Stainless Steel Bolts Meet and Exceed EB 83A Requirements

In 2014, a runway light came loose as a plane took off and the light was impaled in the plane's underbelly. It was still there when it landed.  Obviously this raised many concerns. The FAA started an inquiry and focused on the bolts that hold the light fixture to the light base. They did testing at Intertek, looking at the bolt strength, the horizontal shear, the compressive loading, vibration and corrosion testing. The results were reported at the 2017 IESALC Fall Conference-Dallas in the Monday Afternoon papers titled Final Presentation 10/23/17 Jeremny-2. On page 34 of this document are the horizontal shear test results:

•  Did not successfully support aircraft loading as a slip-critical connection.

        (None of the bolts tested, Grade 5, Grade 2, F593C (SS), F593P(SS) kept the bolted joint from slipping.)

•  Spacer rings drastically reduce the slip coefficient in all cases.

•  Slip coefficients with aluminum light fixture generally better than with ductile iron ring.

DISCUSSION POINT:

     Is this bolt connection a slip-critical connection or a bearing connection ?

(A slip-critical bolted joint relies on the friction between the faying surfaces to keep the joint from sliding. A bearing joint allows the pieces to slip until they come in contact with the bolts in shear. EB 83A considers the connection to be a slip-critical (friction) joint.)

Though the report presented at the 2107 IESALC Fall Conference, showed all bolts failed as a slip-critical bolted joint,  EB 83A was published on December 26, 2018. 

EB 83A deemed Grade 5 bolts adequate. How could this be ? All the testing showed the Grade 5 bolts inadequate. The FAA ASSUMED a Coefficient of Friction (CoF) of .37. Where did this come from ? There are no standards or organizations that support this.

     µ = .37     Hot Dip Galvanize (HDG)      Assumed value from EB 83A

     µ = .14     Hot Dip Galvanize (HDG)      FAA test results 3 spacers average

     µ = .16     Hot Dip Galvanize (HDG)      American Galvanizes Association

     µ = .19     Hot Dip Galvanize (HDG)      ASTM A 325

     µ = .30     Hot Dip Galvanize (HDG)     Federal Highway Administration 

     µ = .30     Hot Dip Galvanize (HDG)     Research Council Structural Components              

When asked how the FAA came up with .37, the response was we think we can get there.

When asked how ?

We think we can get there.

How ?

We think we can get there.

Round and round we go. 

Over many months and after several phone calls  trying to the get answer to how, Senator Ron Johnson's office transmitted the concerns about EB 83A on July 15, 2019.  For the complete response from the FAA download the file 2019 FAA Response to Senator Johnson from the list below. 

Some interesting statements from this response:

Five  different things must happen simultaneously, and that probability is minimal, so it is OK to assume a slip-coefficient  higher than any other industry. Wouldn't those same five things not happen with the F593C bolt as well ?

Requiring the base can vendors and the light manufacturers to modify their manufacturing process  to increase safety would be too costly.

The FAA is proposing additional testing.

As a result of this exchange of information there were no changes.  And that was the end of that.

ON TO MORE CORRESPONDENCE

On December 13, 2019, we sent another letter with additional research. It was sent to Mr. Khalil Kodsi. See 2020 MCB Letter to FAA Khalil download below. 

This letter was to follow up on the additional testing the FAA was planning to do. Also included was testing done at Intertek in early June of 2018. See Intertek Report No. 103473607CRT-001 labeled Galvanize vs. Thermal Super Grip Coatings in the section labeled Industry Documents.

On Page 17 of this report, the test configuration was HDG faying surfaces and the "SuperBolt". This bolt is manufactured from  a custom stainless steel that exhibits corrosion resistance similar to that of Type 304 but with three times higher yield strength, with an ultimate tensile up to 180,000 psi and a yield strength up to 150,000 psi. Because this bolt is stronger than a the A36 top flange material, we paired it with Grade 8 nuts. The results were disappointing, only achieving 8,500 lbs. of force before slipping.  If a bolt much stronger than the Grade 5 bolt failed how could the FAA consider the Grade 5 bolt adequate ?  (This test information was also shared with the FAA before the publication of EB 83A)

The response from this introduced a new justification. See 2020 FAA Kahlil response to MCB below. 

"In actuality, the full maximum weight of a fully loaded aircraft in a locked wheel situation rarely occurs. Aircraft generate full maximum weights only upon take-offs, which would be half the operations worldwide. Because of modern aircraft Anti-Skid Brake Systems (ASBS's), full application of brake pressure/torque, while being directly located on a light fixture, is an extreme condition, and is applied a very short time duration based on aircraft speed. If the maximum load is applied for a fraction of a second, at a lower friction value, we doubt there will be an issue. This is our justification for allowing lower faying surface "mu" assumption."

Because the calculations are based on a sustained shear force applied to the light fixture, they do not apply ? 

So if a truck is stuck on the train tracks, the train should speed up. If it is fast enough, the truck will never feel the force. This is absurd.

This statement also contradicts the earlier response to Senator Ron Johnson's office. In that response the FAA raised the slip coefficient (mu) to (µ) = .37. In this response they lowered µ = .37. Obviously, µ = .37 is the Goldilocks number, barely high enough for the Grade 5 bolts to show as adequate, and too low for any other bolt to show as adequate. 

Sounds like the FAA admitted that this is not much of a problem. It can't be. It happens very rarely and they fixed it with a bolt that fails.

ON TO MORE CORRESPONDENCE, AGAIN.

In hopes that a new manager, someone without a vested interest in EB 83A, on December 09, 2020 we sent a letter to Michael Meyers, the new manager of AAS-100, Airport Engineering Division. 

(See 2021 MCB to FAA Meyers download below). It restated all of the prior concerns.

On April 20, 2021 we received a response (See 2021 FAA Meyers to MCB download below).  

In the response, the FAA stated:

"The 0.37 value was back-calculated using the Grade 5 bolts to determine the required fraying surface µ." 

BACK-CALCULATED ? Does no one see a problem with this ? Start with the bolts you want and then calculate the friction value you need and then, don't test, just assume that it works ???? 

Using the simple formula:

 Total Bolt Clamp Load = Horizontal Shear Force/µ

Grade 5  Bolts (all figures from EB 83A)

  29400 lbs = 10620 lbs/µ

  10,620 lbs/29400 lbs = .361

  µ = .361

Peanut Butter Bolts example   For smooth peanut butter assume 20 lbs (30 lbs for chunky) clamp force per bolt.  Horizontal Shear from EB 83A 

  120 lbs = 10620 lbs/µ    

  10,620 lbs/120 lbs = 88.5   

   µ = 88.5

Following the FAA example from EB 83A, just assume the coefficient of friction is 88.5, and without any testing find peanut butter bolts adequate. Absurd. 

This response also notes that an airport can use other bolts besides the Grade 5 bolt. As long as the bolt can meet the the same strength considerations of the Grade 5 bolts. Which fail all testing

One note from this correspondence. 

There seems to be some confusion on the salt fog test.   

Reference IESALC  Technical Papers and Documents

     2017 IES ALC Technical Meeting  Dallas, TX

          01-Monday-2017

               Afternoon

                    Final Presentation 10-23-17 Jeremny 2

                          Pages 55-58

In the document 2021 FAA Meyers to MCB a note on the bottom of Page 2 under note 3:

" 3.  The intent of the salt fog test of the scuffed bolts to illustrate the importance of bolt inspections especially for reuse in certain environments. Based on the test results, it is not advisable to use a scuffed, coated bolt for installation."

The intent of scuffing the bolts was to determine how well the coatings stand up to mechanical damage. 

Look at this polymer coated bolt on the right. The loss of coating was due to mechanical damage. Because the bolted joint did not have sufficient friction in the HDG faying surfaces, the spacer rings and light fixture slipped into the bolt, knocking off the protective coating. Once the coating is damaged, the carbon steel bolt is subject to corrosion. Relying on the bi-monthly torque checks will not discover the corrosion to the threads due to insufficient friction in the bolted joint. 

In regards to EB 83A, there are NO tests that support the assumption that Grade 5 bolts are adequate, but there are many tests that refute this assumption. The correspondence relies heavily on the notion that the bolts will not see the forces because the duration is too short. 

To test that theory, a test was developed at Elite Electronic Engineering. The test would more closely reflect the actual condition the light fixture/light base bolted joint would experience if a plane landed and braked with that wheel over the fixture.

This test is referred to as the "Instant Shear Test". It can test different bolts, different shear forces, and different faying surfaces. 

The test reports and the test results are all available from the website.