How close is an obstruction allowed to be in an instrument approach?

Question

Inspired by this answer, and the comments that followed.

City politics aside, i.e. hypothetically, can KBOS rwy 14 be used for landing aircraft that don't comfortably need more than 5,000 feet of runway?

The issue arises from the building at the approach end of rwy 14.

The building is 174 ft. tall, lighted, 1364 ft. from runway, and 70 ft. left of center line.

The RPZ (protected zone for excursions) by FAA standards is 1000 ft., which the runway meets.

FAA also says the OFZ (Obstacle Free Zone, the 3D area covering the approach end) would only be 200 ft. since there is no Approach Lighting System installed.

In summary, imagine the building is a rocky outcrop somewhere in a desert, and someone built a 5000-foot runway the same distance from it as KBOS 14, can it be legally used?

Visually I don't see why not, but I'm not an FAA administrator. And for an instrument approach I also don't see a problem stepping down to a specified MDA, and even at an angle (LDA).

To raise the stakes a bit, I believe a DC-9 can comfortably land on a 5000-foot runway. Let's use a DC-9 or similar type in the example above.

To make the answer less stressful, if you know of a similar airport/obstacle scenario, or can find one, with a published approach, let us know.

Answer 1

FAR Part 77 provides standards for what objects constitute an obstruction to navigation:

• 200 ft. above ground level or 200 ft. above the airport elevation (whichever is greater) up to 3 miles (for runway lengths > 3200 ft.) from the airport.
  • Increase 100 ft. every mile up to 500 ft. at 6 miles from the ARP (airport reference point)
• 500 ft. or more above ground level at the object site
• Penetrates an imaginary surface (a function of the precision of the runway)
• Penetrates the terminal obstacle clearance area (includes initial approach segment)
• Penetrates the enroute obstacle clearance area (includes turn and termination areas of federal airways)

There are 5 imaginary surfaces:

• Primary - aligned (longitudinally) with each runway and extends 200 ft. from each runway end
• Approach - longitudinally centered with the runway and extends beyond the primary surface
• Horizontal - horizontal plane 150 ft. above the established airport elevation. Constructed by swinging arcs around the end of the primary surface
• Conical - 20:1 slope surface extending beyond the horizontal surface
• Transitional - constructed to join approach and horizontal or approach and transitional surfaces

Additional dimensions are also dependent on the types of approaches used at the runway. A precision instrument runway has a primary surface 1,000 feet wide, and a horizontal surface radius of 10,000 feet. Approach surface width at end is 16,000 feet, total approach surface length is 50,000 feet, and slope is 50:1 for inner 10,000 feet and 40:1 for the additional 40,000 feet.

The idea is that the FAA is notified of any new obstacle that meets the criteria above, and it is up to the FAA to provide a determination of how it will affect procedures. If an obstacle cannot avoided, special procedures may be required.

---

For the specific example of Boston's runway 14, using the most conservative visual approach clearance requirements, the obstacle is well within the imaginary surfaces:

• The primary surface is 250 feet wide for a visual runway and gets wider through the approach section, so at 70 feet from center line, this building will be within that zone.

• The approach surface for a visual runway is 5,000 feet long, so at 1364 feet the building will be within that zone.

• The approach slope for a visual runway is 20:1, which is a height of 58 feet at a distance of 1364 feet from the runway, so the building at a height of 174 feet is well into that zone.

The runway would need a displaced threshold of at least 2320 feet to clear the building for a visual approach. This would mean you could only use runway 14 for visual approaches, and there would only be 2680 feet remaining. You could use it for non-precision instrument approaches but only with propeller driven aircraft of 12,500 pounds maximum gross weight and less. Anything more than that, and the approach slope gets even more shallow.

Note that other runways at BOS list obstructions as well, but all others list the slope to clear the obstacle. Runway 14 would have about a 6:1 slope to clear that building. Runway 4R/22L has slopes of 8:1 and 7:1 respectively to obstacles, but is also twice as long, allowing for a displaced threshold.