Why HUDs have the advantage over head-down displays.
By Peter Berendsen
ATP/CFII. Boeing 747, MD11
All Nippon Aiways (ANA) operates the Boeing 787 with dual head-up displays, giving the assisting pilot the same information as the pilot flying.
Dean Schwab used to fly for Alaska Airlines in the challenging environment of the Arctic frontier. Visual approaches through tight valleys, short runways, bad weather—all these and more were routine parts of his daily work. Now he is the senior manager of head-up guidance systems at Rockwell Collins, and when he talks about the use of head-up displays (HUDs) you know right away that he is a believer. He tells me that Alaska Airlines actually has a policy of using the HUD full time, even in cruise.
HUDs have been around for a long time, and many regarded them as a poor man's low-visibility landing system. If you could not afford to install an autoland system, you went for a HUD to land in fog. After all, wasn't automation the future, not skilled human performance? The HUD—an instrument built specifically to enhance human performance in manual flight—seemed to have mostly military uses, although some regional airlines installed them for lower landing minimums.
But with the arrival of synthetic vision (SV), infrared (IR) cameras and precision positioning by GPS, executive jet manufacturers began to integrate HUDs as a landing aid for challenging airports. I flew a Gulfstream G450 out of ISP (Islip NY) on Long Island many years ago for a Pro Pilot article, and Gulfstream's PlaneView avionics suite included a HUD that projected the infrared image of the forward IR camera right in front of the pilot's eye, enabling me to see through clouds and darkness.
The Dassault Falcon 7X, which I flew for Pro Pilot in 2008, has a HUD as part of the EASy cockpit layout. Just placing the HUD's flightpath symbol on the runway beginning made it very easy to steer the Falcon to a precise landing.
But why is it possible to fly so precisely using a HUD? One important aspect is scaling. You may never have noticed, but the angles on your primary flight display (PFD) are scaled down. For example, a symbol movement of 1/2 inch on the PFD (or head-down display, as HUD people like to call it) represents an angle of just over 1° when measured from the design eye point (DEP)—the location in the cockpit where the pilot's eyes are designed to be, often marked with reference points on the glareshield.
But the corresponding movement of that symbol may represent 2° or 3° in the real world.
Since the HUD's symbology is projected and overlaid onto the real outside world, scaling is not an option. This is called the conformal nature of the HUD—everything looks further apart. For example, the outer edges of the airspeed and altitude scales on the PFD are about 5 inches apart—an angle of about 12° from the DEP—while the same symbols on the HUD spread almost 30°.
It has to be mentioned, though, that current HUDs do not cover the whole windshield. So while it is true that the symbology is larger and spread further apart in order to conform with the outside world, the HUD functions like a looking glass, as only a small part of the entire possible display is always shown. This needs some getting used to. Pilots have to learn to avoid tunnel vision and make sure that they always observe their entire environment and not just the HUD area.
But, in the end, the expanded display size of the HUD is a key factor in increased pilot accuracy. Maybe you remember your first flying lessons—hopefully on a clear day with a nice horizon.
The flight instructor most probably told you to look at the horizon, and you didn't understand how you were supposed to be able to gain any meaningful information from the natural horizon. Later, you learned that the natural horizon is much more accurate then the little one your training aircraft had, as you struggled to maintain altitude with a small mechanical horizon.
The HUD solves this problem. Meaningful information is projected on the natural horizon, and voilà! Suddenly, anyone can be a precise pilot.
Coming to terms
Rockwell Collins HGS display with standard symbology.
A head-up system is defined as the entire system that displays flight symbology within the pilot's view on a conformal display through the aircraft windscreen. It includes the display element, sensors, computers and power supplies, indications and controls. It may receive inputs from an airborne navigation system or flight guidance system.
In addition to HUD, the term head-up guidance system (HGS) is often used. The term was invented by Rockwell Collins Flight Dynamics to define a HUD that also provides the pilot with its system-generated guidance to takeoff, touchdown and landing.
The parameter defining the usefulness of a HUD is precision—the ability to conform exactly with the outside environment. Years ago, this was only possible on short final on a precision instrument landing system such as ILS. Huge improvements in positon, heading and attitude accuracy led to ever better required navigation performance (RNP) values.
The integration of advanced flight management systems (FMSs), multimode receivers (MMRs) and new technology navigation sources, such as GPS, provided increasingly precise navigation using multiple navigation sensor inputs.
These technical advances have provided operators and FAA with the ability to find new ways to conduct instrument approaches in safety without being dependent on ground-based navigation facilities such as ILS or a microwave landing system (MLS).
The new concept of "mix and match" evolved. Various sources of lateral and vertical navigation information could be combined and processed for a precise location and orientation of the aircraft. While the flightcrew could be presented with the navigational data in a consistent way, different technological solutions all had the same goal—to ensure a safe and timely arrival at the destination airport.
One sensor that certainly needs perfect precision to be displayed in a HUD is the sensor for an enhanced vision system (EVS). The images from onboard IR cameras have been displayed in executive jet cockpits for a number of years now, both on the HUD and also on flightdeck panel displays as a crosscheck for the other pilot.
The initial benefit of this technology is safety, but added operational capability is also possible. EVS technology made lower landing minima possible for business aircraft. EASA and FAA have issued regulations allowing the use of EVS displayed on a HUD or HGS as a "runway in sight" substitute down to 100 ft RA.
Essentially, this gives Cat II minima to an approach that without HUD and EVS would be Cat I only. Needless to say, the added comfort of seeing terrain through the clouds or at night is a significant operational enhancement and ultimately leads to better flightcrew decisionmaking.
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