Hi guys,

Let me jump into the middle of this with both feet, and hope to escape without
too many flames...

14 CFR part 21 contains the procedural rules for obtaining a type certificate
and approval for changes to a type certificate (i.e., a Supplemental Type
Certificate), and airworthiness requirements for "unique" kinds of aircraft.
An applicant who applies for a Type Certificate or Supplemental Type Certifcate
under this part, is granted approval under part 21 if the FAA finds that the
design or change to a design shows compliance to the applicable airworthiness
requirements.

The airworthiness requirements to which the design, or change to a design, must
show compliance may be in part 21 (for example, primary category aircraft or
foreign aircraft accepted under a bilateral airworthiness agreement), or in one
of the airworthiness standards such as part 23 for normal, utility, acrobatic,
and commuter category airplanes.

The Type Certificate Data Sheet for an aircraft shows the Certification Basis.
The Certification Basis is the set of airworthiness standards to which
compliance was shown. For example, U.S. manufactured normal and utility
category airplanes are certificated to airworthiness standards contained in
part 23 or its predecessor, CAR 3. Primary Category aircraft are certificated
to airworthiness requirements listed in part 21, so part 21 would be listed in
the Type Certificate Data Sheet. However, procedural rules of part 21 are not
shown in the Data Sheet.

So, in following this thread, my impression is that you're both right. The
design approvals are issued under part 21, and the airworthiness requirements
to which compliance must be shown are listed in the Type Certificate Data
Sheet.

Nothin' like muddying the waters....

Cheers,

Gordon.