Boeing Bump Index
This article describes the limitations of using the Boeing Bump Index (BBI). APR has recently developed a new tool that combines the best features of BBI with the best features of APR’s aircraft simulation technology. We call this new tool BBI + and it solves the inherent limitations of BBI. For more information on BBI +, please visit our BBI + page.
The Boeing Bump Index – Additional Methodologies Recommended
By Michael Gerardi
The Boeing Bump Index (BBI) is a tool frequently used to evaluate pavement roughness on an in-service runway. This tool is commonly associated with the FAA Advisory Circular FAA AC-5380-9. This article was written to help readers understand the limitations of the BBI and to demonstrate that relying exclusively on this tool could result in misdiagnosing a potentially serious runway roughness problem. This article will illustrate two examples where the BBI failed to accurately characterize roughness. The first is in a single roughness event that consistently produced pilot and passenger complaints. The second example demonstrates the BBI’s inability to accurately characterize multiple-event roughness. While this article illustrates some critical flaws in the BBI, the FAA’s attempt to provide the industry with some relevant guidance on runway roughness should be appreciated. However, we believe that the results of the BBI are being accepted without further consideration, and as demonstrated in the following article, could lead to additional concerns. When evaluating ride quality issues, the BBI should not be the only method or technology considered.
The basics of the BBI is that it simply compares a roughness event’s wavelength verses its amplitude and categorizes that combination as either “Acceptable”, “Excessive” or “Unacceptable”. This technique was originally used by the Boeing Commercial Airplane Company to help its customers with ride quality issues. In 2009 it was adopted by the FAA as an initial attempt at providing the airport pavement community with guidance on how to characterize airfield pavement roughness.
Due to its nature, the BBI is only effective at characterizing some single-event roughness; single event being a single bump or dip of any wavelength or amplitude. The BBI methodology can be used to categorize that single event in one of the three categories (Figure 1). While this method can be effective in some cases, due of its relative simplicity, it can also misdiagnose roughness events.
Figure 1. The criteria established by the Boeing Bump Index is often represented with this chart. With this chart, the bump’s amplitude is plotted verses its wavelength.
In over 20 years of analysis, we at APR have found that a large number of variables must come together to determine whether or not a runway profile shape can have an adverse impact on aircraft ride quality. For example, let’s look at the single bump/dip event that the BBI is capable of categorizing. We have found that the aircraft’s response will largely be determined by the location of that event on the runway, and the speed of the aircraft when the event is encountered.
SINGLE EVENT ROUGHNESS
To illustrate this point, let’s look at a project in which APR was asked to evaluate an area of roughness that routinely produced pilot and passenger complaints. Figure 2 is a plot of the measured profile, in this case plotting only the first 3,000 feet past the start of the runway. The area of concern is located just past the 1,000-foot mark. When evaluated with the BBI, it was found to be “acceptable” (Figure 3). However, when evaluated using APR’s Aircraft Simulation technology, a Boeing 737-800 simulating a 100-knot constant speed taxi found that this event produced 1.42g at the Pilot’s Station and .79g at the aircraft Center of Gravity (Figure 4). Considering that this event consistently produced pilot and passenger complaints, APR’s simulation results seem to characterize this event more accurately than the “acceptable” rating produced by the BBI.
Figure 2. The plotted profile of an event that consistently produces pilot and passenger complaints.
Figure 4. Aircraft simulation of a Boeing 737-800 performing a 100 knot constant speed taxi.
Figure 3. The Boeing Bump Index as computed within ProFAA characterizing this runway as “acceptable”.
MULTIPLE EVENT ROUGHNESS
It has been APR’s experience that in most cases where aircraft response is most severe multiple event roughness exists whereby there are multiple bumps and or dips in succession. This is best shown in the case where multiple event roughness is embedded in the first 1,000 feet of the runway. In the following video, the aircraft is responding in pitch (illustrated by watching the aircraft’s nose landing gear). When evaluated with BBI, you can see that the roughness is characterized as “acceptable” (Figure 5).
Considering the location and the speed of the aircraft at this location, I would say that this characterization is fair. When assessed with aircraft simulation only mild aircraft responses are predicted; just a few areas exceeding the .40g threshold acceptability (Figure 6).
Notice how the aircraft responds in pitch as it encounters multiple event roughness.
Figure 6. Aircraft simulation predicting mild responses to this area of multiple event roughness.
Figure 5. Multiple event roughness at the beginning of a runway.
EVENT LOCATION IS VERY IMPORTANT
What would happen if that series of roughness events were encountered at a different location on the runway? A spot where the aircraft was traveling at a faster speed? Using APR’s profile editing tool this section of profile was moved down the runway, and was inserted at 2,000 feet past the start of the runway. As you can see in Figure 7, accelerations were predicted to be 1.07g for the aircraft’s Pilot’s Station, and close to .95g at the aircraft’s center of gravity. This shows that the aircraft’s speed at encounter is critical when predicting the aircraft’s response. It has been APR’s experience that this level of aircraft response would certainly produce pilot and passenger complaints. However, as shown in Figure 8, even at this new location the BBI still characterizes the event as “acceptable”.
As this article illustrates, there are some significant issues to consider when using the BBI to accurately assess a ride quality problem. It is our opinion that the BBI should not be the only evaluation method used to quantify suspected ride quality concerns. As demonstrated here, due to its nature the BBI is not effective at accurately characterizing some types of pavement roughness. We applaud the FAA for taking this first step at developing some criteria for roughness, however we do not believe that this is the final solution.
Figure 7. Same multiple event roughness. This time located at 2,000 feet.
Figure 8. Regardless of location, the BBI still characterizes this event as “acceptable“.
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