To avoid any misunderstandings, it should be made clear that in the author's opinion the techniques of Air Traffic Control are not likely to be significantly affected by the future advent of an air-to-air collision avoidance system. This is admittedly a contentious view and this paper is a brief survey of some of the factors which lead the author to hold this opinion. After clarifying what is meant by an air-to-air collision avoidance system, the paper deals first with some of the operational problems, then with some of the technical problems and concludes with general comments.

Static pressure in a moving fluid is usually measured by means of a static hole either in a probe or on the wall of a duct. Such a hole creates a disturbance in the flow and thus has some error associated with it. The determination of the magnitude of this error has been attempted by several investigators and the procedure adopted in each case has been as follows :

The pressure in a region of sensibly constant static pressure was registered using holes of various diameter and a curve of pressure versus hole size plotted. The true static pressure was then obtained by extrapolating this curve to zero hole size, and the errors for the various holes were found. This procedure involves the assumption that, as the hole size tends to zero, the pressure registered tends to the true value. It is usually argued that this must be so because the disturbance caused by the static hole is progressively reduced as the hole size is reduced. Obviously, the validity, or otherwise, of the argument is of fundamental importance in the estimation of static hole error, and a somewhat more rigorous justification of the use of the extrapolation procedure is attempted in what follows.

In the technical note in the August JOURNAL by Morgan and Saunders the values given in the last column of Table I on p. 572 are actually the square roots of the open area ratio.

The greatly increased performance of both civil and military aircraft since the Second World War has raised problems undreamt of at an earlier period and, from the simple measures originally introduced to improve pilot’s well-being, have sprung the complex installations which are now essential for human survival and comfort and to ensure the proper functioning of vital equipment carried in present day machines.

The term “Environmental Control System” is used for convenience to cover collectively the various cabin and equipment bay, pressure, temperature and humidity control systems that are fitted.