I have often heard that the first three digits are the model. 150/172/182 etc. the next 2 are the year, and the last three are the sequential number off the assembly line.
The model year is not indicated for Cessna single-engine models; after the model number prefix the numbers are sequential (with some exceptions, as noted below, and don't necessarily start at '1'). The last 1977 Skyhawk was 17269309, and the first 1978 model was 17269310. But the system is different for twins.
Says Edward H. Phillips in
Wings of Cessna:
Beginning in 1961 the company initiated use of the particular airplane model number to prefix a series of c/n blocks. For example, the Model 172 c/n became 17248500 or a Model 150 was designated as c/n 15059350. New designs begin with a c/n such as 172RG00001 to designate a new block of constructor numbers. [...] Recognizing the need to differentiate between single and twin-engine c/n blocks, Cessna created a variation of the 1961 method for its multi-engine models. Each model year has its own distinct c/n group, such as c/n 310H0001 to c/n 310H0148 for the 1963 model year and c/n 310R2101 to c/n 310R2140 for 1981. [...] The reader is cautioned that it is often impossible to interpolate when a particular model was built or to determine exact production configuration by examination of of c/n blocks exclusively. There are frequent paperwork changes to c/n groups that shift numbers from one model year to another or even one airplane to another.
The first 172 in 1956 was c/n 28000. They continued sequentially (adding the '172' prefix in 1961), but skipped 30000 to 35999, and 37000 to 46000.
The last "legacy" 172P (1986) was 17276673. When 172R production started in 1997, the first c/n was 17280004.
Piper had a more logical system in the 1970s and early 1980s. Beginning in 1971, the format was the basic model number (
e.g., '28' for PA-28), then a dash, then the last two digits of the year, then an arbitrary two digit code for the specific model (
e.g., '25' meant Cherokee 140), then the sequential number. My old 1977 Cherokee 140 was 28-7725043. Thus it was the 43rd Cherokee 140 off the line in the 1977 model year. The model year designation was deleted in 1986.
There will be a quiz.
