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Aviation History
1969
1969 - 3118.PDF
670-671 FLIGHT International. 30 October 1969 AUTOMATIC LANDING The philosophies and progress of three major operators are reviewed in this feature. The three articles underline the very large differences which have always existed between the US and British way of certificating all-weather operations. This difference should be borne in mind when the articles are read. The final article, on head-up displays, will no doubt provoke a reaction from various quarters on the merits of various systems DECISION-HEIGHT TARGET—12ft Progress with the HSA/Smiths system To MANY PEOPLE the progress of the Trident Autoland programme must be closely akin to that well-known "Continuing Story of Peyton Place" TV serial. It seems to have been going on for a very long time without any particularly dramatic result in actual airline service. To those who are close to developments in automatic landing techniques, however, the time-scale is no surprise, and indeed was forecast several years ago by Hawker Siddeley. It must be remembered that Hawker Siddeley, which makes the Trident, Smiths Industries, which makes the automatic system, and BEA, the customer, have 'been leaders in a field where the pitfalls are many and the means to overcome them largely untried. It says a great deal for all three companies that they have persevered for so long without losing too much patience with the certificating authorities, even when progress abroad (under different rules) has brought superficially spectacular results. The process of certification in Britain of automatic landing devices is a much longer and more stringent procedure altogether than the US counterpart and when only a few aircraft are available it becomes a long drawn-out task. It is as well to review briefly the sequence of the Trident Autoland programme. Flight trials in Trident 1 G-ARPB finished in January 1967 and provided data for the certifica tion of the Smiths autopilot and autothrottle in the duplex and triplex mode for automatic landing in Cat 2 and Cat 3a weather minima. The flight manual for the Trident 1 was amended in September 1968 to allow automatic landing, in service, down to the lowest limits of Cat 2. In February 1969 the Board of Trade authorised BEA to operate down to limits in the middle of Cat 2 (150ft decision height and 500m RVR). Clearance to operate to the bottom of Cat 2 with passengers aboard will not be given until BEA can furnish evidence of successful operations at the interim limits and the provision of Cat 2 ground equipment (ITS, lighting, and RVR measuring devices) at a number of airfields. Further clearance to Cat 3a limits (200m RVR) depends on the ARB's acceptance of the performance and fault analysis of the airborne equipment, the fitting of BEA aircraft with a standard of equipment which will result from the fault analyses, and confirmation that in-service performance (as measured by in-flight recording) is comparable to that measured by Hawker Siddeley during development. Clearance is also dependent on the provision of suitable ground equip ment, the problem in this case being the fitting of suitable monitors to ILS transmitters, and duplication of transmitters and power supplies to meet Cat 3 ground equipment stan dards. The 3a clearance, Hawker Siddeley predicts, will not be gained before the winter of 1971-72. particularly in the ILS aspect. But the equipment is in the aircraft being operated by BEA and is used to the limits (150ft and 500m RVR) which have been agreed so far. The second major programme which Hawker Siddeley and Smiths embarked upon took place last autumn and winter (July 1968-April 1969) when Trident 2 G-AVFA. was flown from Hatfield on a series of development trials which had two objectives: to repeat the Cat 3a clearance for the different characteristics of the Trident 2; and to extend the clearance for the Trident 1 and 2 for certification to Cat 3b limits (in 50m RVR). The Cat 3a RVR limit of 200m (laid down by ICAO) assumes that after touchdown there will always be enough visibility for the pilot to be able to control the landing run by visual reference alone. Unfortunately visibility of this order is notoriously unstable and can vary rapidly between 50m and 400m, with the result that airline operations in these conditions are frequently impractical. BEA, from the outset, has required a clearance for which the lowest limit is set by the pilots' ability to taxi visually from the embarkation area to the runway and back—in practical terms an RVR of 50m. In these conditions "blobs" of thicker fog on the runway may
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