Sci-Paper@Computational_Analysis_of_Aircraft_Postcrash_Fire

Open Journal of Technology & Engineering Disciplines (OJTED)

Vol. 1, No. 1, December 2015, pp. 14~29

ISSN: XXXX-XXXX

Journal homepage: http://ojal.us/ojted/

14

COMPUTATIONAL ANALYSES OF AIRCRAFT

POSTCRASH FIRES

Eng Mohamad A Othman and Prof. Dr. Essam E Khalil

Mechanical Power Engineering,

Faculty of Engineering,

Cairo University, Egypt.

Email: [email protected]

Article Info ABSTRACT

Article history:

Received Oct. 5th

, 2015

Revised Nov.7th

, 2015

Accepted Dec 14th

, 2015

Along the days air transportation has been subjected to many

accidents. One of the most serious factors that can affect air

transportation is the fire .Fire either formed from an accident or

formed by it. Fire behavior always complicated and unexpected it

was important to find ways to understand it. Many full-scale tests

have been executed by air transportation safety agencies i.e.: US

FAA. In this study FDS is used to simulate one of these full-scale

tests that performed by FAA on C-133.The software is shown to be in

good agreement with the experimental data, producing reasonable

agreement with the fire dynamics prior to flashover and producing a

reasonable prediction of the onset of flashover .The onset of

flashover occurs at approximately 230 seconds while the predicted

onset of flashover happened at 261 seconds. The paper then used the

mathematical model to examine carbon monoxide concentration, fire

spread, smoke spread and visibility.

Keyword:

CFD, Post-Crash, Fire,

Evacuation.

Copyright © 2015


All rights reserved.

Corresponding Author: Essam E.Khalil

Professor of Mechanical Engineering

Cairo University, Cairo, Egypt.

Email: [email protected]



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1. INTRODUCTION

Commercial passenger aircraft carry large quantities of highly flammable fuel in their

comparatively fragile wing structures and operate at high speeds. Thus, all commercial flight

operations inevitably involve a risk of incurring catastrophic fire accidents. These events can be

initiated even before an aircraft has left the ground. The nature of aircraft fires varies widely.

Crash forces tend to release and spread fuel in an unpredictable manner, and can render an

aircraft’s wreckage distorted beyond recognition. Usually, cargo and cabin furnishings will

become involved in a fire, creating complex chemical reactions which can produce toxic

products. The varied nature of aircraft fires created problems when attempting to analyze and

classify past accidents. Three different categories of fire were eventually identified, namely Post-

crash (Post-Impact) and Inflight. In some cases, fire characteristics appeared to span more than

one category.

From accidents researched it was found that the most prevalent class of fire was found to be the

Post-crash type. By definition, a Post-crash fire is the one where the structural integrity of the

aircraft may be compromised by impact with the ground or any other object before the initiation

of a fire. Post-crash fire covers about 92 percent of aircraft fires [1-4].Most of Post-crash fire

cases, the aircraft’s fuel supply would play a dominant role in feeding and sustaining the fire.

The size of fire present thus depends on the area over which the fuel is spread and the rate at

which flames can propagate across this region. Because of this, it appeared that an open pool fire

analogy might be highly applicable for most accident scenarios. This involves fires being treated

as a radiating flame envelope, arising from a circular area of hydrocarbon fuel situated in an

open space.

2. THE FAA EXPERIMENT

In this section the fire experiment described was undertaken by the FAA .The test performed on

C-133 aircraft [4] (see Figure 1: C-133 Fire test [3] ). The schematic of the experimental test is

shown in Figure 2: C-133 fully-furnished full-scale test arrangement [2]. The total length of

aircraft fuselage was about 23.4 m. The fuselage wide at the floor was 4.52 m and the ceiling

height was 2.44 m. The fuselage forward part was 13.7 m while the rear part length was 9.67 m.

The forward part was completely furnished with 14 rows of seats, in double-triple-double

configuration and single – triple seat in front of galley, resulting 101 seats. Seats were covered

by fire blocking layer. The side walls and storage bins were assemblies constructed of epoxy-

Fiberglas honeycomb panels. The carpet was 90/10 wool/nylon. The ceiling was composed of

flat sheets of epoxy-Fiberglas and epoxy-Kevlar honeycomb panels. To increase the reality of the

test, a small number of luggage’s were placed in storage bins and beneath seats.



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As shown in Figure 2: C-133 fully-furnished full-scale test arrangement [2] the fuselage cabin

rupture which allows the fire to enter the cabin was represented by the forward exit, while the

rear exit was used to provide ventilation for the fire. The outside fuel fire source was located on

the left side of the fuselage in the simulation. A rectangular fuel pan with dimensions 3.05m long

and 2.44m wide was used in the post-crash cabin fire experiments [4]. The theoretical heat

release rate for an open pool fire corresponding to the dimensions used in the experiment is

approximately 11.9 MW according to the empirical correlation for large-area Kerosene pool fires

[5].

Temperatures and heat fluxes were measured inside the cabin. Thermocouples were placed

above the top of the seat back at rows 5, 7, 9 and 15 at an average height of 1.225 m above the

floor. The heat flux transducers were placed in the same height of the thermocouples above the

seat backs of Rows 1, 4 and 13, pointing towards the ceiling. Concentrations of CO, CO2 and O2

were measured at the symmetry plane at station 880 (the center line of the rear exits). The gas

sampling location changed from the three heights 1.67m, 1.07m and 0.45m when the gas

analyzer became saturated at any one location.

Figure 1: C-133 Fire test Facility [3]



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Figure 2: C-133 fully-furnished full-scale test arrangement [2]

3. FDS SOFTWARE

Fire dynamics simulator (FDS) is a computational fluid dynamics (CFD) model of fire-driven

fluid flow. The software solves numerically a large eddy simulation form of the Navier–Stokes

equations appropriate for low-speed, thermally-driven flow, with an emphasis on smoke and heat

transport from fires. FDS is free software developed by the National Institute of Standards and

Technology (NIST) of the United States Department of Commerce, in cooperation with VTT

Technical Research Centre of Finland. Smokeview is the companion visualization program that

can be used to display the output of FDS.

FDS is a computer program that solves equations that describe the evolution of fire. It is a

FORTRAN program that reads input parameters from a text file, computes a numerical solution

to the governing equations, and writes user-specified output data to files. FDS output files could

be reviewed by Smokeview which reads and produces animations on the computer screen.

There is a graphical user interface for the Fire Dynamics Simulator (FDS) called PyroSim.

PyroSim helps in quickly create and manage the details of complex fire models.

http://fire.nist.gov/fds



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4. MODELING AND NUMERICAL INVESTIGATION PRINCIPLES

4.1 FDS Computational Model

Model_1 was built for the validation process with experimental results. Because of available

computer resources and time constraints some simplifications have to be applied. The model_1

description is:

The cabin has a rectangular cross section 24 m× 5 m× 3 m height as shown in Figure 3: FDS

3-D Model.

Only forward part furnishing.

Seats distribution (same to the experimental test).

Seat base area 0.5 m×0.5 m with height 0.5m above the cabin floor, and the top of the seat

back is 1.2 m above the cabin floor [7].

Fire source was modeled to be the forward door with total heat release rate 5000 KW /𝑚2.

Figure 3: FDS 3-D Model

It was difficult to obtain accurate material

4.2 Meshing and Grid Independency Check

A simple comparison is made between CFD

Table: 4.1Computational Mesh arrangements



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Cell Size (m) Number of cells Time elapsed to

0.25 x 0.25 x 23,040 2 hrs. and 10 minutes

0.125 x 0.125 x 184 ,320 6 hrs. and 18 minutes

0.06 x 0.06 x 1,474 ,560 3 days and 5 hrs.

Figure 4: Volume mesh on 3-D view with cell size

Figure 5: Measured and predicted temperatures at seat back top in the 𝟏𝟓𝒕𝒉 row for different cell

sizes.



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Figure 5: Measured and predicted temperatures at seat back top in the 𝟏𝟓𝒕𝒉 row for different cell sizes.

4.3 Comparison of CFD Results against Experimental

Temperature

In the experimental analysis, the onset of flashover was defined as occurring when the

temperatures begin to sharply increase [4]. The measured and predicted temperatures at the top

of the seat back of the central seat in Row 15 are depicted in Figure . The thermocouple at row

15 was set at the same position of the full scale test. As seen from the measured experimental

temperatures, the onset of flashover occurs at approximately 230 seconds [2], after which the

measured temperatures raised rapidly. The maximum measured temperature was 860 ℃ at 271

seconds where max predicted temperature was 640 C at 275 seconds.

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500

Tem

pe

ratu

re (

ᴼC)

Time (S)

Temperature VS. Time

Expri.

0.25

0.125

0.06

Delay of Aircraft Fire Safety Improvements



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Figure 6: Measured and predicted temperatures at seat back top in the 𝟏𝟓𝒕𝒉 row.

5. RESULTS and DISCUSSION

Through this section a new model was built (Model_2). Model_2 was initiated to increase the

similarity with the full scale test. The model was built with the same considerations of Model_1

except that the fire source considered being a block with dimensions 3 m x 2.5 m x 0.3 m and

placed outside the aircraft cabin and adjacent to forward door. The fire source total heat release

was set to be 1300 KW/𝑚2. Mesh size was 24 m x 12 m x 4 m. The grid size chosen for this

model was 0.125 m with total number of cells 589,824 .The thickness of the cabin wall is

assumed to be 0.04m. The wall was considered to be inert surface.

5.1 Temperature

Following temperature profiles of Model_1 and Model_2 .The results were reviewed to spotlight

on the effect of cabin side walls and fire blocking layer on delaying the cabin flashover. The

predicted temperatures were for thermocouple at row 15.As shown in Figure it’s clear that the

predicted results of Model_2 made a reasonable delay in flashover time compared with Model_1

results. The rationalize of this delay thanks to the cabin side walls and fire blocking layer which

made great delay to the cabin flashover. For Model_2 the onset of flashover was 138℃ at 281

seconds while the maximum predicted temperature was 805 ℃ at 296 seconds. The side wall

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500

Tem

pe

ratu

re (

ᴼC)

Time (S)


Experimental

Prediction_ Model 1Delay of Fire Safety Improvements



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panel always can produce great delay of flashover time and there for the improvements in this

way were numerous.

Figure 7: Predicted temperatures with different models at seat back top in the 𝟏𝟓𝒕𝒉 row.

It was reasonable to show a temperature slice at height 1.75 m. Figure shows the temperature contours at the onset

of flashover (t = 281 seconds). The figure shows that the forward furnished cabin totally affected by the fire during

the onset of flashover.

Figure 8: Temperature contours (height = 1.75 m) at 281 seconds.

0

200

400

600

800

1000

0 100 200 300 400 500

Tem

pe

ratu

re (

ᴼC)

Time (S)


Model_1 - With firesafetyimprovementsModel_2



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The whole cabin affected by fire at t = 296 seconds as shown in Figure 9.

Figure 9: Temperature contours (height = 1.75 m) at 296 seconds.

5.2 Carbon Monoxide

Because of his great effect on occupant survivability during post-crash fire it was important to

review Carbon monoxide (CO) concentration inside aircraft cabin. Carbon monoxide has a

harmful effect on human life as it considered the most important toxic gas produced by a cabin

fire. Through modeling CO concentration gas analyzer was set at fuselage station 880 (22.34 m

from the) at height = 1.67 m (The same position as experimental test). The predicted CO

concentration showed a slow build up till 280 seconds then a rapid growth between 281 seconds

to reach the maximum value at 293 seconds. The rapid increase in CO concentration was caused

because of cabin flashover. Co concentrations saturated at a reading of 2 % then it made a clear

decrease after cabin flashover.



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Figure 10: Carbon monoxide concentration at height 1.67 m (Station 880)

5.3 Fire Spread and Smoke Spread

It was important to review the relation between fire spread with cabin flashover. In airplane post-

crash fire situation fire spread from outside to inside the cabin. Fire spread through the cabin was

reviewed making a gradually spread throughout the furnished part of cabin as seen in figure 11.

Figure 11: predicted fire spread through the forward furnished part at 282 seconds.

-0.5

0

0.5

1

1.5

2

2.5

0 50 100 150 200 250 300 350

CO

%

Time (S)

CO % VS. Time

CO % @ 1.67 m



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As shown in figure 12 the fire spread to engulf the whole of the cabin at 285 seconds as flashover has already

occurred.

Figure 12: Predicted fire spread through the whole cabin at 285 seconds.

Also reviewing the smoke spread inside the cabin was important as it is estimated that 50–80% of fire deaths are the

result of smoke inhalation injuries, including burns to the respiratory system [4].As shown in Error! Reference

source not found. and Figure the smoke first rose to the ceiling and then spread backward to fill the forward

furnished part.



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Figure 13 : Predicted smoke spread inside the cabin at 90 seconds

After filled the entire cabin, it began to sink down. The smoke filled the entire after the onset of flashover. See

Figure .

Figure 14 : Predicted smoke spread inside the cabin at 281 seconds

5.4 Visibility

Visibility is a factor directly influencing safety. The visibility has significant effect on passenger

evacuation. Having a good visibility at 1.8 m height (from 5 m to 10 m) is very important for

firefighting and evacuation processes. One should bear in mind that lower visibility means longer

evacuation time, which in turn means longer exposure to hazardous gases and greater inhalation

of harmful substances due to rapid breathing. Also it is important to review the visibility inside

cabin model. The visibility slices have been taken at Z = 2 m (The height of exits signs in real

aircraft cabin). From figure 15 it is clear that there is no good visibility in the region beside

external fire source at 50 seconds. Foggy visibility region enlarged to cover the forward part of

cabin at 90 seconds (max time allowed to evacuate from the airplane).At 239 seconds .The

maximum available visibility inside the cabin was to 5 m ,Figure 16 shows the visibility after

281 seconds.



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Figure 015: Visibility contours at Height = 2 m at 90 seconds.

Figure 16: Visibility contours at Height = 2 m at 281 seconds.

6. CONCLUSIONS

FDS fire simulation was used to simulate a full-scale fire experiment though C-133 aircraft.

Model parameters were obtained from the full-scale test which conducted by the FAA. Also

many considerations were obtained from a previous computational study about aircraft post-

crash fires [6].



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FDS shows a reasonable agreement with experimental results. For simplicity the side walls and

fire blocking layer were excluded during model_1 building .Both of the fire safety improvements

provide a slow fire growth about 140 seconds for cabin side panels [6] and 40 – 60 seconds for

the fire blocking layer[6]. According to the past considerations the delay of predicted results can

be reasonable. The onset of flashover happened at 261 seconds through the mathematical model

while it was at approximately at 230 seconds through the experimental test at the same location.

Then Model_2 was built to examine carbon monoxide concentration, fire spread, smoke spread

and visibility through the aircraft cabin. The predicted CO concentration showed a slow build up

till 280 seconds then a rapid growth between 281 seconds to reach the maximum value at 293

seconds. The rapid increase in CO concentration was caused because of cabin flashover which

started in Model_2 at 281 seconds. CO concentrations saturated at a reading of 2 % then it made

a clear decrease after cabin flashover. The simulation showed a total fire spread at the forward

furnished part at the onset of flashover (281 seconds) then the predicted fire spread through the

whole cabin was at 285 seconds. Little difference happened for smoke spread as the smoke

covered the forward cabin at 90 seconds (The maximum allowed time to escape from any

airplane) and the smoke totally fill the whole cabin at the onset of flash over at 281 seconds.

Visibility was the last phenomena reviewed through this study. The visibility slices taken at

different heights (1.75 m and 2 m) .There was no marked differences between results at different

altitudes. The available visibility in the forward furnished part between 3-6 m at the first 90

seconds then it became from 0-3 m at the onset of flashover (281 seconds).

ACKNOWLEDGMENT

Our deepest gratitude to Dr. Zhaozhi Wang a Senior Research Fellow within the Fire Safety

Engineering Group of the School of Computing and Mathematical Sciences at the University of

Greenwich for his support, interest and truthful advices.

REFERENCES

[1] Paul, M. , "Probabilities Risk Assessment Modeling of Passenger Aircraft Fire Safety”. PhD

Thesis, the University of Cranfield, 1997.

[2] Sarkos C. P., Application of full-scale fire tests to characterize and improve the aircraft post

crash fire environment, Toxicity, Vol. 115, pp. 79-87, 1996.

[3] Sarkos C. P., Hill R. G., Preliminary wide body C-133 cabin hazard measurements during a

post crash fuel fire, Federal Aviation Administration National Aviation Facilities

Experimental Center Atlantic City, 1978.



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[4] Sarkos C. P. and Hill R. G., Characteristics of aircraft fires measured in full-scale tests,

Advisor Group for Aerospace Research and Development (AGARD; Neuilly-Sur-Siene,

France) Conference Proceedings, No. 467 on Aircraft Fire Safety, May 22-26, 1989, Sintra,

Portugal.

[5] Galea, E.R., On the field Modelling approach to the simulation of enclosure fires, J. Fire

Protection Engng 1,11-22, 1989.

[6] Wang, Z., Galea, E. and Jia , F., A computational study of the characteristics of aircraft

post-crash, Fire Safety Engineering Group, University of Greenwich, UK, Presented at the

International Fire & Cabin Safety Research Conference Atlantic City 29 Oct – 1 Nov 2007.

[7] http://en.wikipedia.org/

[8] Evaluation of Aircraft Interior Panels under Full-scale Cabin Fire Test Conditions

(CONSTANTINE P. SARKOS and RICHARD G. HILL)

Sci-Paper@Computational_Analysis_of_Aircraft_Postcrash_Fire

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