The N-Numbers Game: Doing It Right

For decades I worried Wisconsin’s annual vehicle registration sticker would clash with my sports car’s license plate. Worse, what if new, green (green!) plates were issued? They’d clash with my pimento-pigmented car. I don’t worry anymore, having switched to collector plates, which never change. However, that decision was years in the making because Wisconsin’s collector plate is blue-ish with purple-ish numbers. (I’m aware I’m privileged to live a life where such trivial things are a concern.) At least my homebuilt Sonex got to wear a registration number (N-number) in a color of my choosing; a little black number with red accents. It looked good on its tail, though it wasn’t proper—in more ways than I knew.

Turns out, many (I’ll even say, most) N-numbers don’t conform to the FARs that define them. Beyond those that fail at first glance, many more don’t hold up to scrutiny. Most embrace the spirit of the FARs, but few survive a forensic examination.

Digging In

The FARs that define registration numbers (sometimes in excruciating detail, sometimes with confusing vagary, and often with a degree of disorganization) encompass the full spectrum of aviation. With this article I try to plain-language acquiring, formatting, and affixing a registration number for a “typical” Experimental/Amateur-Built (E-AB) category aircraft. I’ve separated the process into three categories:

• The acceptable combinations of numerals and letters that comprise an N-number.
• The acceptable height, proportions, style, and contrast of the characters that make up an N-number.
• Proper placement of an N-number on an airframe.

If your project is anything other than a common Cloud Climber or Sod Statue, or must be licensed outside the E-AB category, dig into the documents listed in the sidebar “The FARs by the Numbers” with both eyes (and a highlighter).

Choosing and Reserving an N-Number

Choosing an N-number is the one category where a deviation—intentional or unintentional—cannot be made. The FAA won’t issue a number that doesn’t conform to the regulations. Only you can screw this step up by choosing a number that proves cumbersome to communicate. I’ll explain …

I chose my N-number based on two—no, three—factors. I forgot the third so I’ll touch on the two I recall. It was popular in the Sonex community to use the aircraft’s serial number followed by “SX.” My serial number pointed me toward N9SX. That number was already assigned to a Swearingen SX-300, so I had to consider options. The Sonex factory aircraft all wore a variation of “2SX,” so I decided I’d tack “12” between my “9” and “SX.” Thus N912SX came to be, and another “2SX” entered the Sonex factory hangar.

I found two downsides to that number. The first was people interpreted it to mean my aircraft was powered by a Rotax 912 (it had a Jabiru 3300). At that time the Rotax 912 wasn’t a Sonex-supported engine option. That became a recurring topic in my conversations with Sonex customers. Far worse, however, was clearly enunciating the number. “Nine One Two Sierra X-Ray” quickly slurred into “Nine One Two See X-Ray,” “Two-See-X-Ray,” or, by the seventh touch and go, “Twosee X-Ray.”

“Aircraft calling tower, please repeat your N-number.”

When noodling your N-number options, be sure to say each out loud a number of times before committing to one. Luckily for you, N999NN is already assigned.

Here are the acceptable formats for an N-number:

• The first character (after the “N”) cannot be a zero (0) or a letter.
• The last character may not be the letter “I” or “O,” to avoid confusion with the numbers one and zero.
• The N-number may consist of one to five numbers (e.g., N1, N12, N123, N1234, N12345).
• The N-number may consist of one to four numbers plus a letter (e.g., N1A, N12A, N123A, N1234A).
• The N-number may consist of one to three numbers plus two letters (e.g., N1AB, N12AB, N123AB).

You can reserve an N-number through the FAA’s website for $10 per year, even if you’re only dreaming of building. (But please don’t. Give someone who is closer to flight as many choices as possible.) If your desired N-number is reserved, the reservation’s expiration date is shown. If it’s not renewed by that date, the number becomes available at 12 a.m. Central Time the following day. Good luck!

N-Number Physical Characteristics

FAR Part 45, Subpart C, 45.29 Size of Marks, sharply defines the acceptable characteristics for an N-number’s style and proportions, and breezily mentions that it must contrast with its background. It is in these arenas many N-numbers fall short of the standards.

Character Style

FAR 45.23 (a) Display of marks; general, states “an aircraft must display … marks consisting of the Roman capital letter “N” … followed by the registration number of the aircraft. Each suffix letter … must also be a Roman capital letter.” Advisory Circular 45-2E expands, noting the characters be “written in capital Roman style letters without any ornamentation.”

Paradoxically, “Roman” letters have serifs, which could be considered ornamentation. Roman letters are also comprised of narrow strokes, which are counter to the FAR’s defined proportions for the characters. It would be more accurate to define the approved letter style as sans serif, or “block” lettering.

I had to call a trusted expert to be directed to the language that allows slanted (italic) N-numbers. He pointed me to Order 8130.2K, Airworthiness Certification of Aircraft, where the five words “the font may be slanted” reside. Specifically, Order 8130.2K (2-3) (e.) (1) (a) states, “When verifying an N-number is the proper … height … the marks are measured vertically. The font may be slanted, but do not use the length of the slant line to determine the height of the numbers.” Nothing comes easy.

Typefaces With FAR-Conforming Proportions

I hoped to identify specific, common typefaces which meet the FAR’s requirements. Out of a library of hundreds, I only found two, and they fall short, literally. They are:

• Arial Narrow, bold
• Helvetica New, condensed bold

Both of those typefaces use numerals that are shorter than the letters, which violates the requirement that all characters be uniform in height. To be correct, the numerals need to be set in a larger size to match the height of the letters. I’d share how much larger, but the numerals within each typeface also vary in height from one another.

The characters also do not space correctly for an N-number when typed consecutively in a word processing or graphic design program. If you have your N-number custom cut with vinyl letters, they will need to be spaced manually, either on the computer before the number is cut (recommended) or by placing each letter on the aircraft individually (which is cumbersome).

I noticed that the registration numbers on aircraft prior to the 1970s seemed to adhere to the FARs far more than aircraft that came later. That may be because those numbers had to be laid out by hand (drafted, if you will) before they were painted or stenciled on the aircraft. As such, the FAA’s defined proportions were easier to match.

Height and Proportions

Most Experimental/Amateur-Built category aircraft can be fitted with a 3-inch-tall N-number. Here I paraphrase FAR 45.29, Size of marks, though I retain as much original text as possible.

The [height of the] … registration marks must be … at least 3 inches high on an aircraft for which the FAA has issued an experimental airworthiness certificate … to operate as … an amateur-built aircraft … when the maximum cruising speed of the aircraft does not exceed 180 knots (207 mph) CAS.

Note the mention of maximum cruising speed. Twelve-inch-tall numbers are required on aircraft that exceed 180 knots (207 mph) CAS in maximum cruise. Twelve-inch-tall numbers are also required on aircraft penetrating an ADIZ or DEWIZ. (Though N-numbers must be affixed in a permanent manner, aircraft with a 3-inch number may wear temporary 12-inch numbers to penetrate an ADIZ or DEWIZ.)

An N-number’s dimensional characteristics are tightly defined by ratios. Each ratio is based on the height of the N-number. The ratios are the same whether the N-number is 3 inches or 12 inches tall.

Character Width. Characters must be two-thirds as wide as they are high, except the number “1” and letter “I,” which must be one-sixth as wide as it is high. The letters “M” and “W” may be as wide as they are high.

Translation: For a 3-inch N-number, the “N” must be 2 inches wide. The number “1” and letter “I” must be a half-inch wide. The letters “M” and “W” may be 3 inches wide. All other letters and numbers must be 2 inches wide.

Stroke Thickness. Characters must be formed by solid lines (strokes) one-sixth as thick (wide) as the character is high.

Translation: A 3-inch-tall letter must have half-inch-thick strokes.

Character Spacing. The space between each character may not be less than one-fourth of the character width.

Translation: Three-inch-tall characters must be spaced a half-inch apart.

Character Uniformity. The characters that comprise an N-number must be the same height, width, thickness, and spacing on both sides of the aircraft.

Translation: Get two identical N-numbers whose characters are all the same height and put one on each side of your airplane. If your N-number slants, it’s typical for it to slant aft on both sides.

On occasion, I have seen the characters of an N-number separated by a dash (e.g., N-12345) or the “N” separated from the remainder of the number, with a space or with the “N” above the remainder of the number. While I couldn’t find any words in the regulations that expressly prohibit that, there is enough other verbiage to give an inspector cause to deny separating the characters of the registration number in any way. Advisory Circular 45-2E 7.1.1 states, “You may not place any text or graphic between any of the letters or numbers in the N-number unless the aircraft is an antique aircraft.”

Contrast (Color)

FAR 45.21, General, states the registration number’s color must contrast with the background and be legible. It allows the use of shading or a border only if it makes the number more legible. Achieving this can require creativity when the N-number falls across stripes, swoops, checkers, or elaborate, airbrushed designs. An easy way to handle that is to interrupt the graphic so the N-number sits on a solid background color.

Beyond the requirement that a number contrast with its background, nothing within Part 45 defines “contrast.” However, AC 45-2E, 7.3.2, N-number Legibility, includes this summation: “The FAA considers a 12-inch number legible if you can read it from 500 feet away, in a horizontal line, and perpendicular to the side of the aircraft during daylight hours, without using an optical aid such as binoculars.” By extrapolation, it can be assumed a 3-inch number must be legible from 125 feet away.

Advisory Circular 45-2E also references 14 CFR 25.811, Emergency Exit Marking, which quantitatively defines what “contrast” means to the FAA. I’d be surprised if any homebuilder has ever gone that deep, but it’s comforting to know emergency exit markings have to meet a specific contrast value. What a fun icebreaker on your next commercial flight. “Did you know the luminous flux … “

Placement on the Airframe

Temporary 12-inch numbers to penetrate an ADIZ or DEWIZ notwithstanding, an N-number must be permanently affixed to the airframe. Vinyl graphics are considered permanent; tape is not.

The approved locations for an N-number are clearly defined, yet frequently violated. In brief, the registration number must be displayed on the vertical tail or the sides of the fuselage, between the leading edge of the horizontal stabilizer and the trailing edge of the wing. Now for the details.

If placed on the vertical tail, the numbers may be horizontal or vertical, but they must be the same on each side. On aircraft with a multi-surface vertical tail (think Ercoupe), the number is affixed to the tail’s outside surface on each side of the airframe.

Canards and V- or T-tail aircraft are not specifically addressed in the FARs. Canard-configured aircraft (VariEzes, Cozys, and whatnot), which lack a vertical tail altogether, often wear their number on the outside of each winglet, either vertically or horizontally, as space permits. Y- and V-tail aircraft have no true “vertical” tail; therefore, the N-number must be placed on the fuselage.

Registration numbers on the fuselage must be fully forward of the leading edge of the horizontal stabilizer and fully aft of the wing’s trailing edge. Simple enough, yet that is where a high percentage of aircraft fall into violation.

Canards throw us a wrinkle. It’s safe to substitute “canard” for “horizontal stabilizer” and conclude an N-number placed on the fuselage of a canard-configured aircraft must be forward of the wing’s leading edge and aft of the canard’s trailing edge.

If you have a unique situation (such as a canard that requires 12-inch numbers but lacks room on its winglets or fuselage), give your FSDO a call to discuss your options. Get their guidance in writing.

Licensed Does Not Make It Legal. Illegal Does Not Make It … Illegal?

Casual observation reveals a high percentage of aircraft wear nonconforming N-numbers. Infractions include numbers that are too frilly, improperly positioned, improperly proportioned, lacking in contrast, faded or failing, and purposely camouflaged. Still, I believe the majority of N-numbers embrace the spirit of the FARs, if not the substance.

It’s hard, I know, to compromise your vision of how you want to present your aircraft to the world. When I applied my N-number, I knew the “SX” was nonconforming. I didn’t know the “N912” was also nonconforming. My airworthiness certificate did not make it legal; it simply meant the inspector didn’t look at the number or didn’t care.

It’s my opinion that somewhere between the stark, black letters on white paper that define the stark look of FAA-authorized N-numbers and the, shall we say, “creative” styles some builders employ to make their N-number vanish, should lie a common-sense compromise. Based on the large population of nonconforming N-numbers, licensing authorities may feel the same, turning a blind eye to even the most egregious violations. Until the regulations are rewritten, however, registration numbers that don’t comply can delay your airworthiness certificate or ground your airplane until they are corrected. (As an aside, while writing this article, I was ticketed for violating a parking limit near my coffee shop. I’ve done it for well over a year—as do many others—but that day it was enforced.) That’s something to think about before incorporating an expensive, airbrushed N-number into an elaborate paint scheme.

Earning the “X”

There’s no question the old-timey “NX” is a romantic nod to the Golden Age of aviation. Advisory Circular 45-2E, 7.1.2, Placing Other Text or Graphics between the Two Parts of an N-Number, gives hope to those wanting to punctuate their Pietenpol with an eX-y N-number. Paraphrasing its content, it asserts you may insert the symbol “X” (for “experimental”) between the “N” and the registration number (e.g., NX1234) on an amateur-built aircraft with the same external configuration as an aircraft built at least 30 years ago. Contact your Flight Standards District Office (FSDO) before you commit to an “X.”