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approach area which may be marked satisfactorily, and a range of hills which obstructs a large part of the approach area and which, even if adequately marked, would constitute a danger. 247. The circuit zone " surfaces " prescribed in the AG A Standards may be the most difficult to satisfy in full in New Zealand. In a number of places it is possible to locate an aerodrome with two diametrically opposed clear approaches fully satisfying the AGA requirements for approach areas, and where local prevailing winds are sufficiently regular in direction, these two directions of approach may be all that are required. At the same time, neighbouring hills on both sides, as in a valley site, or on one side, as at some coastal sites, may project well above the prescribed surfaces in the circuit zone. The horizontal surface, applicable to all aerodromes, at a level of 150 feet above the aerodrome reference point, and above which all obstructions should be removed or must be marked, extends to 2f miles in all directions ; and the " conical surface," applicable to Class C international aerodromes, extends to 300 feet at three miles from the aerodrome reference point. Bearing in mind that marking is not a fully satisfactory method of dealing with anything but isolated obstructions, it is clear that the 1.C.A.0. AGA Standards for circuit clearance cannot be fully met at a number of existing and projected aerodromes in New Zealand. Nevertheless, aircraft with adequate manoeuvrability can operate with safety at such aerodromes in visual flight conditions, and in some cases even in instrument flight conditions with modern instrumental aids and suitable operational procedure. Some sacrifice of regularity —that is, acceptance of a lower standard of usability of the aerodrome —may be necessary. New Zealand is not the only country where such conditions exist and where some departure from the 1.C.A.0. AGA Standards as now proposed will be necessary. It appears desirable that 1.C.A.0. should study the amendment of the draft Standards. We recommend that, while New Zealand aerodromes are planned to secure the nearest approach to compliance with the draft Standards, the problem of amendment should be pursued with 1.C.A.0. 248. When the problem of installing instrument landing aids and prescribing instrument approach procedures is faced, it will be found that they affect areas much more extensive than those covered by the AGA approach areas, and that the procedures have to be designed for each aerodrome and for the particular type of aid installed. 1.C.A.0. have not yet produced any final recommendations on standards concerning the application of instrument landing aids, it being a subject of peculiar technical difficulty on which to obtain international agreement. The Ministry of Civil Aviation in the United Kingdom, however, have given the matter considerable study and have produced a Notice to Airmen (No. 117/1948) setting out the instrument approach and holding