Aircraft seating today (military or civilian) usually suffers from two complex but solvable problems: comfort (lack of) and impact or ejection protection (lack of).

“...aircrew suffer from backache twice more frequently than groundcrew, pilots suffer more than other aircrew members and that ejection seats caused more backache than other types of seats.” (ref. 8)
This discomfort can also add up to more than a temporary inconvenience.

“...a potential cause of flight error or accident as a result of inattention or mental diversion is suggested by descriptions such as ‘fatiguing’, ‘irritating’, 'distracting’, which are frequently used by aircrew when describing their complaint.” (ref. 9)
Part of the problem can be explained by the materials used.

“Excessive use of soft cushioning is a common fault in flightdeck and helicopter seats. This type of seat may appear to be comfortable to the casual occupant, but after an hour or so the material begins to ‘bottom’ under the load and the pelvis gradually sinks towards the floor of the seat pan.” (ref. 9)

The main explanation for low back pain, sciatic pain and leg and feet numbness experienced in aircraft (or any other) seating is the poor posture that results from poor seating design. Most seating forces the lower lumbar curve flat, which is not its natural, optimum position. This flattening of the lumbar curve (decrease in lordosis) creates an uneven pressure on the discs in the lumbar region of the spine and hence, the pain.

“It is now recognized that a common cause of this postural low-back pain is posterior protrusion of a degenerated fourth or fifth lumbar intervertebral disc, which tends to develop when the lumbar curve is flattened,...” (ref. 13)
"...the change in disc pressure was due to a deformation of the disc when the lumbar curve was flattened.” (ref. 12)
The reason behind the flattening of the lumbar curve is a downward and back rotation of the pelvis that is forced by the configuration of the seat.

“When sitting, the glutei and posterior thigh (hamstring) muscles are stretched and they tilt the pelvis rearwards. This flattens the lumbar curve, reduces the lordosis and directs pressure on the anterior portions of the intervertebral discs.” (ref. 8)

“The vertebral column in a standing subject is straight in the anteroposterior aspect and curved in the lateral; there is a cervical lordosis, a thoracic kyphosis, and a lumbar lordosis. When sitting down, the knees and hips are flexed, the pelvis rotates backward, and the lumbar lordosis flattens. At the same time, there is an increased load on the spine as indicated by measurements of the intervertebral disc pressure.” (ref. 12)

“When moving from a standing to an unsupported sitting position, the lumbar lordosis decreases by an average 38 degrees. This occurs mainly by rotation of the pelvis, on average 28 degrees.” (ref. 12)

“The most important postural factor of low-back pain in sitting is decrease of the trunk-thigh angle and consequent flattening of the lumbar curve.” (ref.13)
The differences in disc pressure are caused by the flattening of the lumbar curve and result in low back pain which can be quantitatively measured.

“In standing, the disc pressure is about 35 per cent of the pressure in relaxed sitting without back support.” (ref. 11)

What is the solution to this seating design problem?

“The ideal seated posture of the lumbar spine would be the physiological posture assumed by an adult asleep on his side, with the trunk-thigh angle approximately 135 degrees.” (ref. 9)

“Disc pressure was considerably lower in standing than in unsupported sitting. Of the different unsupported sitting positions investigated, the lowest pressure was found in sitting with the back straight.” (ref. 11)

“An increase in lumbar support also resulted in a decrease in pressure.” (ref. 11)

“In the office chair, the disc pressure was also found to decrease when the back was moved towards lordosis.” (ref. 11)

“It is a common observation that people who sit in this type of almost right-angled chair tend to slide forward in the seat for comfort. This is because sliding forward increases the trunk-thigh angle, restores the lumbar curve, and reduces posterior protrusion strain on the lower lumbar intervertebral discs.” (ref. 13)

However, the above statements have met with some disagreement upon further investigation. Other studies have shown that while returning the curve to the lumbar region with pelvic rotation is essential, there is more to the solution than backrest angle or a lumbar pad.

“When the backrest inclination was increased, there were only minor alterations of the lumbar curve. The decrease in myoelectric activity previously recorded and the decrease in disc pressure are therefore probably mainly due to the increasing transfer of body weight to the backrest.” (ref. 12)

“...rotation of the pelvis by the posterior thigh muscles in the right-angled sitting position is a greater determinant in obliteration of the lumbarsacral curve than is the absence or presence of low-back support or the position at the knees.” (ref. 13)

Oregon Aero® Seating Systems achieve the appropriate pelvic rotation without relying on a trunk-thigh angle of 135°, which is physically impossible in most aircraft without major modifications to the seat frame or even airframe. Such modifications are prohibitively expensive. Oregon Aero® Seat Cushion Systems attain the desired posture correction with a pelvic wedge at the aft section of the seat bottom cushion. The wedge rotates the pelvis forward in pitch approximately 19° without any alterations to the seat frame. The results produced by this rotation can be seen in X-ray films taken with a standard equipment ACES II ejection seat cushion and the Oregon Aero® APECS® I Seat Cushion System for the ACES II seat. The radius of curvature of the lumbar spine with the original equipment seat cushion is 76 cm. With the Oregon Aero® Seat Cushion Systems installed, the radius dropped to 24.5 cm, which is much closer to the curvature of the erect standing spine. Thus, the lumbar curvature is restored, relieving the pressure on the lumbar discs and removing the lower back pain. The lumbar pad available with Oregon Aero® Seat Cushion Systems provides some support, but the majority of the work is performed by the seat bottom cushion.

Proper posture enhances safety along with comfort by increasing the spine’s ability to withstand the vertical loads generated during a crash event or ejection.

“...the importance of maintaining the curvature of the lumbar spine. This enables the spine to maintain its natural resilience and resist considerable vertical force....strengthens the spine in compression by 50%.” (ref. 7)

Of course, seat frame construction and cushion materials play important roles in impact protection. Oregon Aero has conducted and participated in hundreds of sled tests to help define the parameters for a General Aviation Aircraft seating system that will meet the FAA’s 19G crash specification. We have found one overall design principal that works; remove all materials or structures that have any spring at all. Any foam or structural material/design that has any spring to it will amplify impact loads.

“...the instant that a triggered ejection seat, for example, starts accelerating upward, the pilot does not start to move because the (cushion) foam is not yet compressed. By the time the seat reaches the pilot, it already has considerable upward velocity (with respect to the vertical axis of the aircraft). Thus, the pilot is impacted by a moving object, instead of being accelerated smoothly along with the object from the moment of rocket fire.” (ref. 5)

“...because the foam is resilient (fast-recovery), the pilot is then accelerated out of the ejection seat slightly by the ‘spring’ action of the foam. Once the foam is fully expanded, the pilot is no longer accelerated. However, since the seat is still accelerating, the pilot is impacted a second time, perhaps harder than the original impact. This jack-hammer effect may continue for several more impacts until either equilibrium is reached or seat acceleration is terminated.” (ref. 5)

“The torso, head and arms mass of a 50th percentile Type II Test Dummy (as required by 14 CFR (FAR) 23.562) totals approximately 72 pounds. When this figure is multiplied by 19Gs, the result is 1368 pounds of force, which explains the survivable, no cushion result. The laws of physics dictate that the result has to be 1368 pounds. The only possible conclusion is that a polyfoam cushion amplifies the loads transmitted to the test dummy...” (ref. 1)

Oregon Aero® Seat Cushion Systems are constructed with a variety of materials including visco-elastic rate damping foams. These materials act like shock absorbers as compared to common polyfoams that behave like springs. Visco-elastic materials slowly return to their original shape after an impact. The time required for the return is much longer than the 50 to 200 milliseconds of a crash or ejection. Polyfoams return instantly, creating the “jack-hammer” effect mentioned above. A visco-elastic cushion also demonstrated a 2°F temperature increase resulting from a 50 millisecond pulse, indicating that the cushion actually converts some of the kinetic energy of the impact to thermal energy. This process removes more potentially harmful force from the event.

“The results were 1428 pounds of force when the data was corrected to 19Gs. This eleven pound decrease in load from the 1439 result of the no cushion test demonstrates that the Oregon Aero conformal foam cushion actually absorbs some of the load even when used with a frame design that has a small degree of spring rebound.” (ref. 1)

“ ...a 19.2G impact and resulted in a temperature change from 69.8 to 71.8°F.” (ref. 1)

Test data is available which depicts impact test results with Oregon Aero® Seat Cushion Systems. Every shot resulted in lumbar loads of less than 1500 lbs when corrected to 19Gs. (ref. 2)

In summation, Oregon Aero^®SM seat cushion systems successfully improve comfort and safety through selection of materials and design for improved posture.
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“No adverse effects were noted by the pilots who flew the improved seat cushion in a tactical environment. The seat cushion did provide a dramatic improvement in comfort over the current cushion when used for extended periods of time. This is significant tactically in that the ferry legs could be compared to long point/area defense missions where CAPS could be established for extended periods of time. The recommendation from the pilots who flew the seat cushion is to equip all of our aircraft with them.” (ref. 4)

References
1) Oregon Aero, Inc. General Aviation Seat Design by Michael Tucker.
2) Preliminary Data Release, Foam Sandwich Seat Cushion Study by Dr. Steve J. Hooper and Mr. Dave Ellis, National Institute for Aviation Research
3) APECS® I (Advanced Performance Escape Cushioning System) Ejection Seat by Michael Dennis, President, CEO, Oregon Aero, Inc.
4) F-15 Flight Test Memo by Edward A. Klein, ACES II Engineering Manager, McDonnell Douglas, Inc. (ret)
5) Improved Comfort, Safety, and Communications for Aviators by Michael Dennis and Philip H. Mandel, Oregon Aero, Inc.
6) Increased Safety & Comfort for Aircraft Seats by Michael Dennis, Oregon Aero, Inc.
7) Pilot Safety and Spinal Injury by Dr. Tony Segal, M.B., B.S. Lasham Gliding Society, England
8) Backache in Aircrew by Wing Commander D.C. Reader, RAF Aviation Medicine Training Centre
9) An Approach to the Problem of Backache in Aircrew by Dr. J.G. Fitzgerald, RAF Institute of Aviation Medicine
10) Disc Pressure Measurements by Alf L. Nachemson, MD, University of Goteborg, Department of Orthopaedic Surgery I
11) Title Unknown by B.J. Gunnar Andersson, et. al., Sahlgren Hospital, Department of Orthopaedic Surgery I
12) The Influence of Backrest Inclination and Lumbar Support on Lumbar Lordosis by G.B.J. Andersson, MD, PhD, R.W. Murphy, MD, R. Ortengren, PhD and A.L. Nachemson MD, PhD., University Of Goteborg, Department of Orthopaedic Surgery I
13) Alterations of the Lumbar Curve Related to Posture and Seating by J. Jay Keegan, MD, University of Nebraska College of Medicine, Department of Surgery, Division of Neurological Surgery
14) Biomechanical Analysis of the Dimensions of Pilot Seats in Civil Aircraft by R.H.M. Goossens, C.J. Snijders, T. Fransen, Department of Product and Systems Ergonomics, Faculty of Industrial Design Engineering, Delft University of Technology, The Netherlands and the Department of Biomedical Physics and Technology, Faculty of Medicine and Allied Health Sciences, Erasmus University Rotterdam, The Netherlands