Nice big old debate on the topic going on at Tam's -- and let me reiterate, this is not about celphones! (Trouble there is on the ground, not on the plane: your phone lights up too many cells, moving too fast. The system can freak out.) So: WiFi, Bluetooth, incidental radiated yeech: do they or do they not have significant potential to interfere with avionics during takeoff and landing?
Jury's still out, which is why FAA is revisiting the issue. What I'm not hearing from anyone is the other problem with having your toys out during those operations most likely to include some violent maneuvers: they can become projectiles. Me, I am not wanting to see what happens when your ultra-slim laptop slides across the aisle, catches air and hits my head; YMMV. (And willya please put away that fat hardback book, too?)
3 weeks ago