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76 Guidebook for Managing Small Airports · Conical Surface. The conical surface extends upward and outward from the periphery of the horizontal surface at a slope of 20 feet horizontally for every 1 foot vertically (20:1) for a hori- zontal distance of 4,000 feet. Height limitations for the surface range from 150 feet above the air- port reference elevation at the inner edge to 350 feet at the outer edge. · Approach Surface. The approach surface is longitudinally centered on the extended runway cen- terline and extends outward and upward from the end of the primary surface. The approach slope of a runway is a ratio of 20:1, 34:1, or 50:1 depending on the approach type. With the recent development of new approach procedures such as GPS, RNAV, and Lateral Precision with Vertical Guidance approaches, there is a greater degree of flexibility in the defini- tion of nonprecision and precision instrument approaches. The FAA has not altered the text related to FAR Part 77 to reflect these changes to date. Aeronautical Study. Under FAR Part 77, the FAA is authorized to undertake an aeronauti- cal study to determine whether a structure or vegetation is or could be a hazard to air navigation. A hazard determination may result in a change to one or more instrument operating procedures (e.g., raising the instrument approach/departure minimums to maintain the required obstacle clearance) or other changes, such as displaced thresholds. However, the FAA is not authorized to regulate tall structures nor is there specific authorization in any statute that permits the FAA to limit structure heights or determine which structures should be lighted or marked. In fact, in every aeronautical study determination, the FAA acknowledges that state or local authorities control the appropriate use of property beneath an airport's airspace. For this reason local land use con- trols are needed to support the findings of the FAA. Notification Requirements The proponent of construction activities or alterations must notify the FAA by completing FAA Form 7460 as outlined in the "Other Supporting Documents" subsection of "Design Standards" in this chapter. In addition, many states impose notification or restriction requirements to areas surrounding airports, which are tied to the imaginary surfaces defined in Part 77. TERPS FAA flight procedure specialists design protected airspace corridors in accordance with FAA Order 8260.3B, United States Standards for Terminal Instrument Procedures, and related FAA orders, commonly referred to as "TERPS." Simply put, TERPS employ a concept of "required obstacle clearance" for determining minimum altitudes along various flight segments. The amount of required obstacle clearance is dependent on the type of flight segment (e.g., en route, transition, circling, initial, intermediate, final approach, missed approach) and the accuracy and integrity of the electronic navigational aids being used. TERPS clearance surfaces can be flat (such as an en route airway) or sloping (such as during departure or a missed approach). Required obstacle clearance is reduced during the approach and departure phases. This reduced separation necessitates maximum navigational accuracy for continued operations during inclement weather. In most cases, protecting against penetrations of the civil airport imaginary surfaces defined in FAR Part 77 is sufficient airspace planning. However, FAR Part 77 does not encompass offset approaches (i.e., those not aligned with the centerline), missed approaches, aircraft climb gradi- ents, or instrument departures. Therefore, it is possible for an object to penetrate a TERPS clear- ance surface, causing an increase in the departure minimums (for example, without penetrating an imaginary surface of Part 77). This possibility is why the notification requirements of Part 77 are more stringent than the identification (a.k.a. charting) requirement. Increasingly, airport sponsors, local governments, and state governments are incorporating TERPS surfaces and other surfaces when developing easements, zoning, or land use policies surrounding airports.