Building height & FAA Part 77 obstruction analysis near Chicago Midway International (MDW)

Which Part 77 surfaces apply near MDW?

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 Chicago Midway International, the surfaces most likely to govern a nearby structure are:

• Primary surface. Centered on each runway at Chicago Midway International; 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 620 ft field elevation — roughly 770 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 Chicago terrain means the Part 77 surfaces, not ground relief, govern everything near MDW — and because the runways are shorter, the approach slopes are steeper (50:1 within 10,000 ft) rather than the 100:1 used for major-runway approaches, making the geometry slightly different from the other Chicago-area airports.

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

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

• More than 200 ft above ground level, anywhere; or
• Within 20,000 ft of a runway longer than 3,200 ft (all of MDW'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 Chicago Midway International has 5 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 MDW 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.