Flying KLM on a NW issued ticket:

We used our son's American branded convertible carseat w/ FAA approval sticker with no significant problems. The only issue with the FA's was I think every single one of them checked the seatbelt to make sure it was tight enough.

That was a forward-facing carseat. However, on both domestic US and US-Canada flights we did get some problems using a rear facing carseat, as that can cause issues with the passenger ahead of it For a 14 month old, *I personally* would install the seat forward-facing.