'Pear-shaped' is an RAF euphemism for 'tits-up', which
is itself a euphemism for 'dead'.
The Oxford dictionary definition cites an RAF source
in 1983. I was an RAF pilot in 1983. The reason no-one will
confirm the meaning officially is that it's rude, hence the rubbish about aerobatics. Loops that go wrong
are egg-shaped, and have always been
referred to as such.