Commuter Category is defined rigorously,
https://www.law.cornell.edu/cfr/text/14/23.3
But I was unable to find an equivalent definition for Transport Category in the regulations (not Wikipedia or the FAA website, haha). Nor was I able to find anything like "if requirements not met for Commuter, then the vehicle is in Transport Category."
Could somebody point me in the right direction?
Thank you.
Transport airplanes are airplanes for which a type certificate is applied for under part 21 in the transport category and that meet the transport category airworthiness requirements. Multi-engine airplanes with more than 19 seats or a maximum takeoff weight greater than 19,000 lbs must be certificated in the transport category.
In other words, there are two possible reasons why an aircraft is transport category: the manufacturer applied for a transport category type certificate, or the aircraft has more than 19 seats or MTOW greater than 19,000lbs.
The first reason refers to 14 CFR 21.21:
An applicant is entitled to a type certificate for an aircraft in the normal, utility, acrobatic, commuter, or transport category, or for a manned free balloon, special class of aircraft, or an aircraft engine or propeller, if—
[...]
(b) The applicant submits the type design, test reports, and computations necessary to show that the product to be certificated meets the applicable airworthiness, aircraft noise, fuel venting, and exhaust emission requirements of this subchapter
The "applicable airworthiness, aircraft noise, fuel venting, and exhaust emission requirements of this subchapter" for transport category aircraft are in part 25, AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES. If your aircraft meets those requirements and you get a type certificate from the FAA, then you have a transport category aircraft.
The second reason refers to 14 CFR 23.2005(a):
Certification in the normal category applies to airplanes with a passenger-seating configuration of 19 or less and a maximum certificated takeoff weight of 19,000 pounds or less.
That means, if your aircraft exceeds 19,000lbs then you can't certify it in the normal category (part 23), which leaves only transport (part 25).
By the way, the 23.3 regulation you quoted doesn't exist any more because part 23 has been completely rewritten.
I'm not sure there is an explicit definition, but the transport category is basically supposed to be "anything too big to qualify for Part 23." There are no general restrictions on what aircraft can be transport category, except that it basically has to have at least two engines. But even that could be open to interpretation. The requirements are generally more strict, so for the most part an aircraft will use Part 23 if it can.
