After playing around in SL with various aircraft, I have come to the conclusion that the most stable platforms for flight are the helicopter and various forms of blimps.
The biggest problem is that flight is really only practical when you have the ability to cross sims at will. Anything that crashes more than fifteen percent of the time is useless as a vehicle for crossing sims. The fact of the matter is that most airplanes will fail this. Ideally, to cross a sim you want to reduce the number of prims and scripts making the crossing and to do so at a speed which will not cause SL to flip out. It's really hard making an airplane that can do this. My best attempt at this doesn't look much like an airplane and relies upon a stripped down version of Cubey's freebie plane script.
So airplanes are useless. The chances of failure to cross (hereafter referred to as %FTC) are too high. When you're rocketing across the landscape SL can't handle it most of the time.
There's another problem, and it involves the time it takes to rezz something. Once again, in real life an airplane's speed is what gives it many advantages, however in SL that is a huge weakness because not only do you not give the server time to hand you off between sims, but you also run the huge risk of smacking into the broad side of a skybox, skyscraper, mountain, or any number of things that involve user created prims. Unless you have a very good lay of the land, something that is hard to accomplish when you're scouting over unknown lands, you're going to hit something. Throw in the fact that land is transient and can be bought and sold fast with new owners changing the landscape and it's a recipe for disaster.
I've encountered this. I've hit parcels where the new owner erected banlines or built something new and I hit it. When you scream across at Mach 2, it's fairly easy for SL to not keep up with how fast you whiz by.
You could negate this by extending the draw distance so as to give the grid a longer range and time with which to rezz something. Then you hit the problem of lag. And it doesn't change the fact that ban lines can hardly been seen these days until you're on top of them (which I wouldn't change for all the money in the world. Banlines are ugly enough as it is without them being seen across the sim).
Airplanes just won't work well in Second Life. Unless you are willing to crash a lot and deal with the consequences of that.
Helicopters and blimps solve these problems by having such reduced speeds. Blimps are notoriously slow, limping along at a crawl and giving the servers ample time to pass you around the metaversal canyons. Helicopters can be much faster (almost as fast as aircraft) but can be reduced to a crawl fast and without the problem of stalling like an airplane or a jet would at such low speeds.
This ability to reduce speed to an acceptable point for crossing sims is their greatest strength. In my entire time flying such vehicles, their most prominent problems are due to pilot error than the craft themselves. Such as when I push a chopper to maximum speed across the land (which can be faster than you would expect). They lack the sheer speed of airplanes and jets but have a much lower %FTC when flown right. This gives them a larger operating zone since they can then cross multiple sims whereas a jet pilot might operate over only two or three to avoid the risk of crashing.
There's also a host of minor annoyances when flying airplanes and most are due to the fact that you cannot divert attention away from the aircraft itself. I can't really answer IMs when flying because if I do I run the huge risk of not being able to switch from the IM box to the aircraft in time to avoid something awful. This isn't so bad over Linden Seas where I can fly level for a stretch to utter a few sentences but is a serious issue over land where hitting banlines and prims is a concern.
In this case, helicopters and blimps soundly crunch airplanes. Both have the wonderful ability to hover in place which allows one to pause to answer and respond to IMs. I can stop right then and there without fear that the helicopter or blimp will continue beyond my control and blow itself to hell. It's a minor problem because, perhaps, a pilot just won't have or wish to answer IMs or perhaps won't care. I care because I don't like to leave people hanging and waiting for my reply.
It isn't all good on this side of the rainbow. Unlike airplanes, helicopters and blimps can be prim and script heavy, a necessity of the fact that you're making something much more complex than what is, in Second Life, a flying car. Most airplanes in Second Life are simple car scripts with the ability to lift off the ground thrown in. For Choppers and Blimps, this is much harder to accomplish. I have seen a few great builders who have pulled it off but for someone constructing their own it is not an easy skill.
While Blimps are easy to learn, helicopters are much harder to get trained on. A tiny mistake or over adjustment on the controls and you can spiral out of control straight into the pavement. And due to the fact that a scripter has to create something that both flys and hovers, the controls to accomplish this can vary wildly and you really have to relearn how to fly a helicopter between different builders. Some may use simple keyboard controls, some may use a form of mouselook, and some may mish-mosh the two. The steep learning curve is a major turn-off for casual flyers and I do not blame them.
But for my money, I'd prefer helicopters and blimps, despite their faults, over airplanes these days. I like to explore SL in an aircraft beyond the local sim or two and I have time to burn learning how to fly. I like airplanes but these days considering the stability of SL (nonexistent) I like to spend more time online than in limbo waiting to red map. Your mileage may vary.