distance

[zinman48]

What is the minimum distance that aircraft must maintain during flight?

[1739T]

Section 91.119: Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:

(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.

(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.

(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

(d) Helicopters. Helicopters may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section if the operation is conducted without hazard to persons or property on the surface. In addition, each person operating a helicopter shall comply with any routes or altitudes specifically prescribed for helicopters by the Administrator.

[Wildcat81]

I believe that zinman48 is refering to minimum distances between two aircraft not minumum safe altitudes.

zinman48 The only regulation that I am aware of that refers to your question is 91.111.
(a)Not so close as to create a collision hazard.
However, for formation flight all is required is arrangement between the PIC's and no passengers on board for hire.

Hope this is what you were looking for!