Most pilots would agree that collision avoidance is desirable. The FAA agrees, and long ago made the desirable mandatory. FAR 91.113 states that all pilots have the responsibility to see and avoid each other regardless of flight plan or aircraft type, when weather conditions permit.
IFR and VFR pilots in VMC have the same responsibility to ensure that no two aircraft occupy the same space at the same time. No excuse will suffice if metal meets metal when visual meteorological conditions prevail. The FAAs logic is similar to that expressed in signs common on Texas ranches that proclaim that trespassers will be shot, and survivors prosecuted.
With this heavy burden upon all pilots, looking out the window remains today the primary means of avoiding other traffic in the general aviation community. Student pilots are usually taught proper scan technique, but those skills start to deteriorate as soon as the instructor is out of sight.
In fact, proper scanning is difficult. Since the human eye tends to focus at a point, even in open sky, pilots must shift glances and refocus in intervals to see incoming objects. An effective scan requires a series of short, evenly spaced movements of the eye. Each segment of sky, taken in at about 15 degrees at a time, should be observed for at least one second. The reality is that most pilots fall short of the ideal. And most IFR pilots focus more on the instrument panel, even in VMC.
The task of seeing other traffic is demanding even with a perfect scan, regardless of whether youre plying the crowded skies of major metropolitan areas or an uncontrolled field on a clear Sunday morning. Consider that a typical jet and light twin would approach each other at almost 800 mph. In a typical scenario, two planes converging head-on at that speed will be less than 10 seconds apart when either pilot would first detect the conflict. The problem is that the most agile and alert pilot would require the full 10 seconds to identify traffic, recognize the conflict and react to avoid a collision.
The imperfections of the see-and-avoid approach are evident in the number of mid-air collisions found in the NTSB database involving general aviation aircraft. In a quick perusal of the data, I counted over 200 such accidents listed since 1980. The vast majority happen in the airport traffic pattern area.
The story unfolds differently for the airlines. A dramatic airborne disaster over the Grand Canyon in 1956 led to the realization that a system needed to be developed to prevent future collisions. In 1978 over San Diego, a small plane collided with an airliner, again prompting calls for more effective collision avoidance. In 1982, the FAA formally launched the Traffic Alert and Collision Avoidance System (TCAS) program.
Competing airborne and ground-based systems were in development when, in 1986, an Aeromexico DC-9 and a Piper collided over Cerritos, Calif. For airliners, full reliance on the human eye to avoid traffic came to an end in the aftermath of this accident.
Congress passed a law requiring the FAA to make TCAS mandatory. The ultimate result through various acts of legislation was a law stating that any airplane with more than 30 passenger seats must be equipped with TCAS II, along with the appropriate class of Mode S transponder. The rule became effective for all affected aircraft on January 1, 1994, and two years later for cargo airplanes.
The Trickle Down Theory
Like many of the innovations aboard mega-buck airliners, TCAS bears only passing relevance to general aviation. However, recent and future developments in avionics may make TCAS-like devices a realistic option for at least some owner/pilots.
Two versions of the full-blown TCAS are operational. TCAS I indicates bearing and relative altitude of all aircraft within a selected range, generally up to 40 miles. A graphic display, usually using color, indicates if an aircraft poses a threat. This gives the pilot a traffic advisory.
Once provided with a traffic advisory, the pilot must identify the threat visually and decide what evasive action is necessary based on the view through the window and the data on the screen. Once a traffic advisory is received, a pilot is authorized to deviate plus or minus 300 feet in altitude, but lateral deviation is not authorized. If in IMC, the pilot must notify ATC for assistance in resolving the problem.
TCAS II goes further by offering the pilot a resolution advisory when appropriate. The system determines the course of all aircraft in range, and if each is descending, climbing or in level flight. The unit then issues advice to the pilot on what evasive maneuver will avoid a collision.
If both aircraft are TCAS II-equipped, the system provides coordinated advice to both pilots that will ensure no conflict ensues. TCAS III, which is under development, will provide traffic advisories in horizontal and vertical dimensions to avoid conflicts.
Since inception, several generations of software and hardware have evolved to improve the system. The FAA has stated that the most recent version 7.0 software will be the final logic for TCAS. The upgraded software significantly reduces unnecessary resolution advisories and improves performance in several other critical areas, including surveillance and limiting interference in high-density areas. But where do these developments leave general aviation?
The first problem in adapting TCAS to general aviation is the fact that the GA fleet is equipped with Mode C transponders, not Mode S. The difference is critical.
Mode C transponders encode altitude, providing key data to ATC trying to separate IFR traffic. Mode S transponders are far more elaborate. Each Mode S transponder-equipped aircraft is assigned a unique code. Using this code, electronic interrogations can be directed toward an individual airplane. Its response (including azimuth and altitude data) is unambiguously identified with that aircraft.
Advances in computing have allowed a form of TCAS to be developed for general aviation using only Mode C. The two primary competing systems on the market are Skywatch by BF Goodrich, and TCAD by Ryan International. Prices vary considerably, ranging on the order of $5,000 to $25,000, depending on the unit and if a separate dedicated display is included in the price.
In addition to these, the Monroy ATD-200 by Monroy Aero offers a low-cost alternative at about $800. In the priciest models, traffic information and conflict advisories are displayed on a CRT as the system surveys surrounding airspace for potential conflicts.
All three systems share the common operating principle of relying on the Mode C transponder of nearby aircraft to identify threats. But the differences in approaches and capabilities between systems are significant, as the price range would imply.
Determining which unit makes the most sense for a given application depends on how price sensitive the buyer is. As with any avionics, you get what you pay for. However, the cost/benefit analysis may leave you scratching your head over whether the systems provide a good value for the dollar.
Ryan International offers the 8000 and 9900 series, with a total of six models ranging in price from $5000 to $20,000, not including installation. The latter is not trivial, however, and can add significantly to the cost. The manufacturer says a typical installation requires between 50-100 hours ($3,000 to $6,000 at a shop rate of $60 an hour). It includes a transponder coupler that periodically suppresses your transponder so the unit can receive transponder replies from other aircraft, a panel-mounted indicator, top and bottom antennas to avoid airframe shadowing, a visual traffic warning light and a connection to the audio system. The higher-end models also require installation of a remote-mounted processor.
Nor can you forget the FAA, which can create an additional burden, depending on the attitude of the local FSDO toward avionics installations.
The Ryan 8800 series TCADs are entry-level units. The 8800 Silver (about $5,000) alerts pilots to traffic threats within a maximum radius of 3 miles and a minimum of 1.5 miles by showing altitude separation, altitude trend and range on an integral alphanumeric display. It does not provide the direction of the threat.
The display unit is small and requires some interpretation, but pilots experienced in its use seem to adjust accordingly. The unit passively tracks up to 50 aircraft and displays the three closest threats. The display also indicates altitude changes of threat aircraft and whether the traffic is converging or diverging.
The 8800 Gold (about $6,000) adds an altitude alert function, providing a warning prior to arrival at an assigned altitude, or if the altitude changes inadvertently in cruise. In addition, the 8800 Gold displays the N-number of mode S-equipped aircraft within range. The 8800 series cannot be upgraded to provide information on the direction of the traffic threat, but the Silver is easily beefed up to the Gold.
The 9900 series is more expensive and more capable. The basic 9900 (about $8,000) is similar to the 8800 Gold, but is remote-mounted and therefore can later be upgraded to the top-of-the-line units. The 9900A (about $10,000) introduces a female voice warning, Traffic instead of a simple beep and allows the pilot to customize the threat envelope range.
The high-end 9000B is offered at the real-money cost of about $14,000, but is the only model that provides direction-of-threat data. The effective range in the 9900B is also upgraded to a maximum of 6 nautical miles and a minimum of 0.5 miles. Perhaps equally important, the 9000B can display threats on an Argus moving map, Avidyne Flightmax or the Garmin 430/530 GPS. All of the lower-end 9900 units can be upgraded to the 9900B.
The most recent Ryan TCAD product is the 9900BX ($20,000), a factory upgrade of the 9900B. This unit enables the TCAD to actively interrogate aircraft up to a maximum radius of 10 nm, using all of the same wiring and antennas of the other 9900 series units. With active interrogation, the technology moves closer to the approach taken by BF Goodrich in the Skywatch.
BF Goodrich offers fewer low-end options than Ryan. The base Skywatch model is about $20,000 before installation, and going full-boat escalates the price tag to a numbing $31,000 before installation. Skywatch data can be displayed on a WX-100 Stormscope or a Garmin or Avidyne display.
The large display significantly improves situational awareness compared to the alphanumeric approach of the Ryan units, and obviates the need to purchase an expensive MFD with the 9900B for those who want a big display. On the other hand, with the growing popularity of the Garmin units, that advantage will diminish with time.
The Skywatch has a selectable range of 2 to 6 nautical miles, and displays the eight most threatening aircraft (compared to three in the Ryan). The biggest difference – besides cost – comes down to the fact that Skywatch is an active interrogator and all the Ryan units except the 9900BX are passive. Proponents of both systems claim superiority.
While BF Goodrich and Ryan International duke it out, a low-priced third choice has entered the ring, the Monroy ATD-200 by Monroy Aero. The device is portable, requiring for its operation only a cigarette lighter and a pilots headset jack. No installation is necessary, which makes this unit attractive to renters and pilots flying multiple airplanes.
Traffic range is indicated on a five-segment red LED bar instead of an alphanumeric display. When traffic is at 4 nm, one LED segment is lighted. As the traffic closes in, successive segments are lighted. When the traffic is closer than 0.5 nm, the bar is fully lighted.
If the intruder happens to be Mode S or TCAS II equipped (the big boys), a yellow LED indicator is lighted. Somewhere close to 3 nm a female voice intones, Traffic through the pilots headset, and at about 1 nm, the voice gives a more emphatic, Traffic nearby.
What the unit does not provide is bearing to the threat or altitude information. You simply know that somewhere within the indicated range there is an airplane that must be identified with the old stand-by eyeballs. Still, for only $800 you have an assistant reminding you to keep your head out of the cockpit and looking for traffic.
Cost vs. Benefit
Now that traffic avoidance systems are available at a price many small-aircraft owners can afford, pilots are faced with a new dilemma – assigning a priority to such a system when there are so many competing demands for the checkbooks attention. Lightning detection, color radar, a backup electric attitude indicator or a new GPS unit with a large color moving map all beckon owners looking to buy additional capability and safety for their airplane.
In the absence of competing priorities and budget priorities, the rationale for installing collision avoidance gear is fairly strong. I recently flew right seat in a King Air C-90B equipped with a Skywatch system and was duly impressed.
I am ashamed to admit that I often do not see targets ATC calls, even with a careful scan. This is a source of frustration, not to mention potential danger. On the clearest of days, I strain to see the traffic, but often with no success. Yet with the help of Skywatch in the King Air, I visually identified every aircraft ATC called out – and many that they did not. The aircraft that we saw that ATC failed to mention were real eye-openers.
Perhaps more compelling is the fact that greater than 95 percent of all mid-air collisions happen in VFR conditions, usually with no ATC traffic advisories being issued to either aircraft. The danger zones include areas of flight training in congested airspace, busy uncontrolled fields and well-known scenic routes.
The question thats difficult to answer is whether devices that dont provide a bearing to the traffic provide that much help – especially if youre flying in a traffic pattern or other airspace where you already know other airplanes are around.
Consider a typical mid-air collision, this one in Kent, Ohio in 1998. Two Cessna 152s collided while operating in a congested airport traffic pattern. The pilots lost visual contact with each other. Would any of the sub-$20,000 collision avoidance systems have provided enough information to prevent the collision?
Another example shows positional awareness and confusion can lead to bent metal, and where having traffic avoidance gear might have helped. The NTSB report describes a Lancair pilot who had been lost. He entered Class C airspace without contacting ATC and then entered the Chino airport traffic area but contacted the Brackett tower for landing.
Brackett tower determined that the pilot was at Chino and instructed the pilot to change to the Chino tower frequency. He was then given instructions by the Chino tower to land number 3 behind another airplane on a three-mile final. That airplane, a Cessna 172, which was sequenced to land behind a Cessna 152, had reported in the clouds right now.
The Lancair pilot said, I have the Cessna, Im going outside him. There was also another airplane in the pattern, a Scout. The Lancair collided with the tail of the Cessna 152, about -mile from the approach end of runway 26.
While the NTSB found fault with the Lancair pilot for inadequate visual lookout, it also blamed the tower controller for failing to advise the Lancair pilot that the Scout was in the pattern and for calling as traffic an airplane that probably was not visible to the pilot of the Lancair.
Accident reports clearly weigh in favor of using collision avoidance equipment, but still the original question about priorities in selecting new avionics remains on the table. Ten pilots in a room will offer 12 opinions about what is important in the panel, and Ill add my opinion here. My overriding priority is safety in making the following choices, in order of priority: a panel mount IFR GPS, lightning detection, radar, an electrically driven backup AI and collision avoidance equipment.
The argument about lightning detection and radar is long-standing, and each has its proponents. Personally, I need and use both to avoid weather, and would not fly in many circumstances if both were not on board. I believe that these units have clear priority over collision avoidance.
The backup AI has a higher priority because I have had many personal experiences with mechanical and electrical failures in spite of obsessive maintenance. The bottom line is that if your panel is already a poster child for avionics, and you have money burning a hole, then Skywatch or TCAD would be a great addition.
The future of this technology is bright, even if a bit out of focus. On the horizon is what many consider an alternative to TCAS, the Automatic Dependent Surveillance (Broadcast), or ADS-B, system. Each appropriately equipped airplane transmits its altitude and position derived from GPS, and the data from all airplanes are displayed on a MFD in the cockpit, as well as on radar displays in air traffic control.
This allows ADS-B to serve as a surrogate radar where such coverage does not exist. The pilot and controller wind up looking at the same picture, which can include moving maps, Nexrad weather images and terrain data, not to mention traffic advisories.
The hope is that ADS-B may in the future enable IFR pilots to accomplish their own traffic separation, eliminating the need for en route radar. For obvious reasons, the ADS-B system is touted as the cornerstone of FAAs future Free Flight program. ADS-B is being implemented on an experimental basis in Bethel, Alaska, which has no traditional radar coverage.
Under this Capstone project, 150 aircraft (mostly Part 135 single engine planes) have been equipped with avionics appropriate for ADS-B. So far, reports are highly favorable from participating pilots and controllers.
With TCAS operational and ADS-B being evaluated in the field, the two systems can be considered complementary for the moment. TCAS displays the location and relative altitude of targets, and resolution advisories in the case of TCAS II. While ADS-B provides more information (weather, terrain, moving maps, and traffic), and it functions at much longer ranges, TCAS is more widely in use.
Neither, however, is in the immediate future of general aviation. For now, the only realistic options are Skywatch, TCAD or the lower cost Monroy alternative – and then only if collision avoidance is the safety angle thats highest on your list.
Also With This Article
Click here to view “Active or Passive, Neither Way is Perfect.”
-by Jeff Schweitzer
Jeff Schweitzer is an aviation writer, editor of the Malibu-Mirage Owners and Pilots Association magazine, and owner of a Piper Mirage.