Airport obstacle surfaces

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

1. Introduction

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. The most commonly used criteria to determine the complicated airport obstacle surface are FAR Part 77 imaginary surfaces [FAA(1993)], TERPS [FAA(2002a)], and the one engine inoperative obstacle identification surface for air carriers [FAA(2004a,b, 2005, 2006a)].

Terminal instrument procedures ensure the safety of aircraft during approaches and departures. The obstacle clearance surfaces used to develop these procedures are in Order 8260.3B [FAA(2002)], U.S. Standard for Terminal Instrument Procedures (TERPS) in FAA(2002), and associated orders. If an obstacle is allowed to penetrate these surfaces, it will affect approach and departure minimums, which the FAA considers to be a hazard to air navigation. As such, TERPS is one of the FAR Part 77 standards for determining obstructions to navigable airspace. Another FAR Part 77 standard for determining obstructions is the more commonly known imaginary surfaces.

The other pertinent design criterion is the one engine inoperative (OEI) obstacle identification surface (OIS) as described in FAA Order 8400.10 [FAA(2004a)], FAR PART 25 [FAA(2004b)], FAR Part 121 air carrier [FAA(2005a)] and FAR Part 135 air taxi operations [FAA2006]. The purpose of the OEI OIS sets the maximum departure weights for air carriers, which have a great impact on airline revenues.

For runway threshold siting, there are certain approach and departure obstacle clearance requirements that must be met. These requirements, or so-called "threshold siting surfaces", include three visual approach surfaces unique to the Airport Design AC 150/5300-13 [FAA(2006b)]] along with supplemental criteria from TERPS and the OEI OIS.

In this paper, the differences and relationships between these surfaces are addressed. The conditions for the use of each criterion is discussed. In addition, the FAA's Obstruction Evaluation / Airport Airspace Analysis (OE/AAA) process is reviewed.

2. Far Part 77.25 Civil Airport Imaginary Surfaces

This regulation defines the requirements for notice to the FAA Administrator of certain proposed construction or alterations, establishes standards for determining obstructions to navigable airspace, and provides for aeronautical studies of obstructions to air navigation to determine their effect on the safe and efficient use of airspace. The FAR Part 77 standards for obstructions, which incorporate TERPS, also include imaginary surfaces surrounding all airports in the United States.

FAR Part 77.25 defines five imaginary surfaces: primary, approach, transitional, horizontal and conical. These vary based on the type of operating procedures established for the airport. The purpose of these imaginary surfaces is to protect the airspace surrounding an airport from any hazards to air navigation. However, keep in mind that the obstruction criteria set forth in Part 77.25 are just a standard to determine if it is necessary to further study the impact of a construction proposal. If an existing or proposed structure exceeds any imaginary surface, an aeronautical study shall be conducted to identify the effects of the object on the use of navigable airspace.

A hazard to air navigation is defined as any obstruction, natural or man-made, that penetrates a Part 77 surface and is shown by aeronautical study to have a "substantial adverse effect" on air navigation, which means not only having an adverse effect but also impacting regular and continuing operations.

Although the FAA can determine which structures are obstructions to air navigation, the FAA is not authorized to regulate tall structures. Rather, the FAA acknowledges that state or local authorities have control over the appropriate use of property beneath an airport's airspace.

3. Terps

TERPS are procedures for instrument approach and departure of aircraft to and from civil and military airports. Instrument arrival and departure procedures in the U.S. are designed in accordance with Order 8260.3B, U.S. Standard for Terminal Instrument Procedures (TERPS). This technical document provides guidance that is used in the development of instrument approaches. Approach minimums are published for different aircraft categories and consist of a minimum altitude and required visibility. These minimums are determined, in part, by applying the appropriate obstacle clearances outlined within the criteria.

TERPS is used for airport obstruction analysis to protect airspace. TERPS criteria are used to develop instrument procedures and conduct obstacle evaluations for instrument operations. TERPS is used to establish restrictions on the height of buildings, antennas, trees, and other objects as necessary to protect the airspace needed for airborne aircraft during preparation for and completion of the landing or departure phases of flight. For aircraft ground operation, TERPS also requires the appropriate runway/taxiway separation, which is based on aircraft design group and ILS category, as specified in FAA Notice 8260.56 [FAA(2006c)]].