What If 100 People Had A Gpsr On A Plane? in Geocaching topics Posted March 23, 2004 I'll try to answer some of your questions. Above 18,000 we use 29.92 in Hg as the altimiter setting. That way when the pressure changes (and it does all the time) all the planes are using the same setting. Just imagine if two planes were to cross 1000 ft from each other and they were using different altimieter settings they could get too close. The cabin altitude should max out at 8,000 ft. In between takeoff and cruise it is climbing or descending slowly to/from that altitude. On 9/11 the hijackers all used handheld GPS units to find their targets. It was said that they even went to the twin towers before hand to mark them as waypoints so they could find them. As for interference, personally I do not think they are a problem. The only thing I have seen that causes interference is Nextel phones. For some reason if they are left on there is interference with the intercom of my plane. Like someone else said, I think it is best to not allow them while low just in case.