Hyperveloce, you exercised your human prerogative for handwaving there. How do you get the plane outside the 20nm radius without it going faster than anybody thought it was going or was capable of going and still have it go off the air in a fashion indicative of hitting the water within 4.25 minutes?
Actually a 20nm circle is a little off beam. Something more akin to a cardioid might be the correct figure to search within. The initial point for the cardioid would be last known position. Extrapolate what a plane could do in fully controlled flight to push the outside of the rest of the envelope. Then search everything inside that envelope. They are taking the "easy way out" by simply searching the full circle when there's lower probability at 20 nm backwards along their projected flight path. But there's rather low probability they could have pushed the plane past the 20nm forward point.
So let's discuss what the plane is doing to get it out past that 20nm and how it still managed to apparently meet the water or be in such bad condition it ceased transmissions in only about 4.25 minutes.
Please play some time line scenarios by us.
{^_^}