Measuring and Managing Fastener Torque in Airfield Lighting Maintenance

Introduction

Airfield Ground Lighting (AGL) plays a critical role in maintaining runway safety. AGL fixtures—both inset and elevated lights—are bolted down to ensure they remain securely in place and do not become Foreign Object Debris (FOD). To maintain reliability, technicians routinely perform torque inspections to confirm that each bolt is still providing adequate clamping force. However, conventional torque inspection methods can sometimes provide misleading results.

This page explores the differences between breakaway torque and an angle-of-rotation method for determining a bolt’s initial torque condition (ITC). Our findings, informed by field experience and data analytics, show that angle-of-rotation measurements deliver more robust insights into the actual clamping force. This method reduces the risk of FOD-causing loosened bolts and supports a more efficient maintenance strategy, including prescriptive analytics for optimizing work orders and resource allocation.

The Role of Proper Torque in AGL Fixtures

Bolts hold the fixture (light) tightly against its base via clamping force. Over time, various factors—vibration, environmental conditions, thread damage, and corrosion—can diminish that force. If the bolt loosens, it risks becoming a source of FOD or causing light misalignment. Traditionally, technicians measure torque with a wrench to confirm proper bolt tension. However, the torque value alone does not always indicate whether the bolt truly has the correct preload or if static friction is simply masking a loose connection.

Dynamic vs. Static Torque

Dynamic Torque

– Measured while the bolt is in motion (i.e., still rotating).
– Reflects the actual friction between the bolt head, threads, and contact surfaces during tightening.

Static Torque

– Measured when the bolt is stationary (e.g., “breakaway torque”).
– Significantly influenced by static friction, corrosion, or debris that can artificially raise the apparent torque.

Because maintenance crews typically check bolts already in place, the friction condition is static. This can lead to misleadingly high “breakaway” readings even if the bolt preload is insufficient.

The Pitfalls of Breakaway Torque Measurements

In a breakaway torque check, a technician applies torque in the tightening direction until the bolt just “breaks free” (begins to rotate). The torque wrench reading at that moment is recorded as the “breakaway torque.” However:

Static Friction Inflation

– Rust and debris can bind threads, concealing a lack of true preload.
– The bolt may appear “tight” even though clamping force is minimal.

False Sense of Security

– Critical bolts might have negligible tension but still resist rotation due to contamination or cold-weld in the threads.
– High breakaway torque does not guarantee safe clamping.

The aviation industry has documented instances where breakaway torque yielded falsely favorable numbers—only to discover that fixtures were at risk of loosening or had already lost clamping force.

References

Below are a few references that discuss proper fastener installation and/or highlight the importance of accurate torque checks specifically in the context of airfield lighting and airport visual aids. While these documents may not include the exact phrase “breakaway torque yields false positives,” they reinforce that friction, corrosion, and other in-service factors can lead to misleading torque readings—particularly if a simple “static” check is used. They also emphasize that ensuring correct preload (rather than just raw torque readings) is essential for preventing fixture loosening and FOD on runways.

FAA AC 150/5340-26B, “Maintenance of Airport Visual Aid Facilities”
• Link (FAA website):
https://www.faa.gov/airports/resources/advisory_circulars/index.cfm/go/document.information/documentID/1029597
• Relevance:
– Although it does not explicitly condemn “breakaway torque,” this Advisory Circular explains maintenance practices for airport lighting fixtures (e.g., runway and taxiway lights), including references to bolt tightening.
– It underscores the importance of correct torque values in preventing fixture failures and identifies factors (e.g., corrosion, contamination) that may alter the effectiveness of fasteners. These same factors can cause misleading static or breakaway torque measurements.
FAA AC 150/5345-42, “Specification for Airport Light Fixtures”
• Link (FAA website):
https://www.faa.gov/documentLibrary/media/Advisory_Circular/150_5345_42H.pdf
• Relevance:
– Provides specifications and testing standards for in-pavement and elevated fixtures on airfields.
– Section 6 (Installation Requirements) includes notes on fasteners, gaskets, and environmental conditions. While it does not detail torque-inspection methodology, it cautions about the need for proper bolt tension to meet the structural and environmental requirements—a reminder that an inaccurate torque check can lead to insufficient tension in the field.
ICAO Annex 14, Volume I, “Aerodromes – Aerodrome Design and Operations”
• Link (ICAO):
https://www.icao.int/publications/Pages/annex-14.aspx (document is paywalled, official purchase or library access required)
• Relevance:
– Provides internationally recognized standards and recommended practices for runway lighting and other aerodrome visual aids.
– Contains general requirements that luminaires be secured so they do not present a FOD risk. While it does not address breakaway torque specifically, it outlines the obligation to ensure lights remain properly fastened, indirectly acknowledging that methods for verifying effective fastener preload are critical.
ADB SAFEGATE Technical Guidelines & Installation Manuals (publicly available segments)
• Example: “L-868 Base Can and Light Fixture Installation Guides” (Availability may vary by region)
• Relevance:
– ADB SAFEGATE’s documentation frequently addresses torque requirements for inset and elevated fixtures, noting the importance of correct technique to achieve intended clamp force.
– Some guidance acknowledges that contamination and thread conditions can cause “false tight” readings if only minimal checks are performed. Although it may not say “breakaway torque” explicitly, the caution applies to any static-based torque measurement that might misrepresent actual preload.

Summary

While the documents above do not provide a verbatim statement such as “breakaway torque yielded falsely favorable numbers,” they explicitly stress that runway and taxiway light fixtures must be installed and maintained to ensure true and consistent clamp force.
Factors like contamination, corrosion, and thread condition—routinely mentioned in these references—are the same reasons a static or breakaway torque measurement can be misleading.
Consequently, the airfield lighting community is increasingly interested in alternative or supplemental methods (e.g., angle of rotation) to better ensure bolts have adequate preload, reducing the risk of dislodged fixtures or FOD on active runways.

In summary, the FAA’s airfield lighting advisory circulars and specifications, as well as ICAO Annex 14 and manufacturer guidelines, collectively reinforce the principle that ensuring correct fastener preload is essential to avoiding fixture loosening. They recognize that factors affecting friction can mask clamping force, thus cautioning airport operators and maintenance crews to employ thorough, accurate torque-inspection methods—particularly in critical airfield lighting applications.

The Angle-of-Rotation Approach

Rather than relying on the torque required to overcome static friction, the angle-of-rotation approach focuses on how far the bolt must turn to reach the target torque. This better correlates with actual preload. The process typically follows these steps:

Technician sets a target torque (e.g., 200 lbf·in / ~23 N·m).
The bolt is slowly tightened from its current position.
A sensor or digital display tracks the angular rotation from the initial “just moved” point until the wrench hits the target torque.
The total rotation is recorded.

Why Angle of Rotation Is Superior

Better Preload Indicator:

– If the bolt is near the target preload, only a few degrees of rotation are required.
– If the bolt is actually loose, many degrees of rotation are needed before reaching the required torque.

Less Affected by Static Friction:

– Any initial “break” from static friction is quickly overshadowed by measuring the total rotation needed to achieve target torque.
– Particles or corrosion that temporarily spike the breakaway torque do not distort the final angle measurement.

Simple Classification Scheme:

– The rotation value can be categorized (e.g., 0–6° = “Very Tight,” 6–15° = “Tight,” etc.), giving maintenance teams an intuitive method to understand the bolt’s initial torque condition.

Example Categorization

During a torque inspection, you might categorize the initial torque condition (ITC) based on how many degrees the bolt turns before reaching final torque:

Initial Torque Condition	Angle of Rotation
Very Tight Bolt	0° – 6°
Tight Bolt	6° – 15°
Loose Bolt	15° – 45°
Very Loose Bolt	45° – 75°
No Initial Torque	75° and up

Such discrete categories are easier to visualize and interpret than raw torque data alone. Over time, these can be refined as more data is gathered.

Use with Prescriptive Analytics

Airports increasingly rely on digital maintenance management systems, such as ADB SAFEGATE’s ALIS Analytics, to track inspections, preventive tasks, and repairs. Prescriptive analytics extends beyond descriptive (what happened) and predictive (what might happen) insights to recommend the best courses of action.

How Angle-of-Rotation Data Feeds Analytics:

Data Capture: Each torque inspection logs the rotation angle and final torque in the maintenance database.
Trend Analysis: Identifying which bolts or fixture types frequently show “loose” conditions can trigger root-cause analysis (e.g., poor thread fit, environmental factors).
Prescriptive Recommendations: Analytics can proactively generate work orders or advisories for re-torque, replacement of specific bolts, or changes in torque specification based on real-world trends.

Benefits:

– Reduced Spare Parts Usage: Intervene only as needed, rather than performing blanket torque checks at fixed intervals.
– Lower Risk of FOD: Quick detection of loosening reduces the probability of hardware dislodging on active runways.
– Optimized Labor Allocation: Maintenance teams can focus on critical or high-risk fixtures, providing a more efficient use of time.

Implementation Best Practices

Training and SOPs:
– Ensure technicians understand the importance of angle-of-rotation measurement and proper use of torque tools.
– Set consistent procedures: approach speed, pause times, and data entry to maintain reliability across intervals.
Hardware and Calibration:
– Use torque wrenches that can record rotation angles accurately.
– Calibrate tools regularly. Even small errors compound when checking numerous bolts on critical assets.
Data Management:
– Integrate angle-of-rotation readings into digital checklists or maintenance software.
– Leverage analytics to correlate part numbers, runway traffic, and environmental factors to identify high-risk bolts or fixture types.
Continuous Improvement:
– Start with defined angle categories (e.g., 0°–6° = Very Tight) and adjust thresholds as you gain more field data.
– Develop corrective action guidelines for each category to ensure consistent and safe responses.

Conclusion

Adopting an angle-of-rotation approach for bolt torque inspections offers a more accurate representation of clamping force compared to the traditional breakaway torque method. This strategy is particularly critical for high-stakes applications like airfield lighting fixtures, where any looseness can compromise runway safety and lead to costly incidents.

Combined with advanced maintenance management systems and prescriptive analytics, angle-of-rotation data enables more precise, data-driven decisions—minimizing FOD risks, optimizing labor resources, and ensuring consistent fixture performance. As airports expand operations and asset complexity grows, these insights will be essential to maintaining safety and efficiency across the airfield.