Head-up benefits

Why HUDs have the advantage over head-down displays.

HGS on a Challenger 601 uses a unique fusion/vision feature. SVS marks the airport location using a dome shape. The dome starts out in a solid color and gradually becomes transparent as the aircraft approaches the airport.

On top of that we can put a synthetic vision system (SVS). SVS builds a database-derived synthetic image of important features of the aircraft's surroundings and displays the resulting landscape on the PFD and the HUD. As this image is overlaid on the EVS image, it can help the crew to identify key features of the environment as well as navigating through different challenges, including airspace restrictions, weather and other aircraft.

This concept is sometimes called "highway in the sky" or even "tunnel in the sky"—a name that I don't like much personally, because general situational awareness of the crew should never be reduced to a supposedly safe tube to fly through. There have been too many sensor failures and errors to rely completely on that.

But it's certainly a concept that is studied. Bear in mind that not all areas of the world have a ready supply of highly qualified and able pilots. As the airspace environment gets more crowded, restricted and complex, and the pilot profession competes with other field for talent, technology has always been seen by aircraft manufacturers and operators as a "simplifier" that might enable a less qualified individual to fulfill the pilot's role.

This is an old and oft-repeated story. Just recall the introduction of autopilots, electronic flight displays (called EFIS), flight management systems or fly-by-wire. Professional pilots know that these never worked out quite as planned.

While new technology brought significant safety enhancements and operational benefits, what always came with it were new and previously unknown ways to crash an aircraft. Only qualified pilots with good training are able to work responsibly and safely with any new technology, use its full potential and avoid pitfalls.

Display standards

It should be noted that manufacturers now seem to have achieved a level of conformity (in the form of geometric agreement between HUD and outside world) that is quintessential for full benefits of a HUD installation—very short crew reaction times. In a 1993 NASA study, the scientists McCann, Foyle and Johnston found that HUD displays that appear "different" from the world behind do not eliminate transition times between instrument processing and world processing—in other words, they make no difference compared to regular instrument flying.

They predicted that future HUDs should be developed with an eye toward removing the cues that cause the visual system to segregate HUDs from the world. The current success of HUDs is due to the fact that this conformity has finally been achieved, leading to greater situational awareness in general.

While in the future even airspace management could be integrated into the HUD by displaying other aircraft's ADS-B data, current benefits are plenty—more precise landings, better energy management, better engine-out recovery, to name a few. Even flare guidance to prevent hard landings is possible. Honeywell even developed a special feature for the MD11 HUD that warns about a bounced landing and gives immediate go-around guidance.

This image from an actual flight over Seattle WA using HGS shows how SVS helps pilots to identify key features. Flight guidance symbology is also shown.

Situational awareness is provided on the HUD through various symbols that present airplane flightpath, inertia, airplane energy state, etc. Some of this information is not available on the head-down displays, but when integrated consistently into the pilot's scan it helps the pilot to have a better picture of his vessel's performance state.

A stable aircraft flightpath depends on the pilot knowing the real energy state of the airplane. This is why Dean Schwab recommends that pilots should be trained to use the HGS as a full-time flight display or as a primary flight reference in all phases of flight. Watching videos of his visual approaches in Alaska certainly made me understand his point.

While civil HUD installations feature indications that are familiar from the PFD—such as speed tape, altitude tape, horizon and heading bug—there is also new information. The aircraft reference symbol (boresight) represents the projected centerline of the aircraft, while the flightpath symbol is inertial-derived and provides instantaneous indication of where the aircraft is going relative to the outside world.

The zero degree pitch reference line is positioned relative to the aircraft's current pitch attitude and conformal to the outside world and, if the flightpath symbol is placed on the horizon, level flight results.

Speed, or energy, is managed with the speed error tape—a dynamic bar that displays the difference between actual and selected airspeed—and the flightpath acceleration symbol. Wind speed and direction are displayed, as well as the angle of attack (AOA) and the resulting pitch limit. A slip/skid indicator and a heading bug are also available.
The goal is to create a display that allows complete situational awareness just by looking at the landscape outside—but through the HUD.

This may help pilots to learn the characteristics of a new airplane sooner, and reduce transition training time and costs. In difficult situations, such as close to or even in stalls, as well as after engine failures, HUD guidance helps the crew to recover better and faster.

After crews learn how to interpret the HUD properly, the aircraft handling characteristics could even be learned "on the job," as proper guidance is always given. At least, that's the theory. So far FAA has not given any transition training credit to HUD users.

Most aircraft offer the HUD only on the captain's side. While the captain, using the HUD, gets the privilege of seeing and knowing more then his first officer, he is also suffering from restricted headroom, as HUD projector installations tend to be quite large and need to be right above the pilot's head.

New technology HUDs project the image from the top side into the screen, using up less headroom, but they still have limited functions.
And wasn't there something called crew coordination, with the pilot flying (PF) supported by the pilot not flying (PNF)?

The problem of monitoring by the other pilot has been recognized by manufacturers and operators, and there are 2 solutions. One is to have a panel display with a repeat of the HUD display for the PNF. (On Gulfstream PlaneView installations, this is done with the IR image.) The other is to have 2 HUDs—1 for each pilot.

But then there are other messages that the pilots need to see too, such as flap and gear position or reverser status. And system warning messages are usually shown on system displays or central warning displays. So designers, engineers and test pilots still have a lot of work to do before we can all just look straight ahead and be in complete control and fully aware.

I fly one of the larger aircraft my company operates—a Boeing 747-400. We are certified to Cat IIIc with no decision height. That means that, if everything goes well, we touch the runway in thick fog in an automatic landing, and I never even saw the runway. We do not have HUDs installed, even though they are available for all airline jets. But it certainly would be nice to have an EVS image as we attempt to bring 400 people safely back to Earth.

Peter Berendsen is a Boeing 747-400 captain for an international airline. He writes regularly on aviation-related subjects.


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