Turbine Pilot

Electronic antidotes for midair collisions

Ever since the 1970s the search has been on for more effective collision avoidance avionics. This push was fueled in large part by a number of high-profile midair collisions, one of the worst being the August 1986 crash involving an Aeromexico DC-9 and a Piper Archer over Cerritos, California. That crash also set in motion today's mandatory Mode C altitude-reporting transponder provisions within a 30-nm ring around Class B airspace.

But even if every airplane had a Mode C transponder, there would still be no guarantee of midair- or near-miss-free flying. After all, ATC is obliged to monitor and separate only those aircraft on IFR flight plans, or in certain terminal airspace. ATC can suppress VFR radar returns. ATC also can fail to notice traffic conflicts. And in visual meteorological conditions, even those of us who fly on IFR flight plans still have to rely on visual separation as the prime means of collision avoidance.

However, times are changing. Today's technology offers general aviation pilots a range of collision-avoidance equipment that couldn't have been imagined 30 years ago. Here's a brief rundown of current collision avoidance hardware and technology.

Ryan TCAD (Traffic and Collision Alert Device)

These devices are the lowest-cost units on the market. The Model 8800 Silver ($4,995, uninstalled) is the basic model, and the 8800 Gold ($5,995) adds an altitude alerter. The information is displayed in text format on a small vacuum fluorescent screen, along with symbols that indicate whether detected aircraft are higher or lower than your altitude, their approximate ranges, and if the offending target is closing on your altitude. Up to 50 targets can be monitored simultaneously, but only the three most imminent are displayed. The pilot can select the dimensions of protected airspace. The en route shield alerts the pilot when an aircraft ventures within a three-nm radius and a vertical depth of 2,000 feet above and below your altitude. A terminal setting narrows the alert to a 1.5-nm radius and plus or minus 500 feet. There's also a shield for use on the ground, which is helpful in looking for traffic prior to takeoff. The 8800 Gold has an additional "unlimited" shield setting — 5,000 feet above and below, out to three nm.

Next up the product line are Ryan's 9900 series of TCADs — the 9900 ($7,995), the 9900A ($9,995), and 9900B ($13,850). These have smaller displays and take up less panel space than the 8800s. The 9900 is the basic model, while the -A version includes voice annunciation of potential traffic conflicts, and the -B also provides bearing information to the nearest target. Bearings are shown by means of small arrows that point in the direction of the threat. Bearings are not precise, because they only point in one of eight directions.

The 9900B can be adapted to feed traffic information to an Eventide Argus 5000 or 7000 color moving map display. This modification (it's called the 9900B+) requires a $500 transponder coupler interface.

The Argus display screen can then show TCAD returns in a thumbnail-view inset in the lower left corner. A full-screen view of the traffic situation can also be called up, and in this mode the TCAD symbols will show a target's relative altitude and bearing, along with any altitude convergence or divergence information. You have to depress a button to call up the bearings of other nearby traffic conflicts.

TCAD's sales pitch revolves around altitude separation. If you can keep other airplanes above or below your altitude, you can't have a midair. Add range information, and you know how close your threats are. But unless you have a 9900B, you won't know where to search the sky for bogeys.

BFGoodrich Skywatch SKY497

Next up the scale in collision avoidance hardware is BFG's Skywatch. The Skywatch has many more modes of operation than the TCAD.

It displays the eight most threatening targets. Skywatch information can be displayed in plan view on an Avidyne or Archangel multifunction display, an EFIS display, various radar displays, and BFG's own WX-1000 Stormscope, using the same target symbology as that of much more expensive TCAS units.

Skywatch takes all that information, determines whether another aircraft will be a collision threat, then displays it with standardized target icons. Aircraft within 30 seconds of a possible collision, or within a one-half-nm radius and plus or minus 800 feet, will generate a traffic advisory (TA) icon on the visual display and a "Traffic, traffic" aural message over the airplane's audio system. The TA appears as a filled-in circle. Non-threat aircraft show up on the screen as open diamonds. Both types of symbols will also show relative altitude and vertical trend (e.g., "+100" means that a target is 100 feet above your altitude; an arrow pointing downward means that the target is descending). Altitude resolution accuracy is within plus or minus 200 feet.

Skywatch operates on one of two sensitivity levels. With landing gear down, it gives traffic advisories if intruders venture within a 0.20-nm radius and plus or minus 600 feet, or if conflicting traffic will converge within 15 to 20 seconds. With gear up (or with fixed gear) the advisory search area is expanded to a 0.55-nm radius and an altitude range of plus or minus 800 feet relative to your altitude. If your airplane is equipped with a radar altimeter, the advisory area narrows below 2,000 feet agl. The intent here is to reduce the number of nuisance traffic advisories during takeoff and landing where there are numerous other aircraft nearby.

There's also "look up" and "look down" display modes. This lets you scan for traffic up to 9,000 feet above or below your altitude. This feature is helpful in the takeoff or descent phases of flight. The normal mode looks at airspace 2,700 feet above and below your altitude.

Skywatch sells for $24,265. That price includes a standalone display. If you already have BFG's WX-1000 Stormscope the information can be displayed on that, for a savings of about $5,000. While its accuracy and 6-mile range make it fall short of TCAS standards, many pilots and operators find that the combination of the Skywatch's price, operating modes, symbology, and informational content make it a very desirable alternative to going with a bona fide — and much more expensive — full-blown TCAS.

TCAS I

TCAS (Traffic Alert and Collision Avoidance System) equipment comes in two levels of sophistication: TCAS I and TCAS II. TCAS I is required equipment for all FAR Part 135 and 121 aircraft seating 10 to 30 passengers, but many corporate operators have voluntarily elected to go the TCAS I route.

With TCAS I, up to 35 targets can be tracked out as far as 40 nm. Pilots can select various ranges using the TCAS display's control panel. Accuracy of the depicted target's bearing is within five degrees; altitude resolution is within plus or minus 100 feet. The symbology used in TCAS I and TCAS II displays is standardized. An open white diamond depicts other traffic within the selected display range. A solid white diamond indicates proximate traffic within four nm of your position and within plus or minus 1,200 feet of your altitude. Should proximate traffic come within 15 to 30 seconds of colliding with a TCAS I-equipped aircraft, or within 0.2 to 0.55 nm and plus or minus 800 feet, the solid white diamond turns into a yellow circle and a TA is generated. This includes the "traffic, traffic" aural message.

AlliedSignal, BFGoodrich, and Honeywell all manufacture TCAS I. AlliedSignal's Bendix/King CAS 66A ranges out to 40 nm, and can interface with a dedicated traffic display (the TID 66A) or a weather radar; EFIS; or a Bendix/King IVSI/TA display, the IVA 81A. The IVSI/TA (instantaneous vertical speed indicator/traffic advisory) is a combination electronic VSI and TCAS target display. The basic CAS 66A goes for $66,683 and is upgradable to TCAS II capability; the TID 66A is another $6,000, and the IVA 81A is $16,800.

The BFG model — the TCAS 791 — sells for $56,520 for the basic hardware, which includes a dedicated color CRT display. Should you want your TCAS advisories displayed on an already-installed EFIS or MFD, the price is $48,480. Maximum target range on the display is 20 nm. Honeywell's TCAS I, the model 1500, is a new arrival on the market, and sells for $75,000.

TCAS II

TCAS II gives you all the features of TCAS I, and more. It's required equipment on all airliners having more than 30 seats. It doesn't just say "traffic, traffic" when an intruder is near, it also gives climb and descent commands so that pilots can maneuver away from any immediate threats. These commands, called resolution advisories (RAs), come via depictions on electronic VSI displays, such as the IVA 81A. The commands are either corrective — calling for a change in altitude — or preventive, advising against an altitude change.

TCAS II calculates these vertical evasion maneuvers by computer, using signals from other Mode C or Mode S (the S is for selective addressing) transponder returns.

Traffic heading for a collision with a TCAS II-equipped aircraft progress from being portrayed as open diamonds to solid white or cyan (on Rockwell Collins displays) diamonds (à la TCAS I), to yellow/amber circles (TAs), to red squares (RAs). RAs come when TCAS II determines that there's anywhere from 20 to 30 seconds to collision. That's when the VSI display will show a green arc — indicating the commanded climb or descent rate, and a red arc — the vertical speeds that must be avoided.

Should a TCAS II-equipped aircraft come in conflict with another TCAS II aircraft, the computers will coordinate RAs via datalink. For example, one aircraft will be instructed to climb, the other to descend. In this way, the aircraft are prevented from making evasive moves that themselves could result in collisions.

AlliedSignal, Honeywell, and Rockwell Collins all sell TCAS IIs. AlliedSignal's CAS 67A includes dual IVA 81As and goes for $167,243. Honeywell's new TCAS 2000 is priced at $146,596. This unit can interface with many display options, including MFDs or EFIS. The TCAS 2000 has some advanced features, such as the ability to display aircraft 100 nm away, and provides enhanced escape maneuvers — commands to increase rates of climb or descent, or reverse them. Rockwell Collins' latest — the $143,000 TCAS 4000 — has similar capabilities.

Currently, TCAS II is the state of the art in collision avoidance. But technology is advancing at a terrific pace. A few short years ago, the focus was on a next-generation TCAS — a system that would provide a combination of lateral and vertical evasion commands. This was to have been TCAS III, then TCAS IV. Now the spotlight has shifted away from TCAS altogether.

The future

The FAA has yet to officially define the methodology or specifications for the next collision avoidance technology. But at this juncture it certainly looks as though Automatic Dependent Surveillance-Broadcast (ADS-B) is the wave of the future. ADS-B is essentially an airplane-based surveillance technology that can both broadcast and monitor the GPS positions, speeds, altitudes, tracks, and predicted positions of participating aircraft. Each airplane broadcasts its GPS navigation information in a constant stream of digital data, and others process that data. ADS-B doesn't just represent a replacement or enhancement to existing TCAS systems. It's also the cornerstone of the proposed free-flight concept of point-to-point navigation.

Instead of relying on short-range TCAS warnings or less-accurate ground-based radar surveillance for traffic separation, the pilot will able to see a much more complete and precise depiction of traffic — right in the cockpit. So will controllers, who will see the same broadcast data on their screens. TCAS-type software can be interfaced with ADS-B, and because of GPS's greater precision and ADS-B's higher data update rate, traffic advisories can come with far more advance warning.

Because of ADS-B's open system architecture, digital datalinks and communications protocols can also be part of the system. This opens the way to in-flight air-to-ground datalinks for weather graphics, ATC clearances, and other flight-plan-related information, and even fax and other telephone services.

So far, II Morrow and the Cargo Airlines Association (CAA) have been conducting successful in-flight trials of ADS-B.

The problem with the ADS-B concept is that no one can agree on the method of data transmission. The II Morrow/CAA tests have been using Mode S transponders, which fit today's conventional mindset. But some experts worry about the Mode S frequency bandwidth's susceptibility to hit data overload when ADS-B functions are added. Another method — VDL Mode 4 — involves using a wider, VHF-frequency bandwidth for datalink in the 108.00- to 117.975-MHz band. VDL is now undergoing tests in Sweden, but it's unclear whether VDL would be suitable for ADS-B applications. The third alternative has been developed and tested by the Mitre Corporation. Dubbed the Universal Access Transceiver (UAT), this approach uses the 966 MHz frequency and technology that merges aircraft-to-aircraft and air-to-ground data for both traffic information and weather products. On its display, traffic, weather radar, and lightning detection returns are presented simultaneously. Mitre believes that its technology would be the least expensive to implement on general aviation aircraft.

We've come a long way from the Cerritos days. Although future collision avoidance technologies have yet to gel, a broad range of increasingly accurate equipment is available today — and at a range of prices that puts it within the financial grasp of many high-end general aviation operators.

E-mail the author at [email protected].