Curv Aero Part 77 Imaginary Surfaces Explained
Every public-use runway is surrounded by five invisible, three-dimensional surfaces that the FAA uses to decide what counts as an obstruction to air navigation: the primary, approach, transitional, horizontal and conical surfaces. This guide explains each one, gives every dimension from 14 CFR 77.19 in a single table, and works through the math for real sites.
Updated November 7, 2026 · Based on 14 CFR 77.15, 77.17 and 77.19
First, don't confuse these with the notice criteria
Part 77 has two different sets of surfaces. The notice surfaces in §77.9 (the 100:1, 50:1 and 25:1 slopes) decide whether you must file Form 7460-1. The imaginary surfaces in §77.19, covered here, are obstruction standards the FAA applies once it studies your filing. If you're asking "do I need to file?", start with the 77.9 notice criteria instead.
Sizing
How the surfaces are sized
The size of each surface depends on the runway's category, which comes from the most precise approach existing or planned for it:
Utility runway
Built for propeller aircraft of 12,500 lb maximum gross weight or less.
Visual runway
Only visual approaches, with no straight-in instrument procedure.
Non-precision instrument
An instrument approach with horizontal guidance only (for example RNAV LNAV or VOR), split by whether visibility minimums go as low as ¾ statute mile.
Precision instrument
An approach with vertical guidance such as ILS or PAR, existing or planned.
Two consequences matter for anyone siting a structure. Planned approaches count, so a runway's surfaces can grow before a new procedure is published. And each runway end can carry a different approach surface, while the primary surface takes the width for the most precise approach at either end.
The surfaces
Five surfaces, one runway
Primary surface
A flat surface centered on the runway. On a paved runway it extends 200 ft beyond each end; on an unpaved runway it ends at the runway ends. Its elevation at any point equals the elevation of the nearest point on the runway centerline, so on a sloped runway it slopes too.
Width is 250 ft for utility runways with only visual approaches, 500 ft for utility non-precision runways, other visual runways and non-precision runways with minimums above ¾ mile, and 1,000 ft for non-precision runways with minimums as low as ¾ mile and for precision runways. In practice, almost anything standing inside the primary surface is an obstruction.
Approach surface
A trapezoid that starts at the end of the primary surface, at the primary surface's width, and rises along the extended runway centerline while widening. Its slope and length depend on the approach:
- 20:1 for 5,000 ft: utility and visual runways.
- 34:1 for 10,000 ft: non-precision instrument runways other than utility.
- 50:1 for 10,000 ft, then 40:1 for another 40,000 ft: precision instrument runways, reaching nearly 9.5 miles from the primary surface.
The approach surface is usually the controlling surface for sites off the runway ends, which is where buildings, cranes and highway signs most often run into trouble.
Transitional surfaces
Sloped surfaces rising 7:1 (1 ft up for every 7 ft out), measured at right angles to the runway centerline, from the sides of the primary and approach surfaces. They stop where they meet the horizontal or conical surface, except beside the outer part of a precision approach surface, where they extend 5,000 ft beyond the approach surface's edge.
Transitional surfaces are steep and close in, which makes them the surface most likely to catch hangars, FBO buildings, fuel farms, light poles and parked equipment along the side of a runway.
Horizontal surface
A flat plane 150 ft above the established airport elevation. Its outline is built by drawing arcs from the center of each end of every primary surface and joining them with tangent lines, giving a stadium shape. The arc radius is 5,000 ft for utility and visual runways and 10,000 ft for all other runways.
Conical surface
A surface rising 20:1 outward from the edge of the horizontal surface for 4,000 ft. It therefore tops out 200 ft above the horizontal surface, or 350 ft above airport elevation.
Fixed for every runway type
- Transitional
- 7:1
- Horizontal
- 150 ft above airport elevation
- Conical
- 20:1 for 4,000 ft
All dimensions in one table
| Runway type | Primary width | Approach slope / length | Approach outer width | Horizontal radius |
|---|---|---|---|---|
| Utility, visual only | 250 ft | 20:1 / 5,000 ft | 1,250 ft | 5,000 ft |
| Other than utility, visual only | 500 ft | 20:1 / 5,000 ft | 1,500 ft | 5,000 ft |
| Utility, non-precision | 500 ft | 20:1 / 5,000 ft | 2,000 ft | 5,000 ft |
| Non-precision, minimums above ¾ mi | 500 ft | 34:1 / 10,000 ft | 3,500 ft | 10,000 ft |
| Non-precision, minimums as low as ¾ mi | 1,000 ft | 34:1 / 10,000 ft | 4,000 ft | 10,000 ft |
| Precision instrument | 1,000 ft | 50:1 / 10,000 ft + 40:1 / 40,000 ft | 16,000 ft | 10,000 ft |
All runway types: transitional surface 7:1; horizontal surface 150 ft above airport elevation; conical surface 20:1 for 4,000 ft.
Worked examples
A hangar beside a precision runway
A new hangar's roof peak will sit 600 ft from the centerline of an ILS runway, opposite a point on the centerline at 5,200 ft elevation. The site is level with the runway.
- Precision runway, so the primary surface is 1,000 ft wide: its edge is 500 ft from centerline.
- The hangar is 600 − 500 = 100 ft outside that edge.
- Transitional surface at the hangar: 100 ÷ 7 ≈ 14.3 ft above 5,200 ft.
- A 25 ft hangar penetrates by about 10.7 ft. It's an obstruction. Moving it 175 ft farther out (to 775 ft from centerline) raises the surface to about 39 ft and clears it.
A building off the end of a non-precision runway
A 90 ft office building is planned on the extended centerline, 3,000 ft beyond the end of the primary surface of an RNAV runway (non-precision, not utility). Ground and runway end are both at 1,100 ft.
- The approach slope is 34:1, so the surface is 3,000 ÷ 34 ≈ 88.2 ft above the runway end.
- At 90 ft, the building penetrates by about 1.8 ft.
- Losing two feet of parapet keeps it clear of the approach surface. It still needs a 7460-1, though. Assuming the airport's longest runway is over 3,200 ft, the 77.9 notice surface 3,200 ft from the runway end is only 32 ft high.
A tower 2 miles out
A 180 ft monopole sits 12,000 ft from the nearest primary surface end of an airport whose runways all have 10,000 ft horizontal radii. Airport elevation is 900 ft and site ground is 880 ft.
- The site is 2,000 ft beyond the horizontal surface, inside the 4,000 ft conical surface.
- Conical surface height: 150 + (2,000 ÷ 20) = 250 ft above airport elevation, or 1,150 ft MSL.
- Tower top: 880 + 180 = 1,060 ft MSL, 90 ft below the surface. It doesn't penetrate the conical surface. It still needs notice under 77.9, though: the site is about 12,200 ft from the runway end, where the 100:1 notice surface is only about 122 ft above a 900 ft runway, or 1,022 ft MSL.
What happens if a structure penetrates a surface?
Under §77.17(a)(5), anything taller than an imaginary surface is an obstruction, and §77.15(b) says obstructions are presumed to be hazards until further aeronautical study concludes otherwise. That presumption is the start of the analysis, not the end. The FAA then looks at actual effects: whether the object would raise instrument approach minimums, affect departure procedures or VFR traffic patterns, or interfere with radar and navigation aids.
Many obstructions end in a Determination of No Hazard, usually with marking and lighting requirements. Others get a Notice of Presumed Hazard (now Notice of Preliminary Findings), and sponsors resolve it by lowering or shifting the structure.
The other obstruction standards
The imaginary surfaces are one of five standards in §77.17. An object is also an obstruction if it is taller than:
- 499 ft AGL anywhere;
- 200 ft AGL (or above airport elevation, whichever is higher) within 3 nautical miles of the reference point of an airport whose longest runway is over 3,200 ft, rising 100 ft per additional nautical mile up to 499 ft;
- a height that would cut into the required obstacle clearance of an instrument approach, departure or circling area;
- a height that would raise the minimum obstacle clearance altitude of a federal airway or approved route.
The last two are why a structure miles from any runway, well clear of every imaginary surface, can still draw a hazard finding.
Military airports and heliports use different surfaces
DoD airports follow §77.21, with a 2,000 ft primary surface, a 7,500 ft inner horizontal surface, a 50:1 approach clearance surface extending 50,000 ft, and an outer horizontal surface 500 ft above airfield elevation. Heliports follow §77.23: an 8:1 approach surface for civil heliports (10:1 military) extending 4,000 ft, and 2:1 transitional surfaces for 250 ft. If your site is near either, the civil table above doesn't apply.
Run every surface for a site at once
Building all five surfaces for every runway end by hand means finding each runway's category, threshold coordinates and elevations, then doing the geometry. The CurvAero Part 77 Report builds the surfaces from FAA airport data, tells you which surface controls at your coordinate, and gives the clearance or penetration in feet, alongside the 77.9 notice check.
Frequently asked questions
What are the five Part 77 imaginary surfaces?
Primary, approach, transitional, horizontal and conical, defined for civil airports in 14 CFR 77.19.
If my structure clears every imaginary surface, do I still need to file?
Possibly. Filing is decided by the 77.9 notice criteria, which are deliberately more conservative. See Do I need to file Form 7460-1?
Are trees and terrain covered?
Yes. The obstruction standards apply to objects of natural growth and terrain as well as construction (§77.13). Airports routinely track trees that grow into their approach and transitional surfaces.
Where do runway categories come from?
From the airport's published and planned approach procedures. The airport's Airport Layout Plan shows planned approaches, which also count.