July 2013-
Avionics is where the action is (and has been!) for some time in aviation. It seems as if each month brings a new, relatively low-cost gadget or app designed to increase situational awareness, monitor aircraft systems or otherwise improve the lot of pilots.
This month, we’ll look at a product that’s been around for a few years and it’s a device that fits the “big bang for the buck” paradigm: Zaon’s passive collision avoidance system (PCAS), the PCAS MRX.
But why PCAS when there’s ADS-B and TCAS?
Portable ADS-B devices are a more accurate traffic depiction system but they are also more expensive, especially when you add in the cost of a tablet computer and mounting device. Also, the ADS-B infrastructure won’t be totally in place until the year 2020 when all aircraft have ADS-B Out capability. TCAS can be expensive and it’s not portable, which may eliminate it as a choice for many GA pilots. This leads us to the Zaon PCAS MRX—a product for aviators that want collision avoidance protection on a budget.
In 2010, (the most recent reporting year), midair collisions occurred at a lower rate than weather-related accidents or accidents occurring while maneuvering at low altitude.
Still, the thought of running into another airplane strikes fear in the heart of every pilot, and for good reason since these types of accidents are generally fatal. Fortunately, for less than the cost of fuel consumed on a long cross-country trip in a typical single engine airplane, you can avoid attempting to occupy the same exact point in space as another aircraft.
The Zaon PCAS MRX provides proximity and altitude readouts and alerts for targets that get a bit too close for comfort. The product retails for $549; street prices are around $100 lower if you do a bit of skulking around the Internet.
Displaying threat aircraft altitude and distance relative to the host aircraft, plus altitude trends (ascending or descending when referenced to the host aircraft), the Zaon device is an excellent tool, particularly when in busy airspace.
It’s battery powered and has an output jack so you can interface its audio alert output with your intercom. And it occupies a very small amount of real estate on the top of an instrument panel—something that’s becoming more and more important with the plethora of portable devices now available.
Using the MRX
The MRX constantly scans for altitude reporting transponders installed on nearby aircraft. By decoding altitude information from the Mode C signal of these aircraft and comparing it to the host aircraft’s Mode C output, the device can approximate the distance and altitude relationship of several nearby aircraft compared to the host aircraft.
It prioritizes the threats according to distance and altitude difference and also determines if the altitude of the threat is increasing, decreasing or staying the same compared to the host aircraft’s altitude.
It issues audio warnings for moderate threats and audio alerts for serious threats in the form of a series of distinct beeps, and it also shows the relative distance and altitude difference of the threat on an LED display (which also shows whether the threat is climbing, descending or staying at a constant altitude).
Finally, it spells out the word “ALERT” on the LED display for a few seconds when things are getting particularly interesting and then reverts to the display showing the distance, altitude difference and altitude trend as it continues to track the threat.
Zaon’s XRX model provides bearing information but its cost is more than double that of the MRX. Because MRX lacks bearing information, it requires some interpretation to gain the greatest functionality. For example, a pilot can watch the target’s proximity (threat distance) over a short period of time to determine closure rate.
If the closure rate is relatively fast, the targets are generally out in front of you, or slightly off to one side—and headed toward you. If the distance stays the same, they are most likely off the wing or station keeping above or below you. A negative closure rate means the target is behind you and going away or it’s behind you and you’re outrunning it. And if the closure rate is slow, it’s likely that the aircraft is behind you and a bit faster than your machine. Looking at the altitude trends (descending, ascending or level flight) helps to determine the location of the aircraft in the Z axis.
All that said, you need to immediately go to an “outside the cockpit” mode, checking all quadrants, if you get an indication that there’s a target within a few miles of your aircraft and the aircraft is trending toward your position and your altitude.
The tech stuff
The MRX is battery powered using two standard “AA” batteries which last about eight hours unless the LED display is set to the highest setting—something unnecessary even in bright sunlight. It can also be powered from the aircraft’s electrical system using the cigarette lighter socket.
The MRX only senses transponder-equipped aircraft and the other aircraft’s transponder must be on and replying to an ATC radar query. In this respect, it’s no different then ADS-B or TCAS since all current technology only “sees” transponder-equipped aircraft.
Once the transponder responds, the device decodes the target aircraft’s transponder signal and processes its Mode C altitude information. It then compares your Mode C transponder replies to the target’s Mode C replies and determines the target’s altitude relative to your own. The altitude information is accurate to within a few hundred feet.
With respect to target proximity, you only have one antenna: the one attached to the unit that is sitting on the top of your instrument panel. So, if that antenna is blocked by the aircraft’s metal structure, the signal strength from the target’s transponder will be lower from the blocked direction (principally from targets below and/or behind your aircraft).
The MRX measures the signal strength of the target’s transponder and then infers its distance based on mathematic formulae that express how rapidly signal strength decays with distance.
But a distance assumption based on the amount of signal the target’s transponder is providing to the MRX could be inaccurate, as explained above. Fortunately the device compensates for this by warping the distance-determining algorithm a bit to make weak targets appear closer than they are. It’s not as accurate as ADS-B or TCAS, but the MRX is much less expensive than the alternatives.
Bottom line
A VFR Sunday afternoon in late summer at an uncontrolled airport is statistically the most dangerous time and place for a midair collision. That said, eternal vigilance is always necessary regardless of time, place, or season, and the Zaon PCAS MRX is a vigilance multiplier.
John Loughmiller is a 4,600-hour commercial pilot and CFII MEI-A. He lives in Kentucky with Donna, his wife of 39 years, and often commits random acts of aviation.
Resources
ZaonFlight Systems, Inc.
zaon.aero
