They do exist.
They rely on two methods one fires the propellors and then the seat the other ejects the seat to the side.
(
riverghost servicing your mum since,
Fri 16 Jan 2009, 17:13,
archived)
i think i saw this on qi actually
now you mention it
(
horrid,
Fri 16 Jan 2009, 17:46,
archived)