Right.

This is a big issue for self-flying aircraft. Keeping the aircraft stable requires reliable altitude, airspeed, and orientation data. If you have that, you can stay in the air. AF 447 lost airspeed data, the flight control system dropped to a lower level, and gave up stall protection. It couldn't tell if the aircraft was stalling. That was enough to confuse the pilots so badly that they crashed a working airplane with plenty of speed, power, and altitude into the ocean.

A self-flying aircraft has the same problem, only worse. Without that info, it cannot stay in the air. There are fighters that need so much active stabilization that if you lose air data, you have to eject. Self-flying aircraft thus need really, really reliable air data sensors, and lots of them.

Getting airspeed from GPS has been suggested. There's one bizjet which does this. It can't be flown in areas where GPS jamming is in progress. Over-dependence on GPS is a problem.

All that is just to be able to fly straight and level. That's flight control. Navigation and landing are separate problems.

All this and the code needs to be written correctly especially with regards to edge cases. Thousands of planes have been flying around the world for several decades now and we still see unique accidents or slip-ups that have never occurred before.

Think of all the edge cases that need to be accounted for with all the different variables out there from weather to people misbehaving on the plane. It's only going to take one freak accident to have people not want to board a self-flying craft ever again.

Self flying doesn't mean no security guards or flight attendants, nor does it mean the plane can't make emergency landings