"G.R. Patterson III" wrote in message
...

snip The data plate was located inside the aircraft until the early
'90s. Then the feds passed two regulations; one required that all aircraft
have 12"
high N-numbers and the other required an identification plate be added at
the tail.
These were quickly ammended to allow aircraft with smaller numbers to
retain them
until the next paint job, allow some other aircraft (mainly antiques) to
retain the
smaller numbers, and to allow the registration info to be painted on
instead of
having a plate if the aircraft did not originally come with an exterior
plate.



George Patterson

The 12' numbers are for crossing the ADIZ and/or for aircraft with a cruise
speed that is higher than most piston engined aircraft.

Here's the FAR (pulled from the Matronics RV List Archives - a very handy
source of information): http://www.matronics.com/searching/search.html


45.29 Size of marks.
(a) Except as provided in paragraph (f) of this section, each operator of an
aircraft shall display marks on the aircraft meeting the size requirements
of this section.
(b) Height. Except as provided in paragraph (h) of this part, the
nationality and registration marks must be of equal height and on -
(1) Fixed-wing aircraft, must be at least 12 inches high, except that:
(i) An aircraft displaying marks at least 2 inches high before November 1,
1981 and an aircraft manufactured after November 2, 1981, but before January
1, 1983, may display those marks until the aircraft is repainted or the
marks are repainted, restored, or changed;
(ii) Marks at least 3 inches high may be displayed on a glider;

(iii) Marks at least 3 inches high may be displayed on an aircraft for which
an experimental certificate has been issued under 21.191(d) or 21.191(g) for
operating as an exhibition aircraft or as an amateur-built aircraft when the
maximum cruising speed of the aircraft does not exceed 180 knots CAS; and

(iv) Marks may be displayed on an exhibition, antique, or other aircraft in
accordance with 45.22.
(2) Airships, spherical balloons, and nonspherical balloons, must be at
least 3 inches high; and
(3) Rotorcraft, must be at least 12 inches high, except that rotorcraft
displaying before April 18, 1983, marks required by 45.29(b)(3) in effect on
April 17, 1983, and rotorcraft manufactured on or after April 18, 1983, but
before December 31, 1983, may display those marks until the aircraft is
repainted or the marks are repainted, restored, or changed.
(c) Width. Characters must be two-thirds as wide as they are high, except
the number "1", which must be one-sixth as wide as it is high, and the
letters "M" and "W" which may be as wide as they are high.
(d) Thickness. Characters must be formed by solid lines one-sixth as thick
as the character is high.
(e) Spacing. The space between each character may not be less than
one-fourth of the character width.
(f) If either one of the surfaces authorized for displaying required marks
under 45.25 is large enough for display of marks meeting the size
requirements of this section and the other is not, full-size marks shall be
placed on the larger surface. If neither surface is large enough for
full-size marks, marks as large as practicable shall be displayed on the
larger of the two surfaces. If any surface authorized to be marked by 45.27
is not large enough for full-size marks, marks as large as practicable shall
be placed on the largest of the authorized surfaces.
(g) Uniformity. The marks required by this part for fixed-wing aircraft must
have the same height, width, thickness, and spacing on both sides of the
aircraft.
(h) After March 7, 1988, each operator of an aircraft penetrating an ADIZ or
DEWIZ shall display on that aircraft temporary or permanent nationality and
registration marks at least 12 inches high.

Section iii is for you. Unless you intend to fly a ADIZ or DEWIZ then if
your cruising speed is 180 KNOTS CAS you can use the 3". Note! Canada
doesn't count.

KB