disrupt radar altimeters -- instruments that bounce a radio d beam at the ground to give a hyperaccurate reading of height. pilots call the system essential when they need it most, in poor weather when it's hard to see the runway. >> this one instrument if it gets bad data, sends it to a collection of other systems, flight krols, auto notless, stick shake, i could go on and on. each of those, if they get bad information in, they are going to do bad things. >> reporter: in this simulator of a regional airliner, i saw what happened when a radar al tim ter fails. automatic warnings could stop and flight displays give confusing, mismatched readings. >> right. figure it out. >> verizon and at&t have now agreed to delay their rollout near certain airports. at&t, which owns cnn's parent company, says airlines and the faa have not utilized the two years they have had to