Mine has tried to fly with her harness on, and it was very upsetting to her. Normally she'd take a quick flight between my husband and I, but one time she decided to do a little "exploring." She decided to try to land on the roof of the house, but then she hit the end of the leash, and didn't know what to do, what direction to go, and was really just frantically flying around me. That in itself could be very dangerous, depending on how open the area is - they could easily fly around something and get the leash wrapped around it if you weren't in a large open area. She's never tried to fly with her harness on since then, nor has she even flapped her wings outside. In the video that came with the harness, there are clips of people flying their birds back and forth with the harness, but after my experience, I'm definitely NOT a proponent of that. My bird isn't trained to fly free outside though, and maybe birds with more solid training wouldn't have this experience. I live in a rugged, heavily wooded, very rural area with few roads where I just can't train her to fly outside. (But knowing that behavior is never 100% predictable, I still don't think I'd trust harness flying.) I think harnesses are for when you DO NOT want your bird to fly, and you do need to do everything in your power to ensure that they don't try it, while making sure that your leash is short enough to keep them from harming themselves if they do take off for some reason. I do think though, that with a lot of training and experience, some (maybe most?) birds could learn the difference between free-flight and leashed flight.