DOT/FAA/AM-02/11

Richard DeWeese

Van Gowdy

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

June 2002

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, VA 22161.

Human Factors Associated

With the Certification of

Airplane Passenger Seats: Seat

Belt Adjustment and Release

Office of Aerospace Medicine

Washington, DC 20591
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-02/11

4. Title and Subtitle 5. Report Date

Human Factors Associated With the Certification of Airplane Passenger June 2002

Seats: Seat Belt Adjustment and Release

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

DeWeese, R., and Gowdy, R.V.

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

FAA Civil Aerospace Medical Institute

P.O. Box 25082, 11. Contract or Grant No.

Oklahoma City, OK 73125

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered

Office of Aerospace Medicine

Federal Aviation Administration 14. Sponsoring Agency Code

800 Independence Ave., S.W.

Washington, DC 20591

15. Supplemental Notes

Work was accomplished under approved task AM-B-02-PRS-96.

16. Abstract

Two separate studies were accomplished to investigate human factors issues related to the use of lap

belts. Human performance trials were conducted under two protocols to measure and assess: (1) seat belt

tension adjustment during normal flight and emergency landing conditions, and (2) the effects on passenger

emergency egress performance related to the lift-latch release angle of typical lap belts.

In the lap belt tension adjustment study, subjects were asked to sit in a typical passenger seat and adjust

the lap belts as they normally would for take-off or landing during a commercial flight. Participants were

then asked to adjust the lap belts as if they were anticipating an emergency landing. The airplane seat used in

this study was instrumented to measure the tension in the lap belt, which was recorded for both the normal

and emergency conditions. A total of 1182 subjects participated in this study. An analysis of results indicate

that most passengers (90%) tighten the lap belt to a tension less than 7 lb. during normal flight conditions

and less than 10 lb. for an anticipated emergency. These data indicate that the tension adjustment of lap

belts restraining anthropomorphic test dummies in airplane seat certification tests should not exceed 10 lbs.

to be representative of belt tension applied by a typical passenger. The current standard practice for

adjusting the belt tension prior to these tests was evaluated and found to be adequate in light of these

findings.

The lift-latch release angle experiments were designed to study lap belt restrained human subjects as they

released the belt buckle and proceeded to egress from a typical passenger seat. Some foreign regulatory

authorities require the release angle to be between 70° and 95°, whereas, typical U.S. buckles release between

45° and 60°. Three lap belts with latch release angles of 30°, 60°, and 90° were installed on a triple

passenger seat. Subjects were observed and timed as they tried to release the belts and exit from the seat.

Each was instructed to perform the exercise quickly. A total of 201 subjects participated in this study. No

significant differences in human performance factors related to the lift-latch angle were detected from an

analysis of the data.

17. Key Words 18. Distribution Statement

Lift Latch Buckles, Seat Belts, Transport Airplanes Document is available to the public through

the National Technical Information Service,

Springfield, VA 22161

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 11

F DOT F 1700 7

1

HUMAN FACTORS ASSOCIATED WITH THE CERTIFICATION OF AIRPLANE

PASSENGER SEATS: SEAT BELT ADJUSTMENT AND RELEASE

INTRODUCTION

This report presents the results from two human

subject studies regarding the use of lap belt restraints

on transport airplane passenger seats. The first study

involved the assessment of the belt tension that results

when passengers manually adjust their lap belts. The

second study examined the lift-latch buckle release

angle for lap belts, focusing primarily on the effect it

has on egress from the seat.

Lap Belt Adjustment. The certification requirements for transport airplane passenger seats can include impact tests using anthropomorphic test

dummies (ATDs) restrained by lap belts in the seat.

The amount of pre-test tension in the belts is commonly affected by the judgment, experience, technique, and strength of the technician preparing the

test. This study addressed two questions related to

pre-test belt tension. First, what belt tension is produced when a typical passenger adjusts his or her lap

belt? Second, what is the relationship between the

parameters that are involved when tightening a lap

belt over an ATD in a passenger seat? The parameters

studied were: load applied to the free end of the

webbing, tension produced in the belt, ATD position,

cushion stiffness, and perceived belt tension.

Buckle Release Angle. Although the lap belt buckle

release angle is not specifically addressed by FAA

regulations, it has been recognized as a feature that

varies widely, with no documented rational for its

effects on passenger safety. Discussions with major

aircraft belt manufacturers revealed that while no

guidance exists, the buckles they supply for US aircraft have traditionally been designed to release when

the lever is raised between 30 and 45 degrees. The UK

Civil Aviation Authority requires that lift latches used

on their aircraft release between 70 and 95 degrees (1).

The presumed intent of the larger angle is to prevent

inadvertent release during turbulence or emergency

landing scenarios. Thus, for the purposes of commonality of lap belt operation and harmonization of international policies, an understanding of how buckle

release angle affects occupant egress is necessary.

These two studies were conducted concurrently

using human subjects participating an aircraft evacuation study at the FAA Civil Aerospace Medical

Institute’s (CAMI’s) Protection and Survival Laboratory. Anthropometric data such as age, height, weight,

stature, and girth were gathered from each subject.

Also, experience in commercial air travel as a passenger was obtained from many of the participants.

The following presents the methods, observations,

and results from these two studies.

LAP BELT TENSION STUDY

Trial Setup. As shown in Figure 1, a typical economy

class passenger seat was trimmed down to a single

place and was instrumented with a load cell placed in

line with the fixed length side of the lap belt. The belt

used was a typical lift latch type with a fixed length

segment of 16.5 inches, including the load cell. The

overall length of the belt path was 29.5 inches when

tightened to 10 lb. of tension over the lap of a 50%

ATD. The armrests were tied up out of the way to

Figure 1. Tension Study Seat
2

simplify belt donning and tightening. Figure 2 shows

the pertinent dimensions of the trial seat, and Figure

3 shows the nominal position of a seated 50% ATD.

Instrumentation. A uni-axial load cell of 100 lb.

capacity was installed between the belt hook and the

seat anchor point, as shown in Figure 4. Since the

tension reading tended to vary with time due to

subject motion and settling, the load cell output was

processed by a computer program that reported the

average value over a three-second period.

Protocol. To acquire the tension readings, the

subjects were instructed to:

• Fasten the seat belt, and tighten it the way they

normally would for takeoff or landing.

• Sit still and stop breathing for a few seconds. (The

trial conductor records the load cell three-second averaged measurement of normal belt tension.)

• Adjust the belt in anticipation of an emergency

landing.

• Sit still and stop breathing for a few seconds.

(The trial conductor records the load cell three-second averaged measurement of anticipated emergency

belt tension.)

This procedure provided tension readings for two

conditions. The first reading was recorded after the

subject was told to adjust the belts as if expecting a

normal takeoff or landing. The other tension measurement was recorded after the subject was told to readjust the belts as he/she would in anticipation of an

emergency landing. To reduce the effects of slight

movements by the subject during measurement of the

4.2"

1.8"

CRP

Seat Belt Anchor

11°

10°

Figure 2. Tension Study Seat Dimensions

3.0"

30.0"

4.4"

4.0"

1.7"

19.6"

60°

Belt Angle

H-Point

Knee Pivot

CRP

Head CG

Figure 3. ATD Position in Tension Study Seat

Figure 4. Tension Load Cell

belt tension, the subjects were told to sit still and not

breathe while the computer was recording the tension

reading. The time history of a few subjects was recorded throughout the trial to verify that the protocol

was producing consistent data. As can be seen in Figure

5, the instructions for the subjects to stop breathing had

the desired result of producing a constant belt tension

during the three-second averaging period.
3

Subject Anthropometry. Data were collected from

1182 subjects during this study, 587 males and 595

females. Their anthropometric statistics are summarized in Table 1. As seen in the Table, all average

measurements are larger than the US average (3)(4).

This difference is somewhat explained by the fact that,

unlike the thinly clad subjects from which the national statistics were gathered, all measurements of

these subjects were taken over street clothing, which

added weight and increased the girth and height. The

girth measurement, in particular, is considered to be

only an estimate, since it was not possible to precisely

locate the subject’s waist (at omphalion) due to heavy

clothing.

Human Subject Belt Tension Results. All subjects

were able to apply enough force to the free end of the

webbing to remove slack from the belt. The average

normal condition tension measured was 3.2 lb., with

90% of all readings less than 7 lb. Figure 6 shows the

distribution of tension measurements recorded for

the normal flight condition. The average emergency

landing tension measured was 5.6 lb., with 90% of all

readings less than 10 lb. Figure 7 shows the distribution of tension measurements recorded for the anticipated emergency condition.

If only experienced airplane passengers (more than

1 flight) are considered, the belt tension results from

both the normal and emergency conditions are not

significantly different from the entire group results.

Also, there does not appear to be a difference between

the results gathered from male and female subjects.

When the subjects were asked to adjust the belt as

if expecting an emergency, 14% elected not to adjust

the tension at all, 77% adjusted the belts tighter than

the normal tension, and 9% actually loosened the belt

(inadvertently in most cases). Some subjects indicated

they would need to loosen the belt so they could

assume the brace for impact position. The most

intriguing result was that some subjects stated they

would unlatch the belt if they thought a crash was

imminent, so they could get out quickly afterwards.

ATD Belt Tension Results. To relate the human

subject belt tension data to initial tension used in sled

tests, measurements were also taken with a 50% H-2

Age Height Weight Waist Age Height Weight Waist

Average 37.7 70.6 192.3 38.3 37.9 65.6 168.3 35.8

Max 65.0 79.7 359.0 60.2 65.0 74.8 322.7 53.9

Min 18.0 61.6 89.1 26.0 18.0 57.8 81.4 24.4

US Average - 69.0 (3) 170.0 (3) 33.7 (4) - 63.7 (3) 137.0 (3) 30.8 (4)

Males Females

Table 1. Tension Subject Anthropometry

Figure 5. Typical Lap Belt Adjustment Time History

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35

Time (seconds)

 Normal Condition

3 Second Average

Emergency Condition

 3 Second Average

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Belt Tension (lb.)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

N = 1182

Avg = 3.15

STD = 2.28

Figure 6. Normal Lap Belt Tension
4

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Belt Tension (lb.)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

N = 1182

Avg = 5.55

STD = 3.19

Figure 7. Emergency Lap Belt Tension

Figure 8. Lap Belt Adjustment Force Measurement

0

5

10

15

20

25

0 10 20 30 40 50

Applied Load (lb.)

Floatation Cushion Non-flotation Cushion Rigid Block

Pre-Test Tension 

Setting Derived by

Researcher 2 (4.4 lb.)

Pre-Test Tension 

Setting Derived by 

Researcher 1 (9.2 lb.)

Figure 9. Lap Belt Tension vs. Applied Adjustment

Force

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 5 10 15 20 25

Belt Tension (lb.)

Floatation Cushion Non-flotation Cushion Rigid Block

Figure 10. Lap Belt Tension vs. ATD Pelvis

Displacement

Figure 11. Lift Latch Study Seat Figure 12. Lift Latch Study Target Markers
5

ATD placed in the same seat used with the human

subjects. The purpose was to measure the relationship

between the force applied to the end of the webbing,

the amount of tension produced in the belt, and the

effect on the position of the ATD. Three cushions

were used for this measurement: 1) the firm flotation

cushion used with the human subjects trials, 2) a soft

non-flotation cushion, and 3) a rigid wood block. The

dimensions of the soft cushion and the rigid block

were such that the ATD’s hip joint (H-Point) was in

the same initial position as when sitting on the flotation cushion.

Load was applied with a force gage to the free end

of the webbing in a direction parallel to the entrance

of the webbing from the buckle, as shown in Figure 8.

The tension produced and the position of the ATD’s

hip joint was noted as the load applied was increased

in 5-lb. increments. Fifteen pounds of force was

required to overcome static friction in the buckle

before slack could be removed from the belt. As can be

seen in Figure 9, after the initial compliance of the

ATD and seat cushion, the relationship between applied load and tension produced is linear.

The standard practice for setting the belt tension

prior to a sled test is to tighten the belt until two

fingers can be comfortably placed between the belt

and the ATD’s abdomen.(2) As part of this study, the

belt tension was adjusted by two experienced CAMI

laboratory personnel using the standard practice.

Shown in Figure 9, the resulting tension differed by

almost 5 lb (4.4 vs. 9.2 lb). Clearly, consistency is not

a hallmark of this method.

The relationship between the ATD compression

into the seat cushion and the belt tension is shown in

Figure 10. The total compression into the seat cushion is affected by the combination of the stiffness of

the cushion and the foam that makes up the ATD’s

pelvis flesh. When the belts are tightened with the

ATD seated on the rigid block, the soft foam flesh

under the ATD’s ischial tuberosities compresses until

it bottoms out. Increasing the tension does not further

vertically displace the ATD.

When the ATD is seated on the firm flotation

cushion and the belt tension is increased, the vertical

displacement of the ATD mimics the load-displacement characteristic of the rigid block, up to the

bottoming point of the pelvic flesh. Increasing the

belt tension results in further vertical displacement of

the ATD as the pelvis continues to compress the seat

cushion, which has a stiffness characteristic slightly

higher than the ATD pelvic flesh compliance.

When the belts are tightened with the ATD seated

on the soft non-flotation cushion, the cushion begins

to compress prior to the point at which the pelvic flesh

bottoms out, indicating that the cushion foam is less

stiff than the pelvis flesh.

These measurements indicate that the differences

between the belt tension – ATD displacement characteristics for the rigid block, soft cushion, and firm

cushion are small in the normal tension range. From

Figure 10, the difference in the ATD’s H-Point displacement is less than 0.02 inch when the belt tension

is 7 lb. Although adjustment of the pre-test belt

tension using the “two fingers” method may not

provide consistency in the resulting tension, differences in ATD displacement from the nominal +1-Gz

seated location are insignificant.

Discussion of Results

In general, the short answer to “How tight do

passengers tighten their lap belt?” is “not very tight.”

This conclusion, while not surprising, is important to

seat and restraint designers, as well as those conducting dynamic sled tests. Data from human subjects in

this study indicate that the tension passengers typically produce when adjusting their lap belts in nonemergency situations is less than 7 lb.

The general guidance suggested by this study is that

the belt tension prior to a horizontal dynamic test

should be high enough to remove all slack but should

not disturb the initial position of the seated ATD.

Although the person executing the procedure affects

the lap belt tension arrived at using the “two fingers”

method, the resulting pre-test tension may be considered sufficiently representative for the purpose of

conducting sled tests. Considering that lap belt tension during a 16-G sled test may exceed 2000 lb., the

pre-test tension produced by this method is unlikely

to affect the test results.

LIFT LATCH ANGLE STUDY

Trial Setup. As shown in Figure 11, a typical economy

class triple-place passenger seat was used in this study.

The seat width, belt anchor location, and arm rest

placement at each of the three seat positions were identical. Lap belts with lift latch type buckles were installed

at all three seat locations. The belts had buckles that

released at 30, 60, and 90 degrees. The seat was secured

to the floor, facing a table in front of the seat. A push

button switch was placed on the table directly in front of

each seat place, 48 inches forward of the front edge of the
6

seat and 30 inches above the floor. To reach the switch,

a subject had to release the lap belt buckle, stand up, and

step forward towards the table.

Instrumentation. Two red light-emitting diodes

(LEDs), one mounted on the table in front of the

subject, and one mounted on the back of the seat,

served as “START” indicators. The trial conductor

controlled the illumination of the START LEDs. The

lap belts were instrumented to detect when the buckle

and tang physically separated, i.e. buckle release. This

provided an electronic measurement of the time interval between the START LED illumination and the

release of the buckle by the trial subject. Also, the time

interval between the START LED illumination and

the activation of the table-mounted push button switch

by the subject was measured during this study.

Video Coverage. A normal speed video camera was

placed in front of the trial setup. All three seat positions and the seat back START LED were in the field

of view of the camera. The trial conductor initiated

each trial by pressing a switch that simultaneously

illuminated the two START LEDs, one prompting

the subject to release the seat belt and the other visible

on the video tape for timing synchronization.

Protocol. Each subject performed a trial with each

of the three lift latch configurations. A repeatedmeasures counterbalanced design was chosen for

economy of subjects and to control for the effects of

trial sequence. Three buckle release angles (30, 60,

and 90) were compared in the study; therefore, subjects were assigned to one of six possible experimental

sequences (30-60-90, 30-90-60, 60-30-90, 60-90-30,

90-30-60, and 90-60-30).

In addition to the normal anthropometry gathered

from each subject, the fore/aft and lateral position of

the belt buckle was also measured after each subject

had tightened the belt. A still photo was taken of each

subject when seated with the belt tightened. As

illustrated in Figure 12, calibration targets were placed

on the seat and belt to allow other dimensional data to

be derived from these still photos as necessary.

The human performance factors measured in this

study were: 1) time to release the belt, and 2) time to

egress from the seat and press a button located four

feet from the seat. Any obvious difficulties encountered by the subject were also noted. The same directions were repeated to each subject. For each belt

position, the subjects were instructed to:

• Sit in the seat, fasten the belt, and tighten the belt

snuggly.

• When the START LED illuminates, release the

belt, stand, and press the button on the table as fast as

possible.

Subject Anthropometry. Data were collected from

a total of 201 subjects during this study (107 males, 94

females). The subjects’ anthropometric statistics are

summarized in Table 2. As discussed previously, the

average measurements of height, weight, and waist

girth for this subject pool are larger than the US

population average. Figure 13 shows the relationship

between the measured buckle location and occupant

girth. For reference, Figure 13 includes these dimensions measured with a 50% ATD.

Results. Figures 14 and 15 show a comparison of

results by trial sequence. These figures do not indicate

a learning effect during the three trials with each

subject. When the average release time and average

egress time is compared between the three belt configurations for all three experience levels, there is no

significant difference between them.

This is not to say some subjects did not have some

degree of difficulty. The maximum release time was

2.85 seconds. Fifteen subjects (7% of the total subject

pool) had to try at least twice to get the buckle to

release. Of the 15 subjects with double attempts, 9

occurred with the 90° lift latch configuration. In each

case but one, the second attempt was successful in

releasing the buckle. One subject repeatedly tried to

push, rather than pull, on the release lever, as normally

would be done in an automobile.

Age Height Weight Waist Age Height Weight Waist

Average 37.2 70.4 188.8 37.2 35.4 65.9 165.0 34.9

Max 63.0 77.2 308.4 54.7 65.0 74.8 322.7 49.6

Min 18.0 64.5 120.3 27.2 18.0 60.3 99.4 26.0

US Average - 69.0 (3) 170.0 (3) 33.7 (4) - 63.7 (3) 137.0 (3) 30.8 (4)

Males Females

Table 2. Lift Latch Subject Anthropometry
7

Discussion of Results. Most of the subjects in this

study pulled the lift lever over 90 degrees when attempting to release the buckle in a rapid manner.

Therefore, the effects on seat egress time due to

differences in the lift latch release angles were negligible. It is important to note that this study did not

address the difficulty in releasing the belt if the occupant was in a folded posture due to post crash injuries,

0.00

0.50

1.00

1.50

2.00

2.50

First Second Third

Sequence of Trial

30 60 90

Figure 15. Average Egress Time

0.00

0.50

1.00

1.50

2.00

2.50

First Second Third

Sequence of Trial

30 60 90

Figure 14. Average Latch Release Time

Figure 13. Belt Buckle Location vs. Occupant Girth

R2 = 0.6379

R2 = 0.5171

-10

-5

0

5

10

15

20

0 10 20 30 40 50 60

Waist (inches)

Fore/Aft Position (forward of CRP) Latteral Position (from Seat CL) 

Fore/Aft Position (50 % ATD) Latteral Position (50 % ATD) 

Fore/Aft Positon (Linear Fit) Latteral Position (Linear Fit)

debris, or aircraft inversion. These scenarios could

make it difficult to release a high-angle buckle due to

interference with the abdomen.

Selection of an optimum range for the buckle

release angle requires striking a balance between the

need for easy release, while precluding inadvertent

release during those situations where the lap belts are

necessary for the safety of the passenger.
8

REFERENCES

1. British Civil Aviation Authority. Specification

No. 1.

2. SAE International. Performance Standard for Seats

in Civil Rotorcraft, Transport Aircraft, and General Aviation Aircraft. Warrendale, PA: SAE; 1997.

Aerospace Standard: AS 8049A.

3. Schneider DH, et al. Development of Anthropometrically Based Design Specifications for an

Advanced Adult Anthropomorphic Dummy Family, Volume 1. The University of Michigan, MI;

1983; Transportation Research Institute Report

No. UMTRI-83-53-1.

4. Gordon CC, et al. 1988 Anthropometric Survey

of U.S Army Personnel: Methods and Summary

Statistics. Alexandria, Virginia: Defense Technical Information Center; 1989; Technical Report

No. NATICK/TR-89/044.