I have flown both United and Southwest and they have always provided a thick plastic bag to put over the carseat when I check it with my luggage. ?What they gave me was thicker than a regular trash bag.
There are special bags you can buy to protect it. I've flown a few times with DD and her car seat always came back in fine condition without protecting it.
We bought an Eddie Bauer car seat travel bag (I found it on Ebay) that we use every time and we've never had a problem. One time we forgot it and the airline gave us a thick, clear, plastic bag to put the car seat in which also worked just fine. However, on the way home they didn't have any bags left so the car seat went uncovered. Not much we could do at that point and it was still fine.
That said, don't rely on the airlines to provide a bag. Bring a big garbage bag or buy a bag made for transporting a car seat. Using a black garbage bag does provide a challenge though for the baggage tag to be seen - you can poke a couple of holes through the bag and clip the straps together outside the bag and put the tag there.
Re: Checking in your carseat when flying
We bought an Eddie Bauer car seat travel bag (I found it on Ebay) that we use every time and we've never had a problem. One time we forgot it and the airline gave us a thick, clear, plastic bag to put the car seat in which also worked just fine. However, on the way home they didn't have any bags left so the car seat went uncovered. Not much we could do at that point and it was still fine.
That said, don't rely on the airlines to provide a bag. Bring a big garbage bag or buy a bag made for transporting a car seat. Using a black garbage bag does provide a challenge though for the baggage tag to be seen - you can poke a couple of holes through the bag and clip the straps together outside the bag and put the tag there.