Monday, August 17, 2009

The Big Sky

On August 8, 2009, at 11:53 a.m. EDT, a Eurocopter AS 350 BA (N401LH) operated by Liberty Helicopters and a Piper PA-32R- 300 (N71MC) operated by a private pilot, collided in midair over the Hudson River near Hoboken, New Jersey. The certificated commercial pilot and five passengers onboard the helicopter were killed. The certificated private pilot and two passengers onboard the airplane were also killed. Visual meteorological conditions prevailed and no flight plans were filed for either flight. The local sightseeing helicopter flight was conducted under the provisions of 14 Code of Federal Regulations Part 136. The personal airplane flight was conducted under the provisions of 14 Code of Federal Regulations Part 91.

Following this very prominent midair collision, the media have been talking about procedures, policies, and regulations with which most reporters have little experience or expertise. This is nothing new. Every time I read a news story on a topic about which I'm not familiar, I wonder how accurate that story really is. But I digress ...

One news report seemed to imply that the pilot of the Piper (being a private plane) was at fault because it had run into the Eurocopter. The author of another story focused on the shocking fact that aircraft operating in the thin sliver of airspace over the Hudson River do so without talking to air traffic control and without a flight plan. Other reports tried to compare and contrast the water ditching of a US Airways Airbus with this accident. This compels me to comment on what is known about this accident, provide a pilot's perspective on operating in airspace that has little or no ATC intervention, and talk about just how well the see-and-avoid approach to preventing midair collisions really works. I'll attempt to address these issues so that non-pilots can develop a better understanding of just what pilots of smaller aircraft who fly at lower altitudes have to deal with on a regular basis.

Apples and Oranges
First off, the only thing this midair accident has in common with the ditching of US Airways Flight 1549 is that in both cases, the aircraft ended up in the Hudson River. The US Airways accident involved a bird strike (which I guess is kind of like a midair collision) that resulted in a loss of power to both engines: The Airbus was still be flyable, it just didn't have any thrust to keep it from losing altitude. The fact that the plane was still flyable, combined with the skilled flight crew and a lot of luck, resulted in an amazingly successful water ditching. In contrast, the midair collision between the Piper and the Eurocopter caused catastrophic damage, both aircraft departed controlled flight, and the impact with the water was not survivable. The only thing these two accidents had in common was their location.

ATC's Role
The Piper departed Teterboro Airport and was, in fact, communicating with the tower controller until he was over the Hudson, when he was handed off to the Newark control tower. Talking to an airport's tower controller is mandatory when an aircraft is within that airport's airspace. In these situations, ATC will point out other potentially conflicting air traffic, but this is done on a workload permitting basis. Here's just a bit of what the Aeronautical Information Manual has to say on the subject:

4-1-15. Radar Traffic Information Service

This is a service provided by radar ATC facilities. Pilots receiving this service are advised of any radar target observed on the radar display which may be in such proximity to the position of their aircraft or its intended route of flight that it warrants their attention. This service is not intended to relieve the pilot of the responsibility for continual vigilance to see and avoid other aircraft ...
Many factors, such as limitations of the radar, volume of traffic, controller workload and communications frequency congestion, could prevent the controller from providing this service. Controllers possess complete discretion for determining whether they are able to provide or continue to provide this service in a specific case. The controller's reason against providing or continuing to provide the service in a particular case is not subject to question nor need it be communicated to the pilot. In other words, the provision of this service is entirely dependent upon whether controllers believe they are in a position to provide it.

Some questions have arisen about the Teterboro Tower's handling of the flight. Reportedly the controller was making a "non-business" phone call to the Newark tower which may have contributed to coordination problems with the handoff of the Piper from Tereboro to Newark. A conflict alert indication was shown on the radar displays at both towers as the Piper and the Eurocopter began to converge. Though these alerts usually produce both a visual and audio warning, neither controller recalled seeing or hearing the alert.

Several initial news reports made a big deal of the fact that the Piper's pilot never contacted the Newark tower after being handed off by the Teterboro tower. It's hard to know why that was, but it's also important to point out that a delay checking in after a handoff is quite common. Radio communication in aircraft is somewhat primitive - only one person can talk at a time. Perhaps the Piper's pilot was busy tuning his radio to the new frequency so he could check in, but we don't really know. It does appear that the frequency change came at a very inopportune time and the collision occurred shortly afterward. Remember all those studies that show distractions (like cell phone use while driving) reduce reaction time and situational awareness? The same thing can happen in aircraft and, apparently, in control towers.

Different Frequencies
Aircraft operating over the Hudson usually communicate using a CTAF - common traffic advisory frequency - which is like a party line where only one person can talk at a time. The CTAF is different from the frequencies used by Teterboro and Newark towers. The idea with the CTAF is that each aircraft announces their position, altitude, and intentions so that other pilots can put together a mental picture of where other traffic might be and avoid them. If this sounds primitive, it is! Yet in areas where there is no ATC service (usually at rural airports) and when there's not too much traffic, the CTAF set-up is pretty workable. The thing is that CTAF areas are usually not swarming with the volume of traffic that is seen on a daily basis over the Hudson River corridor. The important point here is that the Eurocopter was probably monitoring and transmitting on the CTAF while the Piper was monitoring and transmitting on the Teterboro Tower frequency.

Big Sky, Little Planes
This brings up the big sky theory of preventing midair collisions: The sky is big when compared to the size of aircraft, so the probability of a collision is reduced by the simple fact that the sky is so much bigger than the aircraft. This is a good theory if you assume that aircraft are randomly or evenly distributed throughout the big sky. Unfortunately, aircraft tend to congregate around certain locations (like around airports, helipads, and land-based navigation transmitters) like bees around a hive and that dramatically increases the probability of a collision.

The situation over the Hudson River adds another wrinkle since the area of airspace used by the sightseeing helicopters and other light aircraft is underneath and physically constrained by an overlying area of controlled airspace called Class Bravo. Entering Class Bravo requires a clearance from ATC precisely because this airspace was created primarily to keep small, slower aircraft away from larger, faster aircraft. When aircraft are cleared to enter Class B, ATC will guarantee separation between aircraft: This separation is not done on a workload permitting basis, it is guaranteed. This dramatically enhances the safety of aircraft operating in Class B, but ironically creates a thin layer of airspace for the smaller aircraft to share, which makes the Big Sky quite a bit smaller, and increases the probability that these smaller aircraft who are not in Class B will come close to one another.

Invisible Hands
So how about separating aircraft with a controller using radar? Air traffic control (ATC) can and does provide many valuable services to pilots by providing traffic advisories when aircraft get close or appear to be converging, but they are not an invisible hand that holds the aircraft and keeps them completely safe. Just because the pilot or flight crew of an aircraft is talking to a controller does not mean they are immune to mechanical problems, bird strikes, or midair collisions. The idea that ATC keeps aircraft safe, while not entirely a fantasy, is a belief that non-pilots may find comforting. Non-pilots need to remember that it's the pilot that is flying the aircraft and there is no invisible shield provided to aircraft that just happen to be talking to ATC.

Another misconception held by non-pilots has to do with flight plans. The idea that an aircraft is operating with an open flight plan is somehow safer than one operating without a flight plan may or may not be true. There basically are two types of flight plans: Instrument Flight Rules and Visual Flight Rules. Non-pilots need to know that the primary purpose of VFR flight plans is so that the appropriate authorities will be notified if you don't call in and close your flight plan when you arrive. In short, filing VFR flight plan helps ensure that if you crash and no one sees the crash, someone will eventually come looking for you.

Rules, Rules, Rules
Some reporters have claimed that aircraft which are not under ATC control are completely unregulated and not following any rules, but nothing could be further from the truth.

The aircraft that fly in any airspace must meet FAA airworthiness requirements including regular maintenance inspections with specific criteria.

The pilots that fly these aircraft must be certificated (we don't call them licenses in the US, but the media can't get that right either), they must hold a medical certificate, and they must meet recency experience to be able to act as pilot-in-command and to carry passengers.

The airspace in which these aircraft are operated have specific flight visibility and cloud clearance requirements. And there are specific right-of-way rules that pilots follow when they see they are getting too close to one another.

To equate these areas of airspace not under air traffic control to the Wild West is uninformed and stupid.

Technology to the Rescue, sort of

Without radar, isn't there someway that technology can keep two aircraft from trying to occupy the same airspace? It's not as if no one has tried to create technology to do this, but the success has been mixed.

Large aircraft are required to have traffic collision and avoidance systems (TCAS), but even with TCAS these planes can run into one another.

Several similar systems are available for smaller aircraft, but they can be expensive and not every aircraft has them. One system is the FAA's Traffic Information System (TIS) where appropriately configured ground radar facilities upload traffic information to appropriately equipped aircraft. This is a common system in many newer general aviation aircraft, but many ATC radar facilities do not support TIS. Oh, and the FAA is planning to phase out TIS. Yes, you read that correctly. The reason is that another system is supposed to replace TIS, even though virtually no small aircraft out there are currently equipped to support the new system. Call me a curmudgeon, but that sounds about right for the FAA ...

Another system for smaller aircraft (that is also expensive) is an Traffic Advisory System (TAS) that actively interrogates other aircrafts' transponders, just like ATC's radar. These systems can be quite helpful, but with some aircraft (like the Cirrus) there is no way to mute the aural warnings and keep them from barking "Traffic! Traffic!" when you're trying to talk to or listen to ATC.

The last system for small aircraft that I'll mention is a class of portable devices that warn of nearby aircraft and are sometimes referred to as Portable Collision Avoidance Systems (PCAS). These devices are not perfect, but they help pilots have an idea when other aircraft are nearby, even if they don't tell you exactly where those aircraft are. As a side note, I always fly with a PCAS unit.

What's a Pilot to Do?

First, scroll back to the top and watch the YouTube video of the Hudson midair. I know it's scary, heartbreaking, and painful, but watch it nevertheless.

Hopefully that video has you in a mood to listen.

Remember that accident statistics indicate that midair collisions tend to occur on clear, sunny days and usually in the vicinity of airports of navigational transmitting stations.

Keep your head on a swivel when operating in crowded airspace.

Fly at an appropriate VFR altitude for your direction of flight. I see at least one pilot violating this simple safety rule every time I fly.

Avoid distractions, like unnecessary conversations or fiddling with your GPS or MP3 player.

Listen up! Poor radio phraseology and technique not only wastes everyone's time, it can actually threaten your life, the lives of your passengers, the lives of other pilots, and the lives of people on the ground.

If you have a traffic detection device, use it.

If you think this sort of collision can't happen to you, watch the video a few more times.


Marquinius said...

I can only say: thank you for pointing it out once more. Hard lesson, scary moment. And yes, it's a funnel we're operating in. Better keep my eyes peeled.

Marco (PPL, part owner of a small SEP)

Steve said...

Great post for both pilots and non-pilots alike. You did a great job explaining all the things us pilots learn in layman's terms, John.

Indeed the big sky theory can be a bit skewed... although I haven't yet been able to buy myself a PCAS this is the reason I use Flight Following whenever possible.

africanbushpilot said...

The thought of a midair collision is a despairing thought at anytime.
But to watch a video of a collision is cold reality and the sader thing is he was a fellow New Zealander(the helecoptor pilot)

Ron said...

I enjoyed your article -- it's good advice!

However, I would take issue with one thing: traffic separation is definitely not guaranteed in class B airspace.

It's true that "traffic separation" is one of the services they provide, but if two airplanes collide in B airspace, the NTSB report will still list a probable cause as "the pilots failure to see and avoid one another".

Air traffic control's whole reason for existing is to keep IFR airplanes from colliding with other IFR airplanes while in IMC. That's it.

The only time a midair would be ATC's fault would be if two IFR aircraft collided in IMC. If VFR targets collide, it's their own fault, regardless of airspace. Even if IFR targets merge, it would be their own fault if they were in visual conditions at the time. The gist of it is that any time VMC is prevalent, pilots hold primary responsibility for avoiding one another.

It sucks, but that's how it is.

John Ewing said...


Here's what the AIM says (emphasis mine):

An ATC clearance is required for all aircraft to operate in the area, and all aircraft that are so cleared receive separation services within the airspace.

Of course there are no guarantees in life, but if a controller loses the required separation between two aircraft in class B there's going to be paperwork to fill out even if there is no collision. Aircraft are supposed to always exercise see-and-avoid, but the separation service in class B is different from class C, D, or E.

That's why when you get a class B clearance and then depart the boundaries of class b the controller should tell you "Barnburner 123, you have exited class bravo." Read between the lines and what they are telling you is "I'm no longer required to guarantee separation."

Ron said...

I just thought it was interesting that you used the word guaranteed. Perhaps I take a different meaning from the word than other people. "Don't worry about traffic here -- separation will never be an issue because it's guaranteed."

I spend much of my day flying back and forth on localizer style lines inside class B airspace, and often find myself very close to other aircraft -- especially helicopters -- which are not called out as traffic by ATC. It might be that I'm at 1000 feet and the helicopter is at 800 feet and outside the Bravo, but that's no consolation when you see how close you are.

At low altitudes, sometimes their radar won't always see targets. Also, transponders (and especially altitude encoders) can be inaccurate. At the end of the day, primary responsibility for separation must reside with the pilot, not the controller.

The one time I'd take exception would be in actual IMC.

From a practical standpoint, LAX tower has a special helicopter freq that we are in contact with, but at times that position is closed or merged with another LA frequency and has become so congested that they couldn't give us all the traffic calls even if they wanted to. It can be similar to the Hudson situation, now that I think of it.

Anyway, just my thought and experience on the matter. :)

Brian said...


I just found your blog by following a link from Steve's site. That was an excellent post. I look forward to reading more of your posts in the future and just placed a link to your site on my blog,