Airport obstacle surfaces

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

Adjustment of Decision Altitude (DA)

Obstacle penetrations to the OCS result in actions to be taken as specified in FAA Order 8260.3B [FAA(2002a)]. One of these actions is the adjustment of the Decision Altitude (DA) and is employed to guarantee the required obstacle clearance, as shown in Figure 2. Ideally the minimum DA is 200 feet above runway threshold. The calculation of the revised minimum Height Above Touchdown (HAT)/maximum ROC is based on the following formula, and once adjusted, is not less than 250 feet.

Min Hat and Max ROC = GPA/3 250, where GPA is glidepath angle.

[FIGURE 2 OMITTED]

Glidepath Qualification Surface (GOS)

Glidepath Qualification Surface (GQS) is the other criterion to restrict the height of object. The GQS extends from the DA point along the runway centerline extended to the area around the runway threshold, depending on the threshold crossing height (TCH). It limits the height of obstructions between DA and RWT to assure a stabilized descent for safe landing. When obstructions exceed the height of the GQS, an approach procedure with positive vertical guidance (ILS, MLS, TLS, GLS, VNAV, etc.) is not authorized.

Precision Obstacle Free Zone (POFZ)

The Precision Obstacle Free Zone (POFZ) [7] is defined as a volume of airspace above an area beginning at the runway threshold, at the threshold elevation, and centered on the extended runway centerline (200 feet long by 800 feet wide). Under prerequisites of a vertically guided approach and an aircraft within two miles, the POFZ is only in effect if the reported ceiling is below 250 feet and/or visibility less than 1/2 statute miles (or RVR below 4000 feet). Otherwise, the POFZ is not in effect. Thus, planning for a POFZ is required for any runway end that has a vertically guided approach procedure with published minimums of either 250 or -3/4, or lower. When the POFZ is in effect, an infringement by an aircraft fuselage is not allowed, but a wing is allowed. The POFZ is applicable at all runway ends including displaced thresholds. The airport sponsor is responsible for maintaining a clear POFZ and if the POFZ is not clear, the minimum HAT/visibility is 250 feet and 1/2 statute miles.

ILS Critical Areas

For ILS flight, the glide slope and localizer have their own critical areas [FAA(2005b)]. These areas must be cleared.

Precision Approach Path Indicator (PAPI) Obstacle Clearance Surface

In addition to instrument navigational aids, a visual navigational aid, such as the PAPI system, plays an important role in the restriction to the height of the obstacle [FAA(1981)]. The PAPI obstacle clearance surface cannot be penetrated. In conclusion, all available criteria, not just a single criterion, must be taken into consideration to determine if a penetration to airspace is acceptable, as even if a penetration is acceptable by one criterion, other criteria may not allow this penetration. Thus, a comprehensive and complicated airspace analysis study is always needed.

3.2 Non-Precision Approach

Where no usable glidepath is available, there are many types of navigation systems available that can provide horizontal course guidance to an aircraft. Non-precision procedures, such as GPS, VOR, NDB, and LOC-only, do not have vertical guidance, other than below the minimum descent altitude (MDA) where a visual glidepath slope indicator system (VGSI), such as a VASI or PAPI, may provide descent rate guidance. As with precision approaches, only that portion of the final approach area that is between the FAF and the runway need be considered for obstacle clearance purposes. The minimum Required Obstacle Clearance (ROC) in the final approach area is 250 feet. In addition, the MDA established for the final approach area shall assure that no obstacles penetrate the 7:1 transitional surfaces, as shown in Figure 3.