NavList:

Ed re your question about MPP calculation as per AFM 51-40 Vol II Apr 1960

This is from an Air Navigator training manual.  Because aircraft go so fast, Air Navigators must be particularly well organised, so said manual is as much about teaching students a practical routine as it is about teaching theory.  When qualified the Navigator will go to a Command and an aircraft where there will be a particular set of rules for calculating MPPs depending upon the fixing aid used and the DR system used.  Also, the technique chosen will be the most practical for the time available to complete the MPP and the best that can be expected to be learned by an Air Navigator, not necessarily the most statically accurate method that a professor of mathematics might come up with after a week to think about it.  

In general, the perpendicular from the LOP to the DR position will be divided in the ratio of the expected error in the LOP to the expected error in the DR position.  The expected error in the LOP will be a fixed amount depending on the aid used, and the expected error in the DR position will depend upon what’s driving it and the time since the last MPP.  Clearly there are many different fixing aids and many different equipments driving the DR position all with different expected errors, so what the authors have done here is construct a quasi-general case using p, the difference between fix and DR position, and t, the time the DR plot has been running.  This makes the student aware that there is such a thing as an MPP and that he has to include its calculation in his operating routine.  D=tp/t+p can also be written as 1/ (1/p + 1/t) and maybe one of you mathematicians can relate this to a ratio calculation.  We did something similar in the RAF except, as far as I recall, we used the 'school geometry' method of dividing the line. Usual caveat; I might be compleatly wrong.  DaveP