What measures are in place to prevent pilots from being put in charge of passenger planes without an appropriate level of training and skill? Are these measures enforced equally by different countries, airlines and aviation authorities?
Put quite simply for this answer - The Licence a pilot holds is the assurance that the pilot has received the appropriate level of training.
Each country has it's own standards for training, their own requirements, and their own limitations on the operation of that licence. In saying that, MOST countries standards are reasonably in-line in regards to Aeronautical Experience required to sit each flight test.
In Australia, the requirement to sit a Commercial Pilot Licence is a minimum of 200 hours Aeronautical Experience (150 hours if ALL training has been conducted on an Integrated Course, now to be recognized under Part 141). From there, there is a break down of what that 200/150 hours must consist of.
Now, having a Commercial Pilot Licence (again, speaking from an Australian point of view) does not give a pilot the right to sit in the left seat of an Airliner. You need further training such as;
- Multi-Engine Command Instrument Rating
- Type Endorsement (for the intended aircraft)
- Crew Resource Management Training
- Dangerous Goods Training
- Senior First Aid (Not mandatory, but common)
- Company, Emergency, Cyclic and Systems checks (per company)
- Air Transport Pilot Licence Theory Exams (in some cases, Airlines may not even look at you if you haven't completed these - In Australia, it is an unwritten must-have!)
And thats just to sit in as a Co-Pilot..
Captains need years of experience, and a lot more training and check to get into that coveted Left Seat.
So again, to bluntly answer your question, regardless of country, there is law in place that stipulates that Pilots must be appropriately trained to fly airliners.
That is the function of a license. Every country has their own standards, and by international treaties most countries must respect the licenses issued by other countries. So, for example, a Malaysian pilot is allowed to fly in the United States, even though he has a Malaysian license and does not have an FAA license. For this reason, it may be slightly more dangerous to fly on foreign air carriers, especially if they are from sketchy, third-world countries.
Carriers have much different standards for who is allowed to captain an aircraft. For example, a big carrier like Delta will usuallly have tougher standards than a regional airline or a charter company, but not necessarily. You can find small charter companies with really tough standards, and others that will let much less experienced captains fly their aircraft.
Do not think that safety is purely a product of training and experience. You can find guys with 10,000 hours who do stupid stuff and make bumpy landings all the time and kids with 800 hours who fly ten times better than them. Skill does not necessarily = experience. Also, performance in a crisis has little to do with experience. Put one guy in a crisis and he might freeze up like a rabbit, and another cooly reacts with correct decisions. It's impossible to predict ahead of time.
In difficult situations often judgement is more important than skill or experience, although experience can help improve judgement. Examiners will try to test a pilot's judgement, but it is hard to do. It's an undefinable quantity. Once again, you can have a guy with 800 hours who has much better judgement than the long-timer Navy pilot with 10,000 hours. It's difficult to predict.