot; Or do both airplanes have to have their transponders on?
The way 91.215 is written, you must operate your mode C transponder in rule airspace unless ATC tells you not to.

The reason ATC tells you not to squawk in the case of a formation flight is to avoid triggering a constant stream of Collision Alerts between members of the formation, which is useless, annoying and may distract them from an actual problem elsewhere.

However, the CA is suppressed if all the aircraft involved are squawking 1200 (VFR) because there's no point warning the controller about a possible collision between multiple aircraft they're not talking to.