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Aviation History
1968
1968-1 - 0233.PDF
1064 Flight-deck Displays FUGHT International, 26 December (968 When will Head-up go Civil? By B. S. WOLFE, CEng, MIERE, AFRAaS, AMBIM* and G. R. SLEIGHT, BSc, Grad RAeSf Straight-through projector mounted overhead, with the combiner on the glareshield, is one of the HUD installation options. This artist's impression shows Elliott-Automation 80- series equipment in a VCIO THE APPLICATION of head-up displays (HUDs) to civil aircraft has been discussed at conferences and symposia for the last four years, but no airline has yet fitted such systems to passenger-carrying aircraft. As there have been, in the civil field, no really fundamental technical breakthroughs in this period to modify the situation, the other factors affecting their possible fitment have to be fully appreciated before any reasoned estimate can be made as to when and if such systems will be adopted. These factors come under a number of headings. Role Definition There has been much confusion over the possible role of a HUD. Although this is still a contentious subject, the situation is very much clearer than it was even 18 months ago. In the main, this has been helped by the flight trials of Elliott HUDs by McDonnell Douglas in a DC-9-30 at Long Beach; by Short Brothers in conjunction with A&AEE on the Belfast and also by the Blind Landing Experimental Unit (BLEU) with their Comet and Varsity at Bedford. Two prime modes of operation have emerged which could technically justify the application of HUD systems in approach and landing. The first application is in the "all-weather" environment, in which the HUD can form a particularly effective monitor for approach performance and autopilot behaviour in Cat 2 and Cat 3a conditions. In the event of a decision to go-around, either because of an autopilot disconnect or for any other reason, go-around guidance information can replace approach information in the head-up display, thus avoiding any transfer of the first pilot's attention from outside to inside the aircraft. The second application for a HUD is to give the pilot, in good visibility, an accurate flight-path vector and desired flight-path-angle presentation. Although this use was first suggested several years ago by Dr J. M. Naish (now with McDonnell Douglas at Long Beach, but formerly with RAE, Farnborough), it has lately been investigated in detail because of the increasing difficulties of judging approach paths with large jet transports and the more serious consequences of any serious misjudgment. The introduction of inertial navi gation systems in commercial aircraft provides both an adequate computational source and vertical reference informa tion accurate to about one tenth of a degree. This is necessary for effective display of flight path vector and desired flight path angle. In short, although the introduction of HUDs has been delayed by the difficulty of defining their role, a fair measure of agreement has been reached during the last year. Certification A second and very important factor following from this lack of firm role definition has been the simple fact that no HUD system on either side of the Atlantic is yet certificated for use in commercial aircraft. The nearest approach to formal certification to date is the clearance given to the Elliott HUDs which have been fitted to all the Belfast transports in RAF Air Support Command. The reasons are twofold. Firstly, because of the inherent •Manager, Airborne Display Division, Elliott Flight Automation Ltd. fProject Manager, Airborne Display Division. flexibility of electronic HUDs it is possible to develop them to fulfil any reasonable requirement. In the absence of an agreed ARINC or customer specification, a manufacturer is faced with real difficulties in determining a standard for full safety and design analysis for certification: much of his work will be invalidated unless something very close to his chosen standard passes into service. Secondly, and more important, is the consideration that HUD will always be a component part of a flight control system. Entirely representative HUD hardware, integrated with a particular Automatic flight control system in a particular aircraft, must be offered to the certification authorities and flight-tested. Furthermore, this exercise must be accompanied by the manufacturer's equipment design and safety analysis. These steps have yet to be taken by either an airline or airframe company in conjunction with an equipment manu facturer. Installation Problems Because all prime cockpit areas are already assigned to traditional equipment, the addition of a HUD projection unit on the panel presents at first sight an impossible problem but, with a little determination and the new small projection units, satisfactory installations can generally be evolved. There are two general areas in which a projection unit can be installed: the glareshield (coaming) or the overhead area. The glareshield is a border area constrained by internal and external vision areas, instrument sizes and clearance for the pilots' knuckles when the aileron wheel is rotated. In addition, neither the projection unit itself nor the combiner glass should project further aft than the rear edge of the glareshield. Even so, it is frequently possible to accommodate a minimal-size projection unit in the glareshield, either directly ahead of the pilot or offset slightly inboard and angled so that the pilot sees the projected image directly ahead through a compound- angle combiner glass. The overhead area is bounded by the roof structure and pilot's head clearance. In general, the space available reduces sharply going outboard and increases going inboard. Overhead switch panels and eyebrow windows often reduce the space available. An overhead installation was used in the BLEU Varsity, with a straight-through projector above the pilot's head and a combiner mounted on the glareshield. Various other overhead installations have been planned in detail for transport aircraft. Technical Reservations The airlines' principal technical reservations on HUDs were perhaps best summarised by Capt F. L. Wallace of PanAm, chairman of the ATA Head-up Display Working Group, in a paper presented at the ALPA Forum in Seattle this year. The main items were: uncertainty as to the effect on visual cue-acquisition of the light loss at the combining glass; uncertainty over whether the system should synthesise the real world or flight director guidance; reservations about the lack of civil operational experience with HUDs; reservations about the possible dependence of the HUD on ILS signals when used as an all-weather aid; reservations about the integrity of a HUD system as an approach aid; and understandable confusion over display symbol format—a subject on which there tend to be as many experts as there are sheets of paper on which to draw new sets of symbols. Cost Effectiveness Another of the difficulties which has prevented HUDs from entering airline service to date is simply that it is quite feasible to certify a Cat 2/3a system which does not include a HUD. The bulk of HUD development has been done in Britain, but the flight control systems in all modern
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