This report may be regarded as a continuation of that which appeared in Volume 16, No. 1, page 47 of this Journal. The results are based on precisely the same premises and are shown in the graphs as five-year averages. The graphs are drawn somewhat differently but the scales adopted illustrate the trends rather more clearly. As they cover the period of the last assessment as well as the subsequent three years, it will save referring back if the premises are restated.

I was very interested in the recent correspondence regarding a direction indicator to be fitted to ships and while I note the experiments now being carried out by our Dutch friends there are several points which must be borne in mind.

In the first place, whilst sophisticated arrangements like illuminated arrows may be very satisfactory from the operational point of view, this sort of apparatus is just one more piece of expensive equipment that the shipowner must provide and I foresee reluctance on his part if it is intended that this should come into general use.

Already many years ago Captain A. Vreugdenhil proposed the use of visual aids to supplement the sound signals used to indicate changes in heading. His idea of using visual aids was based on the erratic behaviour of sound and the consequent possibility of misinterpretation of sound signals.

Choosing between two possible systems, the directional illuminated arrow and, for example, flashing lights, he preferred and constructed an illuminated arrow, visible as such from a distance of approximately 1½—2 miles. He was against the use of synchronous flashing lights, pointing out the large difference in speed of propagation between sound and light signals and consequent possibility of errors when different sound signals are given in quick succession.

Commander Clissold's article (Journal19, 109) like others before it seems to base the supposition that a new Rule is required on the idea that the collisions occur today because the present Rules are not good enough. I do not think that this contention is borne out by the evidence in cases of collision that come to Court; those are really the only ones on which conclusions should be based as very few people know what happened in the others. There is also a suggestion in his article that, before the days of radar, the Rules were adequate to prevent collisions happening. This is not so either; there were too many collisions in those days and they were caused by very much the same sort of behaviour as they are today. One should not, perhaps, think about the past success or failure of the Rules without remembering that it is not only the land which has become crowded in this century.

1. Hovercraft Environment. With the exception of the sidewall craft which is unlikely to achieve speeds in excess of 30 knots, all hovercraft depend on their ability to rise clear and free from water drag in order to achieve their high economical speeds. Nevertheless a hovercraft will tend to conform with the surface over which it is travelling. A rough sea produces a rough ride which at speed produces a motion not unlike that of a car going over a series of hump bridges with vertical accelerations in the order of 1½ g—one needs to be strapped into a seat. The most efficient means of propulsion is achieved by using air screws; these are noisy and necessitate crew members using an intercom. Control surfaces, rudder and airscrews are aerodynamic and to reduce air drag the hovercraft is likely to be low in silhouette and give poor all-round vision to the command. The biggest single factor is drift; being aerodynamic and free of water drag, the hovercraft is completely wind borne and in moderate beam winds can track with as much as 45° of drift angle on and turns out of wind can be alarmingly large.

In the January 1966 edition of this Journal Captain Wylie has given a lucid summary of the factors involved in the presentation on radar displays of true and relative motion. He also stated a conclusion on the combinations of display and ancillary equipment which he considers best satisfy the requirements for use when plotting to avoid collision at sea. Whilst acknowledging with respect all the work he has done to clarify the issues raised on this important but contentious subject over a number of years, a number of us has found it impossible to agree with his latest findings. As these may influence opinion in the shipping industry, the Board of Trade, the Courts of Law and the radar manufacturing industry, both at official and unofficial levels, they should not go unchallenged. Our disagreement concerns the priorities defined for display equipment and the method to be used for plotting. There can be no disagreement on academic principles.

This and the following paper by Mr. Attwooll are the first of a series which will describe the work of the Royal Aircraft Establishment on various aspects of air collision risk and traffic control. Mr. Reich's paper will be presented in three parts, the last two of which deal with the analytical techniques and how they are applied. Mr. Attwooll deals with methods of costing the effects of air traffic control deviations and shows how these costs are related to the size of separation standards. He discusses systems for both subsonic and supersonic transports.

1. Objectives. The main task of air traffic controllers is to plan traffic flows so that aircraft are allotted sufficient separation to absorb not only systematic differences in speed but also the imperfections of navigation and piloting, which we term flying errors. To this end, they usually work with three separation standards, to be applied in, respectively, the along-track, across-track and vertical dimensions of space. The separation standard for any one dimension is the minimum intended (i.e. planned) separation permitted in that dimension when the intended separations are less than standard in the other two. The problem is to choose standards which are safe enough, but not so large as to lead to unnecessary traffic delays and deviations.

Commander Clissold (19, 109) has produced a very well considered argument to demonstrate that the Steering and Sailing Rules are inadequate for present-day use and that in the future they are likely to fall well short of traffic requirements.

I agree entirely with this assessment and I agree in general with his proposed solution. His suggested Rule requires action to be taken by both parties to every encounter where there is risk of collision, and the most important objection to this is that it implies dual responsibility for manœuvre. This is something which many people consider highly undesirable despite the fact that it apparently succeeds in the case of Rule 18, and to proceed directly from the present Rules to Commander Clissold's Rules would, I believe, be too large a step to be generally acceptable.

In my recent paper I proposed a set of manœuvring rules intended to solve the operational aspect of the collision problem. These rules are simple, but they are different if the bearing is lesser or greater than 90°. The object of this paper is to present a simpler rule valid for every bearing between 0° and 180°.

The Pair Rule. Except for the singular situation of both vessels meeting head-on or head-to-stern, every encounter involving danger of collision is characterized by the fact that the heading of both vessels is on the same side of the sight line. Close-quarter situations also involve, in general, those standing head-on, except for some situations with the heading on opposite sides of the sight line when one or both of them heads close to this line. Consequently, the evading manœuvre must be such that both headings are on opposite sides of the sight line and opening enough to ensure a safe passing.

1. Introduction. An air traffic control system maintains adequate separation between aircraft by requiring some aircraft to deviate from the paths which they would otherwise follow. In the absence of such deviations it must be assumed that civil transport aircraft would choose to operate at the most economical combination of airspeed, track and altitude (within the limits set by inaccuracies of weather prediction); A.T.C. deviations therefore involve economic penalty to the operator. It is important to assess the magnitude of such penalties for two reasons:

(a) To provide a yardstick by which to compare different A.T.C. systems.

(b) To give a measure of how much might justifiably be spent on new systems and equipment, to enable separation standards (and hence the penalty of deviations) to be reduced without reduction in safety.

The January issue of the Journal (pp. 124–5) contained a summary of Professor E. G. R. Taylor's views on the Vinland Map. To the courtesy of the Editor I owe the opportunity both to read this note in proof and to comment on it in the present issue. I have not seen Professor Taylor's complete study nor her drawings. She expressed some of her misgivings to me in correspondence after (in 1962) seeing a photocopy of the map and reading in typescript my study of the map, which is included in the recently published monograph.

In 1964 during the second single handed transatlantic race Colonel H. G. Hasler in Jester lost the correct time and, having no radio, could not get a time check. Thus he could not obtain the longitude but, being a fine seaman and practical navigator, made a landfall on Nantucket light-vessel by a method used by old sailing ship skippers. This was to position himself on the correct parallel of latitude and sail due east or west to the destination.