While congratulating F. H. Robertson on his technical note in the January 1957 JOURNAL (page 52) on “short cut” methods of estimating aircraft direct operating costs, which certainly give a very rapid approximate answer, I am somewhat uncertain as to its practical value. In the analysis of possible projects, having established the general range of operating costs, which can usually be done by a combination of experience and inspection, interest lies primarily in the effect on cost of operation of such factors as change of type of power plant, fuel used, what complexity of structural designs is permissible from a cost aspect, how much is desirable from a weight saving aspect and so on. Unfortunately, Mr. Robertson's formulae do not allow such comparisons to be made primarily because of the assumptions made.