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Even with dedicated cargo aircraft built as cargo aircraft (rather than converted from a passenger-carrying configuration), the cabin is virtually always still divided into an upper and a lower lobe (which would, on a passenger aircraft, be the passenger cabin and the cargo hold, respectively) by a floor located a third to a half of the way from the bottom of the fuselage, drastically reducing the maximum size of the cargo that can be carried and making it much harder to transport round outsize cargo (such as, for instance, large high-bypass turbofan engines as are used on essentially all modern jetliners) by air. Given that the essential structural members of a large modern jet are

  • the skin itself;
  • longitudinal longerons running the length of the fuselage just beneath the skin; and
  • circumferential frames encircling the fuselage just beneath the skin;

it should be possible to remove the floor, and thus dramatically increase the freighter's ability to carry very large objects, without significantly compromising the fuselage's structural integrity (the middle of the cabin would still be partially occluded at the bottom - or the top, or the middle, depending - by the center portion of the wingbox, but there would, nevertheless, be a considerable increase in the outsize-cargo capacity ahead of and behind the wings). Why isn't this done? Is the floor left in to facilitate possible future conversion to a passenger aircraft?

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    $\begingroup$ There are quite a few that have a very "low" floor, remember there is still stuff running through there, hydraulic systems, gear bays, center fuel tanks, fuel transfer lines/pumps, avionics bays, APU compartments, etc. that need to be in the main fuselage as well $\endgroup$ – Ron Beyer Dec 16 '18 at 0:00
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    $\begingroup$ How do you load general cargo on a round floor? Even the Beluga pictured below, though it's carrying round fuselage sections, has a flat floor for the cradle's wheels to roll on. $\endgroup$ – jamesqf Dec 16 '18 at 18:42
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Because most cargo aircraft are derived from passenger aircraft. All dedicated cargo designs have no extra floor: They carry the payload on the fuselage bottom structure.

Comparison of transport aircraft cross sections aft of wing drawn to the same scale

Comparison of transport aircraft cross sections aft of wing drawn to the same scale (picture source). The C-5 and C-17 are dedicated military cargo designs and their round bottom is not only for aerodynamics, but also to fair the structure needed for carrying the cargo floor bending moments. The Boeing 747-400 is a passenger aircraft which is also used as a cargo aircraft.

Even many aircraft which seem to be specially designed for cargo duties have in fact a passenger-carrying origin, and applying more than structurally superficial changes would cost too much for the limited number of aircraft needed. Most freighters started their life as passenger aircraft and only small changes are efficient; redesigning the fuselage is completely uneconomical.

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    $\begingroup$ There is also an associated down-side to having a floor as it needs to be structurally reinforced to carry the weight of the cargo (as opposed to the weight of passengers). That is supposedly one of the reasons that an A380 Freighter is about $1B away from being a reality as discussed here and here $\endgroup$ – nodapic Dec 21 '18 at 16:07
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    $\begingroup$ @nodapic: After that Turkish DC-10 accident, the floor has to carry the full pressure difference in case the cargo area loses pressure. This is easily enough to support the weight of the cargo, too. $\endgroup$ – Peter Kämpf Dec 22 '18 at 0:28
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    $\begingroup$ interesting connection. Does that only apply to older aircraft where that was not part of the original design and you thus have to reinforce the floor during the conversion? Conversion houses talk about structural modifications to the floor being part of most STC, see here and here $\endgroup$ – nodapic Dec 25 '18 at 16:34
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Because the overwhelming amount of cargo is fairly small and floors allow easy loading of both decks at the same time which is more important than the ability to load big cargo on occasion. By having floors you can load standard cargo containers quickly and efficiently. It also gives you the ability to standardize the containers with a reasonable degree of reuse most importantly they can fit into the lower deck of a passenger aircraft with out modification. Passenger airlines make a lot of money moving commercial cargo.

When it comes to flying around big, cumbersome cargo there are simply aircraft designed and built for such a task like the Airbus Beluga

enter image description here

(source)

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On most transport aircraft the floor structure, especially the beams along the sides, is a key part of the fuselage as a load bearing pipe whos lower half is under compression and benefits from the stiffening. Also the space under the floors is full of components, avionics, ducting yadda yadda. Ya gotta put it somewhere. Lots of airliners have underfloor cargo holds though.

The passenger aspect is also a factor. You could design a cargo only airplane with only a very skinny floor section as low as possible that would take specialized circular containers, but then that's all it could do.

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Employees walking through aren't always the most cautious. From experience working at a cargo airline, it is very easy to misstep and fall through and land on something expensive when the floors are up for maintenance.

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    $\begingroup$ I think the question is removing the floor totally, leaving a big round space for cargo. $\endgroup$ – Anilv Dec 18 '18 at 10:05
  • $\begingroup$ @Anilv: Your intuition is correct. $\endgroup$ – Sean Dec 19 '18 at 23:05

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