Comment by Syonyk | Hacker News Reader

Comment by Syonyk | Hacker News Reader

Parent Full thread

No, cargo holds are generally pressurized and have been, as far as I know, since the beginning of pressurized aviation (late 40s to early 50s - there were a few test airframes earlier).

It doesn't mean cargo compartments are not cold, dark, loud, and unpleasant places to be, but they're pressurized.

If you pressurize a cylinder (say, a more or less round airplane fuselage), the stresses become tension stresses around the perimeter - it's trying to inflate the balloon, but the perimeter is designed to handle these stresses. The floor only has to support the weight of passengers.

If you had an unpressurized cargo hold, the floor would now need to not only support the weight of passengers, it would have to support the entire pressurization weight - which is far harder, because floors are typically flat. It would have to be far stronger and heavier.

There have been several aviation incidents from the cargo doors opening in flight - the DC-10 was particularly prone to the problem, because instead of an inward-opening, "plug" type door (where the pressure holds it in place), it had an outward opening door - more cargo space because the door doesn't swing in, but if the latching mechanism fails (which it did), the pressurization tries to push the door open, and it occasionally succeeded.

https://en.wikipedia.org/wiki/McDonnell_Douglas_DC-10#Cargo_...

> If you had an unpressurized cargo hold, the floor would now need to not only support the weight of passengers, it would have to support the entire pressurization weight

Only if the plane is partly a passenger plane. If it would be a pure cargo plane, the whole back could be unpressurized, or am I missing something?

The vast majority of cargo planes are variants on commercial airliners and are pressurized. The other class are military heavy lifters, but they can also be used for troop transport purposes, which means they have to be pressurized as well.

In every commonly flown cargo airframe I'm aware of, the crew has the ability to get into the cargo area if needed, which a pressure resistant bulkhead would make very challenging (you'd have to depressurize the plane or have an airlock, and at that point, see "takes a ton of strength and weight you don't have any reason to haul").

There are a few weird, custom cargo planes that aren't pressurized (the Dreamlifter and Beluga cargo compartments aren't pressurized, though the crew compartments are), but they tend to be really weird, one-off designs where it's more trouble than it's worth to design them for pressurization - or, likely, the weird oversize fuselage would have to be far heavier to handle pressurization loads against the flat sides.

If you point to a random commercial cargo transport, it's almost certain to be pressurized. In the 1960s, there may have been some unpressurized ones still flying around, but they'd be at a low enough altitude for the flight crew that it wouldn't be a problem back in the cargo compartment either.

Speculating, but:

- Most cargo planes are cargo versions of a plane that has a passenger plane design variant. So it probably wouldn't make sense to do a completely different design.

- I imagine that many goods that are shipped by plane have not been tested to be unaffected by significant time at 30K-40K feet air pressures. So now you're introducing an additional shipping restriction for high-value goods that isn't actually necessary.

That is correct. Virtually all cargo planes are pressurized.

However, while the cargo area is almost invariably pressurized, they are often not heated. Some aircraft have designated cargo areas that are heated for transporting e.g. live animals. So if you end up in an unheated cargo area you won't die of asphyxiation, but could easily succumb to hypothermia or frostbite.

> Only if the plane is partly a passenger plane. If it would be a pure cargo plane, the whole back could be unpressurized, or am I missing something?

I'd guess you'd get stress issues in the bulkhead behind the cockpit if you tried that. You'd need to completely redesign the joint between the cockpit section of the fuselage and everything behind it. And then, for your trouble, you've given up the ability to transport anything that can't tolerate the pressure and temperature swings of an unpressurized cabin.

Given that the pressurization systems are already implemented and don't seem to hurt efficiency much (afaik they typically run off engine bleed air), I don't think there's much reason not to just pressurize the whole thing. I guess it probably makes the structure marginally heavier, but at that point you may as well just design a whole new airplane.

Here are some recent interesting videos by "Mentour Pilot" about the DC10 plane accidents caused by a cargo door coming off and depressurising the cargo hold:

https://www.youtube.com/watch?v=Gv5EQlzM1B8

https://www.youtube.com/watch?v=_7rF0wCSpE0

What happened, if you don't want to watch all the video, is that the aeroplane actually collapsed internally. In the second case, some of the passengers were sucked out of the plane.

Reply on news.ycombinator.com