If operating an ultralight with private pilot certificate, which rules apply?

Question

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 "ultralights prohibited" 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 "experimental" 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.)

Answer 1

If the aircraft is not registered, it can only be operated under part 103.

FAR 103.1 states that an ultralight may not "have any U.S. or foreign airworthiness certificate". 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 "prior authorization" 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?

Answer 2

Per 14 CFR Part 103, an ultralight can not be registered as an aircraft under Part 91. But, there is no regulation limiting the weight of an aircraft being operated under Part 91. To do so, an aircraft just must be registered under Part 91 as an aircraft. But, an aircraft can only meet either Part 103 or Part 91. It can not meet both simultaneously. In other words, an ultralight that meets Part 91 regulations is no longer an ultralight. Although, it could be considered a Light Sport Aircraft or experimental aircraft.

If an aircraft were to operate under Part 91, it must meet all aspects of Part 91 regardless of its weight. This includes all documents, inspections, maintenance, and equipment. You will not be able to cherry-pick the ones that suit you. But, for flight time used to meet Part 61 requirements at this time, no time flown under Part 103 can be used.

The better question is whether you can operate your ultralight in the airspace that you want to. Per 14 CFR Part 103.17, you can operate an ultralight in any A, B, C, D and E to the surface airspace for which you have prior ATC authorization. There is no time period or mechanism specified for the authorization. But, if you use a radio onboard your ultralight to ask for authorization from ATC, will it have the wattage to maintain two-way communication?