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					                                                         Applied Ergonomics 31 (2000) 9}14




Biomechanical analysis of the dimensions of pilot seats in civil aircraft
                                    R.H.M. Goossens  *, C.J. Snijders  , T. Fransen
              Department of Product and Systems Ergonomics, Faculty of Industrial Design Engineering, Delft University of Technology,
                                                   Jawalaan 9, 2628 BX Delft, The Netherlands
  Department of Biomedical Physics and Technology, Faculty of Medicine and Allied Health Sciences, Erasmus University Rotterdam, The Netherlands
                                                    Received 22 January 1998; accepted 26 March 1999


Abstract

   The dimensions of pilot seats from "ve di!erent types of civil aircraft were measured and the results compared with existing
standards and biomechanical criteria. It was apparent that these seats failed to meet requirements, particularly in the e!ective depth
and inclination of the seat and in the height of the lumbar support and the armrests. Hence, none of these seats made it possible for the
pilot to establish a comfortable sitting posture. In comparison with aviation standards, the anthropometric dimensions were not
satisfactory, meeting only 4}7 out of 10 requirements. The dimensions based on biomechanics were even less satisfactory, meeting
only between 1 and 3 requirements out of 7.           1999 Elsevier Science Ltd. All rights reserved.

Keywords: Ergonomics; Aviation; Biomechanics; Seat




1. Introduction                                                                      can complaints reported by pilots be ascribed to certain
                                                                                     features of the cockpit seat? To answer this question an
   Complaints of discomfort and low-back pain during                                 anthropometric and biomechanical analysis was made of
middle- and long-range #ights were reported among traf-                              pilot seats in "ve modern middle- and long-range civil
"c pilots (Lusted et al., 1994). These complaints may                                aircraft.
cause a pilot to lose concentration and can thus a!ect the
safety of a #ight. Pilot seats with a range of adjustment
options were designed to guarantee seating comfort. In                               2. Biomechanics: principles of seating
spite of this, complaints continued to be reported (Haw-
kins, 1973; Lusted et al., 1994). To solve these problems                               Parameters which have a strong in#uence on comfort
engineers tried to improve the cushioning of cockpit seats                           in sitting derive partly from anthropometric and party
by modifying the shape and the hardness of seat, and by                              from biomechanical considerations (Wachsler and
covering the seat with sheepskin to improve the circula-                             Laerner, 1960; Drury and Coury, 1982; Snijders, 1988;
tion of air between pilot and the surface of the seat. These                         Zhang et al., 1996). The standard used for pilot seats
changes did not eliminate complaints (Lusted et al.,                                 (Aerospace Standard AS290B, 1965) is based on anthro-
1994).                                                                               pometrics. In addition, we discuss important bio-
   We addressed earlier studies to seating in "ghter air-                            mechanical considerations.
craft (Aghina, 1989; Snijders et al., 1991; Hoek van Dijke
et al., 1993), and to seating in cars, in the o$ce, at school                        2.1. Seat}backrest angle and seat inclination
and at home (Snijders, 1988, Snijders et al., 1995a,b,c;
Goossens, 1994, Goossens and Snijders, 1995; Goossens                                   A biomechanical model (see Fig. 1) (Snijders, 1988)
et al., 1994). This initiated the present study on pilot seats                       demonstrates schematically that when a backrest is used,
in civil aircraft. The question our present research asks is:                        the seat must be inclined backwards at the site of the
                                                                                     ischial tuberosities to eliminate shear forces between skin
                                                                                     and cushion.
                                                                                        Fig. 1A is a free body diagram of the upper part of the
  * Corresponding author. Tel.: ##31-1527-86340; fax: ##31-                          body (including the mass of the arms, head and trunk). It
1527-87179.                                                                          shows the forces that act on the trunk of the sitting

0003-6870/00/$ - see front matter             1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 0 3 - 6 8 7 0 ( 9 9 ) 0 0 0 2 8 - 9
10                                         R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14

                                                                              To prevent sliding when sitting with an inclined back-
                                                                           rest and a horizontal seat, the equilibrium requires
                                                                           a shear force (F ) between seat and ischial tuberosities.
                                                                                              
                                                                           This shear force acts in combination with pressure
                                                                           (caused by F ) and aggravates discomfort during pro-
                                                                                           
                                                                           longed sitting. At high enough levels pressure load can
                                                                           hinder the di!usion of oxygen and metabolites to the
                                                                           cells. We demonstrated that the addition of shear load
                                                                           worsens this phenomenon dramatically (Goossens et al.,
                                                                           1994).
                                                                              In theory, the shear force between seat and ischial
                                                                           tuberosities can be eliminated completely by opting for
                                                                           an angle of 90}1003 between seat and backrest (Fig. 2, ).
                                                                           In that case shear is minimised, because the supporting
                                                                           force (F ) is oriented perpendicular to the seat surface (see
                                                                                    
                                                                           Fig. 1). With such a seat angle there is no tendency to
Fig. 1. Biomechanical model of the upper body. (A) No shear force on
the skin when the seat is perpendicular to F . (B) More backrest           slide. According to Aerospace standard AS290B (1965)
                                                  
inclination goes with a more oblique orientation of F . F "upper body      the backrest angle (Fig. 2, }) during #ight should be
                                                     
weight, F "force acting on the back, F "reaction force acting on the
                                                                          between 65 and 853, consequently the angle of the pilot
ischial tuberosities, F "vertical component of F , F "horizontal           seat (Fig. 2, ) should be between 5 and 153 (Goossens
                                                     
component of F , which is a shear force on the buttocks in case of
                                                                          and Snijders, 1995).
a horizontal seat surface (redrawn from Snijders et al., 1995a).

                                                                           2.2. Lumbar support

                                                                              In order to prevent the pelvis from tilting backward
subject. In a state of static equilibrium (sitting still), the             during sitting, a support force is needed at level of the
lines of action of the force from the backrest (F ),                       posterior superior iliac spine (Goossens, 1994). This lum-
the weight force of the upper body (F ) and the force on                   bar support prevents the generation of a lumbar kypho-
the ischial tuberosities (F ) all three intersect at one point             sis, i.e. it provides the support for the lumbar spine to
                           
(S). Consequently, the reaction force on the ischial tuber-                adopt a slight lordotic curvature. However, compared to
osities (F ) cannot be vertical, but must have a slight                    standing, the lumbar spine will still #atten during sitting
           
inclination when a backrest is used. The "gure on the                      (Keegan, 1953; Andersson et al., 1974; Zacharkow, 1988).
right (B) shows that when the upper body is tilted back-                      Few anthropometric data exist on the height of the
ward, the inclination of the support force (F ) increases,                 posterior superior iliac spine. In a study of 91 sub-
                                                   
because its line of action must go through S.                              jects, Diebschlag et al. (1978) found that for 90% of the




                                    Fig. 2. The measured pilot seat dimensions, see Table 1 for description.
                                           R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14                                11

population the height of the posterior superior iliac spine                2.4. Ewective dimensions
was 18}25 cm.
  More recently, the preferred setting for the height of                      From a biomechanical point of view, only those parts
the lumbar support in o$ce chairs was investigated by                      of the supporting surfaces (seat, backrest, armrest) which
Coleman et al. (1998). They found that the lumbar sup-                     actually support a part of the body are functional. There-
port should be adjustable from 15 to 25 cm (Fig. 2, g).                    fore, in the case of seat depth (Fig. 2, b), e!ective seat
This "nding supports the biomechanical consideration                       depth (Fig. 2, c) is de"ned as that part of the seat which is
on pelvic positioning.                                                     actually used for body support. This also applies to seat
  In order to obtain lumbar support free space between                     width. The measurement technique is explained in
seat and backrest (Fig. 2, i) of at least 12 cm (Zacharkow,                Section 3.
1988) is also needed.
                                                                           2.5. Requirements
2.3. Armrests
                                                                             The anthropometric requirements, as found in Aero-
  Armrests placed at a su$cient height for proper arm                      space standard AS290B (1965), together with the require-
support will considerably reduce loads on the back                         ments obtained from biomechanical considerations are
(Zacharkow, 1988). Armrests placed too low may cause                       presented in Table 1 (de"nitions are shown in Fig. 2). The
people to adopt a scoliotic posture, i.e. with the upper                   two columns in Table 1 are not mutually exclusive. It is
body bent sideways. Then can also cause a kyphotic                         important that the seat inclination (Fig. 2, ) is de"ned as
shape of the lumbar spine (a C-form of the lower back).                    the inclination at the ischial tuberosities.
An armrest should give support below the mass centre of
gravity of the upperarm and forearm. It is a biomechani-
                                                                           3. Methods
cal design mistake to make height adjustment dependent
on tilting the armrest (Fig. 2, ), because then taller
                                                                              The pilot seats were evaluated by measuring their
people are unable to rest their elbows on the supporting
                                                                           dimensions and adjustabilities, and comparing them with
surface. For armrest height (Fig. 2, k) the range of
                                                                           the anthropometric and biomechanical design criteria in
20}32 cm from the Aerospace Standard is used (Table 1).
                                                                           Table 1. Some of the dimensions were characteristic for
Only small angles of the armrest are allowed ( (53), so
                                                                           pilot seats: a column cut out in the front of the seat (Fig.
that the entire arm remains in contact with the armrest.
                                                                           2, f) and an adjustable thigh support (Fig. 2d).
                                                                              Because discomfort most often occurs during pro-
Table 1                                                                    longed sitting, the seats of aircraft operating over middle
Standards of dimensions used for the analysis of cockpit seats.
                                                                           and long distances were considered:
AS290B"Aerospace Standards and biomechanical requirements
                                                                           Boeing 747-400, Boeing 747-300, McDonnell Douglas
      Description                      AS290B      Biomechanical           DC10-30, Airbus A310 and Boeing 737-300.
a     Seat height                      33 min                                 All linear dimensions were measured by means of
                                       51 max                              a ruler (division of scale 1 mm). The height of the lumbar
b     Seat depth                       41 min                              support is de"ned as the height of the most pronounced
                                       45 max                              point of the backrest, measured from the seat surface.
d     Thigh support length             13 max
                                                                           The minimum and maximum positions of the adjustable
f     Column cut out width             10 max
h     Backrest height                  65 min                              lumbar supports were measured. Seat angle, seat height,
j     Backrest width                   43 min                              e!ective seat depth and width, and armrest height were
                                       46 max                              measured, with the seat under a load of 500 N, using
l     Armrest width                    6.5                                 standardized wooden buttocks as used in the Dutch
m     Armrest length                   28 min
                                                                           Standard for o$ce chairs (NEN 1812, 1990).
o     Width between armrests           47 min
      Backrest inclination             65}85                                  Also apparent from Fig. 2 is the fact that some dimen-
                                                                           sions are de"ned with respect to the Seat Reference
c     Seat depth e!ective                          41 min                  Point, according to the Aerospace Standard AS290B
                                                   52 max                  (1965). The term &Seat Reference Point' is de"ned as the
e     Seat width e!ective                          43 min
                                                                           intersection of a line tangent to the surface of the seat
g     Lumbar support height                        15}25
i     Free space pelvis                            15 min                  bottom cushion and a line through the seat back cushion
k     Armrest height                               20}32                   representative of a back tangent line, when in a com-
      Seat Inclination at ischial tuber-           5}15                    pressed state under a load of a 50th percentile person.
      osities                                                              Based on previous measurements (Goossens and Snij-
      Armrest inclination                          0 min
                                                                           ders, 1995), in this study we used 500 N for the P50 load
                                                   5 max
                                                                           on the seat.
12                                         R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14

   The seat angle , backrest angle and armrest inclina-                        No "gures are given for the adjustability of the mea-
tion were measured by means of an inclinometer (Seca,                       sured armrests, because they could only rotate about an
1 degree per division of scale). Five di!erent seats from                   axis in the backrest. Rotation results in the rise of the
four manufacturers were evaluated.                                          armrest at the level of the wrist, but with tall pilots this
                                                                            leaves empty space below the elbow.

4. Results
                                                                            5. Discussion
   In Table 2 all the dimensions which were measured are
listed, as well as the requirements from Table 1. Not                          Some studies use questionnaires, "lled in by users, in
meeting the dimensions in Table 1 leads to asterisks in                     order to evaluate the comfort of seats. Lusted et al. (1994)
Table 2. The cells have asterisks, if the measured para-                    evaluated the seating of Qantas #ying crew by using this
meters fall outside the minimum and maximum limits,                         method. They found that there are certain areas of dis-
and if the measured range does not completely cover the                     comfort, but the reason for the discomfort could not be
required range (see armrest height in Table 1).                             explained.
   No cells have asterisks for the armrest inclination (Fig.                   In a recent study it was shown that some of the causa-
2, ) because all chairs could have an armrest inclination                   tive factors of discomfort can be related to biomechanical
of 03.                                                                      aspects (Zhang et al., 1996).
   The last two rows in Table 2 show the number of                             We, therefore, used a checklist based on anthropomet-
requirements which were met for the anthropometric and                      ric and biomechanical dimensions. The results of the
the biomechanical criteria, respectively.                                   study on the pilot seats presented here, and the results of
   It can be seen that the majority of the parameters                       the questionnaires of the Qantas study (Lusted et al.,
measured were not in accordance with the criteria.                          1994) do not exclude each other. These authors mention


Table 2
Types of aircraft, pilot seats and dimensions measured in situ. Comparison with the standards mentioned in Table 1. An asterisk (*) means that the
considered dimension does not meet the requirements. Length in cm, angle in degrees

          Description                    AS290B            Biomechanical    747-300       747-400       DC10          A310          737
                                                                            WEBER         IPECO         AMI           SOCEA         IPECO

a         Seat height                    33 min                             38}51         34}47         34}50         36}51         38}51
                                         51 max
b         Seat depth                     41 min                             41            45            45            42            40}45
                                         45 max
d         Thigh support length           13 max                             16*           17*           15*           12            16*
f         Column cut out width           10 max                             12*           10            11*           10            11*
h         Backrest height                65 min                             59*           54}63*        63*           67            55}63*
j         Backrest width                 43 min                             37}40*        39}41*        52*           41*           39*
                                         46 max
l         Armrest width                  6.5                                5*            5.5*          6.5           4*            6*
m         Armrest length                 28 min                             45            36            45            48.5          35
o         Width between armrests         47 min                             44.5*         44.5*         46*           47.5          49.5
          Backrest inclination           65}85                              40}85         65}90         50}75*        60}75*        62}85

c         Seat depth e!ective                              41 min           38*           42            40            42            33}38*
                                                           52 max
e         Seat width e!ective                              43 min           46            39*           42*           45            42*
g         Lumbar support height                            15}25            13*           10}19*        13*           9}15*         14}23*
i         Free space pelvis                                15 min           6*            4}13*         0*            4*            0}9*
k         Armrest height                                   20}32            19.5*         19.5*         17*           18*           22*
          Seat Inclination at ischial                      5}15             0*            0*            0*            0*            0*
            tuberosities
          Armrest inclination                              0 min            !5-#5         !25-#25       !25#10        !28-0         !24-#4
                                                           5 max

          Number of measured features that meet the 10                      4             5             4             7             5
           anthropometric requirements
          Number of measured features that meet the 7                       2             2             1             3             1
           biomechanical requirements
                                      R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14                                       13

that their study involved Ipeco chairs. The results of our            from the thighs to allow proper pedal pressure to be
study show that the main areas of discomfort, namely the              applied in emergency situations. However, the #exion of
buttocks and low back as found in the Qantas study, can               the thigh support (Fig. 2, ) to 303 with respect to the seat
be ascribed to a failure to meet the biomechanical re-                has no function according to biomechanics. The surface
quirements in that region.                                            of a seat must be #at in an anterior}posterior direction
   Although the dynamics of #ying will in#uence the forces            (Snijders, 1988). Therefore, the pro"le in the sagittal
acting on the pilot, we decided to use a biomechanical                plane must be straight, since curbs and raised brims do
model in a static seating posture, since civil #ight is, for          not match with human anatomy and will diminish the
most of the time, without extreme accelerations.                      seat depth, and may a!ect lumbar curvature. When
   The biomechanical model we presented is only valid                 a backrest is used, an inclination must to be applied to
for the static situation, and is restricted to the sagittal           the entire seat, to give proper support to the ischial
plane only, and thus does not study the in#uence of                   tuberosities in all positions.
postural changes. Hobson (1992) studied the in#uence                     The impression gained from the results of this study is
of postural changes on the shear force acting on the tissue           that the seats which were evaluated are not able to
of the buttock in the plane of the seat. When the trunk               provide comfortable sitting positions.
was bent laterally 153 to left and right, he found in the                In summary, the following improvements can be sug-
healthy population only little changes in the shear force             gested:
(5 N) compared to the symmetric position. He found that               E Increase the e!ective seat depth by making the seat #at
in healthy people, when the trunk was bent laterally 153                 in anterior}posterior direction.
to left or right, only small changes in the shear force (5 N)         E Raise the lumbar support.
occurred, as compared with those occurring in a symmet-               E Make the armrests adjustable in height by translation
ric position.                                                            instead of rotation.
   According to Hawkins (1973) complaints of discomfort               E Tilt the entire seat to an angle at the ischial tuberosi-
in the cockpit are related to stress, and to the inappropri-             ties up to 7}103.
ate cushioning of pilot seats. Because of the numerous
                                                                         These recommendations for improvement will in no
adjustment options the comfort of cockpit seats is pre-
                                                                      way a!ect the operation of controls or the space available
sumed to be excellent. Some seat dimensions are related
                                                                      in the cockpit.
to the anthropometry of small and tall pilots, for example
seat height, armrest length and width between the ar-
mrests. These dimensions did not compare favourably
                                                                      6. Conclusions
with aviation standards, meeting only 4}7 requirements
out of 10. Furthermore, the dimensions based on bio-
                                                                      E The dimensions of none of the "ve considered pilot
mechanics, which may be related to discomfort, were
                                                                        seats from middle- and long-range aircraft met basic
even less satisfactory, meeting only 1}3 requirements out
                                                                        biomechanical design criteria.
of 7. These were the e!ective seat depth, the lumbar
                                                                      E The majority of the dimensions did not meet aviation
support height, height and position of the armrests and
                                                                        standards.
seat inclination at the contact area of the ischial tuberosi-
                                                                      E The inbuilt adjustment options are insu$cient, in par-
ties. Despite the many adjustment possibilities, impor-
                                                                        ticular, for taller pilots.
tant dimensions, as given in reference literature, could
                                                                      E Identi"ed as most problematic were the horizontal
not be achieved (Coleman et al., 1998; Drury and Coury,
                                                                        seat at the ischial tuberosities, insu$cient e!ective seat
1982; Goossens et al., 1994,1995; Snijders, 1988, Snijders
                                                                        depth, insu$cient height of lumbar support and the
et al., 1991; Wachsler and Laerner, 1960; Zacharkow,
                                                                        absence of height adjustability for armrests.
1988).
   For example, in all the seats the maximal e!ective seat
depth was still too small for the majority of pilots. This
                                                                      Acknowledgements
also applied to the height of the armrests and the lumbar
support. It is curious that the designers claim that the
                                                                        The authors wish to thank N. Anderson, K.J. Bruce,
height of an armrest can be adjusted by rotating the
                                                                      C.H. Draijer, S.V.W. Erftemeyer, H. de Ree and J.
armrest around an axis in the backrest. The above "nd-
                                                                      Steketee for their valuable contributions.
ings indicate that the adaptation of the armrest height to
individual heights is insu$cient.
   A typical characteristic of pilot seats is the adjust-
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