Before embarking upon the business of this paper I should like to make one or two general remarks by way of explanation.

It was suggested by two members of the Council that the present situation with regard to ancillary power services on aircraft was far from satisfactory, and that the question might well form the basis of a lecture before the Society; moreover, that such a lecture might be more interesting and valuable if presented as a joint paper by a member of an engine firm and an aircraft firm respectively. Mr. Nixon and myself have had the honour of being asked to collaborate in this connection and we trust the result will be acceptable to you.