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supersix2 · 06-15-2009, 03:44 PM

After doing some searching on the internet combined with my aerospace engineer powers, I found info that the cargo compartment is pressurized to the same pressure as the passenger compartment. Depending upon the flight altitude, the cabin is pressurized to a 5,000 ft to 8000 ft altitude equivalent. So your pressure in the cabin is between 10.9 - 12 psia. However, the cargo compartment isn't always thermally conditioned the way the passenger compartment is. As a result, the cargo compartment could be colder or hotter than the cabin depending upon the location inside the cargo compartment. It makes sense for them to be at the same pressure, otherwise the floor of the passenger compartment would have to be strong enough to withstand the differential pressure between the passenger compartment and the crew compartment. You would have to have reinforced pressure bulkheads between the two compartments and that would add unnecessary weight. There was an aviation mishap back a while back that involved a cargo hold door losing its seal causing the cargo compartment to depressurize. This caused the floor of the passenger compartment to buckle because of the delta pressure between the two volumes.

If your tires are inflated to 55 psig (~70 psia at sea level) then inside a cargo compartment, your tire pressure could increase as high as ~74 psia. That seems well within the operable range of the tire. I think they ask you to do this as a precaution since some tires may not be as well rated as others. However, if your tire can handle that high of a pressure there should be no problem transporting it with the valve closed. The thing you want to make sure is the units your tire is rated to psig (gauge pressure) or psia (absolute pressure). Since 55 psig and 55 psia are two completely different things. I think this also protects the airline from liability in case any damage occurred to your tires during flight (for any other reasons as well) so they can say "well you didn't follow our policy." So while your tire may be able to handle the pressure, it is probably just a better idea to follow the airline policy to avoid any unneeded fuss.

06-15-2009, 03:44 PM	#10 (permalink)
supersix2 Insane Location: Houston	After doing some searching on the internet combined with my aerospace engineer powers, I found info that the cargo compartment is pressurized to the same pressure as the passenger compartment. Depending upon the flight altitude, the cabin is pressurized to a 5,000 ft to 8000 ft altitude equivalent. So your pressure in the cabin is between 10.9 - 12 psia. However, the cargo compartment isn't always thermally conditioned the way the passenger compartment is. As a result, the cargo compartment could be colder or hotter than the cabin depending upon the location inside the cargo compartment. It makes sense for them to be at the same pressure, otherwise the floor of the passenger compartment would have to be strong enough to withstand the differential pressure between the passenger compartment and the crew compartment. You would have to have reinforced pressure bulkheads between the two compartments and that would add unnecessary weight. There was an aviation mishap back a while back that involved a cargo hold door losing its seal causing the cargo compartment to depressurize. This caused the floor of the passenger compartment to buckle because of the delta pressure between the two volumes. If your tires are inflated to 55 psig (~70 psia at sea level) then inside a cargo compartment, your tire pressure could increase as high as ~74 psia. That seems well within the operable range of the tire. I think they ask you to do this as a precaution since some tires may not be as well rated as others. However, if your tire can handle that high of a pressure there should be no problem transporting it with the valve closed. The thing you want to make sure is the units your tire is rated to psig (gauge pressure) or psia (absolute pressure). Since 55 psig and 55 psia are two completely different things. I think this also protects the airline from liability in case any damage occurred to your tires during flight (for any other reasons as well) so they can say "well you didn't follow our policy." So while your tire may be able to handle the pressure, it is probably just a better idea to follow the airline policy to avoid any unneeded fuss.