Engineering Updates/ Information

EB 83A develops the bolt clamp loads necessary to hold the in-pavement light and base together. As the aircraft get larger and heavier, they cause higher horizontal shear forces on the light fixture. This requires additional forces to keep the light fixture securely fastened to the light base.

Bolted joints are either in tension or shear or a combination of both. The in-pavement bolted joint is classified as a slip critical or friction type joint. The forces that pull apart this bolted joint are the shear forces of the aircraft landing/braking/rolling over the light fixture. The forces that keep the bolted jointed together are the normal forces – the weight of the aircraft provides most of the normal force, leaving the remainder of the clamp load to the bolts. Both of these normal forces exert pressure on the in-pavement light fixture causing friction between the faying surfaces (fixture/pavement ring/spacer rings/light base top flange). The coefficient of friction between the faying surfaces is critical to secure the bolted joint. If this is too low there is not a bolt strong enough to work.

This coefficient is not calculated but both testable and measurable. It does not change with an increase or decrease in horizontal shear. The Research Council on Structural Components (RCSC) has guidelines for galvanized faying surfaces and they must be hand wire brushed to roughen the surface and provide Class A slip coefficient, μ=0.30¹ FAA testing of the slip coefficients are far below the assumed slip coefficient of μ=0.370²   
There are numerous other coatings that will increase the coefficient of friction well beyond the μ=0.370, some as high as μ=0.640 With the additional friction, the load from the bolts is lessened as well.

It is critical that a coating(s) on the faying surfaces on the stack provide adequate slip coefficient. Without this, the bolt clamp load will not be enough to meet the requirements of the in-pavement lights. A minimum slip coefficient should be a requirement in AC 150/5345-42.

Once this is solved, the airport then has to decide between carbon steel coated bolts or stainless steel bolts. Each has advantages and disadvantages. The next step is the torque necessary to develop adequate bolt tension. The 2 part wedge-lock washers cause additional installation friction. Our new Wedgelock™ bolts have the washers integrated to the bolt… or skip the torquing altogether and use our new TightBolts™.

MCB Industries is introducing the Wedgelock™ bolts. Because carbon steel bolts must be coated for corrosion protection, we are developing a coating that will completely cover the bolt and integrated washer. At this time, we will offer these bolts in stainless steel to ensure corrosion resistance.

Carbon Steel

• Mechanical and Material Requirements for Externally Threaded Fasteners
• Grade 2, Grade 5, Grade 8 but many others
• Must have external corrosion protection
  • Zinc corrosion protection
  • Hot Dip Galvanized
  • Mechanical plated
  • Electroplate
  • Flouropolymer coatings
  • Base coat - ceramic metallic
  • Top coat - PTFE
• Will corrode
• Additional corrosion 
• Protection required 
• Requires anti-seize
  • anti-seize pre-applied
• Easier to drill out if broken

Stainless Steel 

ASTM F593

• Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs 
• F593C, F593P but many others
• Will not corrode
• Additional corrosion NOT protection required
• Requires anti-seize
  • Anti-seize pre-applied
• Hard to drill out when broken

On December 26, 2018, the FAA published Engineering Brief 83A (EB 83A) which addressed “standard methods to be employed when using stainless steel or coated carbon steel bolts to secure light fixtures to L-868 light bases”.

MCB Industries, the manufacturer of high quality specialized bolts that fully meet FAA standards, has found that certain aspects of EB 83A are causing confusion among airport professionals. The following paragraphs clarify parts of EB 83A.

Allowable Bolts: EB 83A states that stainless steel and coated carbon steel bolts can be used for connecting extension rings or light bases to light fixtures. Any bolt that meets FAA standards can be used on any airport project.

Clamping Force and Torque Requirements: The value of 4,900 psi of clamp force was used for the example only (which is based on an Airbus A-380). Extensive conversations with the FAA clarified that this example was never intended to create a fixed value. Designing fasteners for an A-380 on every airport results in significant over design, increased costs, and tremendous difficulty in meeting the FAA’s requirement that the airport conduct bolt torque checks. It would also eliminate the majority of currently-approved suppliers and manufacturers from supporting airport projects. Full and open competition has always been the goal of FAA for the airport projects, and the FAA has confirmed that this was not the intent.

Slip Coefficient: EB 83A states a SAE J429 Grade 5 bolt as a given and assumes a slip coefficient of friction of 0.37 in its examples. However, other types of bolts with the corresponding assumed slip coefficient will also provide the required clamp load. The consultant should use the correct slip coefficient for its application (For other bolt consideration, please refer to Appendices C, D and E on MCB Industries website link below). The impact of simply using Grade 5 bolts with an assumed slip coefficient of 0.37 restricts the airport from using other options that would be better suited for that airport. This could result in significantly over designed fastener requirements, eliminating less costly materials that fully meet the design requirement. 

Brand Name Bolts, Washers, Etc.: The EB does not establish a sole-source or limited source for any of the fasteners used in airport in-pavement lighting.  Any item that meets the technical standards are acceptable for use. MCB’s lock washers and integrated WedgeLock™ bolts meet these standards. Using sole-source or limited source materials is not allowed in federally-funded grant projects, and in many cases is prohibited at a state or local level.

MCB Industries, worked with Congressional, FAA,  and industry professionals to resolve the EB 83A areas of confusion. Clarifying these areas of confusion is important because of the inherent risks associated with connection over design, the tremendous cost with maintaining bolts or fasteners that are significantly over designed for an airport. MCB Industries looks forward to the opportunity to discuss this with any member of Airport Consultants Council or other organizations involves within the industry.