After many years of diligent, and what must often have been discouraging, marketing efforts by their manufacturers, sales of head-up displays (HUD) have now taken off and are climbing rapidly. The main impetus behind this is the recognition by the airlines that HUD confers unique operational and cost benefits that are simply not available in the standard flight deck.

Of course, this is not a new discovery. Corporate operators have always been much quicker than the airlines to adopt new technologies, and this has certainly been true of HUD, which is estimated to have been installed in more than 400 business aircraft since the early 1990s.

But when the airlines move, they really move. American Airlines specified a HUD option in 1999 for the 123 Boeing 737NGs it ordered, and for the balance of the optioned 500 NGs that it eventually expects to receive. Earlier this year Delta Air Lines ordered HUDs for its entire MD-88 fleet–all 120 of them–which will join its HUD-equipped 737 fleet. In addition, the Atlanta-based airline announced that it will install HUDs in its 757s, 767s and 777s. Southwest, which acquired a taste for HUDs when it bought Morris Air’s 10 equipped 737s in 1996, is now installing them in its entire 350-plus fleet.

And while the remaining U.S. majors– United, Continental and US Airways–are currently noncommittal on plans for HUDs, industry insiders expect that United could announce a major program before year-end, followed later by Continental. However, US Airways is unlikely to move soon due to a less than rosy financial situation that would make large-scale HUD retrofits, at around $400,000 apiece, a fairly heavy burden.

The reason airlines lag behind business aviation in new technology areas is mainly due to their lengthy flight operations and financial decision-making processes, and this was compounded in the HUD case by the fact that both Boeing and Airbus believed that their autoland systems made HUDs unnecessary. Eventually, Boeing relented under customer pressure and now offers the system as an option on the 737NG series and the BBJ, and the airframer is expected soon to offer HUD options on its other types. But Airbus has still not committed itself, reportedly due to the complexity of integrating the HUD into its highly automated flight-envelope protection design.

A Billion-dollar Business

Nevertheless, the airlines’ HUD shopping bill is now estimated to have gone well beyond the billion-dollar mark, with new orders being announced almost weekly. Yet for most major airlines today the driving force is no longer the attainment of lower landing minima, which was the case in the early days of the system when Alaska Airlines pioneered its use for getting into and out of difficult locations along its home state’s rugged coast. As Delta Air Lines Boeing 737-800 fleet captain Len Goreham explained, “All our new airplanes are certified for Category III autoland, so we’ve already got the limits.” This sentiment was echoed by American Airlines chief pilot Brian Will.

To Goreham and Will, and to others interviewed by AIN, hand-flying the airplane under HUD guidance has proved to be much more precise than autoland, with both operational and financial payoffs. “The HUD,” said Goreham, “turns a good pilot into a great pilot.” AIN readers who routinely fly HUDs already know this, of course, but for those who don’t, a review of the specific HUD features that sold airline management may be useful.

Feature Presentation

Top of the list is the ability to fly stabilized constant-energy approaches with accurate airspeeds, precise ILS tracking and minimal touchdown dispersion. The energy-management concept, coupled with airspeed control within one to two knots, is a unique HUD contribution, where its display screen shows the airplane’s instantaneous inertial acceleration or deceleration state, which can be adjusted by small power changes in the early stages of the approach. Besides saving fuel, the airlines also see this concept as having a calming effect on the passengers.

Precise ILS tracking is similar, since the inertial systems detect and display localizer or glide-path deviations on the HUD well before the conventional ILS pointers move, and which again allows earlier, minor corrections to be made. And touchdown dispersion with the HUD is a non-issue because the inertial systems continuously extrapolate the aircraft’s flight path and indicate, with an appropriate bullseye symbol on the HUD exactly where the wheels will touch down.

This is clearly a useful feature, particularly when breaking out of the clouds on an instrument approach. But it is even more valuable in “black hole” conditions when approaching poorly lit, but clearly visible, runways at night, over water or over featureless terrain, where perspective judgment is difficult. Interestingly, statistics show that in 75 percent of landing accidents, the pilot had the runway clearly in sight but still erred.

It also turns out that inaccurate touchdowns can affect airline schedules. Alaska Airlines subsidiary Horizon Airlines has equipped its de Havilland Dash 8 fleet with HUDs. It operates its turboprops on fast turnaround schedules across a network of local airports in the Northwest, many of which have short runways.

But landing long on these runways would require heavy braking, with the result that the quick turnaround departure could be delayed until the brakes have cooled down; HUDs in this case help reduce heavy braking. The HUDs also allow Horizon to make the unusual claim that, at 300-ft RVR, its Dash 8s have lower takeoff limits than even the biggest airline jet that is not similarly equipped. Tom Gerharter, Horizon’s senior vice president of operations, said, “We won’t buy airplanes unless they are equipped with HUDs.”

Yet Horizon shares with the big leagues the next most important airline concern, which is that of tailstrikes. Its new Dash-8Q-400s are stretched versions and, like the stretched versions of much larger airplanes, are prone to tailstrikes during vigorous takeoff rotations and, potentially more serious, high nose-up attitudes during the flare. Typically, a landing tailstrike by an airliner can take it out of service for up to a week, and even longer if the damage involves the pressure vessel.

Additionally, the repair bill can easily run a couple of million dollars and in severe cases can reach five times that figure. To counter this, HUD manufacturers now program pitch-attitude limits appropriate to the airframe and, when the aircraft’s inertial system senses the risk of exceeding them, an unmistakable tailstrike message appears on a display. As one airline Boeing 777 captain put it, “A bad tailstrike could buy an awful lot of HUD installations.”

Another new HUD display feature that the airlines like is TCAS steering guidance. Here, as soon as an audio TCAS Resolution Advisory is heard, the pilot can immediately “fly” the HUD’s aircraft symbol to a rectangular TCAS steering box that appears instantly on the HUD, without first having to look down at the TCAS display to determine the required avoidance maneuver. Keeping the aircraft symbol centered in the box ensures that an accurate escape maneuver is followed.

And even touchdowns are made softer with the HUD, which provides a flare-guidance cue close to the ground to reduce the vertical speed from the typical three feet per second to one-and-a-half feet per second. This not only pleases passengers but also reduces undercarriage and tire maintenance costs. At the other extreme, recovery from unusual attitudes is also made easier, since the HUD will automatically change its guidance presentation when bank and pitch angles exceed preset limits. At those times, an artificial horizon-like ball appears on the display, providing intuitive guidance to the required recovery maneuver.
HUD Manufacturers

Under today’s booming market conditions, the three incumbent airline HUD manufacturers–Rockwell Collins’ Flight Dynamics in the U.S., BAE Systems in the UK and Sextant in France, but possibly to be joined later by Flight Visions of Sugar Grove, Ill., whose HUD is already certified on the GIII, GIV and Falcon 50–are vying to introduce system features that discriminate their products from competitors. Single-engine Cat III approach guidance, runway distance remaining, braking action and even zero-visibility airport surface guidance using datalinked taxi clearances from the tower are all recently introduced applications. Other developments are in the works, including TAWS-like terrain databases that will automatically display the contours of high ground ahead at or above the aircraft’s altitude, thereby providing additional CFIT situational awareness.

As great as the HUD is, there are two common misconceptions about it. First, you can’t fly a precision approach on the HUD alone. You must have an ILS or its MLS or GPS equivalent for it to reference. Second, having a HUD on the captain’s side will not help him or her see any further in low visibility than can the copilot in the right-hand seat who is simply looking ahead through the windshield. This latter case is where HUD add-on systems–called enhanced and synthetic vision systems (EVS and SVS)–will come into play.

Add Ons

Although their acronyms are sometimes used interchangeably, EVS and SVS are quite different. The EVS uses a forward-looking infrared detector that can “see” through cloud and most types of fog, and produces on the HUD a photograph-like thermal image of the scene ahead. The SVS uses a highly detailed onboard database that, when fed with position, altitude, attitude and heading, projects a computer-generated, correctly oriented, forward view onto the HUD screen. So you could say that an EVS provides a real-time infrared look ahead, while an SVS provides a picture of what the view ahead looked like when its database was last updated.

Both have advantages and disadvantages. In torrential rain and some types of fog, EVS performance weakens, although data from a small rapid-scan radar unit, installed below the weather radar dish, can be “fused” with the infrared signals to restore performance. The SVS database, on the other hand, is unaffected by weather, but it cannot show aircraft and vehicles moving on the airport surface, which according to FAA statistics is now aviation’s most hazardous area.

Gulfstream expects to certify a Kollsman EVS in the GV by year-end, making it the world’s first certified civil EVS installation. The company said 35 clients are already lined up to purchase EVS equipment–at around $500,000 a copy, installed–in their GVs. CMC Electronics, formerly Canadian Marconi Company, is also developing an EVS, with certification forecast for next year.

SVS, on the other hand, is currently undergoing prototype evaluation in NASA’s Boeing 757 flying laboratory and is not expected to be certified for several years.