Helping airport operators make airfields safer
When I first heard the term “airport runway delethalization,” I rushed to my airport design and code manuals to find out what it meant. Are airports lethal? Is this a security issue I’ve overlooked in the past? Fortunately the answer was no to both questions.
Airplane cabins are designed to remain intact when passengers experience crash forces within human tolerance levels. This is called “crashworthiness.” Although crashworthiness is a good thing for aircraft, creating an environment that makes the crash survivable within the airfield design is imperative. Objects above ground, such as lighting or signs, are required to be “frangible”—i.e., have a weak point so they snap easily on impact.
However, buried objects present an entirely different challenge. When an aircraft excursion occurs—those rare but unfortunate situations when a plane skids off the runway before taking off or after landing—the plane’s landing gear can make deep ruts in the grassed area and come in contact with anything underground, such as foundations for signage and lights, stormwater structures, or electrical vaults. Buried objects may tear the landing gear from the aircraft, causing substantial and potentially lethal damage. Airport “delethalization,” then, can be described as making buried infrastructure safe.
A delethalized buried object on an airstrip before topsoil and final seeding.
Delethalization and Airport Design Guidelines
Making buried infrastructure safe has been recognized in the United Kingdom for many years. Delethalization principles are included in their Civil Aviation Authority document CAP 168 “Licensing of Aerodromes.” The graded area of the runway strip, plinths, runway ends, paved taxiway edges, and other elements should be constructed so there is no buried vertical face to arrest aircraft wheels in soft ground conditions. Transport Canada document TP 312 – “Aerodromes Standards and Recommended Practices” refers to this operation as “chamfering” and offers the same design concept. And in the United States, the FAA doesn’t reference delethalization specifically but does outline requirements for runway safety areas.
The International Civil Aviation Organization (ICAO) also considers delethalization a best practice, adopting it for all new construction. Existing structures are grandfathered until such time that it becomes mandatory.
How to Delethalize a Runway
How do we ensure that what we cannot see on our runways and graded strip areas don’t become life-threatening in the event of an aircraft excursion? The specific approach will vary based on local airport conditions and requirements, but generally the process involves
How to “delethalize” a buried object in an airfield.
- Identifying where the need exists. To delethalize a buried object, conduct a careful review of the graded strip area for specific risks such as catch basins, sign bases, intersecting taxiway pavement structures, and more.
- Determining the soil conditions around the buried object. In some cases, the soil will support the landing gear, meaning the wheels won’t sink in and hit the buried object. In other cases, the soil around the object can be replaced with a special soil preparation. The method used will depend on local soil conditions and the mass of the aircraft the runway is intended to serve. A proper geotechnical investigation will help in determining the best method of soil improvement.
The existing soil bearing strength can be determined from site-specific testing and, in Canada, reference to Transport Canada Advisory Circular AC 302-011, “Airport Pavement Bearing Strength Reporting.” Knowledge of the subgrade bearing strength is useful in designing the delethalization. However, it is not at all the same process. Pavement design using the various methods allows for unrestricted use by the design aircraft and/or a fatigue factor. Design of the delethalization area is based on a single use as it will be reconstructed if it ever ends up being used.
- Building a sloped granular soil structure around the buried object if the existing soil is unable to handle the aircraft load. The structure can be typically built from asphalt millings and an optional but recommended geotextile fabric (see diagram). Understanding what types of aircraft use the runway, and reducing the structure’s thickness for single use, can significantly reduce the structure’s design and construction costs.
The finished delethalization is covered with a light layer of topsoil and seeded and becomes a hidden safety precaution. One that we hope is never needed.
About the AuthorMore Content by Paul Walkington