performance of in-pavement runway light fixture assemblies
TRANSCRIPT
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Performance of In-pavement Runway Light
Fixture Assemblies
October 20, 2015
Julia H. Grogan Senior Engineer
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Background
• Incidents involving dislodged in-pavement
runway lights
• Comprehensive engineering assessment
performed in order to mitigate future light
fixture issues
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LPI Evaluation Overview
• Light Fixture and Bolt Failure Analyses
• Sampling Program
• Finite Element Analyses
• Torque Testing
• Instrumentation
• Recommendations and Findings
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Light Fixture Assemblies • 8-in. optical housing with 12-in. support ring
• Type 304 SS, Type 410 SS, coated carbon steel bolts
Optical
Housing Can Spacer
Support Ring Bolt
Flange
Ring
Spacer
Rings
Extension Collar
Base Can
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Incident Light Fixture
Failure Analysis
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Incident Light Fixtures
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Fractured Bolts
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Airport Site Inspections
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Sampling Program
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Sampling Program Protocol
• 30 light fixtures selected per runway for inspection: – Runways: 9
– Fixtures: 270
– Bolts: 1620
• Protocol for Sampling Program included: – Visual examination of light fixture
– Removal of bolts for examination at LPI
– Visual examination and count of spacer rings
– Visual examination of bolt holes in extension or base mounting can
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Sampling Program Findings • 22 cracked bolts found
– Consistent with low-cycle, high-stress fatigue
• 36 bent bolts (Type 304 and Grade 2) found
• Some coated carbon steel bolts exhibited extensive corrosion
• 25 to 50% of support ring holes exhibited thread impressions
• Six extension collars exhibited missing bolts
– Three base cans exhibited ovalization
• 30 to 45% of light fixtures had more than three spacer rings
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Sampling Program Findings
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Thread root cracks
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Sampling Program Findings
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Base Can
Deformation
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Finite Element Analyses
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Finite Element Analyses • Evaluate effect of aircraft roll-over on
light fixture and support ring bolts
• FEA model created for light fixture
assembly
• Loads derived from Aircraft
Characteristics documents
– Considered conservative
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FEA Results
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With Foundation Stiffness Without Foundation Stiffness
Bolt Preload (lb) 2000 3000 5000 2000 3000 5000
Axial [lb] 3029 3849 5033 3311 4067 5156
Shear [lb] 411.3 250.0 71.9 491.0 244.2 117.0
Moment [lb-in.] 476.2 283.4 82.1 558.1 278.1 133.1
Bending Stress [ksi]**
156.5 93.1 27.0 183.4 91.4 43.8
Bolt Z-Disp. [in.] 0.0346 0.0203 0.0065 0.0394 0.0201 0.0103
• Low preload allows slip at Support Ring / Flange Ring interface
** Note: This is an “apparent” stress and is only listed for comparison purposes. Once the bolt starts to yield, the linear relationship no longer holds.
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FEA Results
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5,000 lb
Preload
2,000 lb
Preload
Without
foundation
stiffness:
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Additional FEA Studies
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• Effect of spacer rings
– Bolt displacement and
bending stress increases
with number of rings
• Effect of missing bolt
– One missing bolt results
in higher stresses in
remaining bolts
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Bolt Torque Testing
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Bolt Torque Specifications
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• FAA AC 150/5345-42, 150/5340-26, 150/5340-30
– Torque per manufacturer’s recommendations
– 185 in.-lb (15 ft-lb), 230 in.-lb (18 ft-lb)
• FAA Engineering Brief No. 83
– 18-8 bolt (dry): 185 in.-lb (15 ft-lb) maximum
– Coated Grade 2 bolt (dry): 120-144 in.-lb (10-12 ft-lb)
• FAA AC 150/5345-46
– Light fixture bolts must withstand shear load of 3000 lb
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Design Torque vs. Bolt Preload
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Nut Factor “𝐾”
Experimental constant
• Steel (dry) = 0.2
• Steel (lubricated) = 0.1-0.2
where 𝑇𝑖𝑛 = input torque (in.−lb)
𝐹𝑃 = achieved preload (lb) 𝐷 = nominal diameter (in.) 𝐾 = nut factor
Bickford, John H. An Introduction to the Design and Behavior of Bolted
Joints. 3rd ed. New York: Marcel Dekker, Inc., 1995. Print.
𝑇𝑖𝑛 = 𝐹𝑃 × 𝐷 × 𝐾
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Design Torque vs. Bolt Preload
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• Scatter in preload achieved
at a uniform torque level
Torque-measurement
of unlubricated bolts
Torque-measurement
of lubricated bolts
• Preload uncertainty (Γ)
documented in NASA
NSTS 08307
Γ = ±35%
Γ = ±25%
Bickford, John H. An Introduction to the Design and Behavior of Bolted Joints. 3rd ed. New
York: Marcel Dekker, Inc., 1995. Print.
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Design Torque vs. Bolt Preload
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• 3/8 in.-16 Grade 2 carbon steel
– 75% of proof load = ~ 3200 lb
– Torque required to achieve 3200 lb pre-load (dry bolt) = 20 ft-lb
• 3/8 in.-16 Type 304 stainless steel
– 75% of yield load = 3780 lb
– Torque required to achieve 3780 lb pre-load (dry bolt) = 23 ft-lb
• 3/8 in.-16 Type 410 stainless steel
– 75% of yield load = 5230 lb
– Torque required to achieve 5230 lb pre-load (dry bolt) = 32 ft-lb
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Torque Test Set-up
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• Strain-gaged bolts tested in light fixture assembly
Types of bolts tested:
• Coated Grade 2 carbon steel
• Type 304 stainless steel
• Type 410 stainless steel
• Grade 5 carbon steel
Types of light bases tested:
• L-868 Class IA and IB
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Torque Test Results – Grade 2
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3200 lb
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Torque Test Results – Type 304
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3780 lb
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Torque Test Results – Grade 5
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5230 lb
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Torque to Failure
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Light Fixture Bolt
Recommendations
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Bolt Recommendations
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• Type 410 stainless steel bolts (ASTM F593P)
– 23 ft-lb
– Torque bolts in star pattern
– Loctite marine grade anti-seize
– Two-piece lock washer (Nord-lock type)
• Maximum of three spacer rings
– No silicone sealant
• Check torque after two weeks, then regular maintenance
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Light Fixture Instrumentation
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Instrumentation and Analysis
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Instrumentation and Analysis
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• No indication of direct impact, but good results obtained
from “rolling end” of runway
• Results indicate aircraft does not have to contact light
fixture in order to induce cyclic strain response
• Bolts repeatedly experience 100 to 1500 lb or possibly
more relaxation/unloading
– Utilizing LPI torque vs. preload correlation, 12 ft-lb or more cyclic
relaxation/unloading is occurring
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Instrumentation and Analysis
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• 777 after landing (all wheels down)
– Centerline bolt strain gages
Time (s) Time (s)
1
3 4
6
2 5
Rolling
direction
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Instrumentation and Analysis
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• 737 after landing (all wheels down)
– Centerline bolt strain gages
1
3 4
6
2 5
Rolling
direction
Time (s) Time (s)
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Torque to Failure
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1500 lb
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Instrumentation and Analysis
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• Simultaneous response at centerline and touchdown zone
CL
TDZ
1-V Rolling
direction
Collar Strain
Gage
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Instrumentation and Analysis
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• Centerline horizontal laser transducer
– Approximately 0.006 in. maximum movement
1-V Rolling
direction
Collar Strain
Gage
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Instrumentation – Continued Work
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• Plan to instrument three additional light fixtures
– Two runways, one taxiway
– Data acquisition targeted for late November
• Currently working on pavement and light fixture
FEA to further evaluate/validate instrumentation
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LPI Evaluation Findings
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LPI Evaluation Findings
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• Mechanical performance of fixture assemblies
– Aircraft loading
– Light fixture installation
– Maintenance
• Benefit to increasing torque value (preload) of light fixture
support ring bolts
– Recommended torque value should be determined experimentally
– Maintenance of extension collar to base can bolts
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LPI Evaluation Findings
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• Light fixture and pavement act as a system
– Direct wheel contact not required for strain response
– Continue investigation into light fixture/pavement interaction
• Requires cooperation between airports, FAA, suppliers
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Questions?