Building height & FAA Part 77 obstruction analysis near Detroit Metropolitan Wayne County (DTW)

Which Part 77 surfaces apply near DTW?

Under 14 CFR Part 77, every public-use airport projects a set of imaginary surfaces. An object that penetrates any of them may be an obstruction to air navigation. Near Detroit Metropolitan Wayne County, the surfaces most likely to govern a nearby structure are:

• Primary surface. Centered on each runway at Detroit Metropolitan Wayne County; its width depends on the runway's most precise approach. It establishes the base elevation that the other surfaces rise from.
• Horizontal surface. A plane 150 ft above the 645 ft field elevation — roughly 795 ft AMSL — extending in a radius around the runways. Many mid-rise structures break this surface first.
• Conical surface. Slopes upward and outward at 20:1 from the edge of the horizontal surface for 4,000 ft, catching taller structures farther from the field.
• Approach surface. Extends and rises from each runway end along the approach path. With precision instrument runways here, the approach surfaces are long and shallow — they can be the controlling constraint miles out.
• Transitional surface. Rises at 7:1 from the sides of the primary and approach surfaces, frequently the binding surface for structures close to the runway centerline.

Flat Midwestern terrain around DTW means the imaginary surfaces, not ground relief, dominate the screening — but with six runway ends in multiple orientations, it's common for a single structure to be within the notice radius of more than one runway approach.

When you must file an FAA Form 7460-1 near DTW

You are required to notify the FAA (14 CFR §77.9) when proposed construction or alteration near Detroit is:

• More than 200 ft above ground level, anywhere; or
• Within 20,000 ft of a runway longer than 3,200 ft (all of DTW's primary runways qualify) and exceeding a 100:1 slope from the nearest point of that runway; or
• Within 10,000 ft of a shorter runway and exceeding a 50:1 slope; or
• On the airport itself, or otherwise requested by the FAA.

Because Detroit Metropolitan Wayne County has 6 runways spread across a large footprint, the 20,000-ft notice radius reaches well into the surrounding metro — a structure can sit far from the terminals and still trip a notice criterion tied to a distant runway end.

How LOS Analyzer screens it

Enter DTW and your proposed structure's footprint and height. LOS Analyzer places the control tower and runway ends, pulls terrain elevation along each sightline, and computes two things at once:

1. Part 77 penetration — whether your structure's top breaks the primary, horizontal, conical, approach, or transitional surface, and by how many feet.
2. Control-tower line-of-sight — whether the structure blocks the tower's view of the runway, modeled with earth curvature and atmospheric refraction rather than a flat-earth approximation.

You get an on-screen determination in minutes, a multi-page PDF report, and a pre-filled Form 7460-1 draft — for $499, versus the $5,000–$15,000 and multi-week turnaround of a full airspace study. It is a preliminary screening to inform your filing, not a substitute for an FAA determination or a licensed engineer's review.