Evasive Action

Easing the potential for mid-air collisions around reliever airports means knowing where traffic is bound to be


By Pat Veillette

I just had a near mid-air collision that was far too close for comfort. It all began during a recent flight into White Plains, N.Y., on a relatively clear day.

New York Approach was more than happy to turn us over to the tower controller. The poor controllers in the tower at White Plains were working airliners, business jets and regional airliners on the long runway while also working general aviation aircraft on the intersecting runway.

It was a complex mixture of traffic and, understandably, the radio frequency was a mess. The tower instructed us to extend our downwind and she would call our base leg. I had the TCAS II set down to 6 nm, its lowest range.

We were a good seven miles from the airport – technically outside of the Class D airspace – when we were instructed to turn base. The screen on the TCAS display quickly had five or six VFR aircraft, and the warning system started screaming, Traffic, traffic.

My visual scan for the traffic was in hyperdrive, but I still couldnt see the other aircraft. Our speed of 180 knots was about as slow as we dared go in that configuration. Even though the air was clear, I didnt see the high winged Cessna until it suddenly banked away from us, maybe only 200 feet away. It all happened so fast.

At about that time, the tower controller warned us of multiple VFR targets in your vicinity, cleared to land.

Was this near mid-air collision that unusual? No, it wasnt. I talked about it a day later with the chairman of our safety committee, and he said the number of TCAS alerts they receive each month really seemed concentrated at satellite airports.

I then searched the FAAs database on near mid-air collisions, and soon discovered that this episode fit a rather common pattern. The voluntarily reported data is really just the tip of the iceberg.

Hub Heaven
The most risky areas tend to be where there is so much traffic controllers get overloaded and uncontrolled airports where pilots let see-and-avoid take a back seat to whatever else it on their minds.

Sometimes the scariest place to fly is among the mix of VFR traffic, business jets, commuters and flight training common just outside Class B airspace in many places. Just about every Class B hub has several of busy satellite airports nearby

In the New York area, that describes Teterboro, Morristown, White Plains, Stewart, Islip and the Hamptons. Around Chicago, Palwaukee and Waukegan fall into that category. Orlando has Orlando Executive, Kissimmee and Sanford. The traffic around Atlantas Peachtree-DeKalb can be intense. Within the LA Basin, you could well list every one of the outlying airports.

That there are more NMACs near busy airports seems to be a given, considering thats where the population and therefore the airplanes tend to be clustered. But the dynamic flow of airplanes within the area also has to be taken into account. The airplanes flying into the primary Class B airport are most likely operating under IFR and are under positive control of the tower.

They have been sequenced into the terminal area by controllers whose job it is to move metal. There is a semi-organized flow of traffic through the feeder fixes to the final approach course, and from the runway up through a departure gate.

By regulation, those aircraft are not allowed to drop below the floor of the Class B airspace. In short, the orderly flow of traffic within the Class B airspace, along with all of the aircraft under positive control, equipped with transponders (and often TCAS) has helped to reduce the number of near mid-air collisions in busy Class B airspace.

The situation just outside of Class B airspace and operating into these satellite airports is just the opposite. Not only are you allowed to operate VFR without being in contact with ATC, in many cases you cant get flight following even if you want it. Transitioning VFR airplanes are forced into corridors to navigate around the Bravo, and most tend to follow particular routes that feature prominent landmarks and the least amount of detour possible. Airplanes that are not transitioning are winging about in fairly random directions, with the only common factor being that they tend to avoid the center of the airspace.

A second factor adding to the risk is the variety of traffic. The satellite airports have flight schools, which create a lot of traffic going out to practice areas for their maneuvers, and then back into the satellite airport for multiple touch-and-go landings.

Since the costs at bigger airports tend to be higher than at satellite airports, most owners find it preferable to operate a general aviation aircraft from the suburbs. Besides, what pilot wants to sit No. 20 for takeoff behind a long stream of airliners?

Its simply more convenient to operate at the satellite airports, which are likely closer to where people live anyway. Thus, there is a pretty fair concentration of general aviation traffic for business and personal use at these airports.

Flying into this mix are the business jets and regional jets also operating into those satellite airports. At airports such as Santa Monica business executives from Century City, Beverly Hills and Hollywood can quickly arrive at the airport and be taxiing for departure less than an hour after leaving their offices.

The recent growth in business jet operations can be traced largely to the ability to operate into these airports, which renders significant convenience for the owners. Admittedly, the decline in service on a major airline, the long delays associated with standing in long check-in lines and then waiting in the security lines are significant deterrents.

The benefits of time savings and convenience can be persuasive. The result has been a relative explosion in the number of business jets in recent years, and they find significant advantages operating into the satellite airports.

The result: a hectic mix of traffic. Most of the business jets are operating under IFR, which means strictly following exact routes, headings and altitudes. The flight crews in the jets are busy doing their descent and approach checklists. Many of these aircraft have glass cockpits, and if the air traffic controllers change the arrival or the approach, the flight crew will be very busy programming the next information into the aircrafts flight management system.

Roughly 40 percent of the NMAC reports indicated that the jet was descending during its initial approach to a satellite airport. Roughly one third of the time, the business aircraft was in contact with an operating air traffic control tower. The remainder of the reports were split between contact with approach controllers or en route controllers.

Business jets are built for speed. Many of them cruise faster than airliners, and can cruise higher. The downside is the tendency to require fairly high approach speeds. Maneuvering at speeds around 200 knots is pretty common in many of these jets.

Another complicating factor is the relative size of an airliner. The head-on profile of most business jets is actually very small, not significantly larger than that of other general aviation aircraft. Fast moving, hard to see traffic along these approach corridors certainly contributes to the risk factors.

The vast number of the near mid-air collisions reported to the FAA through the Aviation Safety Reporting System occurred in clear visibility and during the daytime – precisely when we would expect more general aviation traffic to occur. In roughly 30 percent of the reported incidents, the pilot making the report estimated a separation distance of less than 500 feet.

Steering Clear
In order to minimize your risk of a mid-air collision in these areas, the most important step you can take is to avoid the instrument approach corridors, because most of the arriving jets will be using instrument approach procedures, even in clear weather.

The problem is that most VFR-only pilots have never been exposed to the types of flight procedures used during a typical instrument approach. Instrument-rated pilots know about approach corridors, but may ignore them at their convenience. And if you are a transient pilot, you may have no idea of where these routes lie.

As a generalization, a jet will be vectored onto the final approach path about 15 miles from the runway. As you cruise more than 15 miles away from a reliever airport, you may think that you are sufficiently far away from an airport to be outside of its traffic flow pattern. Youre not.

At 15 miles away from the runway, the crew of the jet is busy with configuring the aircraft, making last-minute checks of the systems and running through checklists. In a radar environment, controllers will most often give the arriving aircraft vectors to a long final approach. Cruising at 2,000 to 4,000 feet while 15 miles from a satellite airport can put you right into the path of these jets.

Technically, general aviation aircraft have as much right to fly in the Class E airspace outside of busy areas as anyone else. Hanging around areas of high traffic concentrations is a risky behavior. Your chances of a near mid-air are statistically higher when you operate in an area with a higher concentration of other traffic.

In addition, security concerns are creating a legal environment as well. With terrorist and aircraft inexorably linked in the minds of the general public, any incident in which a GA airplane even inconveniences someone seems to strike fear into the hearts of people everywhere.

The Fishfinder Solution
Many business jets are equipped with TCAS II, which receives a signal from any transponder-equipped aircraft and offers resolutions to threatening encounters. TCAS has a caution area in which it will issue a warning if it senses that another aircraft will come within a certain range within a 40-45 second window. This has saved my bacon more than a few times.

If the potential traffic conflict moves within a warning area on the TCAS screen, meaning that a collision is possible within 20-25 seconds, it then issues a resolution advisory to the pilot, requiring prompt corrective action. If both aircraft are TCAS II equipped, the two TCAS units actually communicate with each other and coordinate preventive actions, directing one aircraft to climb while it directs the other to descend.

The number of resolution advisories in the NASA ASRS database is almost scary. Most of the software bugs have been worked out of the system, and it has been shown to be very effective.

While the vast majority of general aviation aircraft are not equipped with TCAS units, having your transponder turned on can provide you with some protection from fast moving business jets by alerting the TCAS unit in the other airplanes.

The downmarket devices becoming more common on light airplanes, Traffic Collision Avoidance Devices, do not actively interrogate other transponders, as does TCAS, but instead listen to the replies given by those transponders and then process that signal.

The performance of the passive devices is excellent in some cares and execrable in others. Many give no direction or azimuth information, only a signal that theres traffic out there somewhere. Such passive instruments may, in some pilots, contribute to passive brains, with the pilot relying on the device instead of his or her own situational awareness.

When flying in busy airspace, lights on is a good policy. Many jet operators require their landing lights to be turned on when they descend below 10,000 feet. Some operators deliberately put a set of blinking landing lights on their aircraft, based upon some previous human factors research which found that blinking lights are much more likely to get attention than steady lights.

For small airplanes, the light can make the airplane much more visible to fast-moving on-coming traffic, but count on defensive flying to protect you.

Lights on, transponder on, head on a swivel – these about the only real protections a pilot has when flying in congested airspace. It still leaves you very dependent on see and avoid, despite the inherent limitations on its effectiveness.

Your best protection is to minimize your time hanging around such airspace when possible. The problem is, thats often impractical in the real world.

Also With This Article
Click here to view “TCAS and TCAD.”

-Pat Veillette is a transport pilot and aviation safety researcher.


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