Identification techniques have been widely used in the rotorcraft community for some time, but despite good progress, difficulties are often encountered, particularly in the estimation of flight mechanics models. Results are presented on the identifiability of a theoretical six degree-of-freedom flight mechanics model of the SA-330 Puma, in particular on the effect of the control axis used, on the estimation of coupling parameters, and on the importance of low-frequency information. These results form the basis of a proposal that the difficulties encountered in recent AGARD WG18 work can be largely attributed to insufficiently long test records.