"Experimental" is a legal definition in the US which has no direct comparison in the UK or many other countries. Where something is labelled experimental it generally means that:-
- There has been no legal requirement for any kind of independent design review or comparison to an airworthiness standard (having said which the likes of Boeing clearly are incredibly thorough, homebuilders or amateur designers may not)
- The FAA (or their delegate such as an EAA technical counsellor) has carried out a single build quality inspection prior to first flight.
- Initial flying is limited to essential crew only and a fixed, largely unpopulated area.
As an approach it clearly gives great freedom, but at the amateur end, with designs not independently reviewed, no independent inspection during construction of parts that will later be concealed, and pilots who are not flight-test trained, the approach contributes to a greater accident rate than in other countries such as the UK.
Comparing to the system in most European countries, there are marked differences. Over here the aircraft could not normally fly indefinitely under an "Experimental" tag, must must eventually be shown safe and issued with a CofA or local lower standard such as the UK Permit to Fly.
Flight testing in Europe also has normally to be carried out by an "approved organisation", in the UK this usually means that they have "B-conditions" which is delegated flight test authority to work to a pre-agreed rulebook, generally using specific authorised test aircrew. (PFA don't work to B-conditions but to something very similar, BMAA do work to B-conditions as do all the UK aircraft manufacturers).
Of course, the fact that a single seater in this context has suffered a fatal accident may be irrelevant to the experimental status. The Hummel bird is a well established design. Having said that, if it was a first flight FAA publish some excellent guidance in AC90-89a, which sadly does sometimes get ignored - non FT-trained pilots performing first flights are arguably either brave or foolish, and many such first flights in the US are by relatively low-hour PPLs.
My deepest sympathies to all who knew the pilot.
G