Before considering a FOD detection system, thought has to be given to where the system can be deployed on the airfield (if it can be deployed at all). For FOD Detect and FOD Finder this is not an issue, FOD Finder is a vehicle mounted system, and FOD Detect is installed in the place of existing runway lights. For Tarsier and iFerret, finding suitable locations can be difficult, not only due to the strict rules imposed by AC150/5300-13, but also by the availability of power and data-links.

Although the Tarsier and iFerret systems both employ different detection technologies (Radar and camera respectively), they are still both bound by the same requirement, they both need clear line-of-sight to the runway, and must achieve this without breaking the rules imposed in AC150/5300-13, specifically, the Primary Surface, the Transitional Surface and the Taxiway Object Free Area Width.

The Primary Surface: The primary surface is an imaginary surface longitudinally centered on the runway.  The elevation of any point on the primary surface is the same as the elevation along the nearest associated point on the runway centerline.  The primary surface width is 1000 feet and centered on the runway center.

The Transitional Surface: The transitional surface begins at the outside edge of the primary surface at the same elevation as the runway.  The surface rises at a slope of 7:1, up to a height 150 feet above the highest runway elevation.

The Taxiway Object Free Area Width: The area on the ground centered on the taxiway provided to enhance the safety of aircraft by having to be free of objects. For group VI  aircraft this width is 386 feet.

What these rules mean in reality

The image to the left demonstrates what these excluded areas mean on a real airfield, in this case just two runways and the surrounding taxiways at Hartsfield–Jackson International (ATL). It should be noted that the shaded area represents only the Primary Surfaces and Taxiway Object Free Areas, it excludes the Transitional Surface.

In reality these rules result in much of the airfield being out of bounds to non-frangible towers. In order to demonstrate the difficulties I’ve performed a Site Design for deployment of both the Tarsier and iFerret systems at ATL.

Runway 9R/27L

The building rules described above exclude any tower construction to the North of this runway, so both iFerret and Tarsier would have to install to the South. A runway of this length would typically require two Tarsier units, placed 1/4 way from each runway threshold. Tarsier units are typically placed 740 feet from the runway center line. Using this information it is possible to calculate the Tarsier unit locations required in order to provide full coverage. I’ve also shown in the image the approximate coverage (shaded areas). In this example I’ve shown the Tarsier system to have a coverage of around 2600 feet, however it should be noted that detection performance would actually degrade with range.

The iFerret towers at Changi International are located at 820 feet from the runway center (a little farther away than the Tarsier towers), and are separated by 1115 feet. Using this data it is possible to perform a site design  for iFerret for this runway. This is shown in the image to the left, along with the approximate coverage. The iFerret units are assumed to operate out to a range of  990 feet (as described previously). As can be seen, 9 iFerret units are required in total, but this does not provide full coverage since there is a small gap in coverage between iFerret units 6 and 7 that is caused by the taxiways leading to the South cargo ramp.

As with Tarsier, detection performance also degrades with range for iFerret. Coverage does not end at 990 feet, so the system would still cover this area but at a reduced detection capability.

These simple examples demonstrate  typical installation considerations for both the Tarsier and iFerret systems. In these examples both systems provide almost full runway coverage. However these examples used a relatively simple runway configuration, with a relatively uncluttered taxiway system to the South. In the following example I’ll demonstrate one of the main problems facing the installation of the Tarsier and iFerret systems on runways with complex layouts.

High density of taxiways

The building restrictions imposed by having two parallel taxiways on one side of a runway can mean that the nearest a tower could be built is over 1000 feet from the runway center. This range is much farther than the preferred install ranges of both the Tarsier and iFerret systems (740 feet and 820 feet respectively) and is actually greater than the quoted operating range of the iFerret system. Runway 08R/26L at ATL is a good example of such a runway. Not only does it have double taxiways to the South, but installation to the North is impossible due to a taxiway and runway 08L/26R.

To the South East is a double parallel taxiway, the closest a tower could be constructed to the SE is 1010 feet. Due to its long operation range (approx 2600 feet) , Tarsier could be located at this range, but iFerret (with its quoted range of 990 feet) would be excluded. Even at this range of 1010 feet this would place any tower very close to parked aircraft and terminal buildings, and I have my doubts as to whether Tarsier or iFerret could provide suitable locations to cover this runway.

Unfavorable ground height

The reason that both Tarsier and iFerret prefer to operate at a range of approximately 800 feet from the runway centerline is that this is as close as they can place their sensors to give them line-of-sight to the far side of the runway without breaking the Transitional Surface. In order to do this they need to place their sensors on towers that are around 30 feet in height. This height is not measured from the ground, but from above the elevation of the nearest associated point on the runway centerline. In other words, if the ground height at the sensor location is 10 feet below that of the nearest runway centerline, then the required sensor tower would be 40 feet. As far as I am aware no vendor has as yet used a tower that is greater than 30 feet in height. This may not be impossible, but it has not yet been implemented in practice.

Runway 10/28 at ATL is a good example of a runway which has very unfavorable sloping ground conditions. In the image to the left you will see just one of the possible tower locations for this runway (this location would be applicable for both Tarsier and iFerret). As you can see from the image the ground appears to fall off quite dramatically as you move away from the taxiway. This situation is repeated for many of the possible locations surrounding this runway.

Summary

The aim of this article is to highlight just two of the factors that have to be considered when installing a fixed FOD detection system i.e. density of taxiways and unfavorable ground height. There are many other factors that have to be taken into consideration by the vendor’s Site Design engineer. By understanding the issues associated with Site Design a potential customer can be an informed customer, who can ask relevant questions, and better understand any potential risks associated with selecting a particular FOD detection system (e.g whether that customer would be the first to purchase a higher tower, or a frangible tower)

The full Site Design for ATL is available to download as a Google Earth file (.KMZ). This contains all the runways at ATL, and not just those discussed in this article.

This entry was posted on Wednesday, December 16th, 2009 at 4:47 pm and is filed under Editorial, FOD Finder, FODetect, iFerret, Tarsier. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.