Flight testing such as the first flight of a new amateur-built aeroplanes, should be minimum essential crew only. There may be reasons to have the builder there - to act as an FTO, to provide direct feedback on design issues, and so on. However, most people in this line of work would agree that for the First Flight (note capitals), that flight should be solo. If you want to talk to him, radio has been routinely fitted to aeroplanes for some years!
There's a lot of history of builders succesfully doing their own first flights - starting with the Wright Brothers. However, I'd also personally ask two important questions if that is proposed: (1) has the builder being maintaining enough flying currency to do this sort of flight where a lot of things can go wrong (For example, I've flown 5 first flights of homebuilts, 2 of them I suffered engine failures and one an ASI failure), (2) do they have the right sort of specific experience to conduct flight testing - a lot of high hour PPLs, or ATPLs for that matter, do not maintain good recency in low speed handling and PFLs for example.
A couple of other passing thoughts:
- I've heard it suggested that the best person to fly a first flight is somebody who isn't too restrained from trashing the aeroplane if that's needed to survive. I'm not necessarily supporting this rather patronising viewpoint, but thoughts I'd mention it.
- There's a lot of experience in how to safely conduct first flights of light aircraft. Whoever flies needs to be familiar with those lessons learned - realistically, neither normal private/professional flying training covers this, nor TPS. The experience lies mostly in a small number of aircraft manufacturers, and the large sport flying associations such as EAA, LAA/PFA, BMAA... Two very good bits of reading matter also are the FAA's AC 90-89 and Vaughan Askew's "Flight testing homebuilt aircraft".
- In an ideal world, the pilot flying the first flight wants to be current (handling, emergencies, stall recoveries) in something pretty similar.
G