What does the aircraft special use designation G indicate?

The aircraft special use designation G indicates that the aircraft is intended for "Special test, temporary" purposes. This designation is used to identify aircraft that are involved in testing or evaluation activities that are not part of regular operations. Such aircraft may be used for developmental or operational testing, allowing for experimentation with new technologies or procedures.

The other designations are assigned for specific situations. Permanently grounded would not align with the G designation since grounded aircraft wouldn't be involved in special testing activities. Observation refers to aircraft used for monitoring or reconnaissance purposes, which is a different operational category. Cold weather designations relate specifically to aircraft equipped or modified for operations in extremely low temperatures, which is again distinct from the special testing context implied by G. This clarification of designations helps maintain safety and operational efficiency in aviation.