Airport obstacle surfaces

Journal of Advanced Transportation, Fall, 2009 by Scott Litsheim, Xiaosong Xiao

7. Discussions

(1) Differences among TERPS, FAR Part 77 Civil Airport Imaginary Surfaces, and One Engine Inoperative Surface

Figure 8 shows the runway end area of a precision instrument runway and the critical airspace surfaces from Part 77 and TERPS along with the OEI surface. The most critical surfaces are; the 50:1 inner approach surface from Part 77, the 34:1 "W" approach surface from TERPS, the 62.5:10EI surface, and the 40:1 TERPS departure surface. Figure 8 indicates that which surface is lowest depends on the type of operation (approach or departure).

Another complication is that the different non-precision approach surfaces that vary depending upon navigation facility type and location are not indicated. Moreover, Part 77 also includes 7:1 transitional surfaces and TERPS includes both 4:1 and 7:1 secondary surfaces, but these are also not shown because they are much steeper and therefore not as critical.

TERPS surfaces are constructed from the electronic signals transmitted by ground and space based air navigation electronic equipment. These are the instrument procedures that aircraft pilots use to fly between airports and land on runways. TERPS surfaces are different than FAR Part 77, sub-Part C, imaginary obstacle surfaces that surround an airport. These instrument surfaces can extend 10 nautical miles from a runway; whereas obstacle surfaces normally extend only 10,000 feet.

Since airspace protection and obstacle clearance are vital to airport and aircraft operation, restrictions should be established on the heights of buildings, antennas, trees, and other objects as necessary to protect the airspace needed for operation of the airport and aircraft. These restrictions should be primarily based upon the FAR Part 77 imaginary surfaces. However, additions or adjustments that take into account TERPS surfaces should be also made, as necessary, because TERPS requirements are a Part 77 standard for determining obstructions. The OEI surface is also critical, but because it is used to determine takeoff weight, and takeoff weights are also subject to a multitude of other factors such as temperature, altitude, aircraft type, etc., it is often not practical to plan for this surface.

[FIGURE 8 OMITTED]

There are additional airspace surfaces delineated in accordance with the TERPS for the full protection of the initial, intermediate, and missed approach segments that need to be included where applicable. Provisions prohibiting smoke, glare, bird attractions, and other hazards to flight should also be included. Thus, it is evident that a single criterion cannot be used to determine if a penetration is acceptable or to guarantee that no object has an adverse effect on airspace safety or efficiency. As such, a thorough aeronautical study shall be conducted that considers all criteria related to approach and departure procedures.

Furthermore, penetrations to these surfaces may already exist and therefore a new object may not require a further increase in the published decision heights and visibility minimums for existing procedures. In other words, there is a shadowing effect to consider, caused by existing obstacles, whereby application of a surface may not be applicable.