I have them on two of my most used planes. Have not needed them yet but I feel it's the least I can do for the benifit of whoever might help me find-retrieve a down plane as I am getting too old to chase them down myself as they could land out in the boonies and be difficult to get out to, as I am somewhat handicaped. I drill a hole in the side of the fus so the sound emitter is open to the outside of the fus so it can be more readily heard, and I use velcro so it can be removed and switched to another plane easily.