The construction number (cn), also known as manufacturer's serial number (msn), is the permanent identity of an aircraft assigned by the manufacturer. The cn can be found on the cn plate somewhere in or on the aircraft. Except for some total rebuilds the cn is normally not changed during the lifetime of an aircraft, in contrast with the registration. For establishing the history of a particular aircraft through its multiple registrations it is of the utmost importance to know the cn.
For the format of the cn you can consult entries that are already in our database, or you can list them as published in national registers or established reference sources. However, these sources are not always consistent, and the database editors will change them to a standard form if necessary. In the future a Help file will be made listing the standard forms for selected aircraft types.
For some airliners like Boeing and Douglas types, the line number (ln) shall also be included. These line numbers are assigned by the manufacturer to indicate the position of the aircraft on the production line, while construction numbers for these aircraft are normally assigned when a batch of aircraft is ordered, the cns are rarely reflecting the order of production. However, the line numbers are not an official part of the cn, and therefore they are not listed in national registers. When listing a line number use the form cn/ln, as e.g. 43567/234.
Do not put any words in this field like cn, ln, or msn, except for the following: many military aircraft do not have a cn or the cn is unknown. Especially US and British military aircraft often use their military serial number (registration) as their permanent identity. When these aircraft become civil registered the former military serial is often used as the "cn". When the cn is really unknown, these aircraft shall be listed with the serial as cn, but preceded by "sn", as e.g. sn58-4285, or snPP235. And when you are a seasoned aircraft historian and you know that the cn hasn't been discovered by serious researchers, and the aircraft has no US or British military identity, you may insert "unknown".
Filling-in of this field is optional, but it is really appreciated when you do an effort to find the cn and list it, as the cn is a great tool for cross-referencing and for historical research. When you don't know the cn, leave the field blank.