[mikeash]

A typical airliner approach is a 3 degree angle touching down at 1000ft past the end of the runway. That means that they pass over the threshold at about 150ft. Let's say there's another 1000ft past that before things get problematic (at SFO it's considerably less, I believe), that means that the airliner passes over the dangerous bit at about 300ft when operating normally, and 300ft lower would cause a crash.

This isn't normally a problem, because detecting and ensuring that 300ft difference is not really hard.

[momavujisic]

Just a note about approaches and landings – most, if not all airline ops usually say that you should be ~50ft when passing above the threshold.

[mikeash]

My simplistic analysis ignored the flare, thus why the real-world height would be lower. Thanks for pointing that out.