Going Direct: HUD vs. Combined Vision

Settling the head-up versus head-down debate.

(January 2012) The question that was being debated around the halls of NBAA 2011 the other week was about as technical and geeky as it gets: When flying a very low approach, lower even than a decision height of 200 feet, where is the proper place for your “head,” that is, your eyes and your attention, to be? Should it be down, looking at the primary flight display, and then out, looking for the runway environment at decision height, or should your eyes simply be up and outside the airplane all the time?

At first blush, it sounds as though it’s a no-brainer. Why would the pilot be looking down when the runway is out there if he doesn’t have to be? Then again, unless the airplane is equipped with a head-up display, the pilot has to be looking down a good part of the time. Quantifying how much time is spent up and how much time is spent down is not an easy task. Proficient instrument pilots are skilled at monitoring two instruments simultaneously while peeking at a third, and the layouts of PFDs make it easier to see even more than that without shifting focus. I’ve long felt that the way we teach instrument students to scan is a poor imitation of the way we actually take in the flight instrument data and make sense of it, but one does have to start somewhere. The theory behind the HUD is that, by using it, the pilot has everything in the main field of view. There is still a scan going on, though it is a compact, fluid and efficient one, to be sure.

If the whole head-up versus head-down argument sounds purely academic to you, you’re right in a way and wrong in a different, more important way.

Best of Both Worlds?
Honeywell has been working on very cool technology that it calls combined vision, which blends an enhanced vision picture (an infrared look at what’s actually out there) into the center of synthetic vision (a computer-generated view of what the system knows is out there, including terrain and runway environment) on the primary flight display. With combined vision, the pilot uses HUD-style symbology as viewed on the PFD to fly down to minimums.

Today that’s still 200 feet, because the experimental technology gets no credit for lower than that, yet. If and when Honeywell gets certification for the combined vision system and if it gets credit for lower minimums, as can be the case with a HUD, the technique would remain the same.

Once you’ve arrived at minimums, you simply look up to find the runway and land, or, failing that, you initiate the missed approach.

The question is this: When you’re down to very low clearance over the ground and moving at a good clip over it with things happening fast, is the pilot who has to pop his or her head up at that last moment to find the runway at any kind of performance disadvantage compared with the pilot who is looking through the HUD the whole time? The answer seems as though it would be obvious, that the pilot staring at the runway through the HUD would be in better shape to turn the approach into a successful landing. But much to Honeywell’s delight, that seems not to be the case.

Honeywell human factors scientist Trish Ververs has tested the technology quite a bit for the company, comparing the performance of pilots using head-up (through a HUD) and head-down (through the Honeywell combined vision system) technology by plotting where the wheels touch down on landing.

On dozens of flights she tracked the performance down to a meter to see how much better pilots flying with a HUD would do compared with those flying head down on Honeywell’s combined vision system. A chart that showed landing points at the end of approaches flown by pilots using the two systems showed scattershot points with no apparent bias toward either system. Both systems allowed their pilots to fly very accurate approaches.

This result, Ververs says, surprised the pilots in the study who had a lot of HUD time going into the testing. They had all assumed, she says, that the HUD would be the better technology. As far as the testing experience could show, it wasn’t better, Ververs says, and the pilots who participated in the experiment began to look at the subject differently. I flew head-down approaches with Honeywell’s system last year in a Citation Sovereign in Prescott, Arizona, and recently in a Gulfstream G450 in Shenandoah, Virginia. In both cases I found that, when I hit minimums and looked out the windscreen at 150 feet, there was nothing but a huge white stripe staring me in the face.

New HUD Technology
Rockwell Collins isn’t so sure that the technologies are similar. The Cedar Rapids, Iowa-based company, which manufactures head-up displays for Gulfstream and Falcon Jet among others, would seem to have the high ground in this debate. Its argument, and it’s a good one, is that one’s eyes should be looking outside all the time because at some point the pilot needs to see the runway in order to land. Why not just look outside? Because in a jet you need better precision than you can get by looking out the window. The HUD is one-stop shopping for that kind of data.

For its part, Collins is not sitting still. Just this year it announced the HGS-3500, a compact HUD that makes use of a new display technology to allow it to be fitted into a large number of business aircraft, down to turboprop models because it is so much smaller. You even get synthetic vision on the HUD.

The new HUD technology is also less expensive and could conceivably be used even in light twins or singles, though Collins has announced no such plans for it.
Honeywell’s combined vision system, on the other hand, requires an enhanced vision infrared camera, which adds cost and complexity, though it needs only a flat-panel display to work in the cockpit.

Honeywell’s and Rockwell Collins’ new equipment and competing philosophies are battling not only for market share but also for the hearts and minds of decision-makers at aircraft manufacturers.

The battle could have implications, and not just in terms of which company, Honeywell or Collins, wins the business of airplane makers but in terms of the aftermarket as well.

The competition, in terms of technology and consumer choice, will likely be a good thing for all.

High to Low
For the past few years I flew a Cirrus SR22 G3 Turbo with shared ownership company PlaneSmart out of Austin, Texas (KAUS). The airplanes it operated were fully decked-out models, the top of the line in the day, with the last of the Avidyne Entegra PFDs that came off the Cirrus production line before the manufacturer went with the Garmin G1000-based Perspective cockpit as standard equipment. The G3 Turbos were also a throwback in one other way: They were outfitted with the Tornado Alley-modified Continental IO-550 engine. The conversion takes the normally aspirated engine and turbocharges it, while adding Tornado Alley’s GAMI fuel injection system. The engine was smooth, powerful and fuel-efficient — at 17,000 feet and 205 knots true, my fuel burn was about 17.5 gph or less when I had to reduce power to keep the engine cool.

One of the big attractions of flying in the midteens was the hoped-for opportunity to enjoy some big tailwinds when heading east. I had a couple of days when I was seeing 230- or 240-knot groundspeeds, but those were rare events. I usually saw light tailwinds heading east and light headwinds heading west. It seemed as though the headwinds were more common, but it’s human nature to more clearly remember the bad times.

The real advantage of flying just below the flight levels is the fact that there’s very little traffic there, which makes it easy to overfly Class Bravo airspace without getting vectored all over heaven’s half-acre. It also makes it easier to get an altitude change for turbulence or friendlier winds.

These days I’m flying a new airplane, a late-model Cirrus SR22 with the Garmin cockpit. It has synthetic vision, WAAS and an excellent autopilot with extensive vertical-navigation capability, including coupled WAAS approaches.

Perhaps the hardest difference to overlook is the price. The nonturbo model without flight into known icing, outfitted like mine, goes for almost $200,000 less than a fully decked-out bird. You can buy a lot of avgas for that price difference.

Low Down
One of the things the new airplane lacks is a turbocharger. It’s been a while since I’ve flown an ambient air-breathing airplane very much, and I have to tell you, I kind of like it.

The other thing my airplane lacks is ice protection, which sounds like a bad thing but isn’t. My previous Cirrus rides were outfitted with TKS ice protection — the no-hazard kind as opposed to the FIKI variety. My new ride consistently delivers from 182 to 185 knots true at around 7,000 feet at 75 percent power, which you get at 2,550 rpm (with the prop rpm mechanically linked to the throttle position) and around 16.5 gph. I don’t have to mess with the cost or the hassle of the oxygen, and the climbs are quick.

With the nonturbo airplane I get to pick in most cases either 7,000 or 8,000 as a cruising altitude, where I know I can get 75 percent power out of the normally aspirated engine. The downside is, of course, when it’s time to fly very high, which I need to do when I head up to Colorado or the Northwest, or when I travel out to California and hop over the tail end of the American Rockies. The choice comes down, finally, to where pilots live and how they most often fly. For the majority of my flights, the naturally aspirated airplane does the job very well, and, as it is with cars and vacation homes, it makes sense to buy an airplane that fits your needs most of the time and to come up with a different plan those rare times it doesn’t.


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