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Tales from the DAR Side

Joe Norris

In last month’s column I talked about aircraft identification plates, which we commonly call the “data plate.” I received a few questions from readers that I’d like to address before moving on to this month’s topic. First, a member asked if the info in my column applies to experimental light-sport aircraft (E-LSA). Yes, the requirements for an E-LSA data plate are as outlined in my column. Another member asked whether or not he could make his own data plate rather than buying one. Again the answer is yes. As long as the material you use is “fireproof equivalent to steel,” you can make your own. It doesn’t have to be fancy, and it doesn’t have to include the words “builder,” “model,” or “serial number.” It simply has to contain the actual identification data permanently marked on the plate by stamping or engraving.

This month I’d like to touch on another subject that causes almost as much confusion as data plates - the registration marks (which we commonly refer to as “N numbers”). The regulations found in 14 CFR Part 45 clearly define how the N numbers are to be applied to the aircraft. The number size, thickness, spacing, and location are all called out. Just about everyone gets the height correct, but that’s as far as it goes. Thickness, spacing, and location are the areas where applicants run into problems. Let’s take a “plain language” look at the requirements.

First, the easy one: height. 14 CFR 45.29 is where we find the requirements. For amateur-built aircraft and E-LSA, the minimum height is three inches. However, if your amateur-built aircraft has a maximum cruising speed of greater than 180 knots, your N numbers must be at least 12 inches high. There are special rules for aircraft 30 years old or older and replicas that I’ll talk about a bit later. But height is only part of the story. §45.29 goes on to call out other requirements for the layout of the N number. Let’s take a look!

All requirements for the layout of the N number, such as width, thickness, and spacing, are based upon the height. For example, the overall width of each character in the N number is required to be two-thirds of the height. So for a three-inch-high N number, the width of the individual characters must be two inches. The only exceptions to this are the number 1, which must be one-sixth of the height (half an inch for a three-inch-tall N number), and the letters M and W, which are allowed to be as wide as they are high. These requirements catch a few builders, but not nearly as many as the thickness and spacing requirements. Let’s talk first about thickness.

When the regulations mention thickness in relation to N numbers they are referring to the thickness of the lines that make up the characters, sometimes referred to as the “stroke.” If you don’t want your FAA Inspector or Designated Airworthiness Representative to have a stroke when he or she comes to perform your inspection, you need to make sure the thickness of your N number is at least one-sixth of the height! That means that the thickness of the lines making up the characters of a three-inch-high N number must be at least a half inch wide.

Now let’s talk about the one place where most builders run into trouble - spacing. Even builders who get all the other stuff correct; more often than not they don’t have sufficient spacing between the characters of the N number. Regulations being what they are, they lead you down a bit of a winding path to get to the spacing requirement. §45.29(e) tells us that the spacing must be one-fourth of the character width, which in turn was based on character height as discussed above. On our three-inch N number, we determined above that the width of the characters has to be two inches, so the spacing must be one-fourth of two inches, or half an inch. That number should look familiar because it’s the same dimension as the thickness of the characters, which was required to be one-sixth the height! Seems as though they could have made it simpler by combining these two requirements and stating that each should be one-sixth the height, but they chose to make it a bit more complicated. Such are the regulations!

Special Rules for Older Aircraft and Replicas
The rules are found in 14 CFR 45.22(b), which allows any small U.S. aircraft that is at least 30 years old to display N numbers at least two inches high. So if you’re restoring an older homebuilt (or a standard category airplane) that is at least 30 years old, you can use two inches as your minimum height instead of three or 12 as discussed above. Further, §45.22 allows amateur-built aircraft that are replicas of a small U.S. aircraft at least 30 years old to also use two inches as the minimum height. Now, the FAA considers a “replica” to be a full-size reproduction of a preexisting aircraft, so this doesn’t work if you scale the aircraft up or down from the original configuration, or design something that resembles an antique like the Flitzer. What this regulation does allow is the use of two-inch-high N numbers on a number of classic designs that have been around for 30 years or more. I can’t name them all here, but examples that would fit this regulation include Cub replicas, Pietenpols, the Baby Ace, and many others. Even some designs that we think of as “modern” homebuilts, like the RV-3 and RV-4, now qualify for two-inch N numbers!

Now that we know the basic dimensions of the N numbers, we need to talk about where to locate them on the aircraft. That will be the subject of next month’s column. Stay tuned!


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