"With only the Cirrus fitted with a Ballistic shute system from new and designed into the aircraft from scratch, what are the practicallities of retrofitting such a system to older aircraft especially ones used for formation flying or aerobatics?
I would imagine fitting a reliable and tested system would be complicated ?"
Cirrus Design were told for years by the rest of the GA industry they would never get a BRS certified for the SR20. It was an incredible feat. It is probably well known that the desire to add parachute capability to the whole airframe arose directly from Alan Klapmeier having survived fatal midair collision during his flight training.
Flight Safety Foundation 2005 de Florez Award Citation for Alan Klapmeier
Similar BRS can now be retrofitted to other GA aircraft - I have seen photos of them installed in C172 for example. That type has a rear window the rocket can exit from, YMMV in other types.
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A very sad day.