Some airlines will allow use of CARES harness, however it is to the discretion of the air hostess who is responsible for the safety of all passengers and ultimately the pilot.
If the harness interferes with the features of the seat behind, such as the fold up locking table or tv screen it would not be permitted.
You would need in addition to the harness, the 'kangaroo' or baby lap belt to fasten it (I believe).
The air hostess will not assist you with fastening this or any other device.
Some airlines will allow you to use a car seat.
The restrictions are:
Width of the seat, airline policy, pilot/airhostess decision, country of flight carrier's registration.
You can check the policy of your airline online.
Car seat: Combi corroco is the smallest width one I found. (3 years ago/2011) and I paired it with a 'go-go baby' wheel board, which goes in the overhead compartment.
PERFECT!!
The other item I always took was a ring sling, to use as a nursing cover, comfort, going to the toilet together etc. tool.
I suggest a car seat cover for gate checking! - read on...
All but one airline had it front facing (I flew with it about 6times in cabin on trans-atlantic flights, and about 4 times in a medium sized aircraft and about 6times in a small aircraft (two seats one side, one seat the other -sized plane).
It fit in all of the seats in each of the planes.
Some airlines will make you lift the child out and place them on your lap, with or without the lap-extension/baby lap belt if the fasten seatbelt sign comes on. (British Airways for example).
And other airlines will only allow their own seats to be used. (Virgin Atlantic for example).
You cannot win them over with logic, reason, rights, law, common sense, scientific fact or tears, but you can make the journey a whole lot worse by arguing the odds.
You may be asked to check your seat, if you are, insist it is a stroller/pushchair and that you are going to gate check it.
Gate checking is where you keep it to the door of the plane and it is brought to the plane door on arrival, if you must check it, this is how!!
-you must get tags for anything gate-checked!
Otherwise it may well become damaged or lost.
I would certainly use a car seat cover and secure straps as best you can before handing over.
Weither or not you gate check or bring it on board, you will go through security, most are helpful some will try to rush you, so try to make it easy by folding any stroller base you choose or unclipping the seat from the gogobabyz thing. ASAP.
If you're traveling alone, ask for help, they might not notice you're alone and need help.
Most airlines will only allow you to 'gate check' one baby item, and they may not allow you through security with both a stroller and a carseat, again check the policy of the specific airline online.
Calling and asking is a 50/50 thing as to what the person on the phone might know or have access to and as to what they might tell you.
Print off the policy and keep it on hand.