As for planes…. It was some time ago we saw a newspaper that showed an air blimp similar looking to the Hindenburg, so the tech is there for air blimps. But why has planes not been developed? I think we can use some loose (very loosely) logic based on the geography, level of technology, and politics.
Politics= I don't think the OP World Govt. has seen any benefit in planes that would have a hard time crossing the Red Line ...and they would benefit from the isolation of the various islands as there is less of a chance of discontent islands to band together for a joint rebellion if they only had ships and air ballons. LOL Planes would allow quicker commonication, interaction and maybe less dependance on the World Govt. If the World Govt is willing to outlaw the knowledge of the Void Century, which seems to have invovled a great war where the founders of current world rule were the winners, then I can believe they would want to prevent any potential of a future uprising against them. After all they do send citizens of island that refuse to join the World Govt. to be slaves to Tequila Wolf in the East Blue.
Geology= The Red Line is said to be 10,000 meters above the sealevel. 10,000 meters = 32,808.399 feet Lets look at real world geograpy. There are mountains in the Nevadas (forgot the range name) that have such a geography that causes disturbances in air flow and has some dangerous air sheers, causing light planes (that do not go too high) to crash often.
The highest mountain range on Earth, the Himalayas, causes disturbance in air flow so great that on the side facing the oncoming air currents (like India) gets monsoons while the opposite side (such as Mongolia) gets very little rain since it can not pass over. That is taking the majority of the mountain range in question. The highest mountain in that range is Mount Everest, it is with a peak at 8,848 metres (29,029 ft) above sea level, is smaller than the Red Line.
Technology = Plus there is the problem of technology needed for andvanced airplanes. While the islands on same side of Red Line could build light planes, I don't think they have transfered the presurised cabin of submaines to the planes to get high enough to get over the Red Line. We need to presurize the cabins of planes that can reach higher levels, esp the hight needed to go over Mount Everest.
quote below is from 'explain that stuff', link is at bottom=
"Pressurized cabins: Air pressure falls with height above Earth's surface—that's why mountaineers
need to use oxygen cylinders to reach extreme heights. The summit of Mount Everest is just under
9km (5.5 miles) above sea level, but jet planes routinely fly at greater altitudes than this and military
planes have flown almost three times higher!"
A few sites I visted to get info on wind and mountians=
http://forums.cgsociety.org/archive/index.php/t-862457.html
http://www.explainthatstuff.com/howplaneswork.html - simplified, not a long explaination of how planes work
yes I am bored today X_____X