On Display

Chelton's Flight Logic Synthetic Vision

May 1, 2003

Flying through boxes to stay on course

One of the reasons for the FAA's Capstone implementation is based in the turning fjords of southeast Alaska. There pilots have wrestled with the reality of a remote way of living — which relies on air service — that often is at odds with the benchmark of safety. And they've paid the price when these unhappy companions tug apart, and a pilot scud runs to make a freight hop on schedule, darting a workhorse airplane into the hills or cold water.

Contrast this primal world with the promise of synthetic vision, which creates a moving relief map of terrain and obstructions and paves a highway in the sky on a primary flight display for a pilot to follow. Is this glass cockpit a reality for those same Cessna 180s and Piper Cherokee Sixes flying rugged in the Last Frontier? Chelton Flight Systems says yes with its Flight Logic Synthetic Vision — though the price for this capability may limit this equipment to business aircraft for the immediate future.

The principal objective of the Capstone program, created in response to Alaska's high accident rate, is to improve the pilot's situational awareness of the flight environment. This is accomplished through on-board terrain and GPS moving-map information, and traffic information through ADS-B (automatic dependent surveillance broadcast) datalink.

Chelton was awarded part of the FAA Phase II Capstone avionics upgrade, slated for use in southeast Alaska, in 2002 to develop an electronic flight information system (EFIS) that would deliver this critical terrain data along with a full complement of highway in the sky (HITS) and other flight information to the pilot of general aviation aircraft (see " Future Flight: Air Traffic Control's Evolution," October 2000 Pilot).

Chelton, a multibillion-dollar British aerospace conglomerate, purchased Sierra Flight Systems in June 2000. Sierra already had developed an EFIS for the experimental aircraft market; to date more than 100 Sierra EFISs have been delivered and are flying in homebuilts, including several Lancair IV-Ps. With the purchase of the Sierra EFIS, Chelton was poised to tackle the challenge presented by HITS and the incorporation of GPS and free flight concepts; the terrain awareness warning system (TAWS) element 7sked for by Capstone was yet another facet to pursue.

The certified Chelton EFIS and moving-map display, together called Flight Logic Synthetic Vision, imports its critical attitude and heading data from an AHRS (attitude and heading reference system) developed by Crossbow. Crossbow started its work in the AHRS arena with antiskid mechanisms for the automotive industry, which uses similar technology to detect when a car has lost its footing. Though far more complex than these automotive systems, the Crossbow AHRS is a self-reliant, MEMS-based system that doesn't require pitot-static or GPS input for attitude reference, unlike other AHRS used in general aviation applications (see " The Line: Avidyne Entegra Display Suite," December 2002 Pilot). MEMS (micro electro-mechanical systems) gyros measure angular rate much like ring-laser gyros found in commercial jet cockpits, and operate on the same principle: A vibrating structure experiences Coriolis force during rotation, and this is measured. However, in the form used in automotive applications, they are much less accurate. For example, MEMS are highly susceptible to vibration at certain frequencies, such as a power setting that happens to set off a harmonic, leading the gyro to precess at a rate of 10 degrees per second (even the long-toothed directional gyro in an old GA trainer doesn't hit that low note). The key, according to Crossbow, is to thoroughly characterize these weaknesses in each sensor (a typical system has three, along with three accelerometers) and compensate for this within the system software and hardware — in essence, predicting where the sensor stumbles and causing it to perform better.

Sound like hocus-pocus? Crossbow went through FAA testing for the TSOs and STCs along with Chelton, and the program was rigorous. Crossbow uses an Litton LN100 navigation-grade inertial navigation system for flight tests in which the AHRS flies side by side with the INS. The LN100 uses ring-laser gyros with an accuracy of 0.001-degree-an-hour drift — good enough for fighter jets. Crossbow's AHRS rivaled the LN100, according to the company.

It's a long way from the mountainous alleys of Alaska to the flats of Kansas. An icy blanket of clouds 5,000 feet thick below us obscures the land; instead I see the terrain's aspect on the primary flight display (PFD) in front of me in the cockpit of a Cessna 421. The Flight Logic EFIS is nearly certified prior to this flight, only waiting on one last test of a WAAS (Wide Area Augmentation System) GPS software component. The system will be fully WAAS-compatible in accordance with Capstone requirements.

The 421 has four 6.5-inch screens: two on the left side of the cockpit and two on the right. The PFD in front of the pilot stays locked on the system's EFIS screen; the other three screens can be toggled between PFD and moving-map displays as needed. Required backup instruments (a vacuum-based attitude indicator, an electric-driven turn and slip indicator, an airspeed indicator, and an altimeter) line the top-center of the panel. Chelton has chosen to use dual-system backups (vacuum and electric gyros) for extra redundancy.

The PFD shows a traditionally styled EFIS with blue sky and brown terrain. The terrain data is derived from a Jeppesen database updateable when either the mountains move or more practically speaking, when refinements to the data are made. A white aircraft symbol represents the airplane's attitude, and a magenta hoop in the distance gives the desired track. Green boxes represent the HITS — but literally flying through the boxes leads the pilot to overcompensate. Instead, I'm instructed to superimpose the white aircraft over the magenta dot to align the course with my flight path.

A heading tape across the top of the PFD has similar symbology: The white dot shows my heading, the green dot the desired course, and the magenta star my track. While it sounds like a lot to process, over the course of our three-hour flight from Wichita Mid-Continent Airport to Centennial Airport in Denver, I adjust easily to the system. And cross-checking the symbols with a course deviation indicator at the bottom of the screen shows just how accurately the various symbols keep your course. The green HITS boxes represent 400-ft-wide-by-320-ft-high rectangles in the real sky — which results in more than twice the sensitivity of following a localizer course if you stay within them. For en route navigation, simply having the boxes on the screen means you're pretty well centered on an airway.

Traffic information is displayed on both the EFIS and moving-map screens, imported from a Ryan 9900 TCAD (traffic alert and collision avoidance device). Targets are identified in blue, white, and yellow according to their proximity, with relative altitude, distance, and flight-path trend shown — such as a target two miles out and 500 feet above you, descending. During our test flight, one such target appeared and crossed within a mile of us but was never called out by air traffic control.

Airspeed and altitude tapes line either side of the EFIS, and you can set heading and target altitude bugs as desired. All screen functions are accessed via buttons along the sides of the display box, and setup is fairly intuitive. The primary en route tasks — resetting a heading or target altitude, inserting a waypoint, or loading an approach — are relatively easy, involving two or three steps. The most common task of resetting barometric pressure involves pushing a button and twisting a knob.

More complex tasks are, well, more complex. After loading the ILS 35R approach into Centennial, we negotiated with Denver Approach several times to fly the full procedure with no success. (The weather was severe clear with 150 miles of Front Range in sight.) The clearance we ended up with — a direct route to the locator outer marker — required a little more juggling on our part than one might expect for accessing an LOM on a published approach, but we got it to show up correctly after reloading the approach. In the meantime, Denver had us on a good vector. In actual instrument conditions, of course, it is an absolute requirement to know intimately how to work through system nuances such as this.

One instant benefit of flying with the HITS is that there is little or no transition from the en route environment to the approach environment. The boxes, because of their sensitivity, remain identical when you switch to an approach. It felt effortless — I wasn't hunting around for needles, but I had a vague sense of, "Am I really on the approach?" As you intercept the localizer on an ILS, or the final approach segment on a nonprecision approach, the boxes simply curve toward the runway. The runway layout pops up as you enter the airport environment, adding to situational awareness as you transition from the EFIS to sighting the runway.

The vector-based moving-map display repeats some of the information from the PFD, such as a horizontal situation indicator arc and heading and flight-path symbology, with several levels of declutter available. Data blocks show the next waypoint and a destination waypoint if an approach is loaded, with distance and estimated time en route functions. Another data block gives a wind vector with crosswind component (useful on approach), density altitude, outside air temperature, true airspeed, and groundspeed. Terrain is depicted in black, green, and brown shading, and a glidepath range is marked in light blue around the airplane, indicating the airplane's reach at best glide in the event of an engine failure.

Maneuvering in the shadow of the foothills after our missed approach into Centennial, I realized the integrated Class A/B/C TAWS' power. Approaching the hills head-on at 150 knots groundspeed in visual conditions, you don't feel the urgent call to pull up until you are quite close — still far enough out to climb from harm's way, though. But in instrument conditions, or even haze — and covering two or three miles a minute while turning at a standard rate of 180 degrees a minute — you may not see the terrain quickly enough to make a correction. In solid IMC, you must rely on luck alone if you lose situational awareness.

Chelton offers a relatively inexpensive certified TAWS solution (for those Part 91, 135, and 121 operators with turbine engines and six or more passenger seats) wrapped up with the rest of its Flight Logic goodies. A curved flight path projects out from the aircraft symbol on the moving map when in a turn, or a straight flight path connects the aircraft with any possible terrain encroachment. The terrain warning comes at two threat levels, yellow (with an audible "terrain" advisory) and red (with an audible "pull up" command), in accordance with TSO-C151b, with range and time to impact varying with phase of flight. Man-made obstructions are also displayed on both the PFD and the moving map.

Full weather interfaces are planned for depiction on the moving-map display, with the Goodrich WX-500 Stormscope currently offered. Datalink weather from WSI and on-board color weather radar interfaces will be available later this year. An engine display is also available, though it wasn't tested during our flights. We flew the Flight Logic system linked to a Cessna 800-series autopilot; Chelton sells its own digital two- and three-axis autopilot, the AP-2C and -3C, but Flight Logic interfaces with most standard GA autopilots.

The two-screen system weighs about 15 pounds complete with the AHRS, a GPS WAAS receiver for navigation and position determination for TAWS, fuel totalizer, voice warning system, and an air data computer for pitot-static raw data (such as altitude and airspeed) and outside air temperature functions. The basic two-screen system retails for $71,000; a four-screen system such as the one tested in the 421 costs $110,000. Although the STC list for the Flight Logic system is more than 650 aircraft long and includes the venerable Piper Cub, Gordon Pratt, president of Chelton Flight Systems, realizes that the initial market is owner-flown business aircraft, from the Cessna 210 on up to Citations. Chelton is also working with original equipment manufacturers (OEMs) to install the system in new aircraft. "Our first OEM is the Ibis Ae270 from the Czech Republic," says Pratt. (See " Pilot Briefing: Production Ibis Ae270 Flies," page 56.) A University of Alaska Cessna 185 has been flying with a system that conforms to production standards since last fall.

The first customer installation was completed in a Cessna Caravan in March by Yingling Aviation in Wichita — after which the Caravan was scheduled to cross the Pacific to navigate the wilds of Thailand with synthetic vision.

Price: $71,000
Contact: 208/389-9959; www.cheltonflightsystems.com


E-mail the author at julie.boatman@aopa.org.