If operating an ultralight with private pilot certificate, which rules apply? If a holder of a private, commercial, or ATP pilot certificate operates an ultralight aircraft; and that aircraft is also equipped with the radios and transponders required by an airspace for GA aircraft, can this flight operate under standard VFR rules as a GA aircraft rather than operating under ultralight rules? (ie. Using non-towered airports which are marked &quot;ultralights prohibited&quot; in the chart supplement, and controlled airspace airports.)
All of the regulation of airspace that I can find is focused on either the pilot's certificate and rating, or the aircraft equipment, rather than the aircraft category and class. That is a slow &quot;experimental&quot; light-sport STOL bush plane (which could be as slow as an ultralight), a C172, or a King Air 250 are all equally allowed into class E, D, C, or B airports as long as they have the required electronics and a PIC with a private pilot certificate or above.
I ask mainly because I would like to use an ultralight for commuting, and the places I commute are airports of the above nature. (local street traffic is terrible, and I need STOL capability for my home field. Less than 700ft to clear a 50ft obstacle.)
If the aircraft is not registered, it can only be operated under part 103.
FAR 103.1 states that an ultralight may not &quot;have any U.S. or foreign airworthiness certificate&quot;.  So a registered aircraft could not be operated under Part 103.
Your question mentioned transponders.  Bear in mind that FAR 91.1(e) exempts ultralight vehicles from all of Part 91.  Therefore, if operating under Part 103, there is no particular airspace that would become available if a transponder were added to the ultralight, that would not be available otherwise, unless a particular ATC facility chose to grant the &quot;prior authorization&quot; required for Part 103 operations in certain controlled airspace, specifically on that basis.  This seems unlikely.  In fact, it is probably safe to assume that the FAA would attempt to prohibit a transponder from being added to an aircraft operating under Part 103, although it's not clear what regulation this would be based on.
Related:
In the United States, are ultralights legally considered aircraft?
Would FAR 91.227, or any other regulation, forbid use of ADS-B-out in an unregistered ultralight?