So is the plane seatbelt tighter than a car seatbelt? I haven't flown in 10 years and can't remember what the belts are like. I ask because she would slide right out from under a car seatbelt.
I looked into the harness, but they are only approved for up to 40in.
How would she accidentally slide out of a car seatbelt? Oh you mean in a collision?
Regardless, the lapbelt on the plane tightens as much as you need it to. Since she's short she can sit crisscross and tighten that lapbelt up. Since she's older, she's mature enough to know how to behave- when seated the belt goes on (plane rules). That protects from the unexpected turbulence.
I think that the type of motion is different than say being rear ended or slamming on brakes, which might be why I haven't heard of issues with kids slipping under the belt (submarining) on a plane?
Absolutely. As you are saying, we are talking about totally different forces and accidents here. With planes the concern is keeping the kids from going airborne in sudden ups and downs. Lapbelt is good at that. Some kids won't accept the lapbelt or the rules and that's why car seats are good for those kids. One day my flight changed and I was faced with the prospect of getting the Roundabout and 2ish year old and carryons onto an already full, about to take off, plane, and I couldn't handle it. Told him the rules, gate checked (to be picked up at the ramp in order to minimize the forces on the seat) the seat, and had an easy breezy flight with him very safe in his lapbelt. And he felt SO grown up and proud that I believed in him that to this day (he's 11) he clips that belt on even if we are half an hour from takeoff procedures and bugs us to, too.
We didn't start using the bubblebum in cars until he was 6+, and at home he was still in a high back booster (we bought the nautilus for home to keep him harnessed to the max weight but that stinker outgrew the harness by height WAY before his weight hit the harness limit and we couldn't afford to get the Frontier or whatever it was called) and we only used that traveling. Perfect invention. Pops the mature-enough-to-be-in-a-booster kid up for the belts to fit, and weighs a pound so the kid can carry it on his or her own.
In the time we were flying with a seat I never had a FA look at my seat let along look for a sticker. Some FAs have too much time on their hands IMO.