Airport angles and increased risk

SUMMARY: Because this is an unusually long post, you may wish to skip first to the SUMMARY at the end of this post before reading the entire post.

A LOT has been written on the topic of "things you just should never do."  One of these "never do" actions is to build tall structures next to your airport. This is precisely the DOTD plan for extending I-49 through Lafayette.

In an earlier post I talked in general about the problems associated with the Connector plans relative to the Lafayette Airport. In this post. I want to get down to specifics.

Figure 1.  Google Earth image of the northwest end of runway 11-29 in relation to the intersection of Evangeline Thruway, University Ave, and Surrey St.  

Federal safety guidance defines the maximum height that objects should not exceed in the vicinity of airports. This definition is based on a number of imaginary surfaces through which no objects sitting on the ground should penetrate. Keeping aircraft above the imaginary surfaces, and all terrestrial objects below, provides for safe landings and takeoffs.

The lowest of these imaginary surfaces, the primary surface, is a rectangle at the elevation of the runway. The primary surface extends 200 feet beyond each end of the runway and 500 feet on each side of the runway centerline. Beyond each end of the primary surface there is an approach surface. The approach surface begins at the primary surface elevation and rises, for runway 11-29, at a slope of 34:1 (termed a 3% slope). That is, for every 34 feet of center-line distance the surface rises 1 foot.  At their intersection, the width of the approach surface is the same as the primary surface, 1000 feet, and is centered on the runway center-line. The approach surface widens to 4000 feet at 10,000 feet ground distance from the primary surface. That is, the approach surface width is 1000+0.3x, where x is the ground distance along the center-line away from the primary surface. There are other defined surfaces (transitional surface, horizontal surface, conical surface), but only the primary and approach surfaces are relevant to the issue of runway displacement for 11-29.

The 2002 Final EIS asserts in numerous locations that the Lafayette Regional Airport runway 11-29 will need to be displaced 350 feet to the southeast toward Bayou Tortue and Cypress Island Swamp from its present location to meet minimum federal safety requirements for an approach surface slope of 34:1 and a 17 foot margin of safety (FEIS exhibit 4-4). While the FEIS makes the assertion that the 350 foot displacement is required, it does not show the underlying data or rationale needed to support the claim. I have therefore been forced to attempt to recreate these calculations. My calculations, however, do not agree with the conclusion in the FEIS. Lacking documentation of the FEIS methods, I conclude that the 350 foot assertion is likely in error.

A history and general information about the Lafayette Regional airport may be found in the Wikipedia article titled "Lafayette Regional Airport." Additional information on the airport AirNav.com. That web page also includes a link to a useful Airport Diagram. The diagram shows that runway 11-29 is 5401 feet long and 148 feet wide. Elevation at the northeastern end (designated 11) is 37 feet; elevation at the southeastern end (designated 29) is 35 feet.

Figure 1 is an image captured from Google Earth of the northwest end (designation 11) of runway 11-29. It illustrates that the runway does end quite close to the current highway. Measurement shows that the runway currently ends roughly 600 feet from Evangeline Thruway (Hwy 90), and roughly 700 feet from the intersection of the Evangeline Thruway, Surrey St, and University Ave.

Figure 2. This image is extracted from the FEIS Plate 2a2. North in this figure is to the right, and distance along the horizontal extent of the roadway in hundreds of feet is given on the horizontal axis; elevation in feet (NGVD 29 datum) is plotted on the vertical axis. The roadway is charted as the solid black line. The 40 foot elevation is highlighted by a dotted red line. Peak roadway height at the interchange is estimated to be 45 feet. 

Finally, it is necessary to estimate the height of objects above the roadway. This could include signs, streetlights, and aircraft warning lights. The FEIS does mention this, and suggests that special signage and lighting may be necessary. Thus, I will assume that the height of the vehicles on the roadway will be the tallest objects above the roadway. There is no Federal vehicle height requirement for commercial motor vehicles (CMVs). Most eastern states, including Louisiana, set a CMV height limit of 13.5 feet on most highways. Louisiana does allow heights of 14 feet on designated highways, and oversize permits can be routinely issued for heights up to 16 feet 5 inches. Without specific guidance from the Louisiana DOTD, it is unclear what height should be assumed. Here, I will simply assume a maximum height of 15 feet for all vehicles and objects on the roadway.

Assuming the peak height at the interchange structure controls the required runway displacement, the calculation of length for the approach surface is now straightforward.  The interchange height plus object height has an elevation of 60 feet (45+15). Adding the FAA 17 foot margin of safety gives a total elevation of 77 feet. Subtracting the runway height which defines the primary surface elevation then gives a height of 40 feet (77-37). At a slope of 34:1, the length of the approach surface to the primary surface is 1,360 feet (34x40). At this point along the approach surface, the approach surface width is 1408 feet (1000 + 0.3x1,360), or 704 feet on each side of the extended runway center-line (Figure 3). Adding the 200 foot width of the primary surface at the end of the runway gives a total distance form the peak of the interchange of 1,560 feet. The present distance is estimated to be 700 feet, so the total runway displacement required would be 860 feet (Figure 4). This is 510 feet longer than the value asserted in the FEIS. This difference significantly brings into question the economic, environmental, and engineering feasibility of the displacement.

Figure 3. The more northern half of the new approach surface (black outlined trapezoid) begins with a width which is 500 feet on either side of the extended runway centerline, and 200 feet beyond the new runway end (orange line). The distance to the centerline extends to 704 feet at the proposed elevated interchange.

The calculated extension will require very roughly the destruction of 45 acres of the Cypress Island Swamp west of the airport (Figure 4), and more if embankments in the swamp must be longer than existing embankments. This considerably exceeds the 5 acres estimated in the FEIS (p 4-92).

(a)

(b)
Figure 4. The upper figure (a) shows the current airport runway at the southeast end of runway 11-29. The lower figure (b) is the same image with an 860 foot length of runway and associated area is copied onto the current end of the runway. The original image was printed from Google Earth. 

Construction would require significant fill, and consolidation of the underlying wetland soil will further aggravate the existing problems of soil stability at this end of runway 11-29.  The FEIS on page 2-10 states that "a prior runway extension of about 200' constructed in 1967 has subsided up to approximately four feet and has been removed from service." This fill will encounter even greater engineering challenges.

The new extension into the swamp will need to deal with a very significant drop in elevation (Figure 5). As much as 35 feet of fill will be required for the extension. If earthen embankments are used at the sides of the filled area, considerably more than the estimated 35 acre area of wetland may be required in order to accommodate the more extensive embankment areas.

(a)

(b)
Figure 5. In (a), the center line of the runway (red line) is extended at the southwest end of runway 11-29. The green bar indicates 860 feet from the end of the center line, and the thin white lie crossing the center line is 860 feet from the runway end. Figure (b) graphs elevation along the center line from 39 ft to 4 ft.  

Options: What are our options? They include:

1. The No Build alternative should always be considered. If the currently planned I-49 Connector project is abandoned, it could be replaced by upgrades to the current Evangeline Thruway, and bypass to the east along the Teche-Ridge, west using the LRX alignment, or both to form an urban loop.
2. Build the connector project as decided in the FEIS and ROD, and extend runway 11-29 as required to meet minimum FAA guidelines. This will require land acquisition and a Corps of Engineers wetland permit. Likely this alternative will further require wetland mitigation and flood mitigation.
3. Build the connector project as decided in the FEIS and ROD, and request an FAA exemption from airport approach obstacle safety requirements.
4. Abandon use of runway 11-29.
5. Revise the design in of the selected alternative to eliminate roadway elevation in the vicinity of the 11-29 runway approach surface.

SUMMARY: The I-49 Connector FEIS identified unacceptable risk due to failure to meet FAA flight path obstruction guidance, resulting from the proposed interchange construction adjacent to the Lafayette Regional Airport. Without documenting calculations or rationale, the FEIS stated that in order to meet these minimum safety requirements, airport runway 11-29 would need to be displaced 350 feet southeast toward Bayou Tortue and the Cypress Island Swamp.

My calculations, based on FAA guidance, arrive at runway displacement considerably longer than that presented in the FEIS. Here, following FAA guidance, I calculated that the required displacement is 860 feet. This significant difference brings into question the economic, environmental, and engineering feasibility of the displacement. Impact of this displacement on flooding, wildlife, and wetlands should be carefully addressed and documented by DOTD.

The public attitude toward airport safety should always be conservative and circumspect. The Airport's 1975 Master Plan concludes "Conditions at the airport's periphery make expansion of its land area difficult or expensive or both." Even beyond the impacts of runway displacement discussed above, it is simply inappropriate to choose to construct any tall structures on the periphery of our airport which is already severely constrained at its location. Tall structures like the University and Kaliste Saloom interchanges constrain future airport runway alignment adjustments, and impact the ability to meet current requirements and future safety requirements should FAA guidance on safety margins or approach slopes change for any reason.