In the US two people must be in the cockpit at all times. If one of the pilots needs to use the bathroom, a flight attendant has to take their place in the cockpit until the pilot returns. This prevents a suicidal pilot from taking down a plane (or makes it much harder anyway). This wasn't a requirement on Malaysian Airlines at the time. (I don't know if the regulations have since been updated.)
And even that requirement seems to have been dropped. From Wikipedia:
"Aviation authorities swiftly implemented new recommendations from the European Union Aviation Safety Agency that required two authorised personnel in the cockpit at all times, but by 2017, Germanwings and other German airlines had dropped the rule."
While this improves safety, let's not fool ourselves, what are the critical parts of an airtrip?
Takeoffs and landings. What happens if the suicidal pilot act during takeoff or landing?
Plus let's not forget AF447 where a mistake by one pilot (involuntarily) stalling the plane wasn't caught by the other pilots until too late.. To be fair this was during an emergency caused by pitot tube 'failure', so there were lots of things to process.
It feels a little more reasonable to me than removing your shoes or restricting liquids. Even if the shoe bomber had been successful, he wouldn't have been able to take down an entire plane. But it's trivial for a rogue pilot to kill everyone on board a plane. Since 9/11, cockpit doors have been reinforced so that it's basically impossible for anyone in the cabin to force their way into the cockpit. Ensuring that there are at least two people in the cockpit isn't too much of a burden and basically eliminates that attack vector.
Maybe, but it's still trivially easy to break the system. Just slip something in your co-pilot's drink. Or knock him or the (probably female) cabin attendant out with something heavy.
It seems like it's at the same level of the rule that the pilot and co-pilot can't order the same meal at the airport before a flight in case they both get food poisoning simultaneously. Doesn't guarantee that they don't both get sick, but it's a relatively minor imposition and reduces the odds by quite a bit.
