Today avionics manufacturers offer products that meet the FAR 91.175 requirement, which allows pilots to descend below 200 feet during an approach, using a system that displays infrared enhanced vision system (EVS) images on the head-up display (HUD), so the pilot can see the approach lights on the HUD before seeing the runway by the time 100 feet is reached.

There could soon be an alternative, however, and Honeywell has been testing a head-down system, which replaces the HUD with a combined vision system (CVS). What CVS does is overlay EVS onto a synthetic vision system (SVS) view on the pilot’s primary flight display (PFD). In this case, the pilot will use the CVS to look for the runway environment–approach lights–then transition to the outside view at 100 feet and complete the landing. Honeywell is testing CVS and working with the FAA to seek approval for the same credit to descend below 200 feet available in FAR 91.175.

Honeywell Work

“What we’ve done is combined this infrared imagery on our synthetic vision to give you that real-time situational awareness of exactly where you are in the environment,” said Trish Ververs, Honeywell EVS/SVS project lead and human factors expert.

Honeywell has been flight-testing CVS–combining EVS and its SmartView synthetic vision–since June 2010, following laboratory testing using a simulator that found there were advantages to the company’s head-down philosophy. “What we’re trying to prove is that what you have here is at least equivalent to what they currently certify for a lower landing,” she said. But there’s more to what Honeywell is trying to accomplish with the head-down approach, and that is to deliver much more information on the PFD to help the pilot not only interpret what is going on in the outside world during low visibility but also to make the transition from looking head-down at the PFD during the approach then head-up outside the window to make the landing, even at 100 feet height above touchdown.

“What we are generally comparing it to is the system that is currently certified for lower landing minimums,” Ververs said, “and that is the HUD with the infrared camera under FAR 91.175. It says if he’s going to go below minimums, 200 feet, the pilot has to either see the outside environment or, in this FAR, needs to be able to see the image on the display of the runway environment [before taking the airplane] down to 100 feet. At 100 feet he has to transition to the outside environment.

“But we want to enhance a pilot’s ability to detect the environment around him,” she added, “and that’s why we’re adding that real-time data.” This data, by the way, is not only useful in the air but also on the ground; the EVS view helps the pilot see aircraft and ground vehicles and taxiway markings, especially useful in poor visibility.

The Big Picture

Avionics design is moving rapidly toward more representation of the outside world on displays inside the cockpit. That is, after all, what synthetic vision does. This capability is central to what Honeywell is trying to accomplish: elimination of the HUD and use of the CVS to achieve lower approach capability as currently allowed by FAR 91.175 for HUDs.

“There are some people who might believe that you still need a HUD to do that,” said Honeywell chief test pilot Jary Engels. “What we’re finding in our research is that the synthetic vision and the overlay of the EVS on the synthetic vision changes that game.” With modern displays and CVS, he asserts, “We’re presenting the real world. What we’re finding in research is that you’ve already built that real-world picture in your head so that when you make the transition from head-down to head-up, you don’t have to change your context. You see [on the PFD] what you’re going to see when you look out, and when you look out, that’s what you see.

Flight Path Marker

“We recognized the advantages of a HUD are not just the fact that you’re looking through the HUD to the outside world,” said Engels. The flight path marker shows where the aircraft is going as opposed to its attitude. “The pitch attitude is where you’re pointing, the flight path is where you’re going. And that’s true both left and right and up and down. So if I take the flight path marker and put it on the end of the runway, I will end up on the end of the runway.”

With the accuracy of GPS for positioning and a highly detailed terrain and airport database and a high-quality display, Engels explained, “I put the flight path marker on the synthetic runway, and that’s where I will end up. At 100 feet, if I have the flight path marker in the touchdown zone of the runway, when I look up, it’d better be there. And the transition from this to the outside world is seamless.”

Symbology

Honeywell is using a track-centered approach to display information on the PFD. Engels said, “In a HUD if you have a crosswind you may be pointing this way and going this [other] way, and if you get too strong a crosswind, [the runway] can actually move out of your field of view.” On the Honeywell CVS, he added, “we center it on the track.” A small airplane symbol on the PFD shows exactly what the nose of the airplane is doing in a crosswind. If there is a strong wind from the left side of the runway, for example, the airplane’s nose must point to the left to allow the airplane to track straight to the runway. The little airplane symbol makes this crystal clear to the pilot.

Range rings on the SVS display help, too, by giving the pilot a quick aid for depth perception. A line extends from the runway toward the bottom of the SVS display, showing the extension of the runway centerline. Matching the line against the little airplane symbol, the pilot can quickly see the crab angle in a crosswind and, of course, which way to fly to the runway.

Adding EVS

The final key to combined vision is, of course, combining the EVS and SVS views on one screen.

One of the reasons Honeywell prefers the head-down display route is that company designers believe they can do much more with the colorful PFD than with the HUD. “If you’ve ever seen an infrared image on a HUD it’s all green,” said Engels, “as is the image on the HUD that’s drawn in your symbology, so everything kind of blends together.”

Taking the infrared image from the EVS camera, Honeywell designers were able to add color to that image in such a way that it not only blends well with the SVS view but also remains distinct enough for the pilot to recognize it as an infrared image. The top part of the EVS image can be made translucent, so the pilot can see whatever would normally be covered by that part of the infrared view. If there are mountains in the background, for example, why not leave the top portion of the EVS clear so you can see those mountains as depicted by the SVS?

“By incorporating all this information on the head-down display,” said Ververs, “every time you look out the window, it’s a completely unobstructed view of your environment. You’re not having to look through anything [like a HUD] to see the actual environment.”

The Demo Flight

During a demonstration of CVS in Honeywell’s G450 recently, pilot John Tuten flew in the right seat while media pilots in the left seat observed approaches to Shenandoah Valley Airport in Virginia on a clear summer night. The first demonstration was of a normal approach using HUD. A second approach was then flown with the HUD tucked out of the way and looking down at the PFD with the CVS display. Tuten held a piece of paper over the windshield to cover the outside view and instructed the left-seat pilot to keep looking at the CVS and not to look outside until reaching 100 feet above touchdown.

The transition from head-down to head-up was easy and natural. There did not seem to be any difference in workload between the HUD-only view looking outside and the head-down/head-up transition from CVS to looking outside. The outside view perfectly matched what we could see on the PFD, so there were no mental gymnastics required to interpret what the CVS was showing, making the transition very natural.

Another benefit of having the SmartView SVS on the CVS display was evident as we went around and flew the missed approach each time. We could easily see the mountains in the distance and understand why the missed approach procedure included a climbing turn. And when we arrived back at Dulles Airport, the EVS showed a clear picture of the lineman standing in front of the airplane as Tuten taxied into a parking spot.