Final answer:
An aircraft navigating with an IFR-approved GPS on an IFR flight plan is indeed considered an RNAV aircraft, which allows for more efficient navigation within the airspace system.
Step-by-step explanation:
Yes, an aircraft on an IFR flight plan navigating by an IFR approved GPS is considered to be an RNAV (Area Navigation) aircraft. RNAV is a method of navigation which allows an aircraft to fly on any desired flight path within the coverage of ground- or space-based navigation aids, within the limits of the capability of the self-contained system, or a combination of these. An IFR-approved GPS enables precise lateral and vertical navigation and, when used as part of the aircraft's navigation system, supports RNAV operations.
Therefore, an aircraft equipped with this technology would be categorized as an RNAV aircraft, which is significant for adhering to specific air traffic control procedures and separation standards.