As a pilot I would not be happy with a totally isolated inability to access and inspect the rest of the airplane inflight, both interior and exterior, particularly in the case of fire onboard. And I think that you might agree that terrorists, with or without SkyMarshalls on board may simply be able to embark with a pocket-full of hermetically-sealed (undetectable) plastic explosive (which can be moulded into any shape). Plastic explosives can bring down an airliner over a metropolitan area, which although not giving them the satisfaction of a specific target, would be just as devastating down below. They could do this quite passively, without confronting the armed Sky Marshalls. They could also use plastique to blast access into the cockpit. Plastique, with a pencil-fuse, can be used to demolish bulkheads or doors without doing much collateral damage at all (although the overpressure might blow out a few windows). So the enhanced security answer has to be multi-faceted. Cockpit doors should be sturdy, robustly-hinged and secured but the aftermath of explosive depressurisations (decompression) adds a requirement for them to include a large vent. That would have to be a one-way affair and preclude terrorists being able to spray any incapacitating gasses into the flight-deck area. One solution might be to have two entirely separate pressure vessels, with the pilots having the ability to explosively depressurize the cabin, quickly rendering the passengers unconscious and enabling them to enter with a portable oxy bottle, cuff the desperados and promptly repressurize. But would that be an acceptable solution for the travelling public if it was to happen fairly regularly due to air-rage incidents?? What degree of heart attacks would be caused? How many air-rage incidents might actually turn out to be a ploy by suicidal terrorists? How could the pilots be sure they'd got them all - or the right individuals? It wouldn't work on climb or descent with minimal pressure differential. It's suddenly getting complex.
Bona Fide Identities
Primarily you must try to keep them out of aircraft - but sleeper terrorists are called that simply because they reside for long periods and establish bona-fide identities. Even if less than 1% of the passengers boarding an A380 were bad guys, that would still be 4 amongst 500 (not an impossible task for a dedicated band of sleepers I would think). CCTV does offer a whole range of possibilities for exterior and interior scanning. However adding yet another switchable screen that a two-man crew must continually monitor for unlawful interference cabin activity? That's not a goer I'm afraid, even if aircraft were to revert to the old three-man crew concept. The CCTV cameras could be easily blocked off, quite covertly, with a lump of silly putty and then pilots would be blind-sided down the back. The RoboLander concept does arouse in both pilots and passengers a morbid fear of automation failure - however many passengers have been autolanded in fog over the past two decades without knowing about it and the technology is assessed to be both fail-operational and fail-safe because of built-in redundancies and backup systems. It is used by the Space Shuttle and a whole host of military RPV Drones - including the 737-sized Global Challenger. It is admittedly a futuristic concept for airline aviation, yet it also offers non-terrorist capabilities to cover pilot death, incapacitation, bad weather, non-suicidal hijacking and, if utilised in an optional reversionary "monitored mode" - a sensible alternative for a severely fatigued pilot/crew . You must also consider that the sleeper suicide-terrorist may yet find his way into cockpits as a line-pilot. "Never in the US" you may say, however you must also consider all the international airlines that fly into the US. It may become a requirement that they also subscribe to a RoboLander design. If it was standard across Boeing and Airbus and carried a part subsidy from National Governments, well that might make it digestible.
We quite frequently put our lives in the hands of monitored automation, everything from anaesthesia dosimeters, radiation oncology, high-speed elevators, MRI scans, ABS braking and even down to the potential lethalities of a set of computer-controlled traffic lights at a busy intersection. What you have to think in terms of is, if you had been a passenger on one of those four aircraft on 11 Sep, facing what they faced, would a non-autonomous auto-land have been seen as a god-send or just another (albeit lesser) level of terror? If those captains, at the first sign of trouble, had been able to simply lift a guard and push a button irreversibly rendering their aircraft de-weaponized and instead becoming a terrorist delivery platform - well I'd think that the minimal risk associated with that actual process would be no greater than crossing the street on a "Walk" sign. Moreover it would be seen as an unbeatable deterrent. If we don't lean heavily upon the Western capacity for invention and innovative technology, we will end up fighting the Global War against terrorism on their terms - and for me, that is to be avoided at all costs. There's no doubt that any effective war against terrorism will have to be fought very much on our terms and quite ruthlessly. This must include the targeted assassination of terrorist leaders and even those who supply them with arms, funds and provide sanctuary. There should be a policy of "you are either for us or against us", fence-sitting not permitted (but Israelis exempted from military participation). This is the only way in which we can wrest back the initiative, because that is their greatest weapon in the War of Terror. Blockades and embargos will not work on their own. The fundamentalist fanatics want to widen it into a religious war and make it all about oil, so that perilous path must be avoided - by maintaining a cohesive coalition, denying them any morale-boosting successes and by round-the-clock surgical strikes where they live and train. When we eventually get to that point of "Last Man Standing", I'd like to think that Western Democracy will be back in those "broad sunlit uplands" of which Winston Churchill spoke and that the terrorists will be back underground as Trolls - tunneling their frustrations and afraid to surface. No United Nations Resolution is required for any State that needs to protect itself by crossing another state's boundaries in order to bring terrorists to account. Sovereignty is not an issue - it's just that the arbitrary rule of hot pursuit died on 11 September 2001.