Are they?
I'd argue that they generally aren't... except that, over time, cities may grow until they are right next to the airport again. And then new airports get constructed - usually far away.
> terms of freight
Only for certain classes of freight. You distribution center needs ready access to transportation, it doesn't have to be next to your consumers. We are not delivering packages to their destinations with planes, after all. Usually trucks do that.
> If your plane can't add velocity, your ability to plan _anything_ is severely curtailed.
Not an issue with pre-planned routes. There can be contingencies in place for malfunctions at any stage in the flight.
I wouldn't worry too much with engine failures and the like. Worry about sensor problems.
That sounds backwards. Airports get built as close as possible to the cities they serve. For newer airports that ends up being some distance away because they need to avoid existing homes.
> You distribution center needs ready access to transportation, it doesn't have to be next to your consumers. We are not delivering packages to their destinations with planes, after all. Usually trucks do that.
The point of the plane is to save on trucking time though, so you want a distribution center fairly close to population centers.
Airports get built "far away" in the sense that, between land prices and noise abatement, airports are generally on the very outskirts of a city at the time they are built and the city's central train station will be much closer to the city centre.
But apart from military bases on remote islands, no airports are so remote that "don't worry where you ditch the plane if there's a problem, there's nothing near the airport" would be an option :)
I think you immediately have to question how often this has been the source of a disaster. To me, this seems highly unlikely to have been a problem, or to be a problem at all.
Most accidents happen near airports. Most airports are in populated areas, because an airport far removed from a populated area is next to useless in terms of freight or passenger transport.
> An autonomous system doesn't have to do that
Well.. it can't. Which is part of the problem:
> it can spend its remaining time in the air finding and navigating to the most sparsely populated point of impact within its range.
That's a whole separate automation and computer intelligence problem on it's own. There's also no guarantee that a sparely populated area isn't completely empty or even currently unused for any civil or special purpose. Expecting a computer to "solve" this problem is asking a lot.
Just look at the Gimli Glider as an example[1]. Choosing a "safe" place to have an emergency isn't at all obvious and assumptions based upon outdated information are dangerous in and of themselves.
> Find the nearest/largest body of water and ditch into it
Finally.. even if it could make this determination, safely, and with enough sensor observations to reduce human loss of life on the ground, you're presuming a plane without engines can always reach this apparent planned crash site. If your plane can't add velocity, your ability to plan _anything_ is severely curtailed.
[1]: https://en.wikipedia.org/wiki/Gimli_Glider