By STACEY O. MORRIS, P.E.
Tetra what? Tetrahedron.
Per Webster’s Dictionary, a tetrahedron is defined as “a solid contained by four plane’s faces; a triangular pyramid.” If this sounds like it is getting into too much higher math for you, you are in good company.
But, if you are an airplane pilot, especially a pilot with a little more experience (i.e. of more advanced years), you may have seen one of these older style wind-direction/runway-in-use indicators at one of the airports you visited. While airport tetrahedrons were more common in the past, a few airports still have working examples today.
Tucked away in northeast Arkansas is the city of Walnut Ridge. While not a large city, Walnut Ridge has a rich history. The present city was formally established in 1875, not far from a previous area settlement known as Old Walnut Ridge (dating back to about 1860), when railroads began serving the area. Walnut Ridge has remained a rural community since, but has had significant events over the years that have shaped the nature of the community.
One of those events was the construction of an airfield just north of the city in 1942. The airfield was used to train pilots during World War II. After the war, the United States government deeded the airfield to the City of Walnut Ridge. The airfield was designated as Walnut Ridge Regional Airport (KARG) and has been in continuous operation since as an important component of the National Airspace System. In 2017, the airport celebrated its 75th anniversary.
One of the airport’s visual aids for pilots taking off or landing is a tetrahedron located in the infield, just across the aircraft parking apron from the terminal building.
In 2014, the Walnut Ridge Airport Commission entered into a contract with ETI Corporation, of Memphis, Tennessee, to rehabilitate the tetrahedron. The historic tetrahedron had been maintained by the airport, but was in need of a little “tender loving care.”
The Tetrahedron was showing signs of its age. The sheet metal exterior was coming apart and the interior structure was rusting badly.
Once the construction project got underway, C.L. Clark, president of Clark General Contractors, disconnected the tetrahedron from its base and moved it to the West Machine Shop to begin the rehabilitation process.
The first order of business was removing the existing “skin” so the underlying structure could be inspected and repaired or replaced as deemed necessary. It was important to retain as much of the historical structure as possible.
The next step was moving the skeleton inside the machine shop to begin the tedious inspection and repair work.
After the tetrahedron skeleton had been rehabilitated, new aluminum skin was installed and the tetrahedron was moved back to the airport. New bearings were installed on the base and the rehabilitated tetrahedron put back on the base.
Gone were the old concrete cylinder counterweights, replaced with weights specifically designed and produced to be used as counterweights. It is important that the tetrahedron be properly balanced on the base so it will swing freely with the wind.
The new skin for the tetrahedron was painted orange on the left side and light blue on the right side and rear. The purpose of alternating the colors was to make it easier for pilots to identify which direction the tetrahedron is pointing.
Additionally, lighting was added to the tetrahedron. The original tetrahedron depended on lighting provided by area incandescent lighting. The new LED lighting was installed on the tetrahedron itself. Red lights were used along the left side of the tetrahedron and blue along the right side, similar to navigation lights on aircraft.
Rehabilitating the tetrahedron at Walnut Ridge Regional was a unique, rare and unusual project. I am very thankful to have been given the opportunity to be Engineer of Record.
Looking for a good place to chase that $100 hamburger with your friends and family? Besides the tetrahedron, a unique on-field restaurant, and friendly staff, the Wings of Honor museum honoring those who served in World War Two make the visit worthwhile. Check out the Beatles on the Ridge festival as well.