In light of the recent underwear bomber episode, and the equally ridiculous and useless rules set up by our bureauverlords to protect their jobs (the answer to government failure is... more government!), I thought I would weigh in on how to really keep airplanes safe. Allow guns on board.
I am quite certain that terrorist incidents would virtually disappear if we did this - just let anybody on a plane without screening of any kind. On an average flight, you would be almost guaranteed to have several retired military, law enforcement, and good ol' rednecks strapped and ready to waste a prospective terrorist or hijacker.
The first objection is, as usual from people who take what the government says without question, "we can't do that! Everyone would be shooting up planes." Right, and if heroine was legal I guess you would go stick a needle in your arm tomorrow. There is a growing body of research that suggests violent crime rates drop significantly when gun laws are softened. It's the age old truth that criminals with guns can usually avoid cops, and they are not afraid of citizens with knives. Didn't 9/11 show us that our elaborate security theater had done nothing less than make use mortally vulnerable to such treacherous weapons as box cutters? And did it not also show us that, given either a reasonable chance of success, or an overwhelming reason to try, ordinary citizens are our best defense against random acts of violence? Yes and yes.
The second objection is more rational, which is "but don't guns put holes in planes?" That one suggests some caution. After all, if the brave and intelligent Dutch tourist who stopped pantsbomber had been packing a .44, the cure could very well have been much worse than the disease. In that case, how's this for a compromise. Offer a course for military, law enforcement, and maybe ordinary citizens to become licensed "reserve air deputees." Once licensed, these deputees would be allowed to discreetly carry weapons on their person onto commercial airplanes. Of course, there would be a restriction on type of weapon - high-caliber handguns, explosives, rifles, and shotguns would be of little use in a close-quarters airliner fight. However, small caliber and non-lethal weapons like .22's and similar low-power handguns, tasers, mace, etc, would be allowed on. The key word is "discreetly" - the RADs would go through the same screening process as normal passengers, they would be required not to show their weapon to anyone, and they would be required to have their weapons on their persons at all times. The point is to make sure a terrorist can't know how many RADs are on a given flight, or where they are sitting.
The course would teach people the dos and don'ts of airliner combat. The course would cover small-caliber handguns, as well as non-lethal weapons like mace and tasers. Users would be instructed in an airliner mockup about things like where a small caliber bullet is least likely to cause explosive decompression, whether bullets will penetrate different types of partitions, what kinds of proximity effects a cloud of mace has, and so one. It might also cover such things as basic hostage situations, simple forms of non-verbal communication, ways to improvise weapons and/or create diversions with things commonly available in an airliner bathroom or galley, ways to contact the ground from the cabin, and steps to take in case of loss of crew. The class need not be especially long or expensive - probably no more than two days or several two-hour evening sessions should do - one could commandeer a derelict DC-8 in Mojave and shoot it full of any number of holes.
The worst case scenario is that several terrorists would take the course and be allowed on a plane with a gun. However, this is still better than the current scenario. First of all, there would be the requisity background checks. Anyone who took the course would be automatically red flagged by the FBI, and any commonalities where several people on the same flight took the same course, or were funded by the same organization, or have sketchy backgrounds, would come out. This would have stopped at least the Reids and Abdulmutallabs. Second, the worst that would happen is they would get on with a small gun - there would still be screening for explosives, large caliber weapons, etc. All this would not necessarily have stopped 9/11 however. But, potential terrorists would have no way of knowing how many other passengers were carrying weapons, or where they were. This is the most important line of defense. Terrorists are willing to lose their own lives; and if there is a 3.7% chance that they will go to jail because an air marshall happens to be riding then so be it. But if those odds went to close to 100% that there would be at least one and likely several people with the weapons and training to make every attempt at martyrdom as pathetic as pantsbomber, regardless of what kind of heat the terrorists were packing, then they will move on rather quickly.
There could, of course, be automated rules in place to put the odds even more in our favor. There would obviously be flags raised when an aberrant number of RADs chose a given flight. RADs could show up to the airport, and be either deputized (allowed to keep their weapon on their person) or demoted (with their weapon placed in checked bags or in safe storage at the airport police station) at random for their flight that day, so that no one could ever know whether he would be allowed a weapon on board for a given flight. Likewise, we could limit the total RADs on each flight to no more than 2% of passengers, or no more than 5 total RADs, or so on. We could arrange it so that the RADs on a given flight had a wide variety of birthplace, hometown, religion, ethnicity, and education, to reduce the likelihood of collaborators. RADs could be seated together without their knowledge, with air marshalls secretly seated around them, in any suspicious case. But this is all just working the odds. The point is, the program would open the RADs up to so much scrutiny that it would probably be unprofitable for a terrorist organization to attempt infiltration.
Fact is, the cops are never there when you need them. At any level of spending that won't kill mass air travel, we are not going to cover enough flights with air marshalls to make terrorists think twice.