I recently was at the Naval Air Museum in Pensacola Florida. There are lots of models of aircraft carriers, including the latest class (Gerald Ford class as I recall). This latest class has electromagnetic rails instead of steam catapults to launch the jets. However it still has arresting cables. So I got to thinking that with the sophisticated servo systems we have today, it seems possible to have a servo system on rails on the landing strip that follows the jet as it lands and automatically grabs the tail hook and has a controlled deceleration. On the Science channel show "Outrageous Acts of Science" there was an amateur inventor who created a servo system to follow his dart throws to give him a bullseye every time. A servo system would improve the chances of a successful landing. I read astronaut Scott Kelly's autobiography and he said one time as a Navy pilot in bad weather at night he had to try dozens of times to land because his tail hook would not grab a cable. Also, the servo approach would eliminate the scrapping of steel cables. From what I have read, steel cables get replaced fairly frequently due to wear. Am I missing a fundamental flaw for this idea?
As SolarMike points out, there's a ton (pun intended) of energy the arresting cable has to dissipate, along with the springs it's attached to. Remember that the pilot hits full throttle the moment he touches the deck, so that if he doesn't hook in, he can get airborne again -- instead of going for a fatal swim. Your servo would have to be incredibly quick to respond in both horizontal and vertical position to guarantee catching the tailhook. Fast, agile and strong don't really go together.
The tailhook itself, BTW, is mounted to the strongest part of the aircraft frame -- if you thought of a mixed approach such as having the landing gear double as a cluster hook, forget it. They'd snap right off.
Suppose, for the sake of argument, you wanted to go with a SciFi approach of using a mega-electromagnet. It might be fun to calculate the field strength needed to stop the jet, not to mention the hell that would play with all the electronics in a kilometer range!
I don't see any reason the same kind of motor used for launching the aircraft could not be used to decelerate it. The energy would have to be dumped somewhere, but I suspect that would not be a big problem in a ship.
One of the primary advantages touted for electromagnetic launching is that it will increase the lifespan of the airframe due to reduced stress. It's probably less of a problem on landing because the aircraft generally come back lighter than when they left, having burned fuel and dumped their payloads of death and destruction.
For example, take a fully loaded 27000 kg F-35B or Shenyang J-15 or landing at (say) 150 knots (~75m/s), the kinetic energy is about 75MJ. If you allow for heating aluminum by 70°C you'd need 1.2MT of aluminum (and you'd need to cool it down enough before the next landing..). Doesn't sound exceptionally difficult.
I think some automated means of grabbing the tailhook and using the existing cable system or new arresting system could be a viable idea. Dissipating the energy isn't that tricky, and no saying it has to be dissipated by a motor. Remember that the aircraft isnt always perfectly lined up so the system would need to account for a fairly large amount of miss alignment. Also the ship attitude and winds are not constant.
The main issue I see is risk vs reward. How many aircraft have been lost because of the current system. How many aircraft could be lost if this system has glitches during the first year. How many aircraft will be lost with the new system in the long term.
The military isn't concerned with cost or waste, only reliability. Its easier for people to see the reliability in simple springs and cables than in servos and more complicated systems. If you could come up with a system that could leave the existing system in place, that is a much easier pitch. Some sort of computer controlled secondary hooking system, that requires pilots to still practice their skills on the old system, but provides a backup guarantee to get them landed during a combat situation.