NBSIR 75-718

Report of Fire Test on an AM

General Metro Bus

Emil Braun

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

June 1975

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 75-718

REPORT OF FIRE TEST ON AN AM

GENERAL METRO BUS

Emil Braun

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

June 1975

U S, DEPARTMENT OF COMMERCE. Rogers C.B. Morton. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

CONTENTS

Page

1. INTRODUCTION 1

2. TEST RESULTS 2

2.1. Laboratory Tests 2

2.1.1. Motor Vehicle Safety Standard

(MVSS-302) 4

2.1.2. Radiant Panel Test (ASTM E-162). . 4

2.1.3. Standard for the Surface Flammability of Carpets and Rugs (Pill

Test - DOC FF 1-70) 6

2.1.4. NBS Flooring Radiant Panel Test

(FRPT) 6

2.1.5. Smoke Density Chamber (NFPA 258T) . 6

2.1.6. Upholstery Tests on Seats 7

2.2. Full-Scale Tests on the Bus 8

2.2.1. Test Number 1 8

2.2.2. Test Number 2 9

2.2.3. Test Number 3 9

3. DISCUSSION OF FULL-SCALE TEST RESULTS 10

3.1. Temperature Data 10

3.2. Smoke Data 11

4. CONCLUSIONS 11

5. REFERENCES 12

iii

REPORT OF FIRE TESTS ON AN

AM GENERAL METRO BUS

Emil Braun

The Center for Fire Research at the National

Bureau of Standards has conducted a study of the

fire safety of a bus supplied by the Washington D.C.

Metropolitan Transit Authority. The objectives of

the work were: (1) to determine the minimum ignition from source necessary to initiate a fire in

the bus, and (2) to determine the means by which

a fire, once started, is most likely to grow and

spread

.

A series of small-scale laboratory tests were

run in addition to the three full-scale tests. Tests

showed that accidental ignition by a cigarette or

dropped match is unlikely. However, the seat can

be ignited with one or two matches, if applied at

the proper location, as by an arsonist. In fullscale tests, ignition of the seat occurs readily

with the following ignition sources: (1) a small

bag of paper trash on the seat, (2) a newspaper

under the seat, or (3) if the contents of a can of

lighter fluid is poured on the seat.

Fire growth and spread in the bus is primarily

through involvement of the seat cushioning. Fire

spreads from seat to seat with little direct involvement of other interior materials. In all three

tests, between one and two minutes after the urethane ignited, dense smoke filled the bus space

seriously reducing visibility. Spread of fire

beyond the seat of origin is not necessary for the

level of smoke to be formed.

Key words: AM General bus; arson; critical radiant

flux; fire retardant; f lammability ; Flooring Radiant

Panel Test; Metro; Motor Vehicle Safety Standard 302;

urethane

.

1. INTRODUCTION

Beginning in mid-December 1974, the Washington Metropolitan Transit Authority (Metro) experienced a series of

bus fires involving their new fleet of AM General Buses.

1
Newspaper accounts of these fires indicated that ignition

and subsequent flame spread within the bus interior occurred

in a very short time period. The causes of these fires have

never been determined, but the Fire Marshal of the District

of Columbia has listed them as being of suspicious origin,

possibly arson.

After the initial bus fire in December, arrangements

were made to obtain a bus on which the National Bureau of

Standards (NBS) could perform fire tests. On March 26,

1975, Metro delivered to NBS a bus that had been involved in

a traffic accident, but with sufficient undamaged portion { that testing could be carried out to determine: (1) the

minimum heat source necessary to produce an ignition in the

bus interior, and (2) the route by which the fire (smoke and

flame) propagates from the point of initial ignition to the

rest of the bus interior.

NBS conducted a program of laboratory and full-scale

tests to develop this information. Small-scale laboratory

tests describe the performance of the individual materials.

Full-scale testing is needed to determine the performance of

the complete assembly in a fire environment. The results of

the small-scale tests were used to design the full-scale

test sequence. NBS performed three full-scale tests on the

bus supplied by Metro.

2. TEST RESULTS

2.1. Laboratory Tests

^ With the exception of Motor Vehicle Safety Standard 302

[1] which the National Highway Safety Administration requires

be applied to all non-metallic interior materials, only the

tests deemed appropriate for the particular application of

the material in the bus were used.

The floor carpet was subjected to the Flooring Radiant

Panel Test [2] as well as the test prescribed in the Standard

for the Surface Flammability of Carpets and Rugs (DOC FF .1- 70) [3] . Carpeting on the walls and ceiling was subjected

to ASTM E-162 [4]. Smoke density measurements, using a smoke

density chamber (NFPA-2 58T) [5] were conducted on the carpeting, seating materials and other interior components. In

addition several special tests which are not published standards were performed on the seat assembly in order to determine

the minimum energy level required for ignition. Results of

the laboratory tests are listed in table 1.

^Numbers in brackets refer to the literature references listed

at the end of this paper.

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All synthetic materials found in the bus interior were

chemically characterized. These data are listed in table 2.

2.1.1. Motor Vehicle Safety Standard (MVSS-302)

Motor Vehicle Safety Standard 302 is a test for horizontal burn rate. The standard requires that all materials used

in the occupant compartment of a motor vehicle have a horizontal burn rate of less than 4 inches per minute. It can

be seen from table 1 that the advertising signs were the only

materials which failed to meet the minimum requirements set

forth by MVSS-302. The other materials either did not

ignite or flames spread across the surface at a rate of less

than 4 inches per minute.

2.1.2. Radiant Panel Test (ASTM E-162)

This method measures flame spread and rate of energy

release under a varying radiant flux ranging from 4 to 0.3

W/cm^ . The flame spread factor, P. calculated from the

heat flux level and the flame spread velocity, and the rate

of heat release of the burning sample, Q, are combined to

yield a flammability index, I

g

, defined as

I = F • Q

s s The higher the index, the greater is the flammability.

The values tabulated in table 1 represent the flammability

indices found for the two carpet materials and the head pad

at the rear of the bus.

There is no generally accepted level of performance,

based on this test method since it is not a prescriptive

standard. However, results for three materials are listed

below in order to place the Metro bus data in context.

Flexible Polyether Urethane containing: I

a fire retardant

Plywood [7]

Nylon Carpet [7]

[6] 10

196

368
Table 2. Chemical Analysis of Metrobus Components

Material Basic Composition

Dif f user Polystyrene

Poster Styrene/butadiene coated with

Ethylene-ethylaciylate copolymer

Seat Cushioning Polyurethane (Polyether type)

( b

)

Upholstery Fabric Nylon 6/6

Head Pad

Cover material

Foam material

Poly (vinyl chloride-acetate)

Polyurethane

Carpeting

Floor

Wall . . Ceiling K 1

Nylon 6/6

Nylon 6/6

Acrylic Fiber and Polypropylene

Comparable materials taken from another Metrobus; present

materials currently being analyzed.

May be flame retardant based on elemental analysis - x-ray

fluorescence.

5
2.1.3. Standard for the Surface Flammability of

Carpets and Rugs (Pill Test - DOC FF 1-70)

No carpets and rugs sold in the United States are

permitted to spread a flame beyond a radius of 3 inches when

exposed to a small ignition source (a methenamine pill)

.

This standard is intended to eliminate carpets or rugs as an

agent of flame spread from small ignition sources. All the

carpeting found in the bus passed the "pill test."

2.1.4. NBS Flooring Radiant Panel Test (FRPT)

This test method exposes a specimen placed horizontally

to a radiant energy load that varies across a 1 meter length

from a maximum of 1.1 W/cm^ to 0.1 W/cm^ . The specimen is

ignited by a small flame at the high energy end. The distance of the sample from the end at which the burning flooring

material extinguishes itself defines the critical radiant

flux (CRF) necessary to support continued flame propagation.

The higher the CRF, the better is the fire safety of the

carpet

.

The critical radiant flux for the carpet specimen from

the floor of the bus was found to be 0.66 W/cm^ . For comparison purposes, carpeting taken from several large fatal fires

was also tested according to this method and found to have

CRF's of less than 0.1 W/cm^ . A wood floor would have a CRF

between 0.4 and 0.5, while vinyl floor materials have values

greater than 1.1 W/cm^ . Thus, the Metrobus carpeting, by

this test, is comparable to other conventional flooring

materials

.

2.1.5. Smoke Density Chamber (NFPA 258T)

This test method measures the smoke generation of solid

specimens exposed to a radiant flux level of 2.5 W/cm^ . The

smoke produced by the burning specimen in the chamber is

measured by a light source-photometer combination. The

maximum attenuation of the light beam by the smoke is a

measure of the optical density or "quantity of smoke" that a material will generate under the given conditions of the

test

.

Two ignition modes are possible, flaming and nonf laming.

All data presented here were taken under flaming ignition

conditions in both vertical and horizontal positions. The

data in table 1 are the maximum optical density, Dm , found

for the various materials. Dmvalues measured on our

apparatus can range from 0 to 600 units.

6
The values in the Metro bus materials ranged from a low

67 for the upholstery fabric to 354 for the wall carpet. In

order to place these results into proper context, comparable

results for other materials are listed below.

Dm

Plywood [8] 45

Nylon (foam backing) Carpet [8] 270

Red Oak Flooring [8] 300

Glass Reinforced Polyester [8] 720

The values of Dm are useful primarily in ranking relative

smoke production of the materials used, and in identifying

likely sources of severe smoke production in a full-scale

fire. The actual smoke levels produced when bus fire occurs,

however, can only be determined in a full-scale test.

2.1.6. Upholstery Tests on Seats

A test method has recently been developed at NBS to

determine the ignitability of upholstered furniture when

exposed to a lighted cigarette. The test determines if an

assembly ignites or does not ignite. It is performed on

flat surfaces (i.e., seat cushions) and in any crevices in

the entire assembly (i.e., junction of the back and seat

cushions)

.

By this test, the seat assembly did not ignite, when in

contact with a lighted cigarette. In a variation of the

test, the cigarette was replaced by a methenamine pill,

similar to those used in the carpet and rug test (DOC FF 1-

70) . The methenamine pill placed on the surface of the

cushion did not ignite the seat assembly. However, placing

the pill in the crevice between the seat back and bottom

resulted in the ignition of the seat back.

Finally, ignition by matches was attempted. It was

found that the seat would ignite with a single paper match

held under the front seat edge. The fire grew and spread

very slowly. Flaming matches dropped on the junction of the

seat cushion and back did not ignite the seat assembly.

However, if two matches were carefully placed in the crevice

area, ignition of the seat assembly did occur.

7
2.2. Full-Scale Tests on the Bus

The Metro bus was modified by removing the entire

forward compartment and building a wall just forward of the

side exit door. This resulted in a total enclosed volume of

approximately 25 m3 (885 ft^) which represents about onehalf the total volume of a complete bus. Figure 1 shows the

location of various ignition and ventilation points. Also,

a smoke meter was permanently installed near the side exit

door so that it would measure the light attenuation in a

path from floor to ceiling. In addition, thermocouples were

placed in several positions within the bus compartment.

In the first two tests, the fire was permitted to grow

until it could be determined what path the fire was taking

in spreading down the bus interior. Once this was determined,

the fire was extinguished by a trained firefighter. The

third test was permitted to progress to the point of window

failure and then was extinguished.

All tests were photographed and taped by video camera.

Other comments by NBS investigators were tape recorded.

2.2.1. Test Number 1

The ignition point was the center of a complete seat

assembly. A bag comprising a total of 30 g (1 oz) of paper

was placed in such a way that it rested on the junction

between two adjacent seat cushions and touched both seat

backs. A match was used to ignite the paper bag at the top.

Data acquisition began with the ignition of the paper bag.

Ventilation within the bus was controlled by closing

the side exit door and all of the windows except for one.

The window was located on the opposite wall and to the rear

of the ignition point. It was pushed back to give a 7.5 cm

(3 inch) opening. Sometime during the latter part of the

test, this window was unintentionally closed. The total

test time was just over 8 minutes.

At the conclusion of the test, an inspection of the bus

interior revealed that flames had spread to the adjacent

seat behind the ignition point. It was also noted that the

light diffusers above the windows had melted and fallen to

the seats below; they did not appear to have burned in

place. Advertising signs were not included in this test.

8
2.2.2. Test Number 2

The initial source of ignition in this test was .91 kg

(2 lbs) of newspaper placed on the floor below a seat assembly.

The newspaper was ignited in several places on its surface

by a match. Data acquisition began with the ignition of the

newspaper.

Ventilation in this test was accomplished by opening

the forward-most window 4 cm (1-1/2 inches) on the same side

as the ignition source. This window remained open throughout

the test, which lasted 7 minutes.

For tests 2 and 3, a TV camera was placed in the wheelwell cover on the opposite wall so that it was looking up

into the bus.

This test was conducted in the next-to-last row of

seats. Normally a head pad is located just behind these

seats. For this test, it was removed, leaving the metal

back panel exposed. Advertising signs were placed over the

light diffusers as they would be in actual operation.

By the conclusion of the test, the signs and diffusers

had fallen to the seats below. The adjacent seat forward of

the ignition point also ignited. This observation was made

by the second camera and was later substantiated by the

temperature data taken around that seat assembly.

2.2.3. Test Number 3

In this test, the ignition medium was 250 ml (8 1 oz)

of lighter fuel (roughly equivalent in energy content to 0.5

kg of paper) poured onto the seat cushion adjacent to the

wall. A thermocouple was placed just above the seat cushion

(0.3 cm) that had the lighter fuel on it. Ignition was

achieved from the outside of the bus by electrically igniting

a match held just above the ignition point. Data acquisition

began aproximately 2 seconds before the actual ignition of

the seat cushion.

Also, in this test, ventilation was controlled by a

window previously set open to 7-1/2 cm (3 inches) . The test

lasted 10.5 minutes.

For this test, the rear end of the bus was completely

furnished. Diffusers, signs, and head pads were all in

place. Initial flame spread was due to falling material from

the head pad that ignited other seat assemblies. The ignition

point was completely destroyed in addition to the back seat

9
and portions of the next forward seat. The window adjacent

to the ignition point collapsed from its frame and fire

damage spread to the next window pane and the rear window*

The heat trapped in the bus also started to warp the other

window panes.

3. DISCUSSION OF FULL-SCALE TEST RESULTS

3.1. Temperature Data

The temperature data obtained from all three tests at

several locations throughout the bus were compared in order

to determine: (1) the characteristics of the ignition

source, (2) the effect of ignition source on fire spread and

growth, and (3) the probable flow of heat in the compartment

in relation to the open window.

Figure 2 is a comparison of temperatures above each

ignition source. The rate of temperature rise (which is an

indicator of the rate of heat release from the ignition

source) is similar for test 1 and 2. The basic difference

is the induction time that preceded any significant temperature rise. Test 1 had an induction time that was twice as

long as test 2. The induction time for test 3 was too short

to be measured accurately. It is evident from figure 2 that

the initial heating rate was much higher in this test than

the others.

There are several key times marked on the graphs (points

A-D) . The first represents the time when the foam became

involved, based on visual observations. The second and third

numbers represent the time when visibility into the bus fell

to zero and the end of the test, respectively. Window failure

occurred only in the last test and is marked on the plot for

test 3.

Figures 3 and 4 present the aisle ceiling temperatures

fore and aft of the ignition point. A comparison of the

plots for test 1 indicates that there was a uniform buildup

of heat at the ceiling until the test was terminated. It

should be kept in mind that sometime during the test the

ventilation window was inadvertently closed thereby eliminating any airflow bias.

Similar data from test 2 show a definite forward movement of heat. One of the forward windows was open 4 cm (1.5

inches) to provide ventilation to the combustion process.

10

S
The temperature profile for test 3 is very much different

from the previous two tests. It indicates that burning had

reached a quasi-steady state condition after about 3 minutes

and in fact was probably oxygen limited.

3.2. Smoke Data

In addition to the temperature recordings, smoke density

data in the form of percent transmission of polychromatic

light was obtained at the top of the stairs by the exit

door. These readings were taken in the vertical plane and,

therefore, were not sensitive to smoke stratification.

Figure 5 is a plot of the percent of light transmission

as a function of time from initial ignition to the conclusion

of the test. The data indicate that almost total obscuration

occurred between one and two minutes after the ignition of

the urethane foam padding in the seat assembly. Also, the

data indicate that the rate of increase in smoke density,

once the foam had been ignited, was not appreciably different

in the 3 tests. Zero visibility into the bus corresponds to

a decrease in light transmission of about 90 to 95 percent

of the original light level.

In comparing the smoke data from all three tests, the

most critical event is the ignition of the foam padding in

the seat assembly, since zero visibility occurs about a

minute later. Referring to figure 5, the times to ignition

of the foam for tests 1 and 2 were approximately 240 seconds

and 160 seconds, respectively. For test 3, the time was too

short to observe the ignition of the foam and it was arbitrarily set at 5 seconds. No data was obtained on the

toxicity of the combustion gases or the effect of eyeirritation.

4. CONCLUSIONS

The interior furnishings of the bus all have burn rates

below 4 inches per minute, in conformance with the Department

of Transportation's Motor Vehicle Safety Standard 302. The

results of other laboratory tests for the various materials

are generally in line with what is typical for the class

represented. Full-scale tests were necessary to determine

the interaction of materials and their performance under

actual fire conditions.

11
Based on the full-scale bus tests, the seats are the

most probable source of hazard, given an ignition. The

seats can be ignited with a source as small as a single

paper safety match. Accidental ignition, for example via

paper trash, may take a few minutes (2-1/2 and 4 min in the

two tests run) to ignite the urethane foam on the seats.

Use of lighter fluid to simulate arson produced ignition

of the urethane foam padding within a few seconds. Once the

urethane seat padding catches fire a serious smoke hazard

develops quickly. Within one to two minutes after the foam

padding catches fire, visibility is nil within the bus.

Evacuation of the bus would be imperative but would be made

difficult because of the reduction in visibility. The fire

from one seat, fully developed, is substantial and the heat

is a hazard to occupants close by.

The vulnerability of the buses to accidental ignition

and to arson can be reduced by removing the urethane padding,

or preventing its ignition by use of a high-resistant fabric

or by placing a barrier beneath the fabric. The level of

fire safety of the passengers is determined in part by comparing the time required for safe evacuation with the time

between the initial ignition and the ignition of the urethane

paddings. The safety margin for escape, based on the above

results, will not be very large. Clearly, the level of fire

safety, in terms of passenger protection, will be improved in

any case by removing or protecting the urethane padding.

5. REFERENCES

[1] Motor Vehicle Safety Standard - 302, National Highway

Traffic Safety Administration, Federal Register, Vol.

36, No. 5, 70 (January 8, 1972).

[2] Hartzell, L. G., Development of a Radiant Panel Test

for Flooring Materials, Nat. Bur. Stand. (U.S.), NBSIR

74-495 (May 1974)

.

[3] Standard For the Surface Flammability of Carpets and

Rugs, FF-1-70, Federal Register, Vol 35, No. 74,

6211-6212 (April 16, 1970)

.

[4] Surface Flammability of Materials Using a Radiant Heat

Energy Source, E-162, American Society for Testing and

Materials. Philadelphia, Pa. (1967)

.

12
[5] Test Method For Measuring Smoke Generated by Solid

Materials - 258T, National Fire Protection Association,

1974 NFPA Technical Committee Reports, Vol. IB, 1377-1424

(May 20, 1974) .

[6] Robertson, A. F., Surface Flammability Measurements by

the Radiant Panel Method, ASTM Symposium on Fire Test

Methods, Special Technical Publications, No. 344-1962.

[7] Rose, A., Comparison of Flame-Spread Ratings by Radiant

Panel, Tunnel Furnace, and Pittsburgh Corning Apparatus,

National Research Council of Canada, NRC 10788 (June 1969)

.

[8] Lee, T. G., The Smoke Density Chamber Method for

Evaluating the Potential Smoke Generation of Building

Materials, Nat. Bur. Stand. (U.S.), Tech. Note 757

(Jan. 1973)

.

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BIBLIOGRAPHIC DATA

SHEET

1. P.UBLK A HON OH REPORT NO. NBSIR 75-718

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Report of Fire Tests on An AM General Metro Bus

5. Publication Date June 1975

6. Performing Organization Code

7. AUTHOR(S)

Emil Braun

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAMF AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Task/Work Unit No.

/ Q 0 7 7 7 1

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

same as No. 9

13. Type of Report & Period Covered

Filial

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The Center for Fire Research at the National Bureau of Standards has conducted a study of the fire safety of a bus supplied by the Washington D.C. Metropolitan Transit

Authority. The objectives of the work were: (1) to determine the minimum ignition

from source necessary to initiate a fire in the bus, and (2) to determine the means by

which a fire, once started, is most likely to grow and spread.

A series of small-scale laboratory tests were run in addition to the three full-scale

tests. Tests showed that accidental ignition by a cigarette or dropped match is unlikely. However, the seat can be ignited with one or two matches, if applied at the

proper location, as by an arsonist. In full-scale tests, ignition of the seat occurs

readily with the following ignition sources: (1) a small bag of paper trash on the seat,

(2) a newspaper under the seat, or (3) if the contents of a can of lighter fluid is

poured on the seat.

Fire growth and spread in the bus is primarily through involvement of the seat cushioning. Fire spreads from seat to seat with little direct involvement of other interior

materials. In all three tests, between one and two minutes after the urethane ignited,

dense smoke filled the bus space seriously reducing visibility. Spread of fire beyond

the seat of origin is not necessary for the level of smoke to be formed.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

AM General bus; arson; critical radiant flux; fire retardant;

f lammability; Flooring Radiant Panel Test; Metro; Motor Vehicle Safety Standard 302;

urethane

.

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