Archive for August, 2013


Understanding ‘Flight Paths’

Usually before we draw the first wall or column for a new data center, or start demolition in an existing facility, we are asked to aid in site due diligence, which includes evaluating threat assessments for a potential site. These may include nearby hazardous cargo routes, rail ways, nuclear power plants, and the list goes on. Among the most common concerns during this phase is the matter of air travel, which is often brought up as determining ‘flight paths’ or ‘glide slopes’ for airplanes at a nearby airport.

As you can imagine, this is easier asked than answered, and the answers can be quite complicated. I’ll start with the obvious, which I think everyone understands, that airplanes navigate in 3 dimensions and not two. They are physically capable of moving just about anywhere air is and solids or liquids are not. That being said, there are some areas where traffic is more concentrated.

Before I get into where or what those areas are, lets discuss airspace. In the United States, all air above the ground is divided into categories, and in general these categories divide air traffic into different navigation and flight styles (any pilot reading this is probably rolling his eyes right now, but please bear with me, I’m working from scratch here). These categories are assigned a letter, A through G with some more exotic stuff sprinkled throughout. The easiest to understand is Class A, which is a blanket of airspace covering the entire country that starts at 18,000 feet above the ground and extends to the limits of our atmosphere in space. This is reserved for aircraft with instruments and transponders, and is intended to be the highway for air travel.

I should note here that aircraft may travel pretty much anywhere in this space, with the guidance of air traffic control, but there are ‘lanes’ referred to as ‘Victor Lines’ or airways which tend to garner more traffic. These are lines that cross the country and are approximately 8 miles wide, so in this sense, it is possible that your site could be under a ‘flight path,’ and this is probably the closest thing to a defined flight path. It would be good to point out here that every site in the country is under some kind of navigable airspace.

So, if Class A is the ‘highway’ of air travel, where or what are the off ramps? Well, those would be Class B, C and D air spaces. These are ranked in descending order for the volume or quantity of air traffic for a given airport. So for reference, DFW International Airport has Class B airspace around it, Austin has Class C, and Waco has class D. It’s hard to visualize the airspace around an airport, but the best description would be a multi-tiered wedding cake, turned upside down, with the smallest portion extending from the ground at the airport to some altitude above it. The further away you get, the higher the bottom of the tier will be, but usually the top is the same (For example, around DFW, the bottom is at ground level and the top is at 11,000 altitude. If you go out a few miles, the class B airspace would start at 2,000 and go up to 11,000, so forth and so on.)

Class C works much the same way, but usually with only two tiers, and Class D tends to be a cylinder. These air spaces are used to free up the area for traffic that is landing or taking off. It gives planes room to enter a landing pattern around the airport, and keeps the air relatively clear for takeoff. If you are interested in seeing how these spaces are marked off, I recommend checking out some sectional charts that identify these airspaces.

We can start to see that airspace tends to represent a concentration of air traffic at airports. But how do we know how the planes will approach the airport or enter a traffic pattern? The short answer is that we don’t. Where and how planes will fly around airports has a lot to do with conditions at that airport, which can change from day to day. One day, the traffic may land and take off out of the South, the next day, the North. If the airport is exceptionally busy, the pattern may be larger and more expansive.

I should note here that TIA 942 has a recommendation for data centers to be located near, but not in, major metropolitan areas (“Not greater than 16km/10miles” for a Tier 4 Facility) and also provides guidance for proximity to airports (“Not less than .8km/1/2 mile or greater than 30 miles” for a Tier 4 Facility). TIA 942 also says that the facility “should not be in the flight path of any nearby airports.” As we’ve noted, if you are under controlled airspace, or any airspace for that matter, you might be in the ‘flight path’ of a nearby airport, especially if you take the recommendation of the TIA to locate no further than 30 miles from an airport in a big city.

For an area like DFW, the Class B airspace is quite expansive and would certainly cover data centers that were located per TIA. From a due diligence perspective, I think it would be fair to say that you would not want to locate near areas of greater air traffic density, but there is no public site in America that will be free of concerns over air traffic. Furthermore, it would probably be good for organizations like the TIA, and for anyone seeking out a site to develop a more robust understanding of air traffic when trying to assess risk.


August 2013
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