Saturday, May 27, 2006

Little Voice

I was instructing in a G1000-equipped C182 recently and was headed through a local practice area. I had noticed that we hadn't seen any other traffic appear on the G1000's moving map, so I diverted my attention to finding out if the traffic feature had been disabled by the last pilot who flew the plane. Before I got deep in the G1000 menus, I told the pilot I was instructing that my attention would be diverted and that he should keep his eyes outside. Sure enough, the traffic feature had been disabled and when I re-enabled it, a few diamond-shaped targets appeared around us on the moving map. I felt better.

I've flown off and on the last 3 years in various aircraft equipped with TIS (Traffic Information System) or TAS (Traffic Advisory System), but the majority of my flight time has been in aircraft without any such system. What I've learned is that there's a lot more aircraft sharing the sky with you than you'd think if you're just using your eyes. I've also seen aircraft that didn't appear on the TIS or TAS system even though they were clearly visible to the naked eye, so these systems are by no means foolproof.

Most non-pilots don't realize that without an operating transponder, small aircraft don't show up very well on ATC radar. And without a functioning transponder, an aircraft won't show up at all on a TIS, TAS, or airline TCAS (Traffic Alert and Collision Avoidance Detection) units. Without going into a lot of details, a transponder receives the radar sweep from ATC's radar, and responds with a signal of its own. Transponders will also respond to the interrogation signal from a TAS or TCAD system. The transponder reply can contain a unique, ATC-assigned 4-digit code or the standard Visual Flight Rules code of 1200. If the transponder is set to squawk altitude, the signal will also contain the aircraft's altitude. That's right - neither ATC radar nor traffic detection systems can determine an aircraft's altitude on their own: They rely on the aircraft's transponder to tell it the altitude.

I've seen aircraft appear on a TIS or TCAD system with no altitude readout, probably the result of the pilot having the transponder turned on, but not set to squawk altitude. I've even seen this within the San Francisco mode C veil, where all aircraft are required to have an altitude-encoding transponder, to have that transponder turned on, and to be squawking altitude.

Back to my story. A few minutes after re-enabling the G1000's traffic feature, we were passing just east of Mt. Diablo when I saw a target appear on the G1000's moving map, three miles ahead and at our altitude. Then poof! The target disappeared. The pilot I was instructing saw it too and commented "That was strange." We both looked ahead, but saw nothing. After a few seconds, I heard a little voice in my head say "turn 20 degrees left." I made this suggestion aloud and the pilot turned 20 degrees left. That's when we saw the other aircraft, at our altitude, less than a half a mile away, heading directly to where we would have been had we not turned.

Following the regulations, we were east bound at 3500', an odd altitude plus 500'. The other plane was westbound and should have been at an even altitude plus 500', like 4500', but he was WAFDOF - wrong altitude for direction of flight.

My conclusions?
Technology can be a great thing, but it's not always reliable.

Pilots should fly at an appropriate altitude for their direction of flight, but they don't always do so.

With a G1000, you have to fight the urge to stare at the pretty colors on the screen and keep scanning for traffic outside.
And if you hear a little voice in your head, you best listen to it.


Dave said...

YES! to all of your conclusions. I call the GNS 430 the "TV Set". A great device but it while capture and hold your attention like nothing else in the cockpit. ...and I believe in those little voices.


paul said...

OK, verdict then? Is it worth it to spend $1600. on an XRX?

I am leaning towards no. Might tell you about the WAFDOF, but when your queued 1500 ft apart for the local fly-in the box would useless, or worse, diverting your attention from the radio and the scan.

Ask yourself this, would the Bashkirian Airlines Tu154 and DHL 757 have collided near Ueberlingen, Germany without TCAS? Certainly controller error, but from what I read a near miss was turned into a tragety with the help of TCAS.

Great blog BTW - hope things are working out. Too bad your not closer, I need a good IFR instructor.



John said...

My understanding of the accident you cited is that the TCAS systems on the two aircraft provided resolution advisories that would have prevented the collision. The flight crew of the TU-154 decided to obey ATC rather than the TCAS resolution advisory, which was a fatal mistake. Flight crews are always supposed to follow the TCAS RA, then advise ATC of the deviation. ATC was a contributing factor, but TCAS worked as it was designed.

I fly with a pilot who uses a personal collision avoidance device (not the XRX) and he swears by it. You can set the envelope for warning (both distance and altitude) so that it doesn't go off all the time. Used judiciously, I think these devices are pretty darn good. If I could afford one, I'd probably use it.

Anonymous said...

Hi John:

"I heard a little voice in my head say turn 20 degrees left."

Just curious ... why not "20 degrees right?" I thought the rule of thumb (and regs stipulate) that "when aircraft are approaching each other head-on, or nearly so, each pilot of each aircraft shall alter course to the right."

John said...

For the short time that the target showed up on our moving map, it was ahead and to our right. Turning left put the traffic on the right side of the plane where I was sitting. I had more time to look outside since I wasn't the pilot flying. Also, I don't think the pilot of the other aircraft even saw us until we had passed one another and didn't even know there was a conflict.

It's great to know and follow the regulations, but they are no substitute for common sense. Sometimes even intuition comes in handy, too.

Dave Starr said...

Glad to see you back, John. I told you when you were thinking of leaving the last job and wondering about the blog that readers would follow you, and some have already. Keep writing.

Correct on your interpretation of the Überlingen accident. The primary cause was the controller issuing the wrong altitude instructions. Because of various factors the controller issued a descent to the TU-154 crew which would result in them crossing the 75's altitude. The TU-154 complied with the instruction but did not acknowledge and their TCAS immediately commanded them to climb (the correct action that would have avoided the accident). There has never been an accident/documented TCAS 'wrong way' error yet, since the program was initiated. If both aircraft's TCAS are communicating the systems will always cooperate to issue commands that maintain or increase separation. A very good write-up on Überlingen here:
(also includes link to the actual accident report).

For those not familiar with Don Brown's column I highly recommend reading, he explains a lot of things from the controller's perspective that us pilots don't always see in the same perspective.

Nate said...

Excellent post John. Nothing can replace the visual scan. As a student pilot, I thought that I might ease my mind by buying a SureCheck trafficscope micro 200. All this little device did was distract my normal visual scan...and was soon returned. In SF Bay Area airspace (and anywhere else), nothing can replace learning how to scan correctly and being constantly vigilant.

Paul said...

Thanks guys. I read that Ueberlingen accident description again, and stand corrected.

Its wierd. I almost always ask for advisories from ATC (where they will talk to a VFR flight) when I fly CC and most of the time, I don't see the traffic they call out.

I am hoping that Zaon will make a panel mount and that I can afford it...


Christian Goetze said...

I cannot even begin to say how much the Ueberlingen incident - and the responce of ATC people to it - gets me upset. There were three @$#% airplanes in a space of a dozen cubic miles, but somehow the system managed to put them on a collision course.

All the stuff about that one poor controller being overwhelmed just makes my point. Nobody in his right mind would try to drive 20 cars in freeway traffic at the same time, so why are we trying to do that with airplanes?

Greybeard said...

Anyone who has flown a helicopter will know immediately what I'm talkin' about.....

Ya can't see out of a damned airplane! And the bigger they get, seemingly the worse the view out the windshield. I checked out the cockpit on a 737 last week, and the "seeing" is dismal.

My concern about these devices is that they will bring the pilot more into the cockpit, rather than seeing and avoiding!

Lost Av8r said...

Was is a G1000 doing in a 172 anyway?

The little voice in your head that says "something is not right here" is great. Mine has saved me a few times...

Oshawapilot said...

I'm thinking along the lines of Greybeard - shouldn't one be concentrating on looking out the window when flying a little aircraft looking for traffic, instead of staring at (and perhaps overly-relying on) a traffic display?

A traffic display isn't going to pickup that flock of birds you may be headed towards, or a little NORDO ultralight.

Scan, scan, scan was what I was always taught. Anything inside the cockpit is secondary when VFR.

Just MHO...

John said...


I agree that a visual scan is critical, however traffic avoidance systems are quite useful. Fly with one and you'll realize how much traffic is out there - the vast majority of which you never see visually.

Unless you are in class B airspace, ATC is not going to guarantee traffic separation. So anything that helps supplement the imperfect visual scan is potentially a good thing.

The key with this new technology is prioritizing. I've said it before and I'll say it again, don't be seduced by pretty colors on your instrument panel. But if a traffic alert goes off and you can visually acquire the target, this is a good thing I think.

Max said...

Hi John,

great blog. One possible reason for the airplane WAFDOF east of Mt. Diablo is that it is a common practice area -- I've been there many times, for my private as well as for aerobatics. You can't always follow the hemispheric rule when you do maneuvers . For what it's worth, most aerobatic pilots are on 122.75 when they're in the area.

If you fly in the area, don't just look around you -- look above and under too. You might see a Pitts coming straight down in an inverted spin.

And for what it's worth, I do think TIS is a plus. It certainly does not replace a good scan, but many times it has pointed out traffic that I would not have seen until they were dangerously closer. Both man and machine are fallible, but a combination of the two is better than either alone.

Keep up the good work,

-- Max Tardiveau

John said...


Thanks for the observation and I'm glad you enjoy the blog.

Mt. Diablo is indeed a popular practice area and I go there often with students. While maneuvering in the practice area, I ask my students to fly an altitude that is neither VFR nor IFR - 3300', 3700', 4300'. The aircraft I mentioned was clearly straight and level - probably on his way to Hayward, Reid-Hillview, or one of the many other Bay Area airports.

I frequently encounter WAFDOF aircraft while cruising in the Sacramento Valley, so I lean toward clueless piloting as the most likely explanation.