Airport obstacle surfaces

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

TERPS procedures are constructed from the electronic signals transmitted by ground and space based air navigation electronic equipment. The TERPS surfaces therefore protect the instrument procedures that aircraft pilots use to fly between airports, and land and depart on runways. There are four types of terminal instrument procedures: (1) precision approach (including missed approach), (2) non-precision approach (including missed approach), (3) circling (including missed approach), and (4) departure.

3.1 Precision Approach

An instrument approach procedure (IAP) may have four separate segments. They are the initial, intermediate, final, and missed approach segments. Only a portion of the final approach segment needs to be considered as the final approach area for obstacle clearance purposes, which begins at the final approach fix (FAF) and ends at the runway or missed approach point (MAP), whichever is encountered last.

For a precision approach, this means that vertical guidance is available from either a glide slope or GPS and that the final approach course must be aligned within /- 3[degrees] of runway centerline. To protect the final approach segment, there is obstacle protection for the entire final approach area consisting of 34:1 "W" and 4:1 "X" obstacle clearance surfaces (OCSs) for the primary area and a 7:1 "Y" OCS for the secondary area. These surfaces are shown in Figure 1.

[FIGURE 1 OMITTED]

A missed approach procedure [FAA(1994), FAA(2004c)] can be considered as a continuation of the approach procedure, and shall be established for each IAP. The missed approach shall be initiated at the decision altitude (DA) for precision approaches, or from the minimum descent altitude (MDA) at the missed approach point (MAP) in non-precision approaches. The missed approach OCS starts approximately at the DA/MDA altitudes minus the final segment required obstacle clearance (ROC). Therefore, the final segment ROC is assured at the beginning of the OCS and increases as the missed approach route progresses. The OCS is applied until at least the minimum initial or en route value of ROC is attained, as appropriate. Additionally, there are both straight-ahead and turning missed approach procedures, and each type has their own obstacle clearance areas that shall not be penetrated.

There is a minimum visibility requirement for precision approaches. The obstacle clearance requirement to determine the lowest achievable visibility requires a wider area than the 34:1 "W" surface, although it too is a 34:1 surface. A clear 34:1 surface allows for less than 1/2 mile visibility, but then an approach lighting system is also required. If the 34:1 is penetrated then the lowest achievable visibility minimum is 1/2 mile even with approach lights. Moreover, if the 34:1 is penetrated then a 20:1 surface is applied. If the 20:1 is penetrated then the lowest achievable visibility minimum is 1 mile and the obstacle must be lighted or night approaches will be denied.

Restrictions on the heights of objects for a precision approach also depend on the following criteria: