I'll be watching your answers to this one. I've been told 3 things - 1) that any seat rated for airline use should be allowed 2) it's often up to the discretion of the flight attendants if they allow it (for example, if the seats are narrow and you have a wide base like our Marathon, or they might insist that you hold your child on takeoff but be ok with it later) and 3) some airlines have stricter requirements than others.
As far as the keyfit, I found that seat to be incredibly easy to install with both the base and the seatbealt option, so hopefully you won't have a problem if they allow it on the flight or if they make you do the belt vs base install.
It also depends on where to/from. Our short flight from Ireland to Scotland may mean trouble for us with our seat because it's a smaller craft. Still trying to get a straight answer other than "it has to be rated for aircraft." I don't want to be at the terminal shelling out Euros for the carseat because it's over our baggage allowance.
By the way, your Chicco should be airline approved.
Can I take my car seat on an airplane?
The Chicco KeyFit and KeyFit 30 car seats are certified for use in commercial aircraft. Please consult your User Guide for proper installation. Our product manuals are also available on our website.