Head-Up Displays (HUDs) initially found favour in military applications because they enhanced pilots' abilities to fly their aircraft and deliver weapons accurately. The HUD also enabled the pilot to "keep his head out of the office" during high-adrenaline phases of flight by presenting him with the basic parameters he needed while looking at the real world around his aircraft.

HUD technology translated fairly fluently into large-military-aircraft applications. Much work was done on the Lockheed High Technology Test Bed - a significantly modified C-130 conceived to stretch the frontiers of knowledge in many arenas, including aerodynamics, ultra-high-pressure hydraulic flight- controls, voice control, infra-red sensors - and the HUD. In this instance, the HUD was designed to allow the aircraft to be positioned for consistently accurate landings at night, or in poor visibility - mainly for clandestine operations at strips devoid of lights and landing aids.

In turn, it became readily apparent that a HUD capable of being used to guide an aircraft accurately to touchdown in poor visibility could be an attractive proposition for commercial flights. Although Category III landing capabilities, including automatic-landing systems, are proven techniques, they are costly. The Cat III runway has to be expensively equipped with augmented markers and lighting and the aircraft has to have integrated, multiply redundant, autopilot and guidance systems to achieve coupled approaches with precision and reliability. Such operations demand special crew training and currency requirements for both aircraft and crews which can be onerous for an operator to maintain.



Essentially, this is because Cat III capability earns its keep when visibility is bad (when, for example, runway visual range (RVR) is 600ft (185m) and decision height (DH) is 50ft), but is dead weight when conditions are Cat II or better (1,200ft RVR and 100ft DH). Yet pilots have to be practiced in the unique Cat III procedures, and the integrity of the aircraft equipment must be proven by regular use. Thus, crews can find themselves carrying out a Cat III approach into Tokyo, for example, on an "eight-eighths" blue-sky day just to keep current and, arguably, when they could be better employed exercising their manual landing skills.

Also, it usually makes sense to fit Cat III equipment only to new aircraft during manufacture, because of design, production, certification and cost considerations. Many airlines, however, still have older aircraft with years of economical life in them, particularly when fitted with hushkits or re-engined with quieter, cleaner, more fuel-efficient powerplants. United Airlines, for example, still has more than 70 Boeing 727s in service There are numerous airlines, therefore, which would be attracted to a relatively inexpensive HUD that can be fitted to existing fleets, if the cost could be justified through on-time arrivals at destinations often afflicted by mist and fog. (One published estimate says that US airlines suffer $1 billion-worth of direct losses through low-visibility weather conditions each year). An airline which has to delay or divert has to contend with disgruntled passengers, and a parcels carrier such as FedEx or UPS which cannot deliver by its guaranteed hour can face compensation costs.

Additionally, even if a fleet is Cat III-capable, there is a frustratingly large number of airports worldwide not equipped with Cat III runways. Washington Dulles, for example, has three runways, giving six potential landing directions, but only one is Cat III-equipped. Once again this is almost certainly down to economics; if Dulles were persistently affected by fog, equipping more runways might be justified (costs vary from about $600,000, to upgrade an existing instrument runway, to about $2.6 million to equip a virgin strip of concrete). As it is, if a Cat III-equipped aircraft's destination is Dulles on a rare day when the airport does have fog, you can expect to take your turn in the stack for an approach to the one runway in use, with the consequent frustrations of a late arrival.

Clearly, therefore, there is a wide potential marked in the USA alone for a HUD which could give "Cat III capability with Cat I ground equipment".




The benefits of HUDs have already been successfully exploited in the fog-bound reaches of Alaska and in the misty north-west USA. Flight Dynamics HUDs are now providing Cat III minima to operators using them, but only at runways with Cat III ground equipment. This is because US Federal Aviation Administration certification requires the pilot to enjoy the same visual cues at decision height as do his Cat III-equipped brethren - that is, Cat III approach and runway lights - and, in addition, the aircraft must be able to receive the Cat III markers. In simple terms, therefore, the HUD has replaced the Cat III digital autopilot by refining the pilot's handling skills (or those of the Cat I autopilot) to a level where they can closely match the positioning accuracy required for a Cat III approach, but only at Cat III runways.

This serious limitation to the wider use of HUDs has prompted a programme to examine autonomous landing guidance (ALG), by providing "vision enhancement" through on-board equipment - in this case a radar - to give the pilot the visual cues needed at 50ft to complete a landing, as a substitute for the Cat III approach and runway lights. Such a concept would give true Cat III minima with Cat I ground equipment.

Gulfstream has already embarked on a programme to certificate the Honeywell/GEC HUD 2020 HUD on its GIV and GV business jets, with the ultimate aim of providing Cat III capability at Cat I runways (Flight International, 19-25 June, 1996, P31-33). Gulfstream wants a HUD that will give its aircraft a "go-anywhere" capability, almost regardless of the weather, and with enhanced safety. Thus, the system embraces many parameters - global positioning, collision avoidance and windshear, for example - which can be integrated with the HUD display. The pilot can fly a Gulfstream "through the HUD", to marshal it in terminal areas, carry out an approach to Cat III minima and, if necessary, perform a missed-approach procedure - all using the HUD and benefiting from any collision-avoidance or windshear-recovery information superimposed on it. Gulfstream also hopes to gain eventual certification for Cat III approaches without any runway-approach equipment through satellite-landing and enhanced-vision systems and the HUD.

The ALG programme, however, is less wide-ranging in its ambitions. Its aim is to use a HUD only to enable the pilot to fly an accurate approach and land the aeroplane in fog at runways with a Cat I instrument-landing system (ILS) - and that alone. Restricting the aim to conquering fog (not rain or heavy snow) has enabled airborne radar to be a tenable solution, because radar's known performance degradation in rain is not, therefore, an issue.

The interest among US domestic airlines, parcel carriers and the US Air Force was a strong motivator for more research into "autonomous" landing in fog. The US Government, after some vacillation, agreed to provide about half the programme funding, so long as the interested parties found the other half either in cash or kind. The ALG consortium is 13-strong, including elements from the US Government, manufacturing industry, the USAF and airlines. Their brief to date has been to undertake a proof-of-concept programme, examining the vision-enhancement performance of an airborne millimetre-wave radar (MWR) and a forward-looking infra-red (FLIR) sensor. Because there are numerous possible HUD/enhanced-vision applications in large military aircraft, including steep approaches in battlefield conditions and operational air-to-air refuelling, the USAF has a strong participation, providing its so-called "Speckled Trout" Boeing C-135 as a testbed The commercial side of the programme involves a United Airlines 727-200 testbed - just the type which could benefit from enhanced vision, and a logical choice to prove compatibility between modern equipment and the 727's analogue systems.


TESTING the system

Flight International was able to fly this 727 while it was based at Andrews AFB, on the outskirts of Washington, DC. The test team had, by then, completed most of the feasibility trials and were undertaking a tour of the USA, demonstrating the state of the art.

The 727 is a much-used -200, with the passenger cabin stripped of seats save for a few for invited passengers and the essential flight-test observers. Several equipment crates and monitors stand where passengers had previously sat. The only obvious change to the 727's familiar external profile is the addition of a small "chin" to the radome, where the MWR antenna and dual-band FLIR sensors have been installed beneath the weather radar. In the cockpit, all is classic 727, except for a small and neat HUD symbol-generator above the captain's head, and the HUD combiner-glass between the captain and his windscreen. A video recorder films the view of the outside world through the first officer's windscreen.

The monitor screens in the cabin are useful for understanding what is going on at any moment because they display the co-pilot's view, radar image, FLIR picture and HUD display. The FLIR picture is remarkably good, but infra-red (IR) is known to have strictly limited weather penetration and, had the weather been poor, the picture would have been poor.

ALG prime contractor and team leader is Lear Astronics (owned by GEC of the UK), which has provided the radar and the HUD - the latter made by Sextant Avionique of France. Because HUD development was not an objective of the team, a proven HUD available "off the shelf" was chosen; Sextant has made HUDs for 737s operated by Aeropostale of France, for use in similar, foggy, conditions, but without vision enhancement. FLIR Systems has provided the IR sensor. While no-one advocated IR as the most likely medium for airborne vision-enhancement, it could be the answer for taxiing in fog after landing. Part of the ALG brief was to explore "sensor fusion" - basically developing algorithms to display on the combiner whichever is the better picture for the prevailing conditions, radar or IR. United Airlines contributed the 727 (technically leased to Lear Astronics) and its pilot.

As a lead-in to flying the ALG system, I was able to sit in the passenger cabin and watch two approaches on the monitors. The weather was good, with broken cloud at about 2,000ft, hazy sun and about 15km (8nm) visibility. During taxiing, the FLIR picture quality almost equalled that of the video. Taxiways and other aircraft could clearly be seen, but, as the sensors are positioned in the nose of the aircraft, there was little ability to see round corners such as sharp turns on the taxiway. The radar, by comparison, showed indecipherable clutter.

The HUD monitor showed the combiner clearly and it was possible to become familiar with the Sextant symbology. This unit, like most HUDs, is reasonably "user friendly", but, for a pilot without previous experience of HUDs, knowing where to look for the particular information you seek could require practice. Also, like most HUDs, this one is prone to becoming cluttered, particularly during the approach capture when both ILS centreline and glideslope are displayed, but the aircraft is still turning. The object of this exercise, however, was not to evaluate the HUD - merely to use it to fly the ILS approach and be able to evaluate the vision enhancement. It is fair to say, however, that HUDs will be much improved when technology allows symbols to be removed from the combiner, as one or more coincide, so that only the predominantly important symbol survives until after the conflict has passed.


Flying the hud

I was able to fly two approaches to Andrews' ILS-equipped runway 1L, both manually - to ascertain how readily a pilot can "fly the HUD" on approach and absorb the radar information on the combiner. The Andrews surface wind was about 10kt (18km/h) across the runway from the left and the "traffic pattern" at 2,000ft was surprisingly bumpy. United's project pilot for the 727 was Mike Frank, who has been involved with the ALG programme from the start, and his overall knowledge was extremely useful. He had command of the 727 from the right-hand seat. First impressions of flying the 727 were of less-than-crisp roll control below 200kt, fairly large power changes to overcome inertia and maintain speed - and the constant, "vintage", whine-and-clonk noises from the electric pitch trim. These are, however, truly representative handling characteristics for earlier generations of aircraft most likely to benefit from HUD installations.

Even without previous experience of the 727 and with little time to become acquainted with the Sextant HUD, it was possible to fly the aircraft accurately enough for the purpose of the exercise. Dark glasses were provided, which effectively occluded the pilot's view of the outside world, so as to simulate conditions of fog, although engine instruments and the normal instrument panels could be seen readily enough. A "three-balls" eye-position indicator on the windscreen centre pillar facilitated positioning one's seat so as to view the combiner properly. The symbols are crisp and clear (except when clutter occurs) and wearing prescription spectacles proved not to be a problem.

While capturing the ILS centreline and glideslope, the predominant demand information is the flightpath-guidance cue (on the Sextant unit, the symbol is a plain circle) over which the pilot places the flightpath symbol (sometimes called the boresight) - which, frankly, is a bit like a computer game. There is also plenty of orientation information, some shown symbolically (such as heading and speed trend), but much, of necessity, is displayed digitally - such as indicated airspeed, groundspeed, altitude and vertical speed.

Once established on the glideslope, the flightpath-guidance cue becomes vital and is required for the accuracy that this type of approach demands. The other concerns for the pilot to concentrate on are speed control (with power) and height; radio altitude is automatically activated on the descent and appears digitally below the flightpath-guidance cue and between the groundspeed and vertical-speed readouts. At about 700ft above ground-level (AGL), the radar image begins to appear along the lower edge of the combiner (as the terrain comes within range of the radar antenna). Distinctive topographical features, such as a housing estate to the left of centreline, could be made out clearly on the radar image and, from about 400ft AGL, the runway extended-centreline lights and a major public road outside the airfield could be identified, quickly followed by the discernible shape of the runway and taxiways. This is a reassuring cue, allowing the pilot to be certain that he is properly aligned with the runway, and permitting relatively leisurely orientation at about 250ft above DH. In this instance, the approach was continued to the 50ft DH. As the runway radar image became larger and increasingly compelling on the combiner, the point immediately beyond the flightpath-guidance cue indicated the expected touchdown point on the runway.



I was disappointed by the resolution of the radar image. For example, the housing estate was a useful orientational cue, but only because I had seen it on an earlier visual approach and knew what it was. From the radar image itself, it could have been a fuel farm, a boy scouts' encampment or boats on a lake. Arguably, therefore, a good knowledge of the local terrain is an advantage, but, since housing estates do not usually appear on approach charts, no credit towards certification could presumably be claimed for such local features as roads and buildings.

It is, however, the role of the radar just before, at, and beyond DH for which credit is to be claimed in certification. Here, it proved to be a considerable asset, arguably giving greater help even than centreline and runway lights, because it presented an image to the pilot of the threshold, adjacent taxiways, any crossing runway and the other end of the runway in use. Once again, I was disappointed by the poor clarity of the images because the features lacked crispness. The ALG team claims that the system makes it possible to see aircraft and vehicles on the airfield. This would not have been easy from the image that I saw.

I discussed my view with Mike Frank and the on-board team, who said that a transmitter failure the previous day had required a replacement which had not been calibrated, and that the system was only producing about 80% of its usual resolution. Also, they stressed the "breadboard" development nature of the equipment and the room for refinement before production.

Despite my reservations about the quality of the radar image, however, I cannot take issue with its effectiveness as a vision-enhancement device which is both practical as an installation and convincing in giving a pilot autonomous landing guidance, such that he could undoubtedly safely undertake Cat III approaches at Cat I runways. The results of this programme are to be discussed with the FAA. There will also be interest from the Gulfstream project team because its wider ambitions for its HUD could clearly exploit vision enhancement. Progress will need to be rapid, however, to benefit the world's fleets of ageing airliners before they are replaced with new Cat III-equipped aircraft.

Source: Flight International