My 4-year old daughter sits on her car booster seat in the front with me (no seats in the back
). She wears an adult headset set which is not ideal but serves the purpose until I get her a child size set.
The only time she seems uncomfortable is when I swing the prop (she is strapped in and unable to reach the throttle). The longest we've been up is about 45 minutes and she seemed fine and she is always miffed when we land. The Auster has a noisy cockpit (no muffler) and I don't want to batter her tender lugs for too long until I get her a decent headset.
She does seem to find straight and level boring. She loves doing Battle of Britain style breaks. Another favourite is circuits, even when I'm doing Tigger impersonations down the runway.
Good luck with the marathon Fuji, take plenty of drawing material, crayons and emergency drinks/relief bottles and things will be cool.
