How to Prepare an FAA Obstruction Analysis

Two surface families, two purposes

Before evaluating anything, know what each set of surfaces is for — they are not interchangeable.

• Part 77 imaginary surfaces (§77.19). The primary, approach, transitional, horizontal, and conical surfaces. Part 77 is fundamentally a screening and notice regulation: it defines the airspace around an airport whose penetration triggers FAA review of an object’s effect on navigable airspace. It is not, by itself, a design standard.
• AC 13B approach and obstacle clearance surfaces (OCS). The runway design approach surfaces and the obstacle clearance surfaces used to evaluate the runway and its instrument environment. These are design surfaces — a penetration here speaks to the airport’s design and its declared distances / approach capability, not just to airspace notice.

Step 1 — Define the surfaces from the right inputs

The geometry of every surface depends on runway-specific facts, so the analysis is only as good as its inputs. Pull the authoritative runway data — thresholds, runway ends, elevations, and the approach type/visibility for each end — and confirm the runway design code. The surfaces are built from these:

• Part 77 dimensions depend on the runway type (utility vs other, visual vs instrument) and the approach category for each end.
• The Part 77 approach surface anchors its inner edge 200 ft beyond the threshold per 14 CFR 77.19(c)/(d) — a common place to get the geometry wrong.
• AC 13B approach surfaces are selected per the runway’s approach type; the OCS evaluation is keyed to the instrument procedure environment.

Step 2 — Build the obstacle dataset

An obstruction analysis is a comparison between surfaces and objects, so assemble the object set with position and height: surveyed obstacles, terrain, vegetation, structures, and any proposed construction. Each object needs an accurate ground elevation and height so its top elevation can be compared against the surface elevation directly above it.

Step 3 — Evaluate penetrations

For each object, compute the elevation of every surface at the object’s plan position and compare it to the object’s top elevation. Where the object is higher, it penetrates that surface. Record, for each penetration:

• The surface penetrated (which Part 77 surface and/or which AC 13B approach/OCS surface).
• The amount of penetration (how far the object exceeds the surface).
• The object’s identity, position, and elevation for the obstruction data table.

Penetrations feed the ALP obstruction data table and, where an OCS 7 penetration affects a runway end, can drive a declared-distance (e.g., TODA) action. The point is to produce a defensible, itemized list, each entry traceable to the surface definition and the object survey.

Step 4 — Disposition and notice

A penetration is a finding, not a conclusion. For each, identify the disposition: remove/lower the object, light/mark it, adjust declared distances or the approach, or accept it through FAA evaluation. Separately, determine whether FAA Form 7460-1 notice is required — certain construction or alteration near an airport must be filed for an aeronautical study regardless of whether it penetrates a surface, and penetrations of Part 77 surfaces are a key trigger for that review.

What the analysis produces

A complete obstruction analysis hands the ALP and the design team:

1. The defined surfaces — Part 77 and AC 13B — built from confirmed runway data.
2. The obstacle dataset with positions and elevations.
3. An itemized penetration list: surface, amount, and object identity for each.
4. A disposition for each penetration and a determination of whether 7460-1 notice applies.

Built this way, the analysis distinguishes the two questions cleanly — is the airspace clear (Part 77) and is the runway design and approach protected (AC 13B) — and every finding traces back to a surface definition and a surveyed object.

This article is a reference summary for planning use, not a substitute for the governing FAA text. Citations refer to 14 CFR Part 77 (imaginary surfaces §77.19 and notice Subpart B), AC 150/5300-13B (Airport Design — approach and obstacle clearance surfaces), and FAA Form 7460-1 (Notice of Proposed Construction or Alteration). Instrument-procedure obstacle evaluation is governed by FAA Order 8260.3 (TERPS). Always verify surfaces, penetrations, and notice requirements against the current governing documents and your FAA regional office before issuing a design product. See the full airport planning glossary or the AvPlot toolkit.