ComPARISON OF BUCKLE RELEASE TIMING
For Pusu Burton AnD Lirt-LarcH Bett BUCKLES

INTRODUCTION

The most common type of belt buckle release
mechanism for small aircraft passenger restraint systems
is the lift-latch buckle. Other release mechanisms, such
as push-buttons, are less common in aircraft restraint
systems, although they are not explicitly prohibited for
use in small aircraft by the Federal Aviation
Administration’s (FAA’s) standardsand regulations pub-
lished in 14 CFR Parts 23 and 91. In fact, there are no
specific design or performance criteria in FAA advisory
documents or Technical Standard Order approval pro-
cedures for restraint systems that preclude the use of
push-button release mechanisms on aircraft passenger
seat belts. Recent applications seeking approval for the
installation of restraints with push-button buckles on
small aircraft passenger restraints have been submitted
to the FAA. Due to the novelty of push-button buckles
on airplane passenger seats, it is important to examine
the human factors related to the operation of push-
button buckles and egress from a seat in order to ensure
an equivalent level of safety. A key factor in determining
safety equivalence is the time it takes for a passenger to
release the restraint buckle and get out of the seat.

A study of restraint release mechanisms, using hu-
man subjects, was performed at the FAA Civil Aero-
medical Institute (CAMI) to determine if there was a
significant difference between the time it takes a subject
to releasea lift-latch buckle versus a push-button buckle.

Also measured was the time taken for subjects to fully
extricate themselves from the restraint system, and get
out of the seat. The following summarizes the test
procedure and results obtained from this project.

Description of Tests

Subjects. Sixty volunteer subjects participated in the
study. Physical data, including sex, age, height, weight,
waist girth, and handedness were recorded for each of
the human subjects prior to testing. Table 1 contains
the data summary of the subjects used for these tests.

Design. A repeated-measures counterbalanced
design was chosen for economy of subjects and to
control for the effects of trial sequence. 3 restraint
release mechanisms (A, B, and C) were compared in
the study. The subjects were randomly assigned to 1
of 6 possible experimental sequences (ABC, ACB,
BAC, BCA, CAB, and CBA).

Apparatus. As shown in Figure 1, a triple occu-
pant seat bench was configured with 3 restraint
systems installed side-by-side. The bench seat had 2-
inch thick foam cushions covered with a vinyl fabric
installed on the seat and backrest areas. There were
no armrests on the seat fixture. The lap belt and
upper torso strap anchor point geometry were repre-
sentative of a typical aircraft installation.

AGE HEIGHT WEIGHT GIRTH
Mean 39.3 yrs 69.0 in. 170.4 Ibs 34.1 in.
Std Dev 12.1 3.7 38.1 51
Minimum 18 62.3 106.3 26.0
Maximum 69 76.3 330.4 53.0

Total Subjects: 60(28 Males, 32 Females)
Handiness: 57 Right Handed, 3 Left Handed

Table 1. Subject Data.

1



Number of Subjects

Number of Subjects

Button Event A
3-Point, Push Button

12
N=60
10 4 Mean = 3.16
Std Dev = 1.03
84
64
44
24
04
0.0 05 #10 15 #20 25 30 35 40 45 5.0 5.5 6.0
Time in Seconds
Figure 9. Button event egress time for subjects in restraint A.
Button Event B
3-Point, Lift Latch
12
N=60
10 +- - - - - - - - - Mean = 3.14

Std Dev = 1.02

05 1.0 15 #20 25 30 35 40 45 50 55 6.0
Time in Seconds

Figure 10. Button event egress time for subjects in restraint B.

6


Figure 2. Restraint A: 3-point with push button
buckle release.

Figure 3. Restraint B: 3-point with lift latch
buckle release.

Figure 4. Restraint C: Lap belt with lift latch
buckle release.

buckle the restraints and adjust the tension of the belts
without assistance. They were instructed to place their
hands on their knees prior to the test. Each subject was
instructed to unlatch the buckle “quickly” when the red
LED illuminated, then stand up, and quickly move to
depress the push-button switch. The room lights were
turned off, and a small lamp projected against a
laboratory wall was the only illumination during the test.
The lamp was adjusted to provide the equivalent of
minimum aircraft cabin emergency lighting conditions
(0.5 ft-candles), which was measured on the tabletop
next to the LED. The test operator waited a few seconds
to allow the subject’s vision to adjust to the dim lighting
condition, then switched on the red LED to begin the
time interval measurement. After the test, this sequence
was then repeated with the same subject using the other
2 restraint systems.

RESULTS

Figures 5 through 7 present the buckle release time
data. The mean release time for the 3-point restraint
with a push-button buckle (A) was 1.25 seconds, the
mean release time for the lift-latch 3-point restraint (B)
was 1.10 seconds, and the mean release time for the
standard lift-latch seat belt (C) was 0.91 seconds. The
cumulative frequency plot of all 3 restraint systems,
shown in Figure 8, shows the release times for the 3
restraint systems,

Data for the time intervals for subjects to release the
buckle, exit the seat, and then depress the push-button
switch are presented in Figures 9 through 11. The mean
times for buckles A, B, and C were 3.16 seconds, 3.14
seconds, 2.55 seconds, respectively. The cumulative
frequency plot of times for button pushing for the 3
restraint systems is shown in Figure 12.

DISCUSSION

The small differences discovered in the mean times
needed to release the buckles of the 3 restraint systems
(ranging from 0.91 to 1.25 seconds) do not raise a serious
concern related to the effect on occupant egress of
restraint systems with a push-button, as compared with
the lift-latch type buckle. As with the buckle release
times, the small differences in times for an occupant to
release the buckle and egress from the seat (ranging from
2.55 to 3.16 seconds) do not raise concerns about the
abilities of aircraft occupants to egress from seats outfit-
ted with these particular restraint systems.


Photos of the 3 restraint system buckles are shown in
Figures 2, 3, and 4. Restraint A was a 3-point restraint
with a push-button buckle on the left side, and an upper
torso strap over the right shoulder into an inertia reel.
The push-button buckle was similar to ones used in
contemporary automobiles. The force required to re-
lease the restraint by pressing the push-button was mea-
sured to be approximately 20 N (4.5 Ib), and the button
travel was approximately 6.5 mm (.25 in.)

Restraint B was a 3-point restraint with a lift-latch
buckle common to many aircraft restraints. It was
similar to the Restraint A, having the buckle on the
left side and an upper torso restraint over the right
shoulder into an inertia reel. The lap belt tension for
Restraints A and B were manually adjustable by pull-
ing the webbing on the right belt through an adjuster
mechanism. When adjusted to a snug fit, the buckles
on Restraints A and B were positioned on the left side
of the subject’s pelvis.

Restraint C was a typical transport passenger seat
lap belt with a lift-latch buckle. The buckle was
approximately centered over the pelvis of an average-
size adult passenger.

Electrical wires were connected to the metal insert and
to the buckle receptacle hardware on each of the 3
restraint systems. Electrical continuity was established
when the restraint was buckled, and an open circuit

Figure 1. Test Apparatus.

indicated the buckle had been unlatched. The wires
from the buckle were connected through a simple inter-
face circuit to an electronic interval timer.

A bright red light-emitting diode (LED) was posi-
tioned at table height, 80 cm (31 inches), approxi-
mately 102 cm (48 in) in front of the bench seat. The
LED was illuminated by means of a toggle switch
under the control of the test operator. Wires from the
toggle switch were connected to the same interval
timer as the wires attached to the buckles. The
interval timer was set to start measuring time when the
LED was illuminated by action of the toggle switch.
The time interval measurement stopped when electri-
cal continuity of the wires connecting the buckles was
interrupted by the action of the subject unlatching the
buckle. A 40 mm (1.5 in) diameter push-button
switch was placed next to the red LED approximately
102 cm (48 in) forward of the bench seat. The button
was mounted horizontally, and wires from the push-
button were connected to a second time interval
counter. The second interval counter was set to start
measuring time when the red LED was illuminated
and stop when the subject depressed the push-button
switch.

Protocol. Subjects were given brief, individual oral
instructions describing the task to be performed. After
being seated on the test seat, the subjects were allowed to


Buckle Event C
Lap Belt, Lift Latch

12
N=60
10 + - - Mean = 0.91
Std Dev = 0.39
gy
g
>
a
6 67
3
Bat
24]
24
o4
0 0.5 1 1.5 2 2.5 3 3.5 4
Time in Seconds
Figure 7. Buckle release time for restraint C.
Cumulative Frequency
Buckle Event Time
100%
> 80% +-
i=
oa
=
£ 60% +-
g
sg 40% +-
=
fo)
20%
0%

Time in-Seconds

—=— Buckle EventA —*— Buckle EventB —-*— Buckle Event C

Figure 8. Cumulative frequency for release times of restraints A, B, and C.

5


DOTIFAA/AM-99/5 Comparison of Buckle

coffee of Aviation Medi Release Timing for Push-
ice 01 viation edicine e
Washington, D.C. 20591 Button and Lift-Latch

Belt Buckles

Van Gowdy
Mark George
G. A. McLean

Civil Aeromedical Institute
Oklahoma City, Oklahoma 73125

February 1999

Final Report

This document is available to the public
through the National Technical Information

Service, Springfield, Virginia 22161.

U.S. Department
of Transportation

Federal Aviation
Administration


Variables not included in this limited study were arm
rests and seat width. In narrow width seats, arm rest
interference with access and/or release actions could
affect the time for an occupant to release the buckle, and
should be considered in the design of passenger seats
with 3-point restraints. Another important variable not
addressed in this study was the push-button release force.
Although design and performance specifications are
imposed for automobile push-button restraints (1,2),
previous studies indicate significant variation in the
relationship between age and strength capability to press/
release a push-button buckle (3,4).

It must be emphasized that this study was intended to
address factors associated with the use of push-button
buckle restraint systems in small airplanes. Any consid-
eration of the use of push-button buckles on commercial
transport aircraft passenger seats should include data on
a broader range of human factors, such as the effects of
age, familarity with operation of the buckle, cultural
differences, physical limitations, and contrariety with
the universal use of lift-latch buckles in air transport
operations.


=

=

Number of Subjects

Cumulative Frequency

Button Event C

Lap Belt, Lift Latch

2
N=60
o+ ed Mean = 2.55
Std Dev = 0.76
et - - - - - - ~
et - - - - - - - - = - - 4

05 #10 #15 20 25 30 35 40 4.5
Time in Seconds

Figure 11. Button event egress time for subjects in restraint C.

5.0 5.5 6.0

Cumulative Frequency

Button Event Time

:

8
x
t

8
R
t

20% +

Time in Seconds

—*— Button EventA —*— Button EventB —*—- Button Event C

Figure 12. Cumulative frequency for button event egress times for

subjects in restraints A, B, and C.

7


Number of Subjects

Buckle Event A

3-Point, Push-Button Buckle

12
N=60
10 + - Mean = 1.25
Std Dev = 0.67
84
6 4
at
24
o4
0 0.5 1 1.5 2 2.5 3 3.5 4
Time in Seconds
Figure 5. Buckle release time for restraint A.
Buckle Event B
3-Point, Lift Latch Buckle
12
N=60
10+ - - Mean = 1.10
Std Dev = 0.46
2
8 87
roy
>
a
3 64
3
bad
24
24
o4

0.5

1 1.5 2 2.5 3 3.5 4
Time in Seconds

Figure 6. Buckle release time for restraint B.

4




Technical Report Documentation Page

1. Report No. 2. Government Accession No.

DOT/FAA/AM-99/5

3. Recipient's Catalog No.

4, Title and Subtitle 5, Report Date
Comparison of Buckle Release Timing for Push-Button and Lift-Latch February 1999
Belt Buckles
6. Performing Organization Code
7, Author(s) 8. Performing Organization Report No.

Gowdy, V., George, M., and McLean, G. A.

9. Performing Organization Name and Address
FAA Civil Aeromedical Institute

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

P.O. Box 25082,
Oklahoma City, OK 73125

12. Sponsoring Agency name and Address
Office of Aviation Medicine

18. Type of Report and Period Covered

Federal Aviation Administration 14. Sponsoring Agency Code
800 Independence Ave., S.W.

Washington, DC 20591

15. Supplemental Notes

16. Abstract

Small aircraft passenger restraint systems most commonly use lift-latch type buckle release mechanisms. Push-
button buckle release mechanisms, similar to those used in contemporary automobiles, have rarely been used on
passenger restraints. Although push-button buckles are not explicitly prohibited by Federal Aviation Administration
(FAA) regulations, the human factors aspects of introducing push-button buckles in an aircraft environment are
important considerations from the standpoint of safety. A test program was conducted by the FAA Civil Aeromedical
Institute (CAMI) with volunteer human subjects to measure and compare the times it takes a passenger to release a
push-button buckle on a 3-point restraint, a common lift-latch buckle on a 3-point restraint, and a lift-latch buckle on a
common lap belt. Sixty subjects were tested in a repeated-measures counterbalanced test protocol, which included
instrumentation to measure the response times to release the buckle. Response time for the subjects to exit the seat and
press a remote button was also acquired. This report includes the physical profiles of the subjects, the test protocol, and
a statistical summary of the results.

Based on the data acquired in this project, there was no major difference in the response times of the human
subjects to release or egress from a 3-point restraint with a push-button buckle, compared with a lift-latch buckle on a 3-
point or a common lap belt restraint.

This study was intended to address factors associated with the use of push-button buckles restraint systems in small
airplanes. Any consideration of the use of push-button buckles on commercial transport aircraft passenger seats should
include data on a broader range of human factors.

17, Key Words 78. Distribution Statement
Push-Button Buckles Document is available to the public through the
Small Aircraft Restraints National Technical Information Service,

Springfield, Virginia 22161

79. Security Classit. (of this report)
Unclassified

20. Security Classi. (of this page)
Unclassified

21. No. of Pages 22. Price
ll

Form DOT F 1700.7 (8-72)

Reproduction of completed page authorized



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