Are you sure your kiddo still fits in the Scenera ff? It’s generally outgrown both rf and ff around the same time, generally around age 3, as it has extremely low top harness slots (the harness must come from above the child’s shoulders for ff). If your kiddo is at least 30 lbs and 32”, a Cosco Finale is an inexpensive ff only seat that would be a good travel option and would likely be low profile enough that the tray table could be used. Regardless, a 3 year old definitely needs to use a car seat on the plane.
Here’s a very informative post about safe plane travel with kids from Car Seats for the Littles. They also have a very helpful Facebook group for asking questions about specific situations.
https://csftl.org/leaving-on-a-jet-plane-the-csftl-guide-to-safe-air-travel-with-children/