Over the last several years, much has been made of the FAA’s first Advisory Circular on long-wavelength roughness, AC 150/5380-9. It strongly recommended, or even required, that the Boeing Bump method of roughness investigation be used on new airfield pavements receiving federal money. APR has assessed a number of runways using this method.

The Boeing method has been used over the last 20 plus years, primarily by Boeing, as a way to locate specific bumps or dips on a runway that may cause problems for operating aircraft. Using design limits of aircraft, fatigue life, aircraft response, and several other factors, Boeing derived this chart:

As shown in the chart above, bumps are classified as acceptable, excessive, or unacceptable based on bump length and height. How the bump length and height are calculated requires some understanding. First an imaginary straightedge is drawn between two points of the survey data. The bump height is the maximum deviation from that straightedge line. The bump length is the shortest distance from the point of maximum deviation to either end of the straightedge. The straightedge length begins at twice the survey increment (survey increment is a minimum of .25 meter or .82 feet) and goes up to 120 meters (394 feet). The figure below graphically represents how a bump is measured.

The Boeing method is helpful because it does not simply look at sections of pavement or the runway as a whole, but rather finds the exact location of the bump. It is also helpful because it considers long wavelength bumps, the type of roughness that impacts aircraft response. Finally, it is helpful because it is available in easy-to-use programs that calculate the Boeing Bump Index (among other things), such as ProFAA, which is free to download.

Where the Boeing Bump Index falls short is the fact that it only looks at single bumps. There is no scale for multiple bumps in succession. An aircraft will not see multiple bumps as single, separate events, but will actually respond to the events as a whole - encountering one event after another without the ability to dampen out its response. This multiple-event roughness causes the aircraft to respond non-linearly, and is the culprit of many pilot complaints of severe roughness on a runway.

In any case, the Boeing Bump Index and Advisory Circular 150/5380-9 are good ways to get a glimpse of roughness on a runway. It is also a good first step in determining, from a regulatory standpoint, “how rough is too rough?” To calculate the Boeing Bump Index, it is important to understand that a true profile is necessary. Inertial profilers (sometimes called profilometers) are meant to calculate indexes such as California Profilograph (PI) and International Roughness Index (IRI). In order to perform a study on a runway that examines bumps of longer wavelengths, it is necessary to capture all grade changes and wavelengths in the profile collection process. Many walking profilers, such as APR’s Auto Rod and Level, achieve the long wavelength accuracy necessary for the Boeing Bump method.

Once the data has been collected and examined, it can be used to calculate the Boeing Bump Index for each bump on the runway or taxiway. This can be done by plotting the bump on the chart above based on the method of measurement discussed, or ProFAA can be used by importing the file and clicking on Boeing Bump. A right click at any point of the Boeing Bump Index results will provide the user with the specific bump measurements and the Boeing Bump Index for that bump. It will also show the user if the bump is acceptable, excessive, or unacceptable.

To summarize, the following tools and knowledge are necessary to capture the Boeing Bump Index for an airfield.

Necessary:
  -  Equipment such as a walking profiler or rod and level that captures all wavelengths and grade   changes.

  -  Knowledgeable technicians to capture the profile quickly and efficiently during a runway or taxiway closure.

  -  Software, such as ProFAA to calculate the Boeing Bump Index.

  -  A meaningful way to present the results to the pavement owner and or contractor.

Helpful:
  -  An understanding of the potential impact of multiple roughness events in succession.

  -  Available tools to get a deeper understanding when roughness is discovered.

  -  A method of optimizing repairs to areas that are discovered to be too rough.


Please contact us with questions or to have your runway analyzed using the Boeing Bump method.