Part 77 vs TERPS: Which Surfaces Apply to Your Airport
Two of the most-cited surface frameworks in airport planning — 14 CFR Part 77 and TERPS — are routinely confused, and the confusion produces bad conclusions: that a penetrating object is automatically illegal, or that Part 77 sets approach minimums. Neither is true. The two frameworks have different shapes, different origins, and different jobs. This is a planner’s guide to which surfaces apply, when, and why they are not interchangeable.
What Part 77 actually is
Part 77 — 14 CFR Part 77, “Safe, Efficient Use, and Preservation of the Navigable Airspace” — is a regulation, not a design manual. It does three things. It sets the standards for determining obstructions to air navigation (§77.17). It establishes the civil-airport imaginary surfaces (§77.19). And it defines the notice requirements for proposed construction or alteration near an airport, filed on FAA Form 7460-1 (§77.7 / §77.9).
The critical sentence is in §77.17(b): objects that are obstructions under these standards are presumed hazards to air navigation unless further aeronautical study concludes that the object is not a hazard. In other words, Part 77 is a tripwire. Crossing a surface starts a process — it does not, by itself, decide the outcome.
The five imaginary surfaces of §77.19
The civil-airport imaginary surfaces are anchored to the runway and sized by the most precise approach existing or planned for each runway end. There are five:
- Primary surface — centered on the runway, extending 200 feet beyond each end for runways with a specially prepared hard surface. Its width is 250 ft (utility, visual approaches), 500 ft (utility non-precision; visual non-utility; non-precision with visibility >¾ SM), or 1,000 ft (non-precision as low as ¾ SM, and precision instrument runways).
- Horizontal surface — a plane 150 feet above the established airport elevation, formed by swinging arcs of 5,000 ft (utility / visual) or 10,000 ft (all other runways) from each primary-surface end and joining them tangentially.
- Conical surface — extends outward and upward from the edge of the horizontal surface at a slope of 20 to 1 for a horizontal distance of 4,000 feet.
- Approach surface — centered on the extended runway centerline, beginning at the primary-surface end. It expands from the primary width to 1,250 / 1,500 / 2,000 / 3,500 / 4,000 / 16,000 ft depending on approach type, at slopes of 20:1 (5,000 ft, visual/utility), 34:1 (10,000 ft, non-precision non-utility), or 50:1 for 10,000 ft plus an additional 40:1 for 40,000 ft (precision).
- Transitional surface — rises at a slope of 7 to 1 at right angles from the sides of the primary and approach surfaces up to the horizontal surface.
Because the primary surface itself begins 200 ft beyond the threshold on a hard-surface runway, the approach surface — which starts at that primary-surface end — effectively anchors 200 ft beyond the runway end. This is the same runway-end-plus-200-ft convention that governs the inner edge of the approach footprint.
What TERPS actually is
TERPS — FAA Order 8260.3, “United States Standard for Terminal Instrument Procedures” — is the criteria set for designing instrument procedures: approaches, departures, and missed approaches. Where Part 77 screens objects, TERPS builds procedures. Its core construct is the Obstacle Clearance Surface (OCS): a set of sloped evaluation surfaces tied to a specific procedure’s geometry. When an obstacle penetrates an OCS, the procedure designer must respond — typically by raising the minimums (a higher decision altitude or minimum descent altitude) or by redesigning the procedure.
The OCS shapes are not the §77.19 surfaces. They originate from procedure design segments, follow different slopes and widths, and exist to answer a different question: what instrument minimums can this runway actually achieve given what is in the way?
The key distinction
This is the whole article in two sentences. Part 77 is a screening and notice regulation — it identifies objects that may be obstructions and triggers an aeronautical study. TERPS is a procedure-design standard — its OCS determine the minimums a runway’s instrument procedures can support.
It follows that penetrating a Part 77 surface does not automatically make an object an obstruction to air navigation, and it does not automatically prohibit the object. And it follows that Part 77 surfaces are not TERPS OCS — same airport, different geometry, different purpose.
| 14 CFR Part 77 | FAA Order 8260.3 (TERPS) | |
|---|---|---|
| Type | Regulation (screening / notice) | Order (procedure-design criteria) |
| Surfaces | Imaginary surfaces (§77.19) | Obstacle Clearance Surfaces |
| Purpose | Flag possible obstructions; trigger study | Set achievable instrument minimums |
| On penetration | Presumed hazard → aeronautical study | Raise minimums or redesign procedure |
| Applies | Essentially every airport + nearby construction | When a procedure is designed or amended |
When each one applies
Part 77 applies broadly. Its surfaces exist at essentially every airport, and its notice requirement reaches any proposed construction or alteration near an airport — cranes, towers, buildings, even some vegetation — through the Form 7460-1 filing that triggers an FAA aeronautical study. If something is being built in the vicinity, Part 77 is the framework that decides whether the FAA looks at it.
TERPS applies when a procedure is in play. It is engaged when an instrument approach, departure, or missed-approach procedure is being designed or amended for the runway. If no procedure is being created or changed, TERPS is not doing anything — but the moment a runway pursues lower minimums, its OCS govern what is achievable.
How they interact
The two frameworks overlap on the same airfield without being the same thing, and that produces the cases planners actually have to reason about:
- An object can clear Part 77 yet penetrate a TERPS OCS — it raises no obstruction flag, but it forces higher minimums on a procedure.
- An object can penetrate Part 77 yet be found not to be a hazard after aeronautical study — the tripwire fired, the study cleared it.
Because of that, sound obstruction analysis for planning evaluates both. You check the §77.19 surfaces to understand notice and obstruction exposure, and you check the relevant OCS to understand minimums impact. AvPlot’s obstruction analysis does exactly this — it evaluates Part 77 imaginary surfaces and Obstacle Clearance Surfaces together in a single pass (POST /analyze-obstructions), so a penetration is reported against the right framework rather than collapsed into one ambiguous “obstruction” verdict.
Three misconceptions to retire
- “It penetrates Part 77, so it’s illegal.” No. A penetration makes the object a presumed hazard and triggers an aeronautical study; the study, not the surface, determines whether it is actually a hazard (§77.17(b)).
- “Part 77 and TERPS are the same surfaces.” No. The §77.19 imaginary surfaces and the TERPS OCS have different shapes, origins, and purposes. One screens; the other designs.
- “Part 77 sets approach minimums.” No. Minimums come from TERPS procedure design. Part 77 never publishes a decision altitude.
This article is a reference summary for planning use, not a substitute for the governing FAA text. Surface dimensions and obstruction standards are cited to 14 CFR Part 77 (current as of 6/09/2026); instrument-procedure criteria are cited to FAA Order 8260.3, U.S. Standard for Terminal Instrument Procedures (TERPS). Always verify values against the current source documents before issuing a design product. See the full airport planning glossary or the AvPlot toolkit.