Boeing 737-300SF/-400SF

Updated: October 2026 – It is just after midnight, the widebody freighters stand in a row on the apron in Leipzig, and between them rolls a small 737 without a single window. A large door is outlined at the front left, and the engine inlets are flat on the bottom: that is a Boeing 737-300SF or -400SF, a passenger jet of the 1980s and 1990s in its second life. Here you learn how leisure and scheduled airliners became parcel freighters, what is behind the abbreviations SF, BDSF and QC, which airlines fly the aircraft in Europe and how to reliably tell the -300 and -400 apart.

What are the 737-300SF and 737-400SF?

Boeing never built the 737-300 and -400 as freighters at the factory. Both belong to the 737 Classic, the second generation of the 737 with CFM56-3 engines. 1,113 of the 737-300 were built, and 486 of the stretched 737-400. Most flew passengers for decades, in Europe mainly with leisure airlines and network carriers.

When the Next Generation and later the MAX pushed the Classic out of passenger fleets, hundreds of still usable airframes were available. For express and mail traffic on short and medium routes, their size fits well: enough room for a night’s load of parcels between two sorting centers, but not so big that a widebody freighter would pay off. Specialized shops have therefore converted many -300s and -400s into freighters.

The “SF” stands for Special Freighter. The designation is not an official Boeing name, though, but depends on the particular conversion program. Depending on the converter and source, you read:

• 737-300SF / 737-400SF – conversions by Aeronautical Engineers Inc. (AEI) from the USA
• 737-300BDSF / 737-400BDSF – conversions by Bedek, the maintenance and conversion arm of Israel Aerospace Industries (IAI)
• 737-300F / 737-400F – generic designation in many fleet lists, without reference to the converter
• 737-300QC – Quick Change versions whose cabin can switch between cargo and passengers

Pemco in the USA has also converted Classic 737s. For you as a spotter, the exact designation is secondary: from the outside the programs look very similar, and only the fleet list for the registration settles the difference.

How a passenger jet becomes a freighter

A freighter conversion is more than removing seats. The airframe gets a new job, and structure, systems and interior have to be adapted for it. The basic steps are similar at all converters.

At IAI Bedek, a 737 conversion takes about 90 days according to common figures. One advantage of converted freighters is obvious: the development costs of the base type were paid long ago, and used airframes were available in large numbers and cheaply. A used 737-400 could be had for about two to three million US dollars as early as 2018 – before conversion. The downside: the aircraft bring the technology of their generation and burn more than younger types.

A pioneer was Alaska Airlines: it was the first airline to convert a 737-400 into a freighter with room for ten pallets and later turned more -400s into combis with cargo in front and passengers in back. A financing from November 2018 shows how widespread the -400 freighters became afterward: at that time, VX Capital brought a security to market that was backed by 35 leased 737-400 freighters.

Recognition: Classic freighters in three steps

The good news: the Classic freighters are among the types you can call quite reliably even at night and from a distance. The bad news: among the freighters, a -300SF looks quite similar to a -400SF, and the 737-800 conversions are windowless too.

Why the CFM56-3 inlets look so flat is explained in the article How to recognize engines: the large engine had to fit under the 737’s low wing, so accessories moved to the side and the underside of the inlet was flattened. You get an overview of all generations in How to tell the Boeing 737 Original, Classic, NG and MAX apart.

Feature	737-300SF	737-400SF	737-800BCF/SF
ICAO type designator	B733	B734	B738
Length	about 33.4 m	about 36.4 m	about 39.5 m
Engine	CFM56-3	CFM56-3	CFM56-7B
Inlet	flat on the bottom	flat on the bottom	larger, also slightly flattened
Winglets	none from the factory	none from the factory	often fitted
Tail skid	no	yes	yes

You know the length of the 737-800 from the passenger version. More about the younger competition in the freight business is on the page for the 737-800BCF.

The operators in Europe

In Europe, the Classic freighters almost always fly for parcel services and postal companies. The airlines often do not appear on the market under their own name but fly with aircraft and crew on behalf of an integrator. How this business works is explained in the article Express integrators DHL, FedEx and UPS. The following details are based on fleet lists from 2024 to 2026 – numbers change quickly.

Swiftair

Founded in 1986, Swiftair is based at Madrid-Barajas Airport and is one of the most important partners of DHL Aviation. According to fleet lists as of August 2025, it had six 737-400SFs, some of which flew for DHL. Several 737-300BDSFs were also in service earlier and were retired by 2016. Meanwhile, 737-800 freighters complement the fleet.

The Swiftair aircraft EC-MFE gained sad notoriety: on November 25, 2024, the 737-400SF crashed onto a house just short of the runway on approach to Vilnius Airport, about 1.3 kilometers north of the airport. It had taken off from Leipzig/Halle on behalf of European Air Transport Leipzig. The captain was killed, and the other three crew members were injured.

ASL Airlines group

The Irish holding company ASL Aviation Holdings bundles several cargo airlines, all of which have experience with the Classic:

• ASL Airlines Belgium, the former TNT Airways, is based at Liège Airport. Back in TNT days, the 737-300 joined the fleet from about 2003. According to fleet lists, -300BDSFs, -400BDSFs and -400SFs flew later; some of the -400SFs are still active.
• ASL Airlines Ireland, the former Air Contractors, lists hubs such as Dublin, East Midlands, Paris Charles de Gaulle and Shannon. The 737-300 left the fleet in 2019, and according to current lists one 737-400SF is still in it. Customers in the European network include DHL, FedEx and Amazon.
• ASL Airlines France is the successor of Europe Airpost. It flew mail for La Poste at night and passengers during the day – exactly what the QC versions of the 737-300, whose cabin could be reconfigured, were suited for. 737-400SFs were also part of the fleet.

In 2021 and 2022, the group ordered up to 40 737-800BCFs. That shows where things are heading: the Classic is being replaced step by step.

West Atlantic

West Atlantic is headquartered in Malmö and operates a large part of its fleet through West Atlantic UK from East Midlands and Edinburgh. According to its own statements, the company targets express and postal services; its customer list includes Royal Mail and DHL, among others. According to a fleet list as of September 2024, it flew six 737-400Fs and one more 737-300F.

On January 19, 2021, a West Atlantic 737-400F touched down so hard in Exeter that the fuselage deformed. The aircraft was written off and dismantled on site.

Operator	Country	Classic freighters (selection)	Typical use
Swiftair	Spain	737-400SF	DHL network, Madrid and Leipzig/Halle
ASL Airlines Belgium	Belgium	737-400SF	Express network from Liège
ASL Airlines Ireland	Ireland	737-400SF	European express freight
ASL Airlines France	France	737-300QC, 737-400SF	Mail for La Poste
West Atlantic	United Kingdom/Sweden	737-400F	Mail and express traffic from East Midlands

Where and when you can spot them

Freighters follow the rhythm of the sorting centers. Parcels are collected in the evening, handled overnight and delivered in the morning. That is why the Classic freighters land and take off mainly late in the evening, at night and early in the morning. Where night flight restrictions apply, the times shift; what is allowed at German airports is covered in the article on the night flight ban.

You find more about the hub itself on the page for Leipzig/Halle Airport. How to classify freighters in general is explained in How to recognize cargo aircraft, and strategies for rarities are in Spotting rare aircraft.

Because the type codes B733 and B734 also apply to passenger jets, it is best to filter by airline or callsign of the cargo operators in the tracker. How to set up notifications in common services is covered in Setting up alerts for aircraft, types and airlines.

Photo tips for Classic freighters

A windowless fuselage is a rewarding subject because the livery looks uninterrupted. At the same time, you often work in low light.

How to work cleanly at night is shown in the article Photographing aircraft at night.

Common mix-ups

The shortest Classic, the 737-500, was hardly ever converted into a freighter because demand was lacking. A very short, windowless Classic is therefore very likely a -300SF. More about the manufacturer itself is on the page for Boeing, and about the engine on the page of CFM International.

Conclusion

The 737-300SF and -400SF show how long an aircraft can work. First they carried vacationers to Mallorca and business travelers between major cities, then they got a door, a reinforced floor and a new job in the night networks of DHL, La Poste or Royal Mail. In Europe, Swiftair, the ASL group and West Atlantic keep the Classic freighters alive, but the 737-800 conversions are moving up. If you still want to get the flat engines and the windowless fuselage in your frame, you had better not put off the next freighter night for long.