Most commercial aircraft use their main cabin to transport either passengers or cargo. The onset of the coronavirus pandemic saw the lines between these two uses become blurred, as airlines kept otherwise dormant passenger aircraft busy with improvised cargo services. However, carriers used to be able to compromise with so-called ‘Combi’ aircraft.
What are ‘Combi’ aircraft?
So what makes an aircraft fall under the ‘Combi’ category. Simply put, these are planes that split the use of their main cabin between passenger and freight-carrying duties. This allows airlines greater flexibility, particularly on routes with low passenger and high cargo demand. Alaska Airlines used to deploy such aircraft on its multi-stop ‘Milk Run‘ routes.
The Boeing 747 is an example of an aircraft family that has seen several Combi variants. All of these have an ‘M’ suffix to indicate the mixed-use nature of their main deck. The 747-200M was the first jumbo Combi to hit the skies, and the -300M and -400M followed it. In the case of these aircraft, the rearmost section of the main cabin is set aside for cargo.
Such aircraft typically have an additional side cargo door to assist with loading airfreight into this part of the aircraft. Of course, the 747 is not the only Boeing series with Combis. Indeed, this has also been the case for the 727, 737, and 757 families. In terms of other manufacturers, there have also been ATR 42 Combis, to name just one example.
Falling out of fashion
While a wide range of aircraft have had Combi variants, this type of plane has experienced something of a fall from grace in recent years. As can be seen in the graph below, it took less than a decade for the number of active 747-400Ms in the world to fall from nearly 40 to single figures. But why has this been the case for such seemingly useful aircraft?
One key factor is age. In the case of the 747-400M, these aircraft were coming towards the end of their services lives anyway when COVID-19 struck. This prompted their early retirements, although certain KLM examples did briefly return to the skies.
The 727 is also now very rare due to its age, hence these Combis have also vanished. Elsewhere, Boeing only produced one 757-200M. Now more than 30 years old, Nepal Airlines has struggled to sell it. Furthermore, some carriers have reportedly found it to be more profitable to convert their Combis to dedicated passenger or cargo planes.
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So why are manufacturers no longer offering production models as combis? It might have to do with the fact that the loading of both passengers and cargo can be time-consuming when considering aircraft weight and balance.
Some combi aircraft have their cargo sections at the rear of the aircraft. If this section were to be loaded first, a tail stand might be required to prevent the aircraft from tipping back and ‘tail sitting.’ However, a tail stand for larger aircraft is a fairly specialized piece of equipment, not always available.
If passengers were loaded up front first, then they would have to wait for cargo to be loaded before taking off and unloaded first after landing. Ultimately, it seems that airlines and planemakers have decided that it’s better to have an aircraft serve a single purpose during a flight, instead of two.
Not dead yet
It is clear that Combi aircraft no longer play quite such a significant role as they once used to. However, that isn’t to say that this genre of aircraft is entirely dead. Indeed, Simple Flying recently took a look at which airlines are still flying these versatile planes.
Our research had a particular emphasis on Boeing 737 Combis. Overall, we found that 36 of these multi-use twinjets remain in operation today across 13 operators. Of these, the US Navy flies around half. Elsewhere, the 737 Combi continues to ply its trade in locations as widespread as Canada, Congo, the Philippines, and South Africa.
What do you make of ‘Combi’ aircraft? Have you ever flown on one? Let us know your thoughts and experiences in the comments.