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Biomechanical analysis of the dimensions of pilot seats in civil aircraft
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 Complaints of discomfort and low-back pain during middle- and long-range #ights were reported among traf"c pilots (Lusted et al., 1994). These complaints may 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 spite of this, complaints continued to be reported (Hawkins, 1973; Lusted et al., 1994). To solve these problems engineers tried to improve the cushioning of cockpit seats by modifying the shape and the hardness of seat, and by covering the seat with sheepskin to improve the circulation of air between pilot and the surface of the seat. These changes did not eliminate complaints (Lusted et al., 1994). We addressed earlier studies to seating in "ghter aircraft (Aghina, 1989; Snijders et al., 1991; Hoek van Dijke et al., 1993), and to seating in cars, in the o$ce, at school and at home (Snijders, 1988, Snijders et al., 1995a,b,c; Goossens, 1994, Goossens and Snijders, 1995; Goossens et al., 1994). This initiated the present study on pilot seats in civil aircraft. The question our present research asks is:
* Corresponding author. Tel.: ##31-1527-86340; fax: ##311527-87179.
can complaints reported by pilots be ascribed to certain features of the cockpit seat? To answer this question an anthropometric and biomechanical analysis was made of pilot seats in "ve modern middle- and long-range civil aircraft.
2. Biomechanics: principles of seating Parameters which have a strong in#uence on comfort in sitting derive partly from anthropometric and party from biomechanical considerations (Wachsler and Laerner, 1960; Drury and Coury, 1982; Snijders, 1988; Zhang et al., 1996). The standard used for pilot seats (Aerospace Standard AS290B, 1965) is based on anthropometrics. In addition, we discuss important biomechanical considerations. 2.1. Seat}backrest angle and seat inclination A biomechanical model (see Fig. 1) (Snijders, 1988) demonstrates schematically that when a backrest is used, the seat must be inclined backwards at the site of the ischial tuberosities to eliminate shear forces between skin and cushion. Fig. 1A is a free body diagram of the upper part of the body (including the mass of the arms, head and trunk). It shows the forces that act on the trunk of the sitting
0003-6870/00/$ - 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
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R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14
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 5 inclination goes with a more oblique orientation of F . F "upper body 5 ' weight, F "force acting on the back, F "reaction force acting on the " 5 ischial tuberosities, F "vertical component of F , F "horizontal 57 5 5) component of F , which is a shear force on the buttocks in case of 5 a horizontal seat surface (redrawn from Snijders et al., 1995a).
To prevent sliding when sitting with an inclined backrest and a horizontal seat, the equilibrium requires a shear force (F ) between seat and ischial tuberosities. 5) This shear force acts in combination with pressure (caused by F ) and aggravates discomfort during pro57 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, c). In that case shear is minimised, because the supporting force (F ) is oriented perpendicular to the seat surface (see 5 Fig. 1). With such a seat angle there is no tendency to slide. According to Aerospace standard AS290B (1965) the backrest angle (Fig. 2, }) during #ight should be between 65 and 853, consequently the angle of the pilot seat (Fig. 2, a) should be between 5 and 153 (Goossens and Snijders, 1995). 2.2. Lumbar support
subject. In a state of static equilibrium (sitting still), the lines of action of the force from the backrest (F ), " the weight force of the upper body (F ) and the force on ' the ischial tuberosities (F ) all three intersect at one point 5 (S). Consequently, the reaction force on the ischial tuberosities (F ) cannot be vertical, but must have a slight 5 inclination when a backrest is used. The "gure on the right (B) shows that when the upper body is tilted backward, the inclination of the support force (F ) increases, 5 because its line of action must go through S.
In order to prevent the pelvis from tilting backward during sitting, a support force is needed at level of the posterior superior iliac spine (Goossens, 1994). This lumbar support prevents the generation of a lumbar kyphosis, i.e. it provides the support for the lumbar spine to adopt a slight lordotic curvature. However, compared to standing, the lumbar spine will still #atten during sitting (Keegan, 1953; Andersson et al., 1974; Zacharkow, 1988). Few anthropometric data exist on the height of the posterior superior iliac spine. In a study of 91 subjects, 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
population the height of the posterior superior iliac spine was 18}25 cm. More recently, the preferred setting for the height of the lumbar support in o$ce chairs was investigated by Coleman et al. (1998). They found that the lumbar support should be adjustable from 15 to 25 cm (Fig. 2, g). This "nding supports the biomechanical consideration on pelvic positioning. In order to obtain lumbar support free space between seat and backrest (Fig. 2, i) of at least 12 cm (Zacharkow, 1988) is also needed. 2.3. Armrests Armrests placed at a su$cient height for proper arm support will considerably reduce loads on the back (Zacharkow, 1988). Armrests placed too low may cause people to adopt a scoliotic posture, i.e. with the upper body bent sideways. Then can also cause a kyphotic shape of the lumbar spine (a C-form of the lower back). An armrest should give support below the mass centre of gravity of the upperarm and forearm. It is a biomechanical design mistake to make height adjustment dependent on tilting the armrest (Fig. 2, d), because then taller people are unable to rest their elbows on the supporting surface. For armrest height (Fig. 2, k) the range of 20}32 cm from the Aerospace Standard is used (Table 1). Only small angles of the armrest are allowed (d(53), so that the entire arm remains in contact with the armrest. Table 1 Standards of dimensions used for the analysis of cockpit seats. AS290B"Aerospace Standards and biomechanical requirements Description
AS290B
a
Seat height
b
Seat depth
d f h j
Thigh support length Column cut out width Backrest height Backrest width
l m o b
Armrest width Armrest length Width between armrests Backrest inclination
33 min 51 max 41 min 45 max 13 max 10 max 65 min 43 min 46 max 6.5 28 min 47 min 65}85
c
Seat depth e!ective
e g i k a
Seat width e!ective Lumbar support height Free space pelvis Armrest height Seat Inclination at ischial tuberosities Armrest inclination
d
Biomechanical
41 min 52 max 43 min 15}25 15 min 20}32 5}15 0 min 5 max
11
2.4. Ewective dimensions From a biomechanical point of view, only those parts of the supporting surfaces (seat, backrest, armrest) which actually support a part of the body are functional. Therefore, in the case of seat depth (Fig. 2, b), e!ective seat depth (Fig. 2, c) is de"ned as that part of the seat which is actually used for body support. This also applies to seat width. The measurement technique is explained in Section 3. 2.5. Requirements The anthropometric requirements, as found in Aerospace standard AS290B (1965), together with the requirements obtained from biomechanical considerations are presented in Table 1 (de"nitions are shown in Fig. 2). The two columns in Table 1 are not mutually exclusive. It is important that the seat inclination (Fig. 2, a) is de"ned as the inclination at the ischial tuberosities. 3. Methods The pilot seats were evaluated by measuring their dimensions and adjustabilities, and comparing them with the anthropometric and biomechanical design criteria in Table 1. Some of the dimensions were characteristic for pilot seats: a column cut out in the front of the seat (Fig. 2, f) and an adjustable thigh support (Fig. 2d). Because discomfort most often occurs during prolonged sitting, the seats of aircraft operating over middle and long distances were considered: Boeing 747-400, Boeing 747-300, McDonnell Douglas DC10-30, Airbus A310 and Boeing 737-300. All linear dimensions were measured by means of a ruler (division of scale 1 mm). The height of the lumbar support is de"ned as the height of the most pronounced point of the backrest, measured from the seat surface. The minimum and maximum positions of the adjustable lumbar supports were measured. Seat angle, seat height, e!ective seat depth and width, and armrest height were measured, with the seat under a load of 500 N, using standardized wooden buttocks as used in the Dutch Standard for o$ce chairs (NEN 1812, 1990). Also apparent from Fig. 2 is the fact that some dimensions are de"ned with respect to the Seat Reference Point, according to the Aerospace Standard AS290B (1965). The term &Seat Reference Point' is de"ned as the intersection of a line tangent to the surface of the seat bottom cushion and a line through the seat back cushion representative of a back tangent line, when in a compressed state under a load of a 50th percentile person. Based on previous measurements (Goossens and Snijders, 1995), in this study we used 500 N for the P50 load on the seat.
12
R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14
The seat angle a, backrest angle b and armrest inclination d were measured by means of an inclinometer (Seca, 1 degree per division of scale). Five di!erent seats from four manufacturers were evaluated.
No "gures are given for the adjustability of the measured armrests, because they could only rotate about an axis in the backrest. Rotation results in the rise of the 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 meeting the dimensions in Table 1 leads to asterisks in Table 2. The cells have asterisks, if the measured parameters fall outside the minimum and maximum limits, and if the measured range does not completely cover the required range (see armrest height in Table 1). No cells have asterisks for the armrest inclination (Fig. 2, d) because all chairs could have an armrest inclination of 03. The last two rows in Table 2 show the number of requirements which were met for the anthropometric and the biomechanical criteria, respectively. It can be seen that the majority of the parameters measured were not in accordance with the criteria.
Some studies use questionnaires, "lled in by users, in order to evaluate the comfort of seats. Lusted et al. (1994) evaluated the seating of Qantas #ying crew by using this method. They found that there are certain areas of discomfort, but the reason for the discomfort could not be explained. In a recent study it was shown that some of the causative factors of discomfort can be related to biomechanical aspects (Zhang et al., 1996). We, therefore, used a checklist based on anthropometric and biomechanical dimensions. The results of the study on the pilot seats presented here, and the results of the questionnaires of the Qantas study (Lusted et al., 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
a
Seat height
b
Seat depth
d f h j
Thigh support length Column cut out width Backrest height Backrest width
l m o b
Armrest width Armrest length Width between armrests Backrest inclination
33 min 51 max 41 min 45 max 13 max 10 max 65 min 43 min 46 max 6.5 28 min 47 min 65}85
c
Seat depth e!ective
e g i k a
Seat width e!ective Lumbar support height Free space pelvis Armrest height Seat Inclination at ischial tuberosities Armrest inclination
d
Number of measured features that meet the 10 anthropometric requirements Number of measured features that meet the 7 biomechanical requirements
Biomechanical
41 min 52 max 43 min 15}25 15 min 20}32 5}15 0 min 5 max
747-300 WEBER
747-400 IPECO
DC10 AMI
A310 SOCEA
737 IPECO
38}51
34}47
34}50
36}51
38}51
41
45
45
42
40}45
16* 12* 59* 37}40*
17* 10 54}63* 39}41*
15* 11* 63* 52*
12 10 67 41*
16* 11* 55}63* 39*
5* 45 44.5* 40}85
5.5* 36 44.5* 65}90
6.5 45 46* 50}75*
4* 48.5 47.5 60}75*
6* 35 49.5 62}85
38*
42
40
42
33}38*
46 13* 6* 19.5* 0*
39* 10}19* 4}13* 19.5* 0*
42* 13* 0* 17* 0*
45 9}15* 4* 18* 0*
42* 14}23* 0}9* 22* 0*
!5-#5
!25-#25
!25#10
!28-0
!24-#4
4
5
4
7
5
2
2
1
3
1
R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14
that their study involved Ipeco chairs. The results of our study show that the main areas of discomfort, namely the buttocks and low back as found in the Qantas study, can be ascribed to a failure to meet the biomechanical requirements in that region. Although the dynamics of #ying will in#uence the forces acting on the pilot, we decided to use a biomechanical model in a static seating posture, since civil #ight is, for most of the time, without extreme accelerations. The biomechanical model we presented is only valid for the static situation, and is restricted to the sagittal plane only, and thus does not study the in#uence of postural changes. Hobson (1992) studied the in#uence of postural changes on the shear force acting on the tissue of the buttock in the plane of the seat. When the trunk was bent laterally 153 to left and right, he found in the healthy population only little changes in the shear force (5 N) compared to the symmetric position. He found that in healthy people, when the trunk was bent laterally 153 to left or right, only small changes in the shear force (5 N) occurred, as compared with those occurring in a symmetric position. According to Hawkins (1973) complaints of discomfort in the cockpit are related to stress, and to the inappropriate cushioning of pilot seats. Because of the numerous adjustment options the comfort of cockpit seats is presumed to be excellent. Some seat dimensions are related to the anthropometry of small and tall pilots, for example seat height, armrest length and width between the armrests. These dimensions did not compare favourably with aviation standards, meeting only 4}7 requirements out of 10. Furthermore, the dimensions based on biomechanics, which may be related to discomfort, were even less satisfactory, meeting only 1}3 requirements out of 7. These were the e!ective seat depth, the lumbar support height, height and position of the armrests and seat inclination at the contact area of the ischial tuberosities. Despite the many adjustment possibilities, important dimensions, as given in reference literature, could not be achieved (Coleman et al., 1998; Drury and Coury, 1982; Goossens et al., 1994,1995; Snijders, 1988, Snijders et al., 1991; Wachsler and Laerner, 1960; Zacharkow, 1988). For example, in all the seats the maximal e!ective seat depth was still too small for the majority of pilots. This also applied to the height of the armrests and the lumbar support. It is curious that the designers claim that the height of an armrest can be adjusted by rotating the armrest around an axis in the backrest. The above "ndings indicate that the adaptation of the armrest height to individual heights is insu$cient. A typical characteristic of pilot seats is the adjustable thigh support, which rests on a spring. We could not determine the exact purpose of this feature. It seems that it evolved from an ancient chair design, in which the front part of the seat had to spring down under pressure
13
from the thighs to allow proper pedal pressure to be applied in emergency situations. However, the #exion of the thigh support (Fig. 2, h) to 303 with respect to the seat has no function according to biomechanics. The surface of a seat must be #at in an anterior}posterior direction (Snijders, 1988). Therefore, the pro"le in the sagittal plane must be straight, since curbs and raised brims do not match with human anatomy and will diminish the seat depth, and may a!ect lumbar curvature. When a backrest is used, an inclination must to be applied to the entire seat, to give proper support to the ischial tuberosities in all positions. The impression gained from the results of this study is that the seats which were evaluated are not able to provide comfortable sitting positions. In summary, the following improvements can be suggested: f Increase the e!ective seat depth by making the seat #at in anterior}posterior direction. f Raise the lumbar support. f Make the armrests adjustable in height by translation instead of rotation. f Tilt the entire seat to an angle at the ischial tuberosities up to 7}103. These recommendations for improvement will in no way a!ect the operation of controls or the space available in the cockpit.
6. Conclusions f The dimensions of none of the "ve considered pilot seats from middle- and long-range aircraft met basic biomechanical design criteria. f The majority of the dimensions did not meet aviation standards. f The inbuilt adjustment options are insu$cient, in particular, for taller pilots. f Identi"ed as most problematic were the horizontal seat at the ischial tuberosities, insu$cient e!ective seat depth, insu$cient height of lumbar support and the absence of height adjustability for armrests.
Acknowledgements The authors wish to thank N. Anderson, K.J. Bruce, C.H. Draijer, S.V.W. Erftemeyer, H. de Ree and J. Steketee for their valuable contributions.
References Aerospace Standard, AS290B, 1965. Seats for #ight deck crewmentransport aircraft. Society of Automotive Engineers, Warrendale, PA, USA.
14
R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14
Aghina, J.C.F.M., 1989. Low back pain and low level #ying. Thesis. Erasmus University Rotterdam, The Netherlands. Andersson, G.B.J., OG rtengren, R., Nachemson, A., ElfstroK m, G., 1974. Lumbar disc pressure and myoelectric back muscle activity during sitting. I. Studies on an experimental chair. Sc. J. Rehabil. Med. 6, 104}114. Coleman, N., Brynley, P., Hull, P., Ellit, G., 1998. An empirical study of preferred settings for lumbar support on adjustable o$ce chairs. Ergonomics 41, 401}419. Diebschlag, W., MuK ller-Limmroth, W., Baldauf, H., Stumbaum, F., 1978. Physiologische Untersuchungen zur SitzmoK belgestaltung. Z. Arbeits Wiss. 34, 89}95. Drury, C.G., Coury, B.G., 1982. A methodology for chair evaluation. Appl. Ergon 13, 195}202. Goossens, R.H.M., 1994. Biomechanics of body support; a study of load distribution, shear, decubitus risk and form of the spine. Thesis, Erasmus University Rotterdam, the Netherlands. Goossens, R.H.M., Snijders, C.J., 1995. Design criteria for the reduction of shear forces in beds and seats. J. Biomech. 28, 225}230. Goossens, R.H.M., Zegers, R., Hoek van Dijke, G.A., Snijders, C.J., 1994. In#uence of shear on skin oxygen tension. Clin. Physiol 14, 111}118. Hawkins, F.H., 1973. Crew seats in transport aircraft. Shell Aviation News 418, 14}21. Hobson, D.A., 1992. Comparative e!ects of posture on pressure and shear at the body}seat interface. J. Rehabil. Res. Dev 29, 21}31. Hoek van Dijke, G.A., Snijders, C.J., Roosch, E.R., Burgers, P.I.C.J., 1993. An analysis of biomechanical and ergonomic aspects of cervical spine in F-16 #ight situations. J. Biomech. 26, 1017}1025.
Keegan, J.J., 1953. Alterations of the lumbar curve. J. Bone J. Surg. 3, 589}603. Lusted, M., Healey, S., Mandrijk, J.A., 1994. Evaluation of the seating of Qantas #ight deck crew. Appl. Ergon 25, 275}282. NEN 1812, 1990. Ergonomic requirements for o$ce work-chairs and o$ce workdesks, Requirements for dimensions and design, Measurement and test methods. NNI, Delft, The Netherlands. Snijders, C.J., 1988. Design criteria for seating based on biomechanics. Proceedings of ICAART 88, Montreal, 25}30 June, pp. 472}473. Snijders, C.J., Bakker, M.P., Vleeming, A., Stoeckart, R., Stam, H.J., 1995c. Oblique abdominal muscle activity in standing and in sitting on hard and soft seats. Clin. Biomech 10, 73}78. Snijders, C.J., Hoek van Dijke, G.A., Roosch, E.R., 1991. A biomechanical model for the analysis of the cervical spine in static postures. J. Biomech 24, 783}792. Snijders, C.J., Nordin, M., Frankel, V.H., 1995a. Biomechanica van het spier-skeletstelsel; grondslagen en toepassingen. Lemma, Utrecht, ISBN 90-5189-278-0 (in Dutch). Snijders, C.J., Slagter, A.H.E., Strik, R., van Vleeming, A., Stoeckart, R., Stam, H.J., 1995b. Why leg-crossing? The in#uence of common postures on abdominal muscle activity. Spine 20, 1989}1993. Wachsler, R.A., Laerner, D.B., 1960. An analysis of some factors in#uencing seat comfort. Ergonomics 3, 315}320. Zacharkow, D., 1988. Posture, Sitting, Standing, Chair Design and Exercise. Charles C. Thomas, Spring"eld, IL, USA. Zhang, L., Helander, M.G., Drury, C.G., 1996. Identifying factors of comfort and discomfort in sitting. Human Factors 38, 377}389.
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 Complaints of discomfort and low-back pain during middle- and long-range #ights were reported among traf"c pilots (Lusted et al., 1994). These complaints may 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 spite of this, complaints continued to be reported (Hawkins, 1973; Lusted et al., 1994). To solve these problems engineers tried to improve the cushioning of cockpit seats by modifying the shape and the hardness of seat, and by covering the seat with sheepskin to improve the circulation of air between pilot and the surface of the seat. These changes did not eliminate complaints (Lusted et al., 1994). We addressed earlier studies to seating in "ghter aircraft (Aghina, 1989; Snijders et al., 1991; Hoek van Dijke et al., 1993), and to seating in cars, in the o$ce, at school and at home (Snijders, 1988, Snijders et al., 1995a,b,c; Goossens, 1994, Goossens and Snijders, 1995; Goossens et al., 1994). This initiated the present study on pilot seats in civil aircraft. The question our present research asks is:
* Corresponding author. Tel.: ##31-1527-86340; fax: ##311527-87179.
can complaints reported by pilots be ascribed to certain features of the cockpit seat? To answer this question an anthropometric and biomechanical analysis was made of pilot seats in "ve modern middle- and long-range civil aircraft.
2. Biomechanics: principles of seating Parameters which have a strong in#uence on comfort in sitting derive partly from anthropometric and party from biomechanical considerations (Wachsler and Laerner, 1960; Drury and Coury, 1982; Snijders, 1988; Zhang et al., 1996). The standard used for pilot seats (Aerospace Standard AS290B, 1965) is based on anthropometrics. In addition, we discuss important biomechanical considerations. 2.1. Seat}backrest angle and seat inclination A biomechanical model (see Fig. 1) (Snijders, 1988) demonstrates schematically that when a backrest is used, the seat must be inclined backwards at the site of the ischial tuberosities to eliminate shear forces between skin and cushion. Fig. 1A is a free body diagram of the upper part of the body (including the mass of the arms, head and trunk). It shows the forces that act on the trunk of the sitting
0003-6870/00/$ - 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
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 5 inclination goes with a more oblique orientation of F . F "upper body 5 ' weight, F "force acting on the back, F "reaction force acting on the " 5 ischial tuberosities, F "vertical component of F , F "horizontal 57 5 5) component of F , which is a shear force on the buttocks in case of 5 a horizontal seat surface (redrawn from Snijders et al., 1995a).
To prevent sliding when sitting with an inclined backrest and a horizontal seat, the equilibrium requires a shear force (F ) between seat and ischial tuberosities. 5) This shear force acts in combination with pressure (caused by F ) and aggravates discomfort during pro57 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, c). In that case shear is minimised, because the supporting force (F ) is oriented perpendicular to the seat surface (see 5 Fig. 1). With such a seat angle there is no tendency to slide. According to Aerospace standard AS290B (1965) the backrest angle (Fig. 2, }) during #ight should be between 65 and 853, consequently the angle of the pilot seat (Fig. 2, a) should be between 5 and 153 (Goossens and Snijders, 1995). 2.2. Lumbar support
subject. In a state of static equilibrium (sitting still), the lines of action of the force from the backrest (F ), " the weight force of the upper body (F ) and the force on ' the ischial tuberosities (F ) all three intersect at one point 5 (S). Consequently, the reaction force on the ischial tuberosities (F ) cannot be vertical, but must have a slight 5 inclination when a backrest is used. The "gure on the right (B) shows that when the upper body is tilted backward, the inclination of the support force (F ) increases, 5 because its line of action must go through S.
In order to prevent the pelvis from tilting backward during sitting, a support force is needed at level of the posterior superior iliac spine (Goossens, 1994). This lumbar support prevents the generation of a lumbar kyphosis, i.e. it provides the support for the lumbar spine to adopt a slight lordotic curvature. However, compared to standing, the lumbar spine will still #atten during sitting (Keegan, 1953; Andersson et al., 1974; Zacharkow, 1988). Few anthropometric data exist on the height of the posterior superior iliac spine. In a study of 91 subjects, 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
population the height of the posterior superior iliac spine was 18}25 cm. More recently, the preferred setting for the height of the lumbar support in o$ce chairs was investigated by Coleman et al. (1998). They found that the lumbar support should be adjustable from 15 to 25 cm (Fig. 2, g). This "nding supports the biomechanical consideration on pelvic positioning. In order to obtain lumbar support free space between seat and backrest (Fig. 2, i) of at least 12 cm (Zacharkow, 1988) is also needed. 2.3. Armrests Armrests placed at a su$cient height for proper arm support will considerably reduce loads on the back (Zacharkow, 1988). Armrests placed too low may cause people to adopt a scoliotic posture, i.e. with the upper body bent sideways. Then can also cause a kyphotic shape of the lumbar spine (a C-form of the lower back). An armrest should give support below the mass centre of gravity of the upperarm and forearm. It is a biomechanical design mistake to make height adjustment dependent on tilting the armrest (Fig. 2, d), because then taller people are unable to rest their elbows on the supporting surface. For armrest height (Fig. 2, k) the range of 20}32 cm from the Aerospace Standard is used (Table 1). Only small angles of the armrest are allowed (d(53), so that the entire arm remains in contact with the armrest. Table 1 Standards of dimensions used for the analysis of cockpit seats. AS290B"Aerospace Standards and biomechanical requirements Description
AS290B
a
Seat height
b
Seat depth
d f h j
Thigh support length Column cut out width Backrest height Backrest width
l m o b
Armrest width Armrest length Width between armrests Backrest inclination
33 min 51 max 41 min 45 max 13 max 10 max 65 min 43 min 46 max 6.5 28 min 47 min 65}85
c
Seat depth e!ective
e g i k a
Seat width e!ective Lumbar support height Free space pelvis Armrest height Seat Inclination at ischial tuberosities Armrest inclination
d
Biomechanical
41 min 52 max 43 min 15}25 15 min 20}32 5}15 0 min 5 max
11
2.4. Ewective dimensions From a biomechanical point of view, only those parts of the supporting surfaces (seat, backrest, armrest) which actually support a part of the body are functional. Therefore, in the case of seat depth (Fig. 2, b), e!ective seat depth (Fig. 2, c) is de"ned as that part of the seat which is actually used for body support. This also applies to seat width. The measurement technique is explained in Section 3. 2.5. Requirements The anthropometric requirements, as found in Aerospace standard AS290B (1965), together with the requirements obtained from biomechanical considerations are presented in Table 1 (de"nitions are shown in Fig. 2). The two columns in Table 1 are not mutually exclusive. It is important that the seat inclination (Fig. 2, a) is de"ned as the inclination at the ischial tuberosities. 3. Methods The pilot seats were evaluated by measuring their dimensions and adjustabilities, and comparing them with the anthropometric and biomechanical design criteria in Table 1. Some of the dimensions were characteristic for pilot seats: a column cut out in the front of the seat (Fig. 2, f) and an adjustable thigh support (Fig. 2d). Because discomfort most often occurs during prolonged sitting, the seats of aircraft operating over middle and long distances were considered: Boeing 747-400, Boeing 747-300, McDonnell Douglas DC10-30, Airbus A310 and Boeing 737-300. All linear dimensions were measured by means of a ruler (division of scale 1 mm). The height of the lumbar support is de"ned as the height of the most pronounced point of the backrest, measured from the seat surface. The minimum and maximum positions of the adjustable lumbar supports were measured. Seat angle, seat height, e!ective seat depth and width, and armrest height were measured, with the seat under a load of 500 N, using standardized wooden buttocks as used in the Dutch Standard for o$ce chairs (NEN 1812, 1990). Also apparent from Fig. 2 is the fact that some dimensions are de"ned with respect to the Seat Reference Point, according to the Aerospace Standard AS290B (1965). The term &Seat Reference Point' is de"ned as the intersection of a line tangent to the surface of the seat bottom cushion and a line through the seat back cushion representative of a back tangent line, when in a compressed state under a load of a 50th percentile person. Based on previous measurements (Goossens and Snijders, 1995), in this study we used 500 N for the P50 load on the seat.
12
R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14
The seat angle a, backrest angle b and armrest inclination d were measured by means of an inclinometer (Seca, 1 degree per division of scale). Five di!erent seats from four manufacturers were evaluated.
No "gures are given for the adjustability of the measured armrests, because they could only rotate about an axis in the backrest. Rotation results in the rise of the 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 meeting the dimensions in Table 1 leads to asterisks in Table 2. The cells have asterisks, if the measured parameters fall outside the minimum and maximum limits, and if the measured range does not completely cover the required range (see armrest height in Table 1). No cells have asterisks for the armrest inclination (Fig. 2, d) because all chairs could have an armrest inclination of 03. The last two rows in Table 2 show the number of requirements which were met for the anthropometric and the biomechanical criteria, respectively. It can be seen that the majority of the parameters measured were not in accordance with the criteria.
Some studies use questionnaires, "lled in by users, in order to evaluate the comfort of seats. Lusted et al. (1994) evaluated the seating of Qantas #ying crew by using this method. They found that there are certain areas of discomfort, but the reason for the discomfort could not be explained. In a recent study it was shown that some of the causative factors of discomfort can be related to biomechanical aspects (Zhang et al., 1996). We, therefore, used a checklist based on anthropometric and biomechanical dimensions. The results of the study on the pilot seats presented here, and the results of the questionnaires of the Qantas study (Lusted et al., 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
a
Seat height
b
Seat depth
d f h j
Thigh support length Column cut out width Backrest height Backrest width
l m o b
Armrest width Armrest length Width between armrests Backrest inclination
33 min 51 max 41 min 45 max 13 max 10 max 65 min 43 min 46 max 6.5 28 min 47 min 65}85
c
Seat depth e!ective
e g i k a
Seat width e!ective Lumbar support height Free space pelvis Armrest height Seat Inclination at ischial tuberosities Armrest inclination
d
Number of measured features that meet the 10 anthropometric requirements Number of measured features that meet the 7 biomechanical requirements
Biomechanical
41 min 52 max 43 min 15}25 15 min 20}32 5}15 0 min 5 max
747-300 WEBER
747-400 IPECO
DC10 AMI
A310 SOCEA
737 IPECO
38}51
34}47
34}50
36}51
38}51
41
45
45
42
40}45
16* 12* 59* 37}40*
17* 10 54}63* 39}41*
15* 11* 63* 52*
12 10 67 41*
16* 11* 55}63* 39*
5* 45 44.5* 40}85
5.5* 36 44.5* 65}90
6.5 45 46* 50}75*
4* 48.5 47.5 60}75*
6* 35 49.5 62}85
38*
42
40
42
33}38*
46 13* 6* 19.5* 0*
39* 10}19* 4}13* 19.5* 0*
42* 13* 0* 17* 0*
45 9}15* 4* 18* 0*
42* 14}23* 0}9* 22* 0*
!5-#5
!25-#25
!25#10
!28-0
!24-#4
4
5
4
7
5
2
2
1
3
1
R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14
that their study involved Ipeco chairs. The results of our study show that the main areas of discomfort, namely the buttocks and low back as found in the Qantas study, can be ascribed to a failure to meet the biomechanical requirements in that region. Although the dynamics of #ying will in#uence the forces acting on the pilot, we decided to use a biomechanical model in a static seating posture, since civil #ight is, for most of the time, without extreme accelerations. The biomechanical model we presented is only valid for the static situation, and is restricted to the sagittal plane only, and thus does not study the in#uence of postural changes. Hobson (1992) studied the in#uence of postural changes on the shear force acting on the tissue of the buttock in the plane of the seat. When the trunk was bent laterally 153 to left and right, he found in the healthy population only little changes in the shear force (5 N) compared to the symmetric position. He found that in healthy people, when the trunk was bent laterally 153 to left or right, only small changes in the shear force (5 N) occurred, as compared with those occurring in a symmetric position. According to Hawkins (1973) complaints of discomfort in the cockpit are related to stress, and to the inappropriate cushioning of pilot seats. Because of the numerous adjustment options the comfort of cockpit seats is presumed to be excellent. Some seat dimensions are related to the anthropometry of small and tall pilots, for example seat height, armrest length and width between the armrests. These dimensions did not compare favourably with aviation standards, meeting only 4}7 requirements out of 10. Furthermore, the dimensions based on biomechanics, which may be related to discomfort, were even less satisfactory, meeting only 1}3 requirements out of 7. These were the e!ective seat depth, the lumbar support height, height and position of the armrests and seat inclination at the contact area of the ischial tuberosities. Despite the many adjustment possibilities, important dimensions, as given in reference literature, could not be achieved (Coleman et al., 1998; Drury and Coury, 1982; Goossens et al., 1994,1995; Snijders, 1988, Snijders et al., 1991; Wachsler and Laerner, 1960; Zacharkow, 1988). For example, in all the seats the maximal e!ective seat depth was still too small for the majority of pilots. This also applied to the height of the armrests and the lumbar support. It is curious that the designers claim that the height of an armrest can be adjusted by rotating the armrest around an axis in the backrest. The above "ndings indicate that the adaptation of the armrest height to individual heights is insu$cient. A typical characteristic of pilot seats is the adjustable thigh support, which rests on a spring. We could not determine the exact purpose of this feature. It seems that it evolved from an ancient chair design, in which the front part of the seat had to spring down under pressure
13
from the thighs to allow proper pedal pressure to be applied in emergency situations. However, the #exion of the thigh support (Fig. 2, h) to 303 with respect to the seat has no function according to biomechanics. The surface of a seat must be #at in an anterior}posterior direction (Snijders, 1988). Therefore, the pro"le in the sagittal plane must be straight, since curbs and raised brims do not match with human anatomy and will diminish the seat depth, and may a!ect lumbar curvature. When a backrest is used, an inclination must to be applied to the entire seat, to give proper support to the ischial tuberosities in all positions. The impression gained from the results of this study is that the seats which were evaluated are not able to provide comfortable sitting positions. In summary, the following improvements can be suggested: f Increase the e!ective seat depth by making the seat #at in anterior}posterior direction. f Raise the lumbar support. f Make the armrests adjustable in height by translation instead of rotation. f Tilt the entire seat to an angle at the ischial tuberosities up to 7}103. These recommendations for improvement will in no way a!ect the operation of controls or the space available in the cockpit.
6. Conclusions f The dimensions of none of the "ve considered pilot seats from middle- and long-range aircraft met basic biomechanical design criteria. f The majority of the dimensions did not meet aviation standards. f The inbuilt adjustment options are insu$cient, in particular, for taller pilots. f Identi"ed as most problematic were the horizontal seat at the ischial tuberosities, insu$cient e!ective seat depth, insu$cient height of lumbar support and the absence of height adjustability for armrests.
Acknowledgements The authors wish to thank N. Anderson, K.J. Bruce, C.H. Draijer, S.V.W. Erftemeyer, H. de Ree and J. Steketee for their valuable contributions.
References Aerospace Standard, AS290B, 1965. Seats for #ight deck crewmentransport aircraft. Society of Automotive Engineers, Warrendale, PA, USA.
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R.H.M. Goossens et al. / Applied Ergonomics 31 (2000) 9}14
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