News  |  Oct 23, 2023  |  Anne Keener

Prioritize Safety with our Key Considerations for L-858 Airfield Signage

In general, guidance signs are crucial no matter if located on a highway, rural road, or at an airport. Signage can indicate our current location, direction of travel, important operational instructions, restrictions, or describe our desired destination. When it comes to airfield guidance signs, L-858 Airfield Guidance Signs provide all this information, and more!

In this blog post, we will explore essential concepts to keep in mind when modifying your airfield signage system.

Pavement Geometry and How They Can Affect Pilot Viewing Distance

Complying with FAA Advisory Circular (AC) 150/5340-18G is mandatory for FAA controlled airports and is recommended at all other airports. This section will delve into the significance of lateral offsets, the need for consistent placement, and the potential for mitigating misalignment issues.

FAA Advisory Circular (AC) 150/5340-18G defines the location requirements for guidance signs. Some values have been recently revised; however, the maximum and minimum lateral offset distances are defined by AC 150/5340-18G Table 1-1 with reference from the defined edge of pavement. The defined pavement edge could be the edge of the aircraft rated pavement (no shoulder pavement) or the outer most side of the edge stripe. Per design guidance within AC 150/5300-13B, tapers are used to widen the useable pavement prior to taxiway turns and intersections to account for main gear path of travel. Using a consistent lateral offset, signs may be placed considerably farther way from the pavement centerline (i.e. pilot’s cockpit perspective).

To account for this discrepancy, we could use the upper lateral offset limit for signs along straight segments of taxiway and a reduced lateral offset at or above the minimum for signs at intersections to account for tapered geometry. It is important to note that review of design aircraft profiles may be required to confirm reduced offset locations will not fall within 12 inches of any part of the critical aircraft when the main gear is located at the edge of the defined pavement.

Future advisory circulars may revise the distance criteria and the reference on how the lateral offsets should be measured. Until then, if we consider the lateral distance criteria from edge of defined pavement as well as distance from the pavement centerline, we may be able to mitigate pilots perceiving signs as being misaligned and located too far away.

Intersections and Wingtip Clearance

Proper positioning of guidance signs at intersections is critical for maintaining wingtip clearance and preventing potential collisions. We will explore the requirements outlined in FAA AC 150/5300-13B and discuss how to address acute angle intersections that pose unique challenges. By considering complex geometry and implementing appropriate signage and markings, you can reduce the risk of “close call” scenarios and enhance overall safety.

FAA Advisory Circular (AC) AC 150/5300-13B Table 4-1 defines the location distance requirements for guidance signs with relation to the intersecting taxiway centerline and the aircraft design group applicable to the airfield. This location may be co-located with an intermediate taxiway hold line (Pattern C). By following this guidance, the pilot using these signs and markings should be rest-assured that the nose of their aircraft will not encroach into the object free area thus not creating a nose to wingtip collision potential. However, this criterion is missing one component.

If your airport has an extremely acute angle intersection (e.g. merging of two parallel taxiways), modeling of aircraft traffic at this intersection may be required to prevent the holding aircraft wingtip from extending into the object free area of the intersecting taxiway even if the nose remains clear. If you have this type of complex taxiway geometry, you should have operational restrictions to prevent such a scenario.

However, as designers and planners we should strive to place signage and markings to prevent potential “close call” scenarios. There is no guarantee a pilot may not accidentally receive or misinterpret instructions to hold too close to the intersection. The important takeaway is to understand all airport operations and plan for the unexpected.

Impact to the Surrounding Grade from Keeping Signs and Foundations Level

Installing airfield guidance signs in compliance with AC 150/5345-44K can present its own challenges. This section will highlight the need for level sign foundations and discuss potential conflicts arising from existing grade variations. We will also explore the possibilities for future revisions to allow for sloped foundations or introducing tolerance in sign alignment. Addressing grading concerns in projects can ensure proper sign installation and minimize conflicts with other structures.

Airfield guidance signs are installed in specific locations defined by the Advisory Circulars, as “Fixed-by-Function.” Unfortunately, these “Fixed-by-Function” signs are required to be installed level with no tolerance provided. Guidance sign manufacturers must fabricate their products to conform with AC 150/5345-44K which does not provide a tolerance to adjust the sign hardware to maintain this requirement. Therefore, each guidance sign foundation must be installed at a true level in all directions.

For projects with new signs and pavement or grading modifications, this requirement can create grading challenges. Existing shoulder grades may have anywhere from a 1.5 to 5 percent slope away from pavement which can create a significant vertical elevation change for a long sign foundation. Regardless, the requirement for a level foundation will leave the farthest end of the foundation partially above grade requiring corrective grading to intercept the foundation elevation.

Luckily, FAA Engineering Brief EB-79 does allow us to slope the grade around the sign foundation to existing grade at a slope of no greater than 10 percent for a limited distance. Even with this generous allowance, long signs, large foundations, or sign arrays may require extensive grading and additional conflicts are likely to arise with other structures.

To mitigate grading concerns, future advisory circulars could revise manufacturer requirements to allow for varied sign leg lengths of sloped foundations to maintain the current requirement or a tolerance could be introduced to allow for signs to slope slightly away from the pavement centerline. Ultimately, with the standards as they are currently written, sign projects should not be simply limited to installation of signs. Care should be taken to address any existing grading and conflict challenges.

In conclusion, airfield guidance signage plays a vital role in ensuring safe and efficient operations at airports. By understanding and adhering to FAA guidelines, such as those outlined in AC 150/5340-18G and AC 150/5300-13B, airports can enhance pilot visibility, ensure wingtip clearance, and address grading challenges. However, it is important to recognize the limitations and potential gaps in these guidelines, especially in complex geometries and existing grade variations.

As planners, designers, and engineers, we should remain vigilant in our efforts to mitigate risks and plan for unexpected scenarios. By doing so, we can create airfield signage systems that not only meet regulatory requirements but also prioritize the safety of pilots, passengers, and ground personnel. At Arora, we strive to continually improve and innovate in the field of airfield design to ensure the highest standards of safety and efficiency in aviation. Contact us today to get expert guidance for your next airfield lighting project!

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