"6 feet across"?? That sounds most unusual if it was actually "ball lightning" - most "ball lightning" globes are no more than 20-30cms in diameter, and many are only tennis ball sized.
There certainly have been extensively witnessed events of ball lightning inside aircraft - including one in 1963, on an aircraft full of scientists!
Ball lightning is a peculiar event that is understudied because it is so rare and unusual and short-lived.
However, there are numerous ball lightning events that have been widely reported and witnessed for many centuries. The old newspapers contain thousands of ball lightning reports, at sea and on land.
For some reason, the ball lightning appearing inside aircraft doesn't appear to have the same strength as ball lightning near the ground - where it has often been reported as having the same vicious strength as a bolt of lightning.
Below is possibly the best article on ball lightning, written by a CSIRO scientist.
Ball lightning exists - but what on earth is it?
My father and mother were witness to an amazing (ground-based) ball lightning event around early 1950. I was there at the event - but unfortunately, only in a cot!
In 1950, our family lived at the top of a sparsely-populated suburban street, near the top of quite a steep, high sandy hill (around 100M high) that had a ridge of very large Tuart trees scattered along the top of the ridge.
Tuart trees grow to around 30M high and can have a girth of over a metre at the butt.
It was late afternoon (probably around 6:00PM) and a sultry, stormy afternoon when Dad alighted from the bus on the main road at the bottom of the hill and commenced the 400M walk up the hill to our house.
It was still very light, as sunset is around 7:45PM at the height of Summer here.
As Dad walked up the hill, he was aware of a buzzing, crackling sound that made him turn around. He was startled to witness an outstanding example of "ball lightning" that became a much-talked about event, locally.
The event was witnessed by numerous people, including my mother, who had her face pressed to a window in the house, watching the event unfold.
According to Dad and Mum, the lightning ball travelled approximately up the centre of the street at about walking pace, at around a height of about 6 to 8M. They stated how the air "fairly crackled with electricity".
As the ball lightning passed each of the scattered houses on the street, it blew all the fuses on each house. Mum stated how she could "feel the electricity" through the window pane - enough to make her jerk back away from the glass.
They watched as the ball lightning passed our house (blowing all the fuses in the main switchboard, as it went) - then watched as it reached the top of the hill, and commenced to veer away from the road.
The ball lightning travelled directly towards one of the largest Tuart trees on the ridge - which it struck, with exactly the same effect as a lightning bolt.
The tree exploded with a massive blast that shook the house, and the tree effectively vanished!
As with any lightning strike, the tree sap had heated instantaneously and the tree virtually exploded. All that was left of the tree was a smoking stump about a metre high - and enough firewood-sized pieces of timber that kept the entire street in firewood for weeks!
The entire event lasted around 5-8 minutes - a long time, according to most witnessed ball lightning events - as most of the ball lightning events seem to only last a very short time.