MSN, c/n and line number: understanding aircraft serial numbers

You stand at the fence, note down a registration – and two years later the same aircraft shows up again with a different registration, a new livery and at a different airline. Whether you have seen it before is then revealed by only one number: the manufacturer’s serial number. In databases it is sometimes called MSN, sometimes c/n, and next to it there is often a line number as well. This guide explains what the three terms mean, how Airbus and Boeing count their aircraft and how to use the numbers in practice when spotting and keeping your collection.

MSN, c/n and line number: the short answer

The MSN (Manufacturer Serial Number) is the serial number that the manufacturer assigns once to an airframe. It never changes. The c/n (Construction Number) is the older term for the same idea and in practice almost always means the same number. The line number (L/N) is a second, independent count: it indicates which aircraft of a program the machine was on the production line.

The registration, by contrast, is a registration number from the state in which the aircraft is registered. How it is structured is explained in the basics article Understanding aircraft registrations. For you as a spotter, the simple rule applies: The registration tells you where an aircraft is currently registered. The serial number tells you which aircraft it is.

The three terms in detail

MSN – Manufacturer Serial Number

The MSN is the number under which an aircraft is tracked at the manufacturer, at the registration authority and in all maintenance records. Airworthiness directives, repairs, conversions and modifications refer to the serial number, not the registration. That is why an aircraft’s history can be followed without gaps over decades, even if it had five different registrations and three airlines in between.

Airbus uses the term MSN officially too and speaks, for example, of “MSN 1” or “MSN 2” for test aircraft. That makes the number especially present among Airbus fans.

c/n – Construction Number

Construction number literally means build number. The term dates from a time when many smaller manufacturers simply counted their aircraft consecutively, and it has persisted among spotters, in historical registers and in many databases. In practice, c/n, MSN and the S/N (Serial Number) common in registers are interchangeable. In German-speaking countries you also encounter the term Werknummer (works number).

L/N – Line number

The line number counts the aircraft of a program in the order in which they pass through the production line. It is therefore the most honest indication of when an aircraft was built – at least relative to its siblings. For a program like the Boeing 747, the line number yields a continuous list from the prototype to the last aircraft.

Term	Long form	Who assigns it?	Does it change?	What is it useful for?
MSN	Manufacturer Serial Number	Manufacturer	never	unique identity of the airframe
c/n	Construction Number	Manufacturer	never	in practice usually identical to the MSN
L/N	Line Number	Manufacturer, per program	never	build position and rough build time
Registration	Registration	The state’s aviation authority	may change with a change of register or owner	current registration, readable on the fuselage
Hex code	24-bit address	State, tied to the registration	usually with the registration	identification in the ADS-B data stream

The last row shows a common mix-up: the transponder code is tied not to the airframe but to the registration. How exactly this works is covered in the article on the ICAO 24-bit address.

How Airbus counts

At Airbus, in everyday life you practically only encounter the MSN. A separately published line number plays hardly any role, unlike at Boeing; databases usually enter only the MSN for Airbus aircraft.

Counting is done per program family. The A320 family has its own series, as do the A350 and the A380. The closely related A330 and A340 ran through the same line and share a common number sequence – how to tell the two apart visually is shown in A330 or A340?. You can also see this in special cases: the first BelugaXL is based on an A330 and carries an MSN from this series.

Because the A320 family is now assembled at several final assembly lines – in Toulouse, in Hamburg-Finkenwerder, in Tianjin and in Mobile (Alabama) – deliveries do not strictly follow MSN order. A higher MSN thus roughly means “built later”, but not necessarily “delivered later”. For a rough age estimate, that is often still enough.

Interesting for spotters are the low MSNs of the test fleets. The A350 prototype with the registration F-WXWB took off on its first flight in Toulouse on June 14, 2013, and the individual test aircraft are regularly referred to only by their MSN in Airbus communications. If you spot at Finkenwerder, you also see the second reason why the MSN is so important: the factory registrations are reassigned again and again. How to follow such factory flights live is explained in Recognizing test flights and factory flights.

How Boeing counts

At Boeing there are two numbers, and that is exactly what causes confusion for beginners.

• The serial number comes from a company-wide number range. Boeing often assigns it as soon as a customer orders aircraft, not only at build. As a result, serial numbers that are close together can have been built years apart – and conversely, two aircraft that roll off the line right after each other can carry completely different serial numbers.
• The line number, by contrast, counts strictly per program in production order. A Boeing 737-800 with a low line number was built earlier than one with a higher number – regardless of when the serial number was assigned.

To estimate the age of a Boeing, you therefore do better to look at the line number than at the serial number. The 747 shows how handy this count is: a total of 1,574 were built, and the last one, a 747-8F for Atlas Air, rolled out of the hall in December 2022 and was delivered on January 31, 2023.

The customer code: the riddle in “737-8AS”

If you read older Boeing data sheets, you come across designations like 737-8AS or 737-330. The last two characters are a customer code: for decades, Boeing appended an abbreviation for the launch customer to the type designation. “AS” stands for Ryanair, “30” for Lufthansa. The code stayed with the aircraft even when it was sold later – a 737-8AS remains a 737-8AS, even if it flies today for a completely different airline. For spotters, this is a nice clue to an aircraft’s origin.

In 2016, Boeing discontinued the system. Newer aircraft carry only the general type designation, and the 787, the 737 MAX and the 777X never received customer codes. If you are interested in the type designations themselves, the overview of Boeing as a manufacturer helps.

And the other manufacturers?

Every manufacturer has its own system. Some count across all types, others per series, still others put a type abbreviation in front of the number, so that you can tell from the serial number alone which family an aircraft belongs to. With older manufacturers such as McDonnell Douglas, you also find two numbers side by side in databases, similar to Boeing.

The rule of thumb for all of them still applies: the serial number is only unique together with the type. Two different manufacturers may well have assigned the same number, and even within one manufacturer there are parallel series. Therefore always note type plus serial number, never the number alone.

Where is the serial number on the aircraft?

Every registered aircraft carries a fireproof data plate. The US regulation 14 CFR Part 45, for example, stipulates that it must show, among other things, the manufacturer, model designation and serial number, and that the plate is attached so that it survives an accident. For commercial aircraft, the rule allows an accessible location near a door. Comparable requirements apply in Europe.

For you at the fence, this means: you practically never see the serial number directly. For exhibition aircraft at airshows or during an apron tour there may be exceptions, but in everyday life you look the number up via the registration.

How to find out the serial number

The route almost always leads via the registration that you photographed or noted.

How to turn this data into a complete life story is described in the article Researching an aircraft’s life story. And if you are mainly interested in how old an aircraft is, continue directly with Finding out an aircraft’s age.

Why spotters need the serial number

For pure photography, the registration is enough. As soon as you keep a collection, though, you quickly run into cases that only work out cleanly with the serial number.

• Re-registrations: When an aircraft changes country or owner, it often gets a new registration. Without the serial number, you would count it as two different aircraft. Background is in the article on re-registrations.
• Leasing and offshore registers: Many aircraft are registered in Ireland, Bermuda or Malta and often change registration when passing to a new lessee. Why that is so is explained in Offshore registrations and leasing.
• Reused registrations: Some registers reassign a registration when the old aircraft has left service. A photo from 2005 and one from 2025 with the same registration may then show two completely different aircraft.
• Test flights: Factory registrations move from new build to new build, as described above for Airbus.
• Storage and scrapping: At aircraft graveyards, aircraft often stand without paint or with a taped-over registration. Databases resolve which airframe is standing there via the serial number – more on this in Aircraft graveyards and boneyards.

Serial numbers in your own log

How exactly you build the number into your collection depends on your system. A simple scheme has proven itself:

• Note the registration as you saw it. The sighting is a snapshot – the aircraft was operating with exactly this registration that day.
• Add serial number and type as a fixed identity. This links all sightings of the same airframe, no matter under which registration.
• Add the line number for Boeing if you are interested in build time or production history.
• Record previous registrations in the note when you notice a change.

You will find more ideas for a structured logbook in Documenting sightings, and if you collect registrations specifically, Collecting registrations gives you tips for handling special cases.

Common misconceptions

• “The serial number shows the year of manufacture.” Only to a degree. At Airbus it is a rough indicator; at Boeing, because it is assigned at order, it can be significantly misleading. Reliable are first flight and delivery dates.
• “Line number and serial number are the same thing.” At Boeing, expressly not. At Airbus you usually find only the MSN; databases sometimes write it in both fields.
• “New registration, new aircraft.” Wrong – that is exactly what the serial number is for.
• “The hex code belongs to the aircraft.” It belongs to the registration and usually changes with it. It is therefore no good for the unique identity of the airframe.
• “The number alone is enough.” Without manufacturer and type, a serial number is not unique.

Conclusion

MSN, c/n and line number sound like database trivia, but they solve one of the most annoying problems in collecting: registrations are fleeting. The manufacturer serial number, by contrast, stays the same from the first rollout to scrapping. At Airbus the MSN is usually enough for you; at Boeing it is worth looking at both numbers – the serial number as identity, the line number as build position. If you note both for your important sightings, you keep track even after lessee changes, re-registrations and test flights. For a look at the programs behind the numbers, see the profiles of Airbus and Boeing.