It has now been disclosed that Asiana Flight 214 was being flown by a pilot who was new to the 777 under the supervision of a pilot experienced on the plane. This is standard procedure when pilots transition from one type of airliner to another. When the transitioning pilot is making the landing, the pilot supervising is ultimately responsible for the plane's safe operation.
If, at any point during the landing, the trainee allows the speed of the plane, or its path to the runway, to exceed established parameters, it is the supervising pilot's job to take control of the plane, correct the situation, and land the plane safely.
These parameters are fairly wide when the plane is one to two minutes from landing, but as the plane nears the runway, the latitude allowed narrows. When five miles from the airport, the gear should be down, the flaps set for landing, the plane on the proper 2.5 to 3.0 degree glide angle toward the runway touchdown zone, the rate of descent 700 feet per minutes (plus or minus 100 feet per minutes) and the speed within ten knots of target.
Once the plane is less than 1000 feet above the runway, the plane's speed and rate of descent should be within narrow parameters and steady. For example, if the trainee allows the rate of descent even momentarily to reach 1000 feet per minute (as read on instruments in the cockpit), the supervisory pilot should take control of the plane. As the plane nears the runway, if the trainee allows the speed to go even one knot below target speed, or the path of the plane to drop even slightly below the proper glide path, the supervisor must take over the controls.
For safety, the supervisor must hold the parameters actively in mind. If a trainee's performance is outside of parameters - whether minutes before landing or seconds before landing - a trainee must not be allowed to take the plane any closer to the ground. Thus, the question that remains to be answered is - not why the trainee pilot flying the plane landed short of the runway - but why the supervisor allowed the trainee to continue when his performance was unsatisfactory.