In-Pavement AGL Bolt Problems: A Review

Current Bolt Problems: A Review

Jaquith Industries Inc.

Current Bolt Problems

Incidents have occurred at certain airports where the in-pavement light fixture bolted connections have

failed resulting in light fixtures completely separating from the light bases or light base extensions

After these incidents, the FAA directed airports to inspect AGL units: FAA Part 139 CertAlert No. 14-03

Resulting Inspections

Inspections at multiple airports showed similar problems

Missing Bolts - Broken Bolts – Missing Lock WashersImproperly Torqued Bolts – Loose Fixtures etc.

Missing Lock WasherMissing Lock Washer

Loose BoltsLoose Bolts


Current Bolt Problems

Loose, missing, or broken bolts place more stress on the surrounding bolts in the bolt circle. This can lead to damage of the light base or light fixture and may lead to the complete failure of the remaining bolts, creating a potential hazard for aircraft

Possible Root Causes•Inadequate clamping forces to resist shear forces generated by modern commercial aircraft

•Improper installation & maintenance of the AGL units

•Reduced maintenance intervals, due to the use of LED fixtures


Current FAA Bolt Guidance Per EB-83

• A maximum of 185 in lbs. of torque for a dry 18-8 SS bolt is recommended

• A torque of less than 185 in. lbs. should be used for a SS 18-8 bolt with anti-seize compound and for coated bolts

• In-pavement fixture bolts should be checked bi-monthly per AC150/5340-26C

• All bolts should be replaced after removal. Bolts should never be reused

• Always replace the two part locking washers every time the light fixture is removed from the light base. Never use a split type locking washer

• Always clean the light base upper flange threaded holes before bolt installation

• Always clean the mating surfaces between the light base and the light fixture before reinstallation


FAA Testing of In-Pavement Light Fixtures Future EB-83 Guidance

Jaquith Industries is currently consulting with the FAA & Intertek to conduct

comprehensive bolt testing and determine changes to EB-83

• Conduct Shear Force, Compressive Load, and Vibration Testing to Evaluate Performance of Light Fixture Assemblies Based on Increased Bolt Clamping Loads

• Determine Strength Limitations of Light Fixture Assemblies for Resisting Increased Bolt Clamping Forces

• Instrument and Install In-Pavement Light Fixtures in Runway and/or Taxiway to Evaluate Performance of Light Fixture Assemblies under Controlled Aircraft Wheel Loading Conditions

• Utilize Instrumented B727 Aircraft and Smaller Size Aircraft to Evaluate Light Fixture Assemblies Under Varying Wheel Load/Tire Pressure Conditions

• Conduct Bolt Corrosion Testing per MIL-STD-810


FAA Testing of In-Pavement Light FixturesRelationship of Shear Forces – Clamping Force - Tension-

Torque

What does it all mean?

Torqueing the bolts in the bolt circle of the light

fixture / light base creates tension (elongation

of the bolt) within the bolt. The tension in each

bolt combines to create a clamping force that

holds the light fixture to the light base and

works to resist the shearing forces placed on

the AGL unit during aircraft loading. These

shearing forces are greatest during aircraft

braking or turning.


FAA Testing of In-Pavement Light Fixtures EB-83 Horizontal Shear Forces

Shear Forces are generated as the result of an aircraft running overtop the light fixture. These forces are the highest when an aircraft is braking or exiting the runway

using a high speed exit taxiway

Clamping forces must be sufficient to resist the shearing forces exerted on the light fixture during

aircraft braking and high speed exits from the runway

FAA Currently Requires: “The light fixture must withstand a shear load of

3,000 pounds (1,360.78 kg) applied to the top of the light in any direction parallel to the mounting surface.”

The FAA will be conducting shear force testing to evaluate shear forces present under various

conditions and determine clamping forces & bolt torque needed to resist those shearing forces. We expect the FAA to require the light fixture withstand shearing forces greater than the current 3,000 lbs.


The FAA will be conducting testing to determine the required clamping force to

withstand higher shear forces placed on the light fixture. The FAA will also determine

clamping forces generated at specific bolt torque values with various bolts, sizes, & materials. We expect the FAA to increase

recommended clamping forces to withstand higher shear forces

FAA Testing of In-Pavement Light Fixtures EB-83 Fixture & Bolt Clamping Forces

Required Fixture Clamping Force = (Shear / K of Friction) x Safety Factor

Required Fixture Clamping Force = (3,000 lbs. / .45) x 2

Required Fixture Clamping Force = Approximately 13,400 lbs.

Required Clamping Force Per Bolt = Fixture Clamping Force / # of Bolts

Required Clamping Force Per Bolt = 13,400 lbs. / 6 Bolts

Required Clamping Force Per Bolt = Approximately 2,300 lbs.

Determining Required Clamping Forces

Clamping forces are the result of tension in the bolt. Clamping forces are generated by the combined tension of each bolt in the bolt circle. Clamping forces must be sufficient to resist the shearing forces exerted on the light fixture during aircraft braking or exiting the

runway using a high speed exit taxiway


FAA Testing of In-Pavement Light Fixtures EB-83 Torque vs Tension

Torque is the amount of effort required to thread the bolt into the top flange. While tension refers to the elongation of the bolt that produces the clamping force to hold

the light fixture to the light base

The FAA will test several different bolts on various light base assemblies to determine the strength limits of various bolts & light

fixture assemblies

The Balancing Act:It is important that the bolt tension

create a high enough clamping force to resist shear forces. However we must not create so much tension

that the threads of the bolt or flange ring are damaged. It is important to

remember fluctuations in bolt tension within a bolt circle may lead to uneven bolt loading, which may damage bolts, fixtures, and light

base flange

T = TorqueK = Coefficient of FrictionD = Bolt DiameterFp = Axial Tension


FAA Testing of In-Pavement Light Fixtures EB-83 Torque vs Tension

Torque is a relatively poor way of estimating clamping force because the value for K can be greatly influenced by the friction present between the top

flange & bolt threads

Torque vs Tension with Changing K What Can Influence K (friction) Value?•Damaged threads on the bolt or top flange

•Use of anti-seize or lubricant compounds

•Rust or elongation of the threads in bolts that have been re-used

•Debris in the bolt or top flange threads

K = .20 K = .10

The FAA will test several different bolts on various light base assemblies to determine K (friction) values. Values will serve as a baseline for calculating required torque. We expect that the FAA will require higher torque values to overcome fluctuations in the K value and produce higher clamping forces


Bolt RecommendationsGrade 5 Coated Bolts

• Coefficient of friction .05 -.10 – Translates to higher clamping force at lower torque

• Bolts already coated- no need to apply anti-seize products like Loctite – More consistent K values

• Reduced risk of galling

• In the event of a “breakage” bolts are more easily drilled out

• Grade 5 bolts are often stronger than stainless steel bolts


Alternative Bolt Recommendation18-8 Stainless Steel Bolts with 51609 Loctite

• Stainless steel bolts have a proven ability to secure the light fixture to the light base

• When used with 51609 heavy duty Loctite the coefficient of friction is lowered – producing higher clamping forces at lower toque

• Stainless steel bolts are cheaper in comparison to coated bolts

Considerations:

• Stainless steel bolts have the potential to produce a galling effect

• Even with the use of Loctite stainless steel bolts have a higher coefficient of friction than coated bolts


Bolt & Washer RecommendationsLOCK Washers

• Prevents vibration from loosening bolt and decreasing clamping forces or creating uneven clamping forces between the light fixture and light base

• 2 piece washer “bites” bolt head and light fixture – securing the bolt, washer and fixture

• Step cam between the 2 piece lock washers prevents bolt from moving

• Lock washers should be changed every time bolts are removed to protect the integrity of the lock washer system


Recommended Best PracticesMaintenance & Bolt Torque

• Ensure bolt & flange threads are clean and not damaged

• Ensure all mating surfaces are clean and free of debris

• Bolts should be torqued with a calibrated torque wrench and never a high speed air impact wrench

• Bolts should be torqued to the bolt or light fixture manufacturers reccomondations (ADB – 500 GBA – 300)

• “Dry” stainless steel bolts should never be used with a stainless steel top ring due to the potential for galling

• Bolt torque should be checked on a regular basis with emphasis on high stress or problem areas

• Replace bolts & lock washers after every use

• Should threads become damaged – they should be repaired with Jaquith’s thread repair kit


Date post:	22-Jan-2018
Category:	Engineering
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In-Pavement AGL Bolt Problems: A Review

Engineering