On Display

The Traffic Picture

March 1, 2005

Assessing the latest traffic avoidance systems

Art as well as science plays an important part in detecting traffic, whether you use a high-dollar system or the eyeballs originally issued to you. And while artists typically like to spread their paint around, the art of collision avoidance you practice in the airplane should keep your paint in one place.

Traffic advisory and avoidance systems create a virtual shell around the airplane through which another aircraft cannot penetrate without triggering an alert. Several different panel-mount systems exist on the market, as well as a couple of portable options, which supplement your ability to identify traffic threats. While your pocketbook may dictate the type of system you can afford, it helps to understand the range of capabilities available and how they differ from each other before putting money down.

In fact, these systems have contributed a great deal to safety for commercial operators. Since the FAA first required installation of traffic alert and collision avoidance systems around 1990, no airline midair collisions have occurred in the United States, according to a report prepared by Mitre Corporation. Mitre's Center for Advanced Aviation System Development (CAASD) first proposed the use of transponders for traffic avoidance in the early 1970s and provided engineering support and development for the predecessors to today's systems as well as their initial deployment.

Traffic hardware

The top-of-the-heap traffic alert and collision avoidance systems come in two flavors, TCAS I and TCAS II. All air-carrier aircraft must be equipped with one or the other. TCAS I displays the relative altitude, distance, and bearing — known as a traffic advisory (TA) — of any transponder-equipped (and operating) aircraft within a given range, typically up to 40 miles. Both TCAS I and II give voice position alerts ("Traffic, 12 o'clock!"). TCAS II comes to mind when we conjure an airline-style system, because it gives a resolution advisory (RA) in addition to the TA. The RA directs the pilot with a vertical evasive action. You can imagine the consequences of two TCAS II-equipped aircraft receiving the same RA to avoid each other, and climbing or diving into the very midair the pilots sought to avoid. So TCAS II can coordinate complementary RAs with a TCAS II installation in an "intruder" aircraft.

TCAS collects traffic data by interrogating Mode C and Mode S transponder-equipped aircraft within range of the antennas. These antennas are mounted on both the top and the bottom of the aircraft to minimize shadowing effects of the fuselage — the system typically interrogates from both antennas. Then TCAS calculates a "closest point of approach," or CPA, to determine the relative threat of the target, allowing for anywhere from a 15- to 30-second warning before your metal encounters someone else's.

TCAS I and II systems are available for commercial aviation and large-aircraft use from Rockwell Collins and Honeywell. Honeywell Bendix/King also offers a TCAS I product, the KTA 970, for use with the KMD 550 or 850 multifunction displays, or the KMH 980, a "multihazard" version of TCAS I for use with Bendix/King's Integrated Hazard Avoidance System (IHAS).

Heeding good advice

Next on the list are traffic advisory systems (TASs) that use detection and modeling algorithms based on their upscale TCAS brethren. In fact, the L-3 Skywatch HP (of the Skywatch line formerly manufactured by Goodrich) can be installed as a TCAS or TAS, depending on the display and antenna, which must conform to TSO-C147 for traffic advisory systems and TSO-C118 for TCAS. Both the Ryan and L-3 TASs use a similar CPA calculation to TCAS. According to Ryan International spokesman Bruce Bunevich, the active interrogation system used by the Ryan 9900BX is technically the same as that driving a TCAS II, and its installation uses top- and bottom-mounted antennas similar to those of the high-end units. An audible warning is standard to both the L-3 and Ryan products, and both display traffic on either a dedicated display or a number of multifunction displays and EFISs. Ryan has as an option its 3ATI multihazard display to display weather, terrain, and traffic imagery, while L-3 offers a dual display for its Skywatch and Stormscope lightning detection equipment.

While the L-3 and Ryan TASs have different ranges, depending on the product, the systems display traffic in the same manner, giving relative altitude, descent or climb if applicable, and imminence of threat (by icon — a solid-yellow circle for targets warranting an alert, and a diamond for those within range but outside the traffic advisory airspace). The symbology is similar to that used in TCAS, so moving up to the higher-level system should be seamless. In addition to the visual warning on the screen, both systems provide an aural warning — the often vital, sometimes aggravating "Traffic!" call-out. Ryan's TAS adds azimuth, bearing, and distance to the call-out: "Traffic! 11 o'clock! High! Six miles!"

There are two schools of thought on the advantages and disadvantages of the aural warnings combined with a display. Bunevich, who is vice president of sales and marketing for Ryan International, contends that its extended aural warning helps pilots keep looking outside of the cockpit, rather than needing to refer to the screen inside. The traffic call-out is designed to be similar to one that a pilot might receive from air traffic control.

However, some pilots find the additional information distracting, preferring a quick glance to the screen — and noting that they often mute such systems in high-density traffic areas. This points to a Catch-22 situation: A midair collision is far more likely to occur in the high-traffic areas surrounding an airport, yet this is where the abundance of traffic calls leads to pilot saturation. On the bright side, most of us are keyed up to look obsessively for traffic nearing the airport.

Ryan has another option for those with smaller budgets, the 9900B TCAD (traffic alert and collision avoidance device). While the 9900B TCAD lacks the TSO certification of the 9900BX, the unit uses similar hardware — to move up to the BX, pilots need only send the processor in for an upgrade; the wiring and antenna installation remain the same.

The TCAD listens positively rather than the active interrogation used by its TAS and TCAS siblings. That means the TCAD receives transponder replies that have been triggered by center or approach control radar, or by TCAS- and TAS-equipped aircraft. If no one else out there is asking, the TCAD won't hear any answers. TCAD also gives an aural warning (a simple "Traffic!" call-out) and a visual indication of traffic bearing, altitude, and distance on its standard display, on the 3ATI multihazard display, and on various MFDs.

Traffic gear to go

A couple of portable devices passively detect transponder replies and gauge your distance from, and Mode C altitude of, those targets. Without an aircraft-mounted antenna to use and specialized installations that adjust for the fuselage challenges of a particular aircraft, the airframe can block signals from the device. But for the price, they represent a valuable extra set of eyes — and a reminder to look for traffic during high-workload times. SureCheck Aviation (see "Pilot Products," April 2002 and September 2004 Pilot) and Monroy Aerospace (see " Pilot Products," July 2004 Pilot) each make a selection of units at different prices.

Transponder plus

The increased demand on communications frequencies over the past 15 years has sprouted several developments that intend to make best use of existing avionics. Paramount of these is datalink, and specifically the FAA's TIS and FIS (traffic information system and flight information system) and ADS-B (automatic dependent surveillance-broadcast). While TIS-equipped transponders make use of Mode S transponder data (which carries aircraft information along with position and altitude) to display traffic information, ADS-B uses multifunction display technology and an airborne transceiver, UAT (universal access transceiver), to uplink and show traffic data between aircraft (position reports, velocity vectors, and aircraft intentions). The UAT also can be used to uplink TIS and FIS data (which includes textual and graphical weather) from the ground, in the form of TIS-B and FIS-B (the B stands for broadcast). ADS-B and TIS-B in particular offer a less expensive way for pilots to equip to a level of traffic awareness they would not have otherwise — because it delivers information that's piped into the ether by the FAA.

TIS-capable Mode S transponders give pilots access to traffic information from approach control radars. If an aircraft has an actively interrogated transponder within range of one of the 112 approach control radar installations in the TIS network, that aircraft's position information is transmitted via datalink to any TIS-compatible transponder in the radar's service area. The information is then displayed either on the transponder itself or on a multifunction display (MFD). Honeywell Bendix/King's KT 73 is TIS-capable, as are Garmin's GTX 330/330D transponders.

ADS-B ups the ante by allowing aircraft to share data independent of ATC or a ground-based facility. The system is used in the Capstone program (see " On Display: Chelton's Flight Logic Synthetic Vision," May 2003 Pilot, and " Future Flight: Air Traffic Control's Evolution," October 2000 Pilot), in which on-board avionics automatically broadcast aircraft position, altitude, and velocity (along with other data) every second or so via the UAT. Other aircraft or ATC can then use the data without the need for radar, bypassing radar's limitations. These limitations can be substantial in remote or rugged areas where radar is unable to penetrate nearby rocks from its limited ground sites.

ADS-B was first deployed in a demonstration where it was used to vector an airplane onto an ILS (instrument landing system) in a nonradar environment. As the technology has developed, the FAA's Capstone program uses ADS-B as part of the total system that combines ADS-B and GPS data for aircraft position monitoring, routing, and communication. Eventually, the FAA intends this as a model for a next-generation National Airspace System (NAS); the current NAS depends upon ground-based systems (such as, you guessed it, ATC radar). Part of this future is already here: A series of ADS-B stations are in place in the mid-Atlantic and Florida, providing traffic (and weather) information in these congested regions. AOPA, in championing a high-value solution for GA, has been heavily involved in testing and evaluating ADS-B technology and participates in the Capstone program.

Operational insights

While traffic systems assist pilots with finding and avoiding other aircraft, the systems work effectively — or at all — when pilots actively seek out the traffic called and supplement its warnings with their own subsequent visual grasp of the offending airplane. There are still aircraft legally and safely flying without transponders; the responsibility to see and avoid all traffic still rests with the pilot, regardless of the equipment on board (see " Scanning for Traffic," October 2004 Pilot).

When using any system that displays traffic, keep in mind that your eyes need time to adjust when shifting focus from inside the cockpit — and on that display — to more distant objects outside the airplane — such as that very airplane or helicopter the box just told you to avoid. Scanning for traffic at night uses more of your peripheral vision, since the rods around the perimeter of your eye are better equipped to sense dimmer light than the cones responsible for your color vision in the central portion.

Operationally speaking, only TCAS II gives pilots a programmed way out. So what do you do when you receive a traffic call-out from any other system? You need to determine evasive action (if necessary) on your own. Normally, dramatic banks away from the direction of the traffic are your last resort, saved for when you see an airplane about to smack you. Instead, pay careful attention to the target aircraft's altitude.

Think about it this way: A pilot runs the greatest risk of a midair when flying formation, something I engage in regularly in my role for this magazine, as well as for fun within the warbird community. If I were to lose sight of a fellow airplane while flying formation, I would first establish the other airplane's altitude and make sure mine didn't match. Then I would sort out where he'd run off to. Your location over the ground means little and can be easily misinterpreted — just witness the varying interpretations of "one-mile final." But you simply cannot hit another airplane if you aren't at the same altitude.

So you'll need to separate yourself by altitude from the target on the display. Usually a few hundred feet is sufficient to make up for discrepancies in altimeter settings and errors in the altimeters themselves. Most traffic systems use Mode C encoder information for their advisories, so the altitude data is typically quite accurate.

Then look for closure in range — range numbers scaling down or closing in position on the display, and how quickly the numbers or position moves — to prioritize traffic call-outs. Also, cuing off the trend in range above all helps to mitigate problems caused by stronger or weaker than normal transponders, or antenna shadowing (reduced ability to detect traffic overall) or patterns (which cause range to read closer than the actual target).

You can also use the scalloping effect to help determine the priority of a threat. Scalloping, or lobing, results from the lack of uniformity in the signal strength received by the antenna. From Ryan's pilot handbook on the 9900-series devices, "Traffic on a collision course does not move laterally, so the antenna pattern does not cross lobes." So the signal received is stable in this case, and the range indication decreases smoothly. When traffic crosses lobes, fluctuation in the signal level causes the range displayed to vary. So a smooth decrease in range warrants more concern than a fluctuating range.

You'll find the techniques that work for you after experience with a given device, perfecting the art of collision avoidance. With the latest developments in traffic systems, you have more tools than ever with which to create the picture.


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