When you sit and think about it, the only real interface a passenger has with the takeoff or landing experience is the ride quality - will it be bumpy or will it be smooth? In most cases, the operation is conducted and nothing is said; it must have been a smooth ride. However, if the pavement produces poor ride quality, pilots and passengers typically speak up. The problem is that there are no official criteria which say what is too rough and what is acceptable. It remains a fairly subjective process.

APR uses aircraft simulation to predict how an aircraft will respond to the measured pavement profile. The aircraft model responds to the variation in the measured profile and reports that response in accelerations (g’s) and dynamic loading onto the pavement.

So, what do we consider too rough? To answer that question, APR uses a study published in Volume III of the Shock and Vibration Handbook, Chapter 44 “Effects of Shock and Vibration on Man” by D. E. Goldman and H. E. Von Gierke. In essence, this study placed human subjects on a shaker table exposing them to a range of vibrations at various frequencies. The examiners asked the subjects when the vibrations were perceptible, when the vibrations became uncomfortable and when the vibrations became unbearable. This study found that .40 g became a common point where vibrations became uncomfortable.

From this study, APR began using .40 g as a threshold for identifying areas of roughness. A typical plotted simulation plots the accelerations predicted for two locations of the aircraft; the Pilot’sStation , identified on the plot’s upper Y-axis with PSA, and the aircraft’s Centerof Gravity, identified on the plot’s middle Y-axis as CGA. Each of these acceleration oscillations are banded with two red lines denoting positive and negative .40 g. When the predicted accelerations reach or exceed those red bands, we consider the area to generate excessive response. Depending upon how great the predicted acceleration was - in terms of duration and peak acceleration, we will classify the response as either mild, moderate or significant.

All accelerations are not equal. That’s to say that it is much more common to experience higher accelerations in the front of the aircraft than it is to over the aircraft’s center of gravity. Because the pilot’s station is at the end of a long arm (distance from the main gear about which the aircraft pivots as it pitches up and down). Those in the front of the aircraft will get a rougher ride than those in the middle and back of the aircraft. As such, if the aircraft’s CG experiences .40 g, that section of pavement that produces that level of response is going to be rated worse than if only the Pilot’s Station exceeded the .40 g threshold.

I hope you found this article informative.  Should you have any questions or comments about APR's aircraft simulation capability, please feel free to contact us.