BasePDH | Course 023 | 3 Professional Development Hours

UAS for Element-Level Bridge Inspection Under the NBIS

1. The Regulatory Boundary: UAS Under the 2022 NBIS

Whether an uncrewed aerial system belongs in a bridge inspection is decided by a regulation that never uses the word. The 2022 National Bridge Inspection Standards final rule, 87 FR 27396, amending 23 CFR part 650 subpart C, took effect June 6, 2022, and its codified text mentions no aircraft of any kind [3]. FHWA's position on UAS lives instead in the rule's preamble, where the agency answered commenters in three separate places: once on what the technology can do, once on who may operate it, and once on how a new method enters practice. The three answers are consistent, and together they draw the boundary this course is organized around. An engineer planning a UAS-assisted inspection needs all three, because each forecloses a different program design, and a program checked against only one of them can still be non-compliant on the others.

1.1 The position statement and the four capability gaps

Seven commenters asked FHWA whether advanced technologies such as UAS or structural monitoring could be used in bridge inspection. The response states the agency's position directly: "FHWA's position is that proven advanced technologies may be used to supplement but not supplant bridge inspection personnel and inspection methods" [3]. The technologies are not a replacement for the personnel performing inspections, and they are not intended to replace visual and physical inspection methods [3].

FHWA then states two conditions under which an advanced technology is useful, and they are alternatives rather than a pair of requirements: its use enables an inspection to be done more efficiently without compromising the thoroughness and effectiveness of the inspection, or visual and physical methods are not able to assess fully a bridge component [3]. The second condition matters as much as the first. A UAS justified only on efficiency has to demonstrate that nothing was given up; a UAS reaching a component that visual and physical methods cannot fully assess is doing work the rule contemplates on its own terms.

The same passage enumerates what a UAS cannot address, and the list is the durable content of the position. FHWA names live load response, auditory cues, and sounding of members as aspects of an inspection a UAS cannot carry, and then elaborates on physical, tactile examination: sounding or hammering on the surface of a member establishes the soundness of the material, and where a defect is present, its dimensions [3]. The elaboration carries the engineering reason. Those dimensions feed two downstream uses, tracking deterioration over time and determining strength or capacity when calculating a load rating [3]. The rule's own definition of load rating closes the loop: it is the analysis to determine safe vehicular live load carrying capacity using bridge plans, supplemented by measurements and other information gathered from an inspection [3]. A task whose output is a defect dimension destined for a load rating is therefore closed to remote capture regardless of how good the camera is. A task whose output is presence, extent, or change is open. That distinction, which follows directly from what FHWA says the tactile examination is for, is the most useful single test in this course, and it reappears in every later section.

FHWA also names three practical constraints on UAS use that are conditions of the site rather than limits of the aircraft: lighting at the portion being inspected, the need for cleaning the portion inspected before it can be assessed, and the potential for driver distraction when flying adjacent to live traffic [3]. Of the three, cleaning is the one with no specification answer. Section 3 of this course treats lighting as a solvable specification problem, and it largely is. No tool on a standard platform can contact the surface to clean it, and Section 7 returns to a fracture-critical inspection where that limitation, not image quality, decided the outcome.

Against these constraints FHWA credits UAS with two specific efficiencies when used effectively: limiting the amount of time access equipment is in use, and reducing time working adjacent to live traffic [3]. The credited savings are in access and exposure only, not in inspection labor, documentation, or qualification. Section 6 shows the field data landing in exactly the same place.

The governing test FHWA states is that UAS may supplement a bridge inspection when its capabilities are able to meet the requirements of a specific task in the bridge inspection [3]. The unit of analysis is the task, not the inspection. A routine inspection is not a UAS inspection or a conventional one; it is a set of tasks, some of which a UAS pass can defensibly carry.

The preamble illustrates the test with a worked example worth restating as the decision sequence it is. For a bridge over water, flying the UAS to take bird's-eye photography of the site is, in FHWA's words, an efficient use of the tool. Qualified personnel observe and document changes in the channel since the last inspection from that imagery. If the photography shows concerning changes, the inspector must utilize physical, tactile techniques to investigate further [3]. Two features of the sequence generalize. The escalation trigger is a concerning change, not a confirmed defect, so the threshold for putting a person on the structure is lower than the threshold for a finding. And the remote observation serves as the observation, never as the disposition.

FHWA closed the passage with a commitment to continue evaluating new tools with stakeholders and to update the Bridge Inspector's Reference Manual (BIRM) to allow technological advances into the National Bridge Inspection Program [3]. The BIRM, not the regulation, is the stated vehicle for admitting new technology. The BIRM is a guidance document, deliberately not incorporated by reference, which is what lets FHWA revise it outside rulemaking [3]. The practical consequence for the reader is that the boundary described in this section can move without a Federal Register notice, and the current BIRM has to be checked alongside 23 CFR 650.

1.2 The qualifications side: there is no drone inspector

A commenter noted that the rule contains no performance-based qualifications for inspectors using UAS and recommended requirements covering training and testing for accuracy, visual acutance, image quality, and documentation [3]. FHWA declined, and the shape of the refusal defines how UAS operations sit inside an inspection program.