Winging It! With Dr. Paul : PWI

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Winging It! With Dr. Paul

PWI

I hear the Friendly Aviation Authority has dropped the other shoe, and mandated altitude-reporting radar transponders for all aircraft operating within or near all Terminal Control Areas and Airport Radar Service Areas. That's a major chunk of airspace regulated in the name of safety. And although the civil libertarians wring their hands over our lost freedom of flight, I must say the idea has merit on the face of it. Transponders on all aircraft will facilitate a proximity warning system for all aircraft, which can't help but improve safety.

Why, then, am I still skeptical?

In the early 1970's, General Aviation Electronics Inc., alias Genave, introduced a low cost aircraft Proximity Warning Indicator. Simply a microwave receiver tuned to the 1090 MHz Air Traffic Control transponder reply frequency, PWI sounded an aural alarm and lit a warning light whenever another transponding aircraft appeared within its range. The sensitivity of the receiver could be varied between "high" and "low" with a panel-mounted toggle switch; a comparison of response in the two positions gave a qualitative indication of the target's distance. Not a bad bargain for $395.

Although not a collision avoidance instrument in the fullest sense because it gave no indication of target direction, the Genave PWI did provide a useful warning as to the presence of another aircraft in the general vicinity. The product achieved only limited commercial success, and disappeared from the market shortly after its introduction. It is important to note that the PWI only indicates the presence of transponding aircraft, and was introduced at a time when only a small fraction of the General Aviation fleet was transponder equipped. Thus I consider the Genave PWI to have been several years ahead of its time.

In the intervening years transponders have become first available, next affordable, then nearly universal, now mandatory. The PWI concept has been upgraded to 1990's technology, and renamed TCAS. Collision avoidance for all, at minimal cost.

Why, then, am I still skeptical?

Actually, there are significant technological objections to the transponder-based CAS approach. Have you ever been flying across the country, either IFR or under VFR advisories, and heard the controller say "radar contact lost, report LOSTU Intersection"? About the same time, you will have noticed that the orange light on your transponder stopped blinking. Now maybe that never happens where you or I live, but there are vast stretches of airspace over our country which are out of range from a ground radar facility. And if nobody interrogates a transponder, it never squawks!

Even if you're flying within a radar environment, transponders are not without limitations. Let's even eliminate the element of human failing by stipulating that everyone in the sky is responsible, thus transponder equipped, and infallible, which means they always remember to turn their switches to Mode C. The transmitters in ATC transponders still depend upon a small cavity oscillator containing a ceramic planar triode vacuum tube. Like all tubes, it has a finite life of perhaps a couple of thousand hours. And when it fails (as it ultimately will), you have no way of knowing it, but you just became invisible!

Why, then, should I not be skeptical?

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