Commercial air transport passenger safety and survivability, in the event of an impact-survivable crash, are subjects receiving increased technical focus/study by the aviation community. A B-720 aircraft, highly instrumented, and remotely controlled from the ground by a pilot in a simulated cockpit, was crashed on a specially prepared gravel covered impact site. The aircraft was impacted under controlled conditions in an air-to-ground gear-up mode, at a nominal speed of 150 knots and 4-1/2 deg glide slope. Data from a number of on board, crash worthiness experiments provided valuable information related to structural loads/failure modes, antimisting kerosene fuel, passenger and attendant restraint systems and energy absorbing seats. The development of an energy absorbing (EA) seat accomplished through innovative modification of a typical modern standard commercial aviation transport, three passenger seat is described

Alfaro-Bou, E.

Eichelberger, C. P.

Fasanella, E. L.

English

NASA Technical Reports Server

N85-33515
DEVELOPMENT OF AN ENERGY-ABSORBING
PASSENGER SEAT FOR A TRANSPORT AIRCRAFT
Charles P. Eichelberger and Emilio Alfaro-Bou*
Edwin L. Fasanella**
INTRODUCTION
Commercial air transport passenger safety and survivability, ir_ the event
of an impact-survivable crash, are subjects receiving increased technical
focus/study by the aviation community. One such study was recently initiated
by a Joint understanding of the Federal Aviation Administration (FAA) and the
National Aeronautics and Space Administration (NASA). A B-720 aircraft,
highly instrumented, and remotely controlled from the ground by a pilot in a
simulated cockpit, was crashed on a specially prepared gravel-covered impact
site. The aircraft was impacted under controlled conditions in an
air-to-ground gear-up mode, at a nominal speed of 150 knots and 4-i/2 ° glide
slope. The flight test was performed at the NASA Ames Dryden Flight Research
Facility at Edwards Air Force Base, California.
Data from a number of on-board, crash-worthiness experiments provided
valuable information related to structural loads/failure modes, artimisting
kerosene fuel, passenger and attendant restraint systems and energy-absorbing
seats. This paper describes the development of an energy-absorbing (EA) seat
accomplished through innovative modification of a typical modern-standard,
commercial aviation transport, three-passenger seat. Values are given in both
S.I. and U.S. Customary units. The EA seat development effort was carried out
at the NASA Langley Research Center and tested on the B-720 aircraft used for
the FAA/NASA Controlled Impact Demonstration Test.
SEAT SELECTION
The commercial transport passenger seat selected by NASA Langley for
modification to an energy-absorbing collapsible seat was a triple passenger
seat manufactured by the Fairchild Burns Company, Winston Salem, North
Carolina.
The seat (trade name Airest 2000 m) was deemed typical of the designs now
employed by commercial airlines on modern air transports. The seat's basic
structure consists of a rectangular shaped frame fashioned from nominal
*NASA Langley Research Center, Hampton, VA
a*Kentron international, _mpton, VA
*Identification of commercial pr.ducts and companies in this paper is used to
describe adequately the EA seat development. The identification of these
commercial products does not constitute endorsement, expressed or implied,
of such products by the National Aeronautics and Space Admlnstration or the
publishers of this paper.
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4.45-cm (1 _/4-in.) outside diameter aluminum tubes. Four forged aluminum
legs, bolted to the frame and stabilized by two aluminum diagonal- and
_a:e-r_ _ r_t_,ch r:e<:_-s, _upport the seat and its occurants. Plastics plus
llght_e:gnt c_a ions and covers complete the outer appearance. Weighing about
25 kg '_,c_lb,,the seat possesses exce]lent strength-versus-weight qualities.
Figure i l!iJ*_t:_,e_-,_ Fronta] view of the Airest 2000, while Fig. 2 displays
the structu,__ :e',tu_es.
DESIGN GOAL
_he deszgn goal in the development of an energy-absorblng passenger seat
for a transport alrcraft was to protect the passengers, simulated by three
75-kg (165 ib) anthropomorphic dummies, against both a vertical and
longitudlnel velocity change of 6.40 m/sec. (21 ft/sec). Each of the three
dummies represents a 50th percentile passenger, defined in reference i.
Figures 3 and 4 present graphic data pertaining to human tolerance
acceleration limits for vertical and horizontal motion (reference 2).
SEAT MODIFICATION DESIGN
Several innovative design changes were featured in converting the standard
triple passenger seat (Fig. 5) to an EA unit. To limit acceleration the seat
was rectified (Fig. 6) so that it would rotate forward under high load. To
allow seat rotatlon, split-sleeve-type bearings were affixed to the upper ends
or the rear legs and a combination nylon bearing, block-steel yoke unit was
installed on the upper ends of the forward legs. All bearings sufficiently
! encircled the seat Frame tubes to allow rotation and yet withstand the impact
loads. Conventional aircraft bolts were employed as hingeplns to permit pivot
: rotation of both the forward and rear legs at their lower seat rail
attachment. These changes effectively converted the standard seat to a
four-bar linkage system. The original flexible membrane seat pan was replaced
: with a O.079-mm (1/32-i[,.) thick aluminum sheet. Both forward and rear legs
were ir_cllned parallel to each other at an angle approximately 65° with the
ho_ Jzontal. Two graphite-epoxy, energy-absorblng tubes, installed diagonally
between the forward and rear seat legs (Fig. 6), ceplaced in the original
diagonal members to stabilize the seat under normal flight conditions. The
tubes _rogre_sively crush as axial columns during the aircraft impact. Figure
5 di_pl_tys a Araphic cross section of the unaltered standard seat. Figure 6
depicts the same cross section of the modified seat for comparison. The
energy-absorbing, graphite/epoxy tubes, associated tube attachment hardware,
and overall seat dimensiol_s are shown. Note that the passenger seat pan
height above the aircraft floor level and buuy posture angle are the same in
both standard and moOified designs. Figure 7 graphically traces the kinematic
stroke of the seat during the energy-absorbing process. The stroke is limited
by hardware constraints to a maximum distance of about 16.5 cm (6.5 in.).
ENERGY-ABSORBING TUBE DESIGN
Various design characteristics were considered when selecting the
energy-absorber device. The energy absorber should be lightweight, small, and
as simple as possiote to oe cost effective. It should possess long-term
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reliability, require minimum maintenance, and be corrosive/envlronmenta t
resistant, and unaffected by vibration. D_ost importantly, the Qevlce must
decelerate the pa._sengers while not exceeding the load limits of human
tolerance.
Graphite epoxy crushab'e tubes were chosen to meet the requirements for
the energy-absorbing process. Since the energy absorption is a function of
the materials (fiber and matrix) and ply orientation, the tubes could be
readily tailored to absorb the dummies' kinetic energy. A series of
developmental tests plus the knowledge gaiped from a recent study (reference
3) resuited in the selection of i0- and 12-ply graphlte-epoxy tubes 21.08 cm
{8.30 in.) in length to balance the uneven seat weight distribution caused by
lack of symmetry between the seat legs and the occupant se_ting positions.
The tubes were fabricated from the prepreg materlal of Thornel 300 graphite
fibers and Narmco 5208 matrix composed of an MY720 epoxide base. F_ch ply had
a nominal O.Ol40-cm (0.0055-in.) thickness and a ply orientation of +60 °.
Data from investigations (refernce _) indicated changes in energy-absorblng
values from 90° to 45° ply orientation; thus, 60 o was selected for size and
strength considerations. A taper and circJiar notches (Fig. 8) were machined
on one end of the tubes to reduce an initial high load spike without affecting
the sustained crushing load.
During the development phase of the composite tubes, static and dynamic
_ests were conducted to determine the crush load-dlsplacement characteristics
and the deceleration-time response of the tube. The static tests wer_
conducted on a 533,786-N (120,OO0-ib) capacit, compresslon-testlng machine.
E_ring the static tests, a cap was placed on the tapered end of the tube and a
plug was inserted in the flat end to simulate the effect of the cap and plug
used in th seat mechanism that holds the tube in place. The crush force-
displacement curve for a 6-ply composite tube is sDown in Fig. 9. Figures I0
and II display the static tests results for the iO- and 12-ply tubes used in
the seat. Although the proper initial force Ievel was achieved, the 12-ply
tube crushing load was found to inc,'ease at a high rate after _ in. of crush
{Fig. ii) because the crushed material filled the remaining volume of the
tube. A larKer-diameter tube with Iess plies wouid have been more deslrabIe
but would have required making new end caps.
The dynamic tests on the tube were performed ,n a drop-tower impact-test
machine. A weighted head of approximately 77 kg (170 lb), guided by two
rails, was dropped vertically on the tube. D_celeration of the head was
measured using accelerometers _,nd was recorded on a strip-chart recorder. A
typical deceleration-tlme curve for a 6-ply-tube dynamic test is displayed in
Fig. 12. Head-impact velocity was 4.52 m/sec (I_.83 ft/sec), resulting from a
drop height of 1.04 m .3.42 ft). Pulse time was about 70 ms. Deceleration
ranged between 6 and 12 g. Compacted crushed material, filling the remaining
volume of the tube during the tube-crushing energy-absorbing process,
accounted for the deceleration g buildup. About 21-era (6.4-in.) length of
tube crushed in absorbing the bead's kinetic energy.
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STATIC AND DYNAMIC SEAT TESTS
Static Seat Test
With the graphite-epoxy tubes in place on the assembled seat, a problem
arose during static loading tests conducted to determine maximu, stroke
di3tance. A local bending condition at the ends of the graphite-epox:, tubes
prevented uniform axial crushlng and resulted in tube failure. 1_e problem
was :_olw;d by the installation of an axial alining pivot cap and plug
attachment at each end of the tube. As the seat stroked do_mward and forward,
t_ pivot attachment mechanisms caused the tubes to crush uniformly in an
axial compressive mode.
lhe utilization of this mechanism has several possibllities. The
energy-absorbing device aligns only in one direction, _ince the seat had a
misalignment problem in only one plane of rotation. A bali Joint could be
used for multldirectlon misalignment if necessary. It should provide a useful
tool to other designers working with compressively loaded energy absorbers
thac require the dual characteristics of movable Joints and fixed axial
aJignment conditions. This application is ideal for composite tubes which
crush in a brittle mode and are thus difficult to control.
In actual use, the seat is unevenly loaded because of lack of symmetry
between t_, seat legs location and the occupant seating posltlons. The
outboard legs sustain twice the laod of the inboard legs because cf the
offset. However, this effect was nullified during static seat test by
locating the hydraulic ram symmetrically between the seat legs. For this
symmetricall} loaded seat, both composite tubes were 6 ply. Four static tests
were ;erformed to check the operation of the seat mechanism. The applied
vertical load (hydraulic ram), and the vertical (Z) and horizontal (X) floor
reaction forces for a typical test are shown in Fig. 13.
_Dynamic Seat Tests
fhe assembled seat, with installed I0- and 12-ply EA tubes, was
dynamically tested at the Langley Research Center's D?namic Impact Test
Facility. A series of dynamic drop tests were conducted to simulate the
vertical and forward expected shock pulse charaet-.ristics. The drop tests
were accomplished by mounting the seat at a 45" tilt angle to the horizontal
flat surface of a steel carriage. The carriage structure basic%fly consisted
of t_o horizontally positioned, built-up beam units, connect..d by _teel pldce
members. The carriage, with the 45" tilted seat containing the three
anthropomorphic dummies, was then raised and dropped vertically. Impact pulse
was controlled by permitting the falling carriage to strike and deform a
series of steel bars at ground level. The dummies were restrained by lap
belts and were positioned leaning forward with their head between their legs
in a crash preparation body posture,. Data acquisition consisted of
accelerometers, load cells extensiomet_rs, and nigh-speed motion picture
cameras. This method of dynamic impact testing was based on knowledge and
techniques established by many previous experiments performed at the Langley
Research Center (see reference #). Figure I_ illustrabes an actual stroked
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1985025199-052
seat which resulted from these shock tests. About 12.7 cm (5 in.) of each
composite tube crushed from the 4.27-m (14-ft) drop height, in absorbing the
dummy passenger kinetic energy. Carriage input pulse plotted with the
resulting pulse on the seat pan is shown in Fig. 15. Accelerat'on levels
remained within the range of human tolerance levels (see Fig. 1 and 2). Data
analysis of the seat from the Boeing 720 crash had not been completed at the
time this paper was prepared.
CONCLUSIONS
This investigation applies the concept of an energy-absorbing, axial,
tube-crushing unit to commerlcal aviation passenger transport seats. The
investigative effort was limited by the primary schedule of the Controlled
Impact Demonstration Project which prevented optimizing the energy absorbers.
For optimum seat performance, further development of the energy absorbers is
recommended.
Such parameters as belt restraihts, varying passenger seat weight loads
and their distribution, seat rebound, and economic aspects must be examined.
Preliminary results from the work conducted so far are encouraging. The
desired objective of demonstrating the concept/feasibility of converting a
standard commercial passenger seat to a axial, tube-crushing, energy-absorbing
unit as an additional crash safety feature has been attained.
REFERENCES
i. U.S. Code of Federal Regulations, Title 49, Chapter 5, P_rt 572:
Anthropomorphic Test Dummy, U.S. Government Printing Office, Wacnington,
D.C., (Rev.) 1978.
2. Eiband, A. M.; Human Tolerance to Rapidly Applied Accelerations: NASA
Memorandum 5-19-59E, June 1959. Data curves as published in Aircraft
Crash Survival Design Guide, Volume II, USARTL-TR-79-22B, January 1980.
3. Farley, Gary L.: Energy Absorption of Composite Materials. NASA
TM-84638, March 1983. Also, Journal of Composite Materials, vol. 17, May
1983, pp. 267-279.
4. Alfaro-Bou, Emilio; Williams, M.S.; and Fasanella, E. L.: Determination
of Crash Test Pulses and Their Application to Aircraft Seat Analysis: SAE
Paper 81061, Business Aircraft eeting and Exposition (Wichita, Kansas),
i April 7-10, 1981.
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Development of an energy-absorbing passenger seat for a transport aircraft

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