Chapter 2: HISTORY OF FIRE

SUPPRESSION IN AIRCRAFTi

Donald P. Bein 

Naval Air Systems Command 

TABLE OF CONTENTS

2.1 Fire Threats to Military Aircraft ........................................................................................20 

2.2 Protected Compartments on Aircraft .................................................................................24 

2.2.1 Engine Nacelles.......................................................................................................24 

2.2.2 Other Powerplant-type Compartments....................................................................28 

2.2.3 Dry Bay Compartments...........................................................................................31 

2.2.4 Cargo Compartments...............................................................................................34 

2.2.5 Other Compartments ...............................................................................................36 

2.2.6 Fuel Tank Ullage .....................................................................................................37 

2.3 Types of Fires Experienced ...............................................................................................41 

2.3.1 Safety-related Fires..................................................................................................43 

2.3.2 Ballistically-induced Fires.......................................................................................45 

2.3.3 Spray Fires...............................................................................................................47 

2.3.4 Pool Fires.................................................................................................................49 

2.3.5 Fire Characteristics..................................................................................................50 

2.3.6 Aircraft Operational Temperature Environment44 ..................................................54 

2.3.7 Reignition ................................................................................................................79 

2.4 Fire Suppressants Used on Aircraft ...................................................................................80 

2.4.1 Powerplant Compartments......................................................................................81 

2.4.2 Dry Bay Compartments...........................................................................................86 

2.4.3 Certification.............................................................................................................86 

2.5 Halon Alternative Technology Development Program (TDP)..........................................92 

2.5.1 Phase 1 – Operational Parameters Study.................................................................93 

2.5.2 Phase 2 – Operational Comparison of Selected Extinguishants..............................94 

2.5.3 Phase 3 – Establishment of Design Criteria Methodologies...................................94 

2.5.4 Impact of Halon Alternative Fire Suppression to the Aircraft ................................96 

2.6 References........................................................................................................................109 

 

i

 The content of this chapter has been approved for public release by the Naval Air System Command (NAVAIR), per NAVAIR 

public release authorization control numbers 063-06, SPR07-005B, and SPR07-005D.
20 History of Fire Suppression in Aircraft 

2.1 FIRE THREATS TO MILITARY AIRCRAFT 

Safety and survivability drive the requirements for fire suppression in aircraft. Whereas safety is 

concerned with mitigation of hazards associated with system or component failures or human error, 

survivability relates to susceptibility and vulnerability to threats directed at the aircraft. A fire is deemed 

a safety-related fire when it results from component failures, which may be due to inadequate design, a 

mechanical failure mechanism such as fatigue, or maintenance error, and results in either a flammable 

fluid contacting an always-present ignition source, or the failures themselves provide both the flammable 

fluid and the ignition source. Fires that relate to aircraft survivability are those that are ballistically 

induced in areas on an aircraft that, if not protected by some means, are vulnerable to fire or even 

explosion. Extensive literature compilations by the National Advisory Committee on Aeronautics 

(NACA) summarize investigations since 1922 of aircraft fire problems, fire prevention measures, fire 

detection, fire suppression, fuel tank explosion hazards, and inerting. These indicate that what was then 

the War Department had been addressing the field of aircraft fire protection at least as early as 1927.1,2

 In 

1938, just prior to World War II, the issue of in-flight power-plant (engine compartment) fires led to the 

Civil Aeronautics Administration (CAA) expanding its fire test program to include fire extinguishing to 

mitigate the hazards associated with these fires.3

 Review of the combat data from World War II indicated 

that, in order, damage to engines, cooling and lubrication subsystems, fuel system, and flight control 

systems were to likely lead to aircraft loss, and that the majority of aircraft lost were on fire.4

 

Surveys conducted after World War II documented the obvious need for lower volatility fuels and 

separation of flammable fluid-carrying components from ignition sources with the engine emphasized as 

the principal ignition source.5

 It was also then recognized that during flight the fuel-air mixture within 

fuel tanks could be alternately combustible and noncombustible, depending upon flight conditions. 

However, engine failures were still found to be the most frequent cause of flammable fluid ignition by an 

ignition source in flight.6

 It is no surprise that years later, analysis conducted by the United States Navy 

(USN) indicated a similar outcome: non-combat in-flight fires, i.e., safety-related fires, occurred 

predominately in engine compartments, as illustrated in Figure 2-1. 

The combat environment exposes military aircraft to ballistic and higher-level threats that, until recent 

years, have not been experienced in U.S. commercial aviation.7

 These threats can be incendiary or nonincendiary. The military conflicts in Korea and Southeast Asia and analysis of aircraft combat loss data 

from those conflicts indicated that the fires due to fuel-system-related damage were becoming the 

predominant fire vulnerability, as aircraft were lost mainly due to hits on the fuel system.4

 Analysis of 

aircraft losses suffered during the Vietnam conflict indicated generally that half were due to fuel fires and 

explosions, and that half of those were attributable to fuel explosions in dry bay areas on aircraft.8

 

As illustrated in Figure 2-2, dry bay areas along with the fuel tank ullage, which can itself contain a 

flammable fuel-air mixture, are vulnerable to ballistic threats.ii From the aircraft survivability design 

perspective, the fuel tank ullage and dry bay compartments contribute to an aircraft’s ballistic vulnerable 

area. Incendiaries released from a ballistic round that penetrate the ullage can result in ignition of fuel 

vapor. A ballistic round entering into a dry bay adjacent to a fuel tank, and that also penetrates a fuel tank 

or other flammable fluid component, can cause fuel to leak and be ignited within the dry bay when 

incendiaries are released. 

 

ii Dry bays are void areas on aircraft adjacent to fuel tanks or can be areas containing flammable fluid-carrying components. 
Fire Threats to Military Aircraft 21

0 20 40 60 80 100 120 140 160 180 200 220 240

Engine

Nacelles

Electrical

Equipment

Auxiliary Pow er

Compartments

Rotor

System

Landing

Systems

Transmission

Compartment

Engine

Tailpipes

Afterburner

ECS

Compartment

Number of Fires

Figure 2-1. Significant Non-combat Fire Locations on U.S. Navy Aircraft 1977-1993.9, iii

Figure 2-2. 

Combustion Threats 

in and Around an 

Aircraft Fuel Tank.4 Ullage - also 

referred to as 

a wet bay (Reprinted by 

permission of the 

American Institute of 

Aeronautics and 

Astronautics, Inc.) 

Dry Bay

Areas 

 

iii Forty-nine percent (49 %) of the fires represented in the electrical equipment bar in Figure 2-1 occurred on one aircraft 

platform type within the aircraft cabin and were readily extinguished by either securing electrical power to that equipment or 

by use of on-board portable fire extinguishers. Also in Figure 2-1, ECS is an acronym for Environmental Control System. 
22 History of Fire Suppression in Aircraft 

Table 2–1 provides a generalized evolution of tactical and rotary aircraft vulnerability based on live fire 

testing (LFT) vulnerability assessments for armor-piercing incendiary (API) threats.10 Though aircraft 

vulnerable areas have decreased over the years, as depicted notionally in Figure 2-3, fuel systems remain 

a significant vulnerability issue and drive the need for vulnerability-reduction measures such as fire 

suppression for aircraft dry bay compartments as well as for engine compartments.

Table 2–1. Evolution of Aircraft Vulnerability Based on Live Fire Test 

Vulnerability Assessments (API Threats).iv

Aircraft 

Category, System 

Vulnerable Area 

Percentage 

Vulnerable Area 

Percentage 

Tactical Aircraft 1960s through 1970s Current 

Crew 8 % 10 % 

Propulsion 54 % 5 % 

Flight Controls 11 % 5 % 

Fuel System 25 % 75 % 

Structure 1 % 2 % 

Other 1 % 3 % 

Rotorcraft 1980s Current 

Crew 1 % 3 % 

Propulsion < 1 % 2 % 

Drive System 15 % 20 % 

Flight Controls 38 % 33 % 

Fuel System 30 % 42 % 

Structure 16 % -

Other - < 1 %

1980’s

Rotorcraft

State of the Art

Rotorcraft

Crew

Propulsion

Drivetrain

Flight Controls

Fuel System

Structure

Others

System Vulnerability:

Figure 2-3. Illustration of Notional Reduction in Overall Rotorcraft Vulnerable Area.

 

iv Figure 2-3 and the percentages in Table 2-1 were provided by the Naval Air Warfare Center Weapons Division, Aircraft 

Survivability, and are printed with permission of the Naval Air Systems Command. 
Fire Threats to Military Aircraft 23

The susceptibility of aircraft to engine fires combined with the combat vulnerability to fuel-systemrelated fires cannot be overstated. Based on United States Air Force (USAF) experience, the combined 

historical cost from 1966 through projected cost through 2025 of aircraft loss due to fire from both

operational and combat losses has been estimated as over $30 billion. The cost to provide fire

suppression for that same period has been estimated at less than $1 billion.11 Chapter 10 provides further

discussion of life cycle cost of aircraft fire protection. 

In terms of personnel safety, the fire risk is obvious. However, in terms of crew survivability in combat, 

the need for fire suppression takes on an added dimension. Figure 2-4 shows known ejection locations of 

U.S. Navy aircrewmen who subsequently became prisoners of war (POW) during the Vietnam conflict, 

and Figure 2-5 shows locations of Navy rescues of Navy aircrewmen during that conflict.12 It had been

estimated that the difference in terms of time was typically 5 min flight time between ejection locations 

and rescue locations. Thus the ability to provide fire suppression capability to address both predominant

operational failure threats as well as combat threats is likely to increase aircrew survival and recovery. 

Figure 2-4. Known Ejection Locations of 

Navy Aircrewmen Who Became POWs 

During the Southeast Asia Conflict.12

Figure 2-5. Locations of Navy Rescues 

of Navy Aircrewmen During the 

Southeast Asia Conflict.12

The remainder of this chapter describes not only the aircraft compartments that were the primary focus of 

the United States Department of Defense’s Next Generation Fire Suppression Technology Program 

(NGP), but also provides general summaries of all the various types of compartments on aircraft for 

which halon fire suppression has been implemented. This has been done so that the reader is reminded

that, although the focus of the NGP addressed those compartments for which, statistically, the fire threat
24 History of Fire Suppression in Aircraft 

has been the greatest, the work of the NGP is likely to influence in the future how fire suppression is 

applied for other aircraft compartments that may require protection. 

The next section includes a brief discussion on various techniques for fuel tank ullage fire suppression, 

which was the subject of an exploratory effort during the NGP. This is followed by a summary of the

NGP’s investigation into the types of fires experienced in the compartments that were the focus of the

NGP: engine nacelles, auxiliary power unit (APU) compartments, and dry bays. In particular, this section 

provides expanded discussion on the topic of temperature and historical experience related to fire 

suppressant releases relative to outside air temperature (OAT) and the nacelle temperature environment.

Next, brief descriptions are provided for the fire suppressants that have been used on aircraft prior to the

efforts to identify halon alternatives that commenced after production curtailments driven by the U.S.

Clean Air Act of 1990 and amendments to the Montreal Protocol. Throughout this entire section, the 

word “halon” is used to denote any of the halogenated fire suppressants that have been used in aircraft fire 

suppression, except where the particular halon type is noted specifically. Finally, a brief overview of the 

Halon Alternative Technology Development Program (TDP) is provided to familiarize the reader with the 

“near-term” halon alternatives identified for use in aircraft fire suppression applications, their benefits, 

and their limitations, which led to the need for the NGP. 

2.2 PROTECTED COMPARTMENTS ON AIRCRAFT 

The predominant implementation of halon-1301-based fire suppression on aircraft has been for fire 

protection of powerplant-type compartments. These compartments are those that contain engine-type 

equipment that require flammable fluids (aviation fuel, hydraulic fluid, and lubrication oil) to generate 

propulsion power, such as a turbojet engine that generates thrust power for a fighter jet or a turboshaft 

engine that generates shaft power to turn a propeller on a turboprop aircraft or turn rotors on a helicopter. 

Powerplant compartments include engine compartments, which are referred to as nacelles, but depending

on the type of aircraft, e.g., transport, cargo, fighter, or helicopter, may also include additional types of

powerplant-type compartments such as APU compartments. Also, depending on the type of aircraft, 

halon 1301 has been utilized to provide fire suppression for other compartments, such as dry bays, cargo 

compartments, avionics compartments, weapons bays, and fuel tanks. Although the final focus of the

NGP was to address halon alternative fire suppression for engine nacelles, APU compartments and dry

bay compartments, the intent of this section is to familiarize the reader with these compartments as well 

as other types of compartments for which fire suppression has been provided on military aircraft. 

From the aircraft designer’s perspective, “fire extinguishing” implies that a system is to be designed to 

extinguish a fire without subsequent reignition, whereas “fire suppression” implies that reignition can 

occur but that the system will suppress the fire until a safe landing can be achieved. However, the phrase

“fire suppression” is used widely throughout aviation as well as in the NGP and its publications to mean 

fire extinguishment, except where specifically explained otherwise. The latter convention is followed in 

this chapter as well, except where noted otherwise. 

2.2.1 Engine Nacelles 

Engine nacelles are the physical compartments on aircraft that house the engines. These compartments 

may be integral with the fuselage or mounted externally to the aircraft’s wings. An example of a 

turboshaft engine is illustrated in Figure 2-6. The engine itself is physically complex, containing ribs, 
Protected Compartments on Aircraft 25

tubing for fuel and bleed air, electrical harnesses, and externally-mounted accessory components (e.g., oil

pump). Integration of an engine into an aircraft nacelle increases greatly the geometric complexity about 

an engine, as components and systems that must interface directly with the engine will also reside within

the nacelle. Adding to the complexity of the installation are the dynamic conditions that exist within the 

nacelle during aircraft operation. Engine surface temperatures can exceed 538 °C. Ventilation airflow is 

required through the nacelle to ensure engine and accessory component performance is not degraded and 

is effected by incorporation of air inlets and outlets on the nacelle structure. Depending on the type of

aircraft and flight conditions, nacelle mass airflow rates for DoD aircraft that have been fielded have

ranged from under 0.23 kg/s (0.5 lbs/s) to in excess of 14 kg/s (30 lbs/s) for a fighter aircraft dash

operation.13,14 When the combined integration is considered, the nacelle becomes a geometricallycomplex, turbulent compartment capable of sustaining fire. Additionally, components within the nacelle 

and the engine components themselves provide for potential flame holders, recirculation zones, and for 

potential areas of stagnation. 

Figure 2-6. Example Turboshaft 

Engine. 

(Reprinted with permission from RollsRoyce Corporation.) 

Figure 2-7 is an example of a military aircraft engine nacelle. As shown in Figure 2-8, the typical nacelle 

fire suppression system installation consists of one or more fire suppressant bottles, located external to the

nacelles, connected to a directional control device that interfaces to distribution plumbing that routes 

discharged suppressant to the nacelles. This system may also be designed to deliver suppressant to an 

APU compartment. Discharge is effected by the pilot in the cockpit: an electrical signal causes activation

of a pyrotechnic cartridge actuated device (CAD) to rupture a burst disk on the bottle to effect suppressant

release. These systems are typically designed to provide a specified suppressant concentration level (6 % 

for halon 1301)v

 throughout each protected compartment for a minimum of 0.5 s. Figure 2-9 shows a fire 

suppression system installation for an engine nacelle in which the suppressant bottle is located in a

compartment adjacent (and external) to the nacelle. Though not typical in currently fielded DoD aircraft,

the fire suppression system bottle may also be located within the nacelle, as shown in Figure 2-10, 

increasing nacelle complexity. 

 

v

 Throughout this chapter, where percentages are indicated in discussions related to suppressant concentrations, they are always 

referring to concentration by volume. 
26 History of Fire Suppression in Aircraft 

Figure 2-7. A Fighter Aircraft Engine Nacelle. 

(Printed with permission of the Naval Air Systems Command.) 

No. 1 

Engine 

APU

Thermal 

Relief 

Outlet

Aft Bottle

Directional 

Control 

Valve

Forward 

Bottle 

No. 2 

Engine 

Figure 2-8. Diagram of an Aircraft Fire Suppression System Installation.8
Protected Compartments on Aircraft 27

Fire Suppressant 

Bottle (Mounted in 

Compartment 

External to the 

Nacelle) 

Distribution Line

within Nacelle 

CAD

Nacelle Air 

Inlet Structure 

Distribution Line

Outlet 

Discharge Line from 

the Bottle 

Figure 2-9. An Aircraft Engine Nacelle Fire Suppression System Installation, Suppressant 

Bottle External to the Nacelle. 

(Printed with permission of the Naval Air Systems Command.) 
28 History of Fire Suppression in Aircraft 

Distribution 

Outlet 

Fire 

Suppressant 

Bottle 

Distribution 

Line 

Nacelle Air 

Inlet 

Figure 2-10. An Aircraft Engine Nacelle Fire Suppression System Installation, 

Suppressant Bottle Within the Nacelle. 

(Printed with permission of the Naval Air Systems Command.) 

2.2.2 Other Powerplant-type Compartments 

In addition to engine compartments, DoD military aircraft may contain other types of powerplant 

compartments for which a fire suppression capability is provided, either dedicated or shared with another 

fire suppression system on the aircraft. The most common compartment that falls into this category is 

that containing the auxiliary power unit (APU). On some aircraft this compartment may also be referred

to as the auxiliary powerplant (APP) or the gas turbine compressor (GTC) compartment. These units may

be miniature turbines or other power generating equipment, but are typically smaller than the normal jet

engine propulsion systems. These units furnish electrical power when engine-driven generators are not

operating or when external power is not available, and they may be used to provide emergency power to 

all or some of the aircraft subsystems in the event of an in-flight engine shutdown.vi These units function 

and generate power independently from the normal aircraft engine systems. On the ground, the power 

output from the APU is used to generate power to drive a starter unit for engine starting. 

 

vi Because aircraft power for the fire suppression system may not be available during startup, one fielded rotorcraft model 

actually employs a manually activated fire bottle for its APP compartment. A cable is run from the cockpit fire bottle actuation 

mechanism, referred to as the T-handle, through overhead cabin compartments, around a two-pulley system, and into the 

discharge port of the bottle.
Protected Compartments on Aircraft 29

In addition to or instead of a dedicated APU compartment, there are fielded DoD aircraft that contain an

accessory compartment or secondary power system (SPS), or in the case of a rotary aircraft, a gearbox

compartment. These compartments may contain the APU or other equipment utilized during ground 

engine starting and aircraft operation such as a Jet Fuel Starter (JFS) or an Airframe Mounted Accessory 

Drive (AMAD). Like engine nacelles, each of these compartments is likely to have ventilation airflow to 

ensure proper component performance and, like the main aircraft engines, components like the APU are 

potential ignition sources and their integration into the aircraft can present geometric complexities with

respect to fire suppressant distribution. Figure 2-11 is an example of an APU installation on a transport 

aircraft. Figure 2-12 is an example of a fighter aircraft accessory compartment. Figure 2-13 is an

example of an aircraft gearbox compartment that also contains an APU. Figure 2-14 is an example SPS 

installation on a fighter aircraft. Integration of an APU or similar component into an aircraft accessory, 

gearbox, or other powerplant-type compartment results in a complex compartment in which fire may be 

sustained, due to either a component failure or, in the case for some aircraft as will be described later, 

because the compartment is vulnerable to ballistically induced fires. 

Figure 2-11. A Transport Aircraft Auxiliary Power Unit Installation.15, vii

(Reprinted with permission of the author.)

 

Exhaust 

Muffler 

Bleed Air 

Manifold 

Oil Tank 

Oil Filter 

Start Motor 

Bleed Air 

Valve

APU Fuel

Line Upper 

Shroud 

Fuel 

Nozzles 

Fuel Filter 

Oil Filler 

Generator 

Light 

Switch

vii Several DoD aircraft models are variants of commercial transport aircraft. Such aircraft are referred to within the DoD as

commercial-derivative aircraft. 
30 History of Fire Suppression in Aircraft 

Figure 2-13. An Aircraft Gearbox Compartment with APU. 

(Printed with permission of the Naval Air Systems Command.)

Fire Suppressant 

Bottle 

APU

Figure 2-12. A Fighter Aircraft Accessory Compartment 

With Fire Suppression System Installation.

(Printed with permission of the Naval Air Systems Command.) 
Protected Compartments on Aircraft 31

Figure 2-14. A Secondary Power System (SPS) Installation.16

(Component callouts spelled out or simplified for clarity.) 

2.2.3 Dry Bay Compartments 

Dry bay compartments, or simply dry bays, are compartments on aircraft adjacent to fuel tanks or are 

compartments in which flammable-fluid-carrying components are located. The com ents adjacent to 

fuel tanks may contain other equipment, such as avionics equipment, or may be, for the most part, empty. 

There may or may not be ventilation airflow. If not empty, a dry bay can be geometrically complex, and 

if there is some ventilation airflow, the compartment may be turbulent. One study identified generally 

four types of dry bays: wing leading/trailing edge, wing midchord, fuselage fuel cell boundary, and 

fuselage forward and aft equipment bays.17 Ballistic threats such as armor-piercing incendiary (API) 

rounds or high-explosive incendiary (HEI) rounds can penetrate a fuel tank or fuel-carrying component, 

effecting release of flammable fluid into a dry bay. The incendiary released by the round from the 

penetration can then ignite the fluid-air mixture. The damage inflicted by a ballistic round may also result 

in damage to an aircraft system that can result in a secondary ignition source, such as heated metallic 

fragments or arcing from a damaged electrical harness. Figure 2-15 illustrates that dry bay compartments 

ay be located within the fuselage as well as in wing areas that surround a fuel tank. Figures 2-16 

throug igure 2-16 illustrates a dry bay 

compa ry bay compartments that also 

contain flammable-fluid carry

partm

m

h 2-18 provide examples of actual aircraft dry bay compartments. F

rtment adjacent to a fuel tank whereas Figures 2-17 and 2-18 show d

ing components. 

AMAD

PC2 

Pump Right 

Utility 

Hydraulic 

Pump

Door 

Frame

AMAD

Door Frame

JFS 

Exhaust 

Duct

PC1 Pump

JFS

Left 

Utility 

Hydraulic 

Pump

Right 

Integrated 

Drive 

Generator

Central 

Gearbox

Left 

Integrated 

Drive 

Generator

Left Access Door

Right 

Access 

Door

AMAD

PC2 

Pump Right 

Utility 

Hydraulic 

Pump

Door 

Frame

AMAD

Door Frame

JFS 

Exhaust 

Duct

PC1 Pump

JFS

Left 

Utility 

Hydraulic 

Pump

Right 

Integrated 

Drive 

Generator

Central 

Gearbox

Left 

Integrated 

Drive 

Generator

Left Access Door

Right 

Access 

Door
32 History of Fire Suppression in Aircraft 

Figure 2-15. Typical Dry Bay Locations in a Fighter Aircraft.18

Figure 2-16. A Dry Bay 

Compartment Adjacent 

to a Fuel Tank; Fire 

Detector and Fire 

Suppressor Installed 

within the Dry Bay 

Compartment. 

(Printed with permission

of the Naval Air Systems 

Command.) 
Protected Compartments on Aircraft 33

Figure 2-17. Top View of a Wing Dry

Containing Drive, Hydraulic, and Electrical 

Systems Components. 

(Printed with permission of the Naval Air Systems

Command.) 

Bay

d Electrical Systems 

ed. 

.) 

Figure 2-18. A Wing Leading Edge Dry Bay Containing Hydraulic an

Components, Leading Edge Structure Remov

(Printed with permission of the Naval Air Systems Command

Powder 

Panel 
34 History of Fire Suppression in Aircraft 

There are active and passive approaches for suppressing dr

system must be capable of detecting a fire on a tim

within a few hundred milliseconds. In the truest sen

for protection against ballistic threats in dry bay

DoD aircraft have such dry bay fire protection sy

to be installed on United Kingdom CH-47 Chin

y bay fires. An active dry bay fire protection

e scale of less than 10 ms and suppressing the fire 

se, fielding of halon 1301 fire suppression systems 

 compartments on military aircraft has been limited: no 

stems, but halon 1301 dry bay fire suppression is known

o 19, viii ed to

protect engine nacelles, the fire bottles used for dr lso

utilize a pyrotechnic device to rupture a burst dis the

bottles are automatically activated upon receipt of a

bottle also effects discharge.20 Figure 2-19 depi

referred to as the COBRA system.

ix The bottles a nacelle fire 

suppression systems, and the time to discharge agent is on the order of 10 ms, compared to typically 1 s 

for a nacelle fire bottle.21 There are, however, DoD aircraft platforms for which halon 1301 is utilized to 

protect a powerplant or other type of compartment but in reality the protected compartment is a quasi dry 

bay compartment that is vulnerable to a ballistically induced fire; i.e., these compartments are either

located adjacent to a fuel tank or they contain flammable-fluid-carrying components. For a few DoD 

platforms, those halon 1301 fire suppression systems are also intended to provide protection against dry 

bay fires.17

Passive techniques have been the predominant fire protection approach for aircraft dry bays on DoD

aircraft, which have been implemented either by installation of a physical material to defeat the flame 

front, such as reticulated polyether foams and rigid foams, or by installation of panels adjacent to the 

ompartment that, if ruptured by a ballistic projectile, release a powder suppressant to quench incendiaries 

erformance limitations of this latter method were investigated under the NGP and resulted in 

development of new design techniques that dramatically improve powder panel protection capability.

22 

his is described further in Chapter 9. 

.2.4 Cargo Compartments 

argo compartment fire suppression systems are typically installed on commercial transport aircraft, 

hough at least one DoD aircraft equipped with this type of system, the C-5, an Air Force cargo

anspo raft.

23 Such systems are designed to provide a minimum knock-down concentration

llowed by a sustained minimum flooding concentration to suppress a fire that continues to burn until a

afe landing is made. Such systems typically provide for 60 min of suppression capability. Thus unlike a 

re suppression system designed to extinguish a nacelle, other powerplant, or dry bay fire, the protection

 for a cargo compartment is true fire suppression. 

ok helicopters. Like halon systems design

y bay fire suppression on the CH-47D helicopters a

k on the bottle to effect suppressant release, but

signal from a pressure transducer – an initiator on the 

cts the components of this type of system, which is

re much smaller than those fielded for

c

released by the projectile. (In Figure 2-18, powder panels can be seen installed on the wing spar.) 

P

T

2

C

t

tr

there is

rt airc

fo

s

fi

philosophy

viii In Reference 32 it is stated “...the F22 dry-bay protection scheme...incorporates multiple ‘bottles’, using halon 1301, to

provide appropriate coverage.” This statement infers that DoD USAF F-22 aircraft use halon 1301 for dry bay fire 

suppression. Space and electrical wiring provisions are installed on delivered F-22 aircraft in the event it is decided in the

at this time with halon 13 ix During research con it be confirmed why

the system was assigned such a name. Literature provided by the system supplier indicates that it simply refers to the snake. 

future to install dry bay fire suppression system components. Though the system was designed for use of halon 1301, 

research conducted for this Chapter confirmed (Reference 19) that, as of the writing of this book, no F-22 aircraft are fielded

01, i.e., no dry bay fire suppression system components are installed. 

ducted for this book, it could not be confirmed if “COBRA” is an acronym, nor could
Protected Compartments on Aircraft 35

Figure 2-19. Non-DoD Aircraft Halon 1301 Dry Bay Fire Protection System.24,25

(Reprinted with permission of Airscrew Limited, U.K.) 

Multi-Extinguisher 

Initiator 

Fire Extinguishers 

Projectile Sensors 

Multi-Extinguisher Initiator: 

Self-Operating, Enables 

Several Extinguishers to be 

Initiated by Sensor(s) Output 

Projectile Sensors: SelfOperating/Fuel Immersed, 

Produces an Output in 

Response to Decelerating 

Strike Projectile 

Up to 10 COBRA 

Extinguishers May Be 

Connected by a Parallel 

Ring Circuit to a 

Common Output from a 

Multi-Extinguisher 

Initiator, which Receives 

Initiating Energy Pulses 

from Similarly 

Connected Sensors. 

The Number of 

Extinguishers Required 

Will Depend on Bay 

olume, Configuration 

ongestion. The 

Number of Sensors 

V

and C

Required Will Relate to 

Fuel Capacity 

Configuration and 

Fire Extinguishers: 

Cartri

Pro

dge Operated to 

vide Protection 

within Vulnerable 

Compartments 

 

Threat. 
36 History of Fire Suppression in Aircraft 

The diagram in Figure 2-20 is an example of a cargo fire suppression system installation on a transport

aircraft. Such systems today utilize halon 1301 for fire suppression. In the case of an engine nacelle or 

APU compartment fire, the fuel source is shut off from either of those compartments prior to effecting

agent discharge; whereas in a cargo compartment, the fire may be deep-seated and not removed from its

flammable material source. The amount of agent installed to provide extinguishing capability to pr

engine nacelles may be relatively small compared to that required for the cargo compartment. For 

ple, on C-5 aircraft, there are four halon 1202 bottles (6.8 kg agent each) for engine fire suppression, 

whereas 17 halon 1301 bottles (31.75 kg each) are installed for cargo compartment fire suppression. 

Though the focus of the NGP did not inc

otect 

exam

ment fire suppression, extensive research and

testing into application of halon alternatives for these compartments has been conducted by the FAA’s 

Fire Safety Branch at the Hughes Technical Center. 

Figure 2-20. Diagram of a Cargo Fire Suppression System Installation 

on a Transport Aircraft.26,27

(Photograph of panel reprinted with permission of the author.15) 

2.2.5 nts

During the development of an aircraft design, safety hazard and vulner entify

potential fire threats aboard the aircraft in compartments other than powerpl ay and, if 

applicable, cargo compartments. (The vapor space within fuel tanks is a separate case that is described in

the next section.) Design solutions are then pursued to mitigate the likelihood of a catastrophic fire and 

plement ballistic vulnerability reduction measures if needed, which typically considers isolating 

potenti not be

mitigated to an accept stalled to protect the 

compartment (or if not, the risk is accepted). Examples of such compartments on DoD aircraft are, or

lude cargo compart

Aft and Bulk Cargo 

Compartments 

Discharge Nozzle (8) 

Forward Cargo

Compartment

Aft In-Line

Pressure 

Switch

Metered Bottle (3)

Filter/Regulator

Forward In-Line

Pressure Switch 

Flow Valves 

Dump

Bottle (2) 

Cargo Fire Control Panel

Other Compartme

ability analyses will id

ant-type, dry b

im

al ignition sources from flammable fluid or other flammable sources. If the risk for fire can

able level, a dedicated fire suppression system will be in
Protected Compartments on Aircraft 37

were, the fuselage fire protection system installed on F-111 aircraft to protect the cheek and stabilization 

areas and weapons bay, the C-5 avionics compartment fire suppression system, and the A-6E/EA-6B aft 

equipment bay fire protection system. All are, or in the case of retired aircraft were, halon 1301 systems.

The certification requirement for engine nacelle halon 1301 fire suppression systems was applied to 

certify the A-6E/EA-6B aft equipment bay system, and that requirement was likely applied to certify the 

other systems on the C-5 and F-111. Thus it follows that certification requirements developed for any 

agent or technology developed by the NGP for engine nacelle fire suppression would likely become 

certification benchmarks, or likely starting points for such benchmarks at minimum, for systems installed

to protect most compartments on aircraft except for dry bays and cargo compartments. The exception for 

dry bay compartments is that although viable technologies for dry bay protection were developed under 

the NGP, certification for those systems will likely still be accomplished through live fire ballistic testing, 

as is done currently in accordance with live fire testing legislation. The exception for cargo

compartments is the unique knock down and flooding suppression requirement, which was not addressed

by the NGP. 

2.2.6 Fuel Tank Ullage 

In contrast to dry ents, fuel tanks are es referred to as wet bays. The fuel tank 

ullage is that volume not o y liquid fuel but contains an air and fuelvapor mixture. From an operational safety perspective, e potential for a fuel tank fire/explosion

hazard exists on every aircraft due to a system or component malfunction or failure, safety

the requirements impetus nting ullage protection on DoD military aircraft. (Relatively

activity has been conducted to reassess and develop such protection for commerc ,29,30) The

case for military arily, like dry bay fire protection, one of vulnerability reduction for 

combat survivabil ere are several military platforms that incorporate some type of active 

ullage protection utilized halon 1301 to provide fuel tank inertion, 

the USAF ircraft and the United States Navy (USN) A-6E, which is no longer in service. 

No research was performed under the NGP to identify or develop potential alternatives to halon 1301 for

fuel tank inerting. However, the NGP did c ppraise techniques for fuel-tank ullage 

entified the following technologies that can be installed within the ullage itself to discharge a 

suppressant linear fire

extinguisher (LFE), and the Parker Reactive Explosion Suppression System (PRESS). Existing design

guidance on fuel technology, the 

Fenwal cylindrical suppressor. Of these, only the cylindrical suppressors utilized a halon fire 

n several aircraft, one of which was the USAF F-105. These 

ercial transport aircraft (Boeing 707 and 747-100 models) in 

 bay compartm

 portion of the fuel tank

sometim

ccupied b

though th

has not been 

for impleme recent 

ial aircraft.

28

aircraft is prim

ity. Though th

system, only three DoD aircraft have

F-16 and F-117 a

ompile and a

s

id

uppression of fire and explosion, which had been previously developed and fielded.31 That work

upon detection of the fire/explosion flame front: scored canister system (SCS),

system fire and explosion suppression also identifies an additional

20

suppressant (halon 1011) and were fielded o

suppressors were also utilized on some comm

surge tanks to prevent ground fires entering the wing. These systems were designed to provide fire

protection, not explosion protection.32, x

 The SCS utilized pentane to create a fuel-rich environment and 

was also fielded on several platforms. The PRESS technology used water plus a brine additive to reduce 

the water’s freezing temperature below -54 °C (-65 °F) and has not been fielded. Figure 2-21 illustrates 

the SCS, LFE, and cylindrical suppressor devices. 

 

x

 Reference 32 suggests that this type of fire suppression was installed to protect against fires initiated by lightning strike.
38 History of Fire Suppression in Aircraft 

(a) Scored Canister31

(Reprinted with Permission of Kidde Aerospace.) 

Figure 2-21. Illustration of Ullage Fire/Explosion Protection Devices. 

(b) Linear Fire Extinguisher33

(Reprinted with Permission of Meggitt Safety Systems, Inc.) 

(c) Cylindrical Suppressor20

(Reprinted with Permission of Kidde Aerospace.) 

Scored 

Hemi/Backplate 

Detonator 

Clamping

Backnut,

Washer, 2X

Distanc

e

Filler 

Plug Seal

Filler 

Threaded 

Stem

Detonator with 

MIL-C-38999 Connector 

Linear 

Shaped 

Charge Tubular 

Container 

Clamp 

P re Switch with 

MIL-C 38999 

ressu

-

Connector

Length as Required 

(Straight Configuration 

Shown)

Pressure 

Relief 

d Fill P t

Extinguishant 

Pyrotechnic Tube 

Attachment Plate 

Base Plate Score for Separation 

Detonator 
Protected Compartments on Aircraft 39

An add

quires an on-board inert gas generation system (OBIG to provide th s. The NIBB 

system has not been fielded on any DoD aircraft, wherea GGS itself is fielded on several aircraft

-17, MV/CV-22, F-22, UH-1Y, AH-1Z, and t inert the fuel

eing considered for the USN’s multi-mission maritime aircraft, to be designated 

s the P-8 aircraft.34 The C-5 and the SR-71 had implemented liquid nitrogen inerting systems for ullage

rotection. In the SR-71 aircraft, a supersoni , the inerting system was implemented because of 

e high skin and fuel tank wall temperatures gener ing supersonic flight, i.e., aerodynamic heating

ffected by supersonic flight could cause walls to ignition source for fue the ullage.

35 

The use of nitrogen as an inerting and fire suppression agent is described later in this chapter. 

The NGP investigation c three categories: active

(system is always providing a required level of 

el exhaust used as an ullage

inerting agent - the Russian-built Ilyushin IL-2 Sturmovik cooled and pip es into its

el ta paces. Som oD aircraft also rated ullage inerting using exhaust gas 

(B-50 aircraf 2 and dry ice (B-47 and B-36 aircraft), but these techniques were 

iscontinued because of technical problems.36 Reactive techniques would integrate a detection system

and a suppression system utilizing the LFE, SCS, PRESS, or cylindrical suppressor technologies. Passive 

chniques involve installing material within the fuel tank itself to prevent an overpressure by extracting 

 a flame front. There are p rials that have either been fielded or at least have

een demonstrated by the DoD as v assive fuel tank ullage fire protection: reticulated

orous) polyurethane foam and aluminu . Polyurethane foam has been used in numerous DoD 

latforms for years (A-7, A-10, F-15, F/A-18, C-130, and P-3), whereas aluminum mesh has not been 

implemented on DoD aircraft. 

During the NGP, no methane (CF3I) as 

replac stem. A diagram of the F-16 inerting

stem is show -22 and is a typical halon 1301 inerting system design implementation. The 

ystem also includes a to maintain a constant halon reservoir pressure over the entire 

operating temperature envelope for the aircraft, as the reservoir was located in a wheel well area. A 

eater had also been part of the inerting system installed on A-6E aircraft. 

reliminary analysis and testing indicated CF3I was a promising alternative to halon 1301, as it was 

itially concluded that only minor airframe system modifications would be required.37,38 However, CF3I 

ad been previously removed from consideration for use in engine nacelle, APU, and dry bay applications

on DoD ai o toxicit patibility concerns, as well as due to its relatively high

oiling point.

xi Subsequent review later concluded that CF3I was inadequate as a replacement for halon 

1301 in the existing F-16 system due to its higher boiling point and resultant reduced delivery pressure at 

low temperatures.39 Additional concl

 

itional technique identified during the NGP investigation was the nitrogen-gas-inflated ballistic 

bladder (NIBB) system, a double-walled fuel cell that is filled with nitrogen. Interestingly, this technique 

re GS) e nitrogen ga

s OBI

(C AH-64) o provide nitrogen-enriched air to 

tank ullage. OBIGGS is b

a

p

th

c aircraft

ated dur

e become an l vapor in

lassified the various ullage protection techniques into

protection), reactive, and passive. Active techniques

fi ngine has also been

ed engine exhaust gas

ded currently are halon-based inerting and OBIGGS. E

nk ullage s

t) as well as CO

4 fu e earlier D incorpo

d

te

heat from

b

rimarily two mate

iable for providing p

(p m mesh

p

n-NGP investigations were performed into the use of trifluoroiodo

a

sy

ement for halon 1301 in the F-16 fuel tank inerting sy

n in Figure 2

s 400-watt heater

h

P

in

h

rcraft due t y and materials com

b

usions were that:

xi CF3I is listed as one of many acceptable halon alternatives for use in unoccupied spaces on the Environmental Protection 

Agency’s (EPA) Significant New Alternatives Program (SNAP) list, and a minimum performance standard of 7.1 %

volumetric c rmined under 

the Engine N performance

standards for use of HFC-125 (17.6 % volumetric concentration) and FK-5-1-12 (6.1 % volumetric concentration). Boiling 

points for HFC-125 and FK-5-1-12 are -48 °C and 49 °C, respectively.

oncentration is being developed for its use for nacelle fire suppression in commercial aircraft, as dete

acelle Halon Replacement Program conducted by the FAA. That program also developed minimum
40 History of Fire Suppression in Aircraft 

Figure 2-22. F-16 Aircraft Inerting System Diagram.40

• More flight qualification testing would be required on the materials compatibility of CF3I 

before it could be recommended as a replacement for halon 1301. CF3I is more chemically 

d provide a more quantitative 

estimate of its toxicity in realistic exposure scenarios.

reactive than halon 1301, which could lead to metal corrosion or elastomer failure. It was 

indicated that it may be possible to specify materials that could be used in service with CF3I 

after such testing.xii

• CF3I is more toxic than halon 1301. It was indicated that under equivalent operational 

conditions, it would be considered unwise to replace a chemical with one that is 20 times 

more toxic, and that more rigorous toxicity testing of CF3I woul

xiii, xiv

 

xii During design evolution of an aircraft and its systems and subsystems, assessment of materials compatibility is typically 

completed well prior to conduct of flight qualification testing. The rationale for recommending materials compatibility 

evaluation for CF3I as part of flight qualification testing was that it could degrade the fit, form, or function of any of the 

components that it contacts, and that it could introduce a contaminant or a degradation product into the fuel tank and fuel that 

produces damage. Due to the number of components involved, the extent of attack required to interfere with the operation of 

different components could vary. Thus it was assessed that it is virtually impossible to account for all possibilities in a 

laboratory and that engineering testing and evaluation would be required before CF3I could be fully qualified for use. xiii An aircraft fuel tank as well as an engine nacelle or an APU-type compartment is a normally unoccupied space. For such 

spaces, this means it is permissible to use fire suppressants whose design or certification concentrations that are in excess of 

xiv

n 

 aircraft, which are still being procured as of the 

the Lowest Observed Adverse Effect Level (LOAEL). 

 The LOAEL, which represents the volumetric concentration of a substance that can induce cardiac sensitization in testing 

using beagle dogs, is reported for halon 1301 and CF I as 7.5 % and 0.4 %, respectively. The 20 factor was derived by the 

ratio of the halon 1301 LOAEL to the CF I LOAEL (i.e., 7.5/0.4). Though cardiac sensitization LOAEL was one of several 

CF I issues reviewed, the fielding of a suppressant whose LOAEL is significantly below a system certification concentratio

is not without precedent in the DoD. USAF C-130J and USN KC-130J

3

3

3
Types of Fires Experienced 41

• The ODP of CF3I is highly dependent upon the altitude and latitude at which it is released. It 

was calculated that CF3I usage onboard an F-16 aircraft could be between 13 % and 167

damaging to the ozone layer than use of halon 1301, or, in other words, ozone depletion from

F-16 application of CF3I could be as small as one-eighth that of halon 130

altitudes) or as large as one and two-thirds times as damaging as halon 1301 (at higher 

altitudes). However, even for conservative choices, CF3I use onboard an F-16

Class I ozone-depleting substance if significant amounts are released above 6.1 km 

(20,000 ft), and the US Clean Air Act bans use of Class I substances. 

Subsequent NGP modeling of ODP associated with the use of CF3I in engine nacelle and APU-ty

applications is described in Chapter 7. This study was conducted using the same chemi

odel, but updated with the latest representation of atmospheric chemistry and dynamics. The results of 

odeling indicated the ozone depletion effects would not be significant for CF3I use in engi

pe applications.41 

TYPES OF FIRES EXPERIENCED 

Both occurrence and suppression of fire on board an aircraft are probabilistic. Although obvi

f such probabilities plays a role in determining whether a fire suppression sy

ented, or if implemented, the type of system, active or passive, and if implemented 

 % as 

1 (at lower 

 would be a 

pe

cal transport 

m

the m ne nacelle

and APU-ty

2.3 

ous, 

assessment o stem is

implem whether that 

ystem will provide protection under all flight and mission conditions. The level of protection 

implemented then becomes a risk trade for the program manager(s) responsible for the development,

roduction, and fielding of a military aircraft. 

Such trades receive a ost often applied when 

conside

specifica

vulnerability

program ma

would be case, since during the formulation of the NGP strategy it was found that majority of the types of

fires exp ie

dry bay m

further the c

The primary cost. Key aircraft performance 

parameters (KPPs) include range, speed, and drag, all of which are impacted by weight. The aircraft 

p any

sy re a

red

ac

ma ude a

er

wo t

s

p

dditional attention from program managers, since they are m

ring the need to implement fire suppression systems beyond those normally required by an aircraft

tion for safety (i.e., engine nacelle and APU compartment) or those necessary to achieve desired

reduction (i.e., dry bay compartments). In short, such trade efforts are difficult to sell to a 

nager, since they increase aircraft weight, cost, and complexity. It is no surprise that this

er nced in DoD military aviation, as listed in Table 2-2, were types related to engine nacelle and 

co partment fires. This provided the NGP with an overarching view from which to investigate 

haracteristics of fires within engine nacelles and dry bay compartments. 

 aircraft design considerations are weight, performance, and 

rogram manager(s) and aircraft design engineers are always seeking the lowest weight possible for

stem on the aircraft. For example, established fire suppression system specifications may requi

undant nacelle fire suppression capability. The reality may be that the aircraft just cannot physically

commodate the installation of an additional fire suppression agent container, or a risk trade analysis

y be performed to assess the risk of not implementing the additional bottle, which will incl

probabilistic analysis for failure to suppress the fire leading to loss of the aircraft and fatalities (in oth

rds, what is the risk of not implementing redundancy to save weight). If it is decided to accept tha

 writing of this book, utilize halon 1211 for engine nacelle and APU fire suppression. The LOAEL for halon 1211 is 1.0 %, or

7.5 times higher than the value for halon 1301. Chapter 6 provides additional information regarding the cardiac sensitization

protocol and the development of physiologically-based pharmacokinetic (PBPK) modeling of potential human exposure to 

halon alternative suppressants. Halon 1211 is also a Class I Ozone Depleting Substance (ODS). 
42 History of Fire Suppression in Aircraft 

risk, the e

analyses tha

– analysis w

assessed fo

perspective 

all sides are 

those sides o

42

 r quirement to provide the redundant capability will be removed or waived. Vulnerability 

t effect whether or not dry bay and ullage fire protection are implemented is also probabilistic 

ill assess probability of kill (aircraft loss) given a hit. Vulnerability reduction techniques are 

r weight and performance, with performance assessed both from the vulnerability reduction 

and aircraft performance perspective (for example, is it necessary to design a fuel cell so that 

completely self-sealing, or can weight be reduced by electing to employ self-sealing only on 

r a portion of those sides having the greatest vulnerability). 

Table 2-2 Tabulation of Aircraft Fire Types.

Service Fire Type Location Fuels 

Suppression 

Time 

Fixed 

System (F) 

or HandHeld (H) 

Occupied 

Compartment 

at Discharge 

(Y or N) 

US Army Fuel spray Engine nacelle JP-4 or JP-5 seconds F N 

US Army Stack fire APU exhaust JP-4 or JP-5 seconds F N 

US Army Turbine jet 

fire 

Helicopter 

engine nacelle 

JP-4 or JP-5 ≈ 5 s F, H N 

TriService 

Explosion Dry bay JP-8, hydraulic 

fluid 

≈ 10 ms F N 

TriService Turbine jet 

fire 

Engine nacelle JP-4, JP-8 ≈ 5 s F N 

USAF Electrical Compartment Plastics < 30 s H Y 

USAF Wall fire Cargo bay Paper, plastics, 

c

≈ 5 min F (H) Y 

hemicals 

USN Fuel spray Engine JP-4, JP-5, 

JP-8, hydraulic 

fluid 

seconds F N 

USN Fuel spray Crew 

compartment 

JP-4, JP-5, 

JP-8, hydraulic 

fluid 

< 30 s H Y 

USN Electrical Crew 

compartment 

Insulation, 

plastics 

< 30 s H Y 

USN Turbine jet 

Fire 

Engine nacelle JP-4, JP-5, 

JP-8, hydraulic 

fluid 

10 s to 20 s F N 

USN Rapid 

growth 

Dry bay JP-4, JP-5, 

JP-8, hydraulic 

fluid 

≈ 50 ms F N 

The relevance of trade analyses that evaluate requirements, weight, and performance relates directly to the 

residual fire threat for the aircraft after all design considerations have been addressed to the extent 

possible or practicable, e.g., drainage provisions for leaked flammable fluid, separation of fuel sources 

from potential ignition sources, tolerance of components to fire or hardening, and fire containment, which 

themselves are subject to weight trades prior to or in consideration with implementing fire suppression. 

As a precursor to its research activities, the NGP conducted a review to characterize the nature, 

fre

fie

co s 

quency, consequences, and severity of fires for which halon-based fire suppression systems have been 

lded on DoD aircraft.43 With the knowledge acquired from this review as well as from various studies

nducted during the TDP, studies performed coincident with the NGP and presented at Halon Option
Types of Fires Experienced 43

Technical Working Conferences (HOTWC), and continuing work by the FAA’s Fire Safety Branch and 

the International Halon Replacement Working Group (IHRWG), the characteristics of all types of aircraft 

fires have been described.xv The remainder of this chapter discusses fire characteristics for areas 

specifically addressed by the NGP, engine nacelles, APUs and dry bay compartments, plus it includes a 

similar discussion on ullage fire/explosion characteristics. 

2.3.1 Safety-related Fires 

As described in the beginn e when it results 

from component failures, which may be due to inadequate design, a mechanic m

as fatigue, or maintenance error, and results in either a fla c n 

ignition source, e.g., a hot engine case, or the f thems e b mm d 

the ignition s se occ r on d. The two most common types 

of fire hazard in the engine dire ce of ns of fuel livery and the failure 

m pe to le ther or a p re.11 A mechanical failure in the 

component path n the ovides fuel to the engine can either result in a fuel spray or the 

p ng of fu les o pes a c el line, ystem 

c nt that was improperly installed and a el to leak. An additional fire hazard associated 

w the aircr ac n af ishment strong potential exists for 

reignition of the fire from hot surfaces on the engine. Hot surface reignition remains a threat as long as 

f or and om ith su y hot surfaces. This concern prevails in flight as 

well as on the ground during ngin fte shutdown, for a brief period, the 

engine case te and free v perature

t ature s ffecte mov n ai at would cur with the engine 

operating. It is possible then that a small le not result in an in-flight fire, could result in a 

nacelle fire o d dur gine shu the co ent is wetted with the flammable 

fluid, or if the fluid has pool ng nt can impart extensive damage.

G that dry conta mable f s, such as l and hydraulic lines 

and hydraulic fluid reservo tial ig ources such as bleed air ducts, safety-related fire 

events may a t ents ugh e major of dry bay fires over 

the lifetime of a particular military aircraft ballistic threats, other events can result in a 

d fire, su ctrica arcing ted by failed electrical circuits or 

wiring, or damage induced by som er fo ch a trike. 

-flight Fires 

ing of the chapter, an aircraft fire is deemed a safety-related fir

al failure

ontacting a

oth the fla

echanism such

always-present

able fluid an

mmable fluid

ailures elves provid

ource. The types of fires ur in flight o the groun

nacelle are a ct consequen the mea de

ode that rmits fuel 

withi

ak, i.e., ei

 nacelle that pr

a spray fire ool fi

ooli

ompone

el. Examp f these ty of failures are 

llows fu

racked fu or a fuel line or s

ith aft engine n elle is that eve ter extingu is achieved, a 

uel vap air can c e in contact w fficientl

 or after e e shutdown. A

olume tem

r engine 

mperature 

oak-back e

nacelle

d by re

 can actually increase due to engine

emper al of ventilatio rflow th

ak, which does

 oc

n the groun ing en

ed, the resulti

tdown. If 

 ground fire eve

mpartm

iven bays may in flam

irs, and poten

luid carrying com

nition s

ponent fue

lso occur in hese compartm . Even tho

will result from

the probabl ity

ry bay ch as an overheated ele l generator, 

rm of impact su

 genera

e oth s a bird s

In

In-flight engine fires can result in catastrophic loss of an aircraft and fatalities if the aircraft has only a 

single engine, or if there are multiple engines and the fire goes undetected and suppression is not 

attempted until the fire has effected damage to the aircraft that results in loss of lift or thrust or the aircraft 

fails to execute a safe landing, or if the nacelle fire suppression system cannot suppress the fire even if 

activated immediately after a fire warning is provided to the pilot and there is subsequent catastrophic 

damage or failure to execute a safe landing. A qualitative assessment of pilot reaction was conducted by 

 

xv As of the writing of this book, information related to work conducted by the FAA Fire Safety Branch and the IHRWG can be 

found at http://www.fire.tc.faa.gov. 
44 History of Fire Suppression in Aircraft 

the NGP and estimated that 95 % of the time pilot reaction was characterized as normal, meaning that the 

pilot response followed typical emergency procedures for effecting agent release without delay. After 

receipt of a fire warning, the pilot isolated the affected compartment to remove fuel flow to that 

compartment, armed the fire suppression system, confirmed that the fire warning persisted and had some 

secondary indications of the fire condition, and then discharged the suppressant into the compartment.44 

During the TDP, extensive review of fire incident and mishap data was conducted to evaluate effectivity 

ession systems aboard USN aircraft.45,46 That review revealed 

that 55 % of fixed-wing fire events occurred in flight, yet for rotorcraft only 35 % had occurred in flight. 

d an overall 

higher percentage of in-flight nacelle fires on aircraft with nacelle fire suppression systems and a much 

engine fires have been experienced within the DoD. An internal engine fire can occur during startup or 

shutdown and may be a result of improper procedures, a component failure, or severe ambient conditions 

s

of currently fielded halon 1301 fire suppr

An additional significant finding was that in-flight effectivity of the nacelle halon fire suppression 

systems was 76 % for fixed-wing aircraft, yet just 47 % for rotorcraft. It is important to bear in mind that 

these percentages are overall numbers for each of the aircraft types. However, they infer that the different 

nature of fixed-wing and rotorcraft designs and the missions these aircraft are required to fly influence 

different fire threats from an operational safety perspective. 

The most surprising finding in these studies was that related to the redundant fire suppression capability 

on most rotorcraft and several fixed-wing aircraft. For rotorcraft, it was found that this capability was not 

utilized frequently, and when utilized it was successful in suppressing fire less than 10 % of the time. 

Usage on fixed-wing aircraft was also infrequent, except on P-3 aircraft.47 In those cases, the reserve 

capability was successful 67 % percent of the time. The rationale derived from the rotorcraft events was 

that as a fuel-fed fire continues to grow after the first discharge, it is not likely that there would be 

successful suppression by the time the pilot effects the second discharge, which is delayed since the pilot 

has spent time coping with the emergency.47 This rationale has been applied to support implementation of 

non-redundant halon-alternative nacelle fire suppression capability on the USN UH-1HY and AH-1Z 

rotary aircraft models. (However, no rationale is provided in the USN fixed-wing analysis that describes 

the 67 % success rate of the redundant capability on P-3 aircraft.) Contrasting the USN effectivity 

analysis was analysis performed by the U.S. Army, where frequency of use and effectivity of the 

redundant fire suppression capability on their rotorcraft between 1985 and 1995 indicate

higher effectivity of the redundant fire suppression capability.48,49 Given that Army and Navy analyses 

included similar model rotorcraft, these outcomes suggest that service-specific missions may have an 

impact on nacelle fire occurrence and the ability to suppress nacelle fires for similar aircraft models. As 

of the writing of this book, it is planned to revisit and update the U.S. Army and USN data in support of 

halon alternative nacelle fire suppression efforts under a joint U.S. Army-USN program for H-60 model 

helicopters (i.e., U.S. Army Blackhawk and USN Seahawk helicopters).50

Ground Fires 

Aviation mishap data from the DoD safety centers typically classify mishaps by whether the mishap 

occurred in flight; if not in flight then whether intent for flight existed; or whether the mishap occurred on 

the ground and there was no intent for flight, such as during a ground maintenance engine turn operation. 

Fire mishaps that fall into the intent-for-flight-existed category typically occur on the ground (e.g., a 

rotorcraft engine nacelle fire that occurs during rotor turns prior to takeoff). Also, fires that occurred on 

aircraft while on a ship’s deck were also considered as ground fires. Both nacelle fires as well as internal 

uch as high winds while at sea. In the case of improper starting procedures or severe ambient conditions,
Types of Fires Experienced 45

the engine does not ignite properly during startup and excess fuel is dumped into the engine combustor. 

The fuel can be blown into the turbine and tailpipe, subsequently igniting. In the case of a mechanical 

failure, a fuel line may rupture, a pressure and drain valve may fail, or the engine bearings may fail. Fuel 

can accumulate in the combustor, turbine, or tailpipe and may subsequently ignite. These internal fires 

are colloquially referred to as tailpipe fires.51

The reviews described in the previous paragraph revealed that 65 % of rotorcraft fires occurred on the 

ground, whereas for fixed-wing aircraft this number was 45 %, and the aggregate effectivity of the nacelle 

halon fire suppression systems was similar for each aircraft category: 65 % for fixed-wing aircraft and 

64 % for rotary aircraft. APU fires occurred primarily on the ground, and their frequency of occurrence 

was noted to be far less than that for nacelle fires. Effectivity of halon fire suppression in APU 

compartments was even higher, though fire events were indicated for two aircraft platforms only – 100 % 

effectivity was found for P-3 APU fire suppression, and 75 % for H-53 APU fire suppression. Similar to 

ground nacelle fire suppression events, there is reduced or minimal airflow within an APU compartment 

to aircraft survivability are those that are ballistically induced, usually during enemy 

combat. That combat projectiles could induce fire within an aircraft dry bay or fuel tank ullage with 

during a fire suppression event. 

Effectivity vs. Optimization vs. Over-design 

The extension of the preceding to the NGP relates to its efforts in the development and validation 

computational fluid dynamics (CFD) models for nacelle fires and nacelle fire suppression. Those efforts 

considered the complex physics of fire, suppressant discharge and transport, the effects of compartment 

clutter on transport and distribution, and the effects of ventilation airflow on the behavior of fire and 

suppressant distribution. That effectivity of current nacelle halon fire suppression systems would be 

lower for in-flight events correlates with the NGP focus to develop CFD modeling capabilities to 

optimize nacelle fire suppression system designs for the dynamics of in-flight conditions. The need for a 

design optimization capability cannot be overstated when considering that even with halon 1301 as the 

suppressant and normal pilot reaction to effect agent discharge, the overall effectivity of halon 1301 

nacelle fire suppression systems was still less than 80 %. It is this dichotomy that challenges an 

overarching assertion formulated during the TDP and that has prevailed during the NGP that halon 1301 

nacelle fire suppression systems are over-designed. Rather, whether a system is either over-designed or 

not optimized is more appropriately assessed on an aircraft-by-aircraft basis. In any event, the CFD 

capability developed under the NGP provides a tool for use in conducting such assessments. 

2.3.2 Ballistically-induced Fires 

Fires that relate 

catastrophic results has never been unique to U.S. military aircraft. Imperial Japanese Navy design 

policies during the later part of World War II not only required carbon dioxide (CO2) for nacelle fire 

suppression, but also for protection of fuel cells and alcohol tanks on aircraft; and structural spaces 

surrounding fuel cells were to be air tight, structural and weight limitations permitting. The CO2 systems 

protecting fuel cells were to be automatic, and because of combat experience in which hits on aircraft 

with alcohol tanks resulted in fires, an automatic CO2 system was to be implemented for the spaces 

surrounding the alcohol tank, i.e., a reactive dry bay fire protection system; a pilot-activated system was 

to be implemented for discharge into the tank itself.52 There is evidence that a concept for implementing 

active dry bay fire protection was being pursued by the Imperial Japanese Navy at the latter stage of 
46 History of Fire Suppression in Aircraft 

World War II (Figure 2-23) and was planned to be an automatic system utilizing CO2 as the fire 

suppression agent. What was being investigated then still applies today, in that aircraft remain vulnerable 

to ballistically-induced fires in dry bays and in the fuel tank ullage. These types of fires are summarized 

below without the intent of describing all of the damage effects that could be effected by a ballistic 

projectile or without describing the functioning characteristics of various projectiles. Though it is 

possible for a ballistic projectile strike into another compartment to effect a fire, e.g., within an engine 

Dry Bays 

nacelle, dry bays and the fuel tank ullage remain the most vulnerable to ballistically-induced fires. 

Taking into consideration that a delay of only several hundred microseconds may occur before an HEI 

projectile functions after impact, the response time of any reactive ullage or dry bay protection system to 

detect the combustion event is typically on the order of microseconds. Whether this delay is longer is a 

function of several variables, including the thickness of the aircraft material that the projectile must 

penetrate and impact obliquity angles.43

Figure 2-23. Imperial Japanese Navy Dry 

Bay and Ullage Fire Suppression System 

Concept, Late World War II.52

During aircraft dry bay live fire testing, the effects of wet-to-dry and dry-to-wet penetrations into the 

aircraft are characterized as follows. The wet-to-dry penetration is one in which the projectile first 

penetrates a fuel cell, travels through the fuel, and exits into dry bay. As the ballistic projectile travels 

through the fuel it creates an overpressure in the fuel cell. During exit, fuel may either leak or mist and 

can be ignited by hot spall; by pyrophoric, incendiary or tracer components of the projectile; or by an onboard ignition source, such as such as hot gases from penetrated bleed air lines, hot metallic surfaces (e.g., 

surfaces of bleed air or other hot gas ducts), or arcing electrical lines. The dry-to-wet penetration is one 

in which the projectile first enters into the dry bay and then penetrates either a fuel cell or a flammable 

fluid carrying component resulting in either leaking or misting fluid, or spall from the projectile will 
Types of Fires Experienced 47

damage such components sufficiently to effect a leak or mist, particularly if the damage to a system or

component is one that operates at high pressures. In either case, fluid entering the dry bay can be ignited. 

Ullage 

Various ideas to describe the ullage combustion by an electrical discharge are all based on the notion of a

critical flame kernel or bubble. Experiments reveal that the initial disturbance created by the electrical 

discharge must exceed a minimum volume in order for the flame to become self-sustaining. Experimental 

observations show that this volume is often toroidal in shape for a spark generated between bare

electrodes. Ullage ballistic testing performed by the USN described the ullage combustion process as 

two-stage, free-radical, branched-chain reactions, depending upon the ignition source. For a spark

ignition source that is produced by a component failure within the ullage, it was described that in the first 

stage a seed of free radicals is produced, which in turn produces a blue flame front throughout the ullage 

y to the second stage in which free radicals 

e ating set of molecules that are the products of

combustion and can lead to explosion. This process can take place within a few hundred milliseconds, as 

depicted in Figures 2-24 and 2-25.

xvi The functioning of an HEI ballistic projectile within the ullage will 

cause a violent reaction that, instead of a two-stage reaction, directly produces the products of

combustion.53 

(a) before spark (b) 17 ms afte gnition (c) 34 ms after ignition 

Figure 2-24. Ignition and growth of a flame in 7 % H2, 1.4 % C3H8 Jet A Simulant Mixture at 

295 K (22 °C) and 84.1 kPa (0.83 atm).54

2.3.3 Spray Fires

fuel vapor/air mixture. That mixture then transitions rapidl

react with one another and produce a stable though acc ler

r i

A cracked high pressure fuel, lubricant or hydraulic fluid line, or a cracked flammable fluid system

component, such as a motor housing, can supply a steady spray of fuel for a fire stabilized behind

obstacles in an engine nacelle. Small droplets quickly evaporate, and the momentum from the spray 

efficiently entrains the air necessary for combustion. The fuel-air mixture is ignited when it contacts a hot 

surface, or the mechanism that effected the failure mode can provide the ignition source, such as arcing

onto a flammable fluid line as can occur when an electrical harness chafes against the line.

 

xvi

 For the process depicted in Figure 2-25, the spark source was a J-57 engine igniter that produced a spark from a capacitance 

discharge of 19 J of energy. The photographs in Figure 2-25 were provided by the Naval Air Warfare Center Weapons 

Division, Aircraft Survivability, and are reprinted with permission of the Naval Air Systems Command. 
48 History of Fire Suppression in Aircraft 

(j) 117.5 ms after ignition (k) 140 ms after ignition (l) 375 ms after ignition 

(a) 0 seconds (b) 7.5 ms after ignition (c) 15 ms after ignition 

(d) 22 ms after ignition (e) 30 ms after ignition (f) 37.5 ms after ignition 

(g) 50 ms after ignition (h) 72.5 ms after ignition (i) 95 ms after ignition 

Figure 2-25. Spark-Initiated Ullage Combustion Process. 
Types of Fires Experienced 49

Ex

co

pray, and the combustion is allowed to starve, or the combination of reduced flammable fluid flow and

ventilation airflow provides sufficient strain. In addition to the flammable fluid system pressure and

entilation airflow, spray fire flame stability may also be influenced by ventilation airflow temperature,

ype of flammable fluid, and the fire suppressant type. 

.3.4 Pool Fires 

ilure of a fl omponen ntity of

flammable fluid, which cannot drain away from the compartment and thus collects to form a pool. 

ode,

e stability of a pool fire can be greatly enhanced if an obstacle at the leading edge of the pool is present. 

In some nacelle configurations, obstacles in the form of structural ribs or other bluff bodies are present at

cations where combustible liquids may puddle and fail to drain in a timely manner. Pool fire flames are 

ixed structure and are more stable than the diffusion structure of spray flames, and

oth types of flame are believed to be less stable (more easily extinguished) at high airspeeds.

igure 2-26 depicts the sequence of events leading to a stabilized pool fire within a dry bay compartment 

aving relatively low ventilation airflow. 

fire can

be very significant. Test results show that under similar airflow and baffle height conditions, a baffleta

uring the TDP has shown that occluded pool fires are more difficult to suppress than spray fires. The 

CAA reported that suppression mass quantity greatly increased with the presence of transverse structural 

bs situated in the lower part of nacelles tested. This was attributed to difficulties in extinguishing pool 

fires between structural ribs. Later testing of three nacelle fire scenarios, fuel spray fire, burning of fuel 

s it flows as a thin film over a hot surface, and a burning pool of fuel, showed that greater suppressant

 and pool fire conditions than for spray fires.55 That

reater suppressant concentrations were required to suppress pool fires was again reconfirmed during the

DP i t stabi d larger ns and

mixin

fires were again fou st program for the 

USAF F-22 aircraft.

tinguishment of the burning spray can occur if a critical amount of suppressant entrains into the 

mbustion zone, or if the flammable fluid flow is cut off, thereby reducing the pressure effecting the 

s

v

the t

2

A pool fire can occur if fa ammable fluid line or c t releases sufficient qua

Ignition may occur by contact with a hot surface or by the mechanism that effected the failure m

which could include a ballistic projectile. If there is ventilation airflow, such as within an engine nacelle,

th

lo

believed to have a prem

b

F

h

Ignition of vaporized and pooled fuel 

and fuel-air mixture combustion 

Ballist Residual stabilized pool fire 

effected by penetration into the dry 

bay

ic round incendiary flash 

Figure 2-26. Stabilized Pool Fire Formation within an Aircraft Dry Bay Compartment. 

Lab-scale fire suppression tests have shown that the stabilizing effect of a baffle in front of a pool

s bilized pool fire is dramatically more difficult to extinguish than a baffle-stabilized spray fire where the

baffle is located in the middle of the flow field. Nacelle fire testing performed by the CAA and then later 

d

ri

a

concentrations were required for the hot surface

g

T n which it was found tha lity of pool fires require suppressant concentratio

longer characteristic suppressant g times to achieve suppression due to recirculation zones. Pool

nd more difficult to suppress than spray fires during a nacelle fire te

56
50 History of Fire Suppression in Aircraft 

2.3.5 Fire Characteristics 

Flame Temperature 

The fire triangle and fire pyramid are often used to describe the basic components of fire. In the fire 

triangle, the three sides are typically indicated as fuel (or substance that will burn), oxygen, and 

temperature (i.e., sufficient heat to initiate and sustain combustion). The four surfaces of the pyramid 

emical chain reactions involving free radicals, which are necessary for the 

reaction (hence combustion) to be sustained. For hydrocarbon fuels (which include aviation fuels), the 

indicate a fourth component: ch

reaction is usually written as Fuel + O2 → H2O + CO2. The net heat release for the reaction is referred to 

as the heat of combustion, and this can be used to estimate the maximum possible flame temperature, or 

the adiabatic flame temperature.57 The net heats of combustion for JP-4, JP-5 and JP-8 aviation fuels used 

on DoD aircraft are within the range of 40 kJ/g to 45 kJ/g. As illustrated in Figure 2-27, the maximum 

flame temperature occurs when the equivalence ratio is one. The equivalence ratio is the actual fuel-air 

ratio divided by the stoichiometric fuel-air ratio. The fuel-air ratio is simply the ratio of the mass of fuel 

supplied to the mass of air supplied. In stoichiometric combustion, there is complete conversion of 

carbon and hydrogen to CO2 and H2O, with no leftover oxygen. Thus, the stoichiometric fuel-air ratio is 

the ratio of the mass of fuel supplied to the mass of air supplied for which the stoichiometric condition is 

satisfied. 

Figure 2-27. Flame Temperature vs. Equivalence Ratio.58

(Reprinted with permission by AFP Associates, Inc.) RICH LIMIT LEAN LI

M

Maximum Adiabatic Flame Temperature

e Ratio Actual F

(Note that the maximum flame

temperature is on the rich side

of stoichiometric.) Adiabatic Flame Temperature CurveIT

at Equivalenc = 1 lame Temperature

>1

Fuel

Rich

<1

Fuel

Lean

Lower 1.0

Flammability

Limit Range

Upper

Flammabilty

Limit Range

Flame Temp

Equivalence Ratio

er e atur

RICH LIMIT

LEAN LI

M

Maximum Adiabatic Flame Temperature

e Ratio Actual F

(Note that the maximum flame

temperature is on the rich side

of stoichiometric.) Adiabatic Flame Temperature CurveIT

at Equivalenc = 1 lame Temperature

>1

Fuel

Rich

<1

Fuel

Lean

Lower 1.0

Flammability

Limit Range

Upper

Flammabilty

Limit Range

Flame Temp

Equivalence Ratio

er e atur
Types of Fires Experienced 51

In reality, the adiabatic flame temperature is not attained, particularly for highly-ventilated compartments

such as engine nacelles as well as for dry bay compartments, as they may also be ventilated and have

ot completely sealed – openings such as drain holes may be present). Also,

leaking fuel is not assured in the event a fire occurs in nacelle or dry bay 

issions of the ultraviolet radiation. In contrast, the reduction in oxygen

pressure, as occurs with increasing altitude, may reduce the flame temperature and thus the intensity of

e and

burnin

temp erature

of 109 2

) for

15 mi thermal 

fire detectors optical fire

2

Deflagratio

 supersonic 

e

characte acelle, dry 

bay, a 

(sustained) fi e in 

ons of

the aircraft agrate with 

ducted at a

simu

easure from film was observed to be 3.35 m/s (11 ft/s).53 If a combustion wave propagates throughout

the ullage with nea kPa (100 psig) or 

leakage (i.e., they are n

complete combustion of 

compartments. It is not uncommon to find residual fuel or hydraulic fluid in these areas after a fire event.

Thus, as illustrated in Figure 2-27, the actual flame temperature will be less than the adiabatic flame 

temperature, but the maximum flame temperature occurs on the fuel-rich side of an equivalence ratio of 

unity, as greater fuel mass available results in greater energy and heat release. 

In work to determine the spectral characteristics for fire detection by optical fire detectors, it was shown

that ultraviolet emissions from JP-4 flames increased with increasing altitude and that the ultraviolet (UV) 

power emitted at 10.7 km (35,000 ft) by the burning fuel was nearly double that emitted at sea level. 

Thus UV detection must be sensitive for the sea level condition. The power emitted at visible and nearinfrared wavelengths decreased at higher altitude. Thus visible/near infrared detection must be sensitive 

at high altitude. The visible/near-infrared emission spectra are due to hot carbon particles in the diffusion 

flame, while the energy emitted at ultraviolet wavelengths is associated with the electronic transitions of 

free radicals in the flame. It is believed that as altitude is increased (and pressure decreases) the reduction

in the available oxygen to sustain the combustion process may result in an increase in the density of these 

radicals, and thus stronger em

the visible/near-infrared radiation emitted by the carbon particles.59 Typical hydrocarbon flam

g JP-4 fuel emission spectra are shown in Figure 2-28. 

The relevance of the preceding discussion is that requirements for fire tolerance, thermal fire detection,

optical fire detection, and ultimately fire suppression are related to temperature (and exposure at that 

erature). Fire zone components are required to withstand exposure to a minimum flame temp

3 °C (2000 °F) and minimum heat input (heat flux density) of 9.3 Btu/ft2

-s (10.6 W/cm

n. This temperature is also the DoD fire warning requirement threshold for continuous-loop

used in many engine nacelles and APU compartments for fire detection, whereas 

detection for dry bay compartment fires are typically sensitive to the 4.3 μm wavelength (i.e., the CO

spike) associated with JP-type aviation fuel fires. 

n vs. Detonation 

Deflagrations and detonations are distinguished by flame front velocity. Detonations involve

burning velocities and are more likely to occur in oxygen than in air, whereas deflagrations ar

rized by subsonic burning velocities.60 Research and testing has clearly proven that n

 and ullage fire events are deflagrations. Detonations and deflagrations may or may not lead to 

re. When the combustion rate of energy release is rapid and there is a large increas

pressure, referred to as overpressure, it may be sufficiently large enough to damage or destroy porti

structure and is referred to as an explosion. Aviation fuels typically defl

overpressures normally less than 1380 kPa (200 psi).4

 In one series of ullage fire testing con

lated high altitude, with a pressure of 58 kPa (8.4 psia) and 21 % oxygen, an initial flame velocity

m

r stoichiometric fuel/air mixture, a pressure increase of over 790
52 History of Fire Suppression in Aircraft 

eight times atmospheric pressure is theoretically possible.43 Photographic evidence from full-scale dry 

bay testing indicates that turbulent flame speeds are below 300 m/s (984 ft/s).xvii 

 (a) Typical Hydrocarbon Fire Emission Spectrum61

 

(b) Emission Spectrum of Burning JP-4 Fuel above 35,000 ft59

Figure 2-28. Hydrocarbon Flame Emission Spectra. 

(1800 K = 1527 °C = 2780 °F) 

xvii The speed of sound is 340.29 m/s at sea level. 

 0.2 0.29 0.4 0.8 2.7 4.3 

Wavelength, micrometer (μm) 

Intensity 

Visible

Light 

Infrared

(IR) 

CO2

Spike 

UV Detection Region 

adiant Intensity 

(Wμm

Spectral R

-1s-1) 

 0.2 0.4 0.6 0.8 1.0 1.5 2.0 4.0 6.0 8.0 10 15 

Wavelength, micrometer (μm) 

1800 K

Blackbody 

10 

1 

10-1

10-2

10-3

10-4

-5 10

10-6

10-7
Types of Fires Experienced 53

Turbulence 

Turbulence within an aircraft engine nacelle or dry bay compartment is influenced by a variety of factors. 

he obvi artment will also 

, recirculation, or 

tagnation. Structural members (ribs, es, longerons), subsy s equipment (engine and accessory 

mponents, bleed air ducts, flammable fluid components and lines, electrical components and 

arnesses), and mounting hardware (clamps, brackets), collectively referred to as bluff bodies or clutter, 

ill effect complex flow fields within nacelle or dry bay compartments and also provide locations for 

ame holders or pooled fuel. Estimation of the increase of the Reynolds Number as change occurs in a 

circulation z of premixed gases burning in the wake a bluff body was known previously to support 

ons.55 (The Reynolds Number is used for determining whether flow is laminar or 

rbulent.) Es tion of the Reynolds Number at the air inlet of an engine nacelle indicates this area to

y turbulent even eration of clutter-enhanced mixing.62 Though ventilation airflow

rough an e be assumed to be longitudinal throughout the annular volume, it is also 

possible that ventilation airflow will in some places be found to be circumferential about the engine and

ven contain areas of reversed flow. An engine nacelle fire is typically a turbulent diffusion flame

stabilized behind an obstruction in a moderately high-speed airflow that was typically found to range 

om 0.57 kg/s (1.25 lb/s) to 1.25 kg/s (2.75 lb/s).43 

he primary design purpose of ventilation is to provide cooling for the engine to 

ensure desired engine performance and mponents located in

the nacelle, though in the cas directional ventilation

irflow to also prevent flame propagation as well as remove flammable vapors.63 The number of air 

xchan (volumetric air flow divided by net volume) depends on the aircraft design and

ay be as high as one per second.43 ough nacelles are unique for each aircraft, the intended forwardairflow characteristic is typical. Dry bay compartment sizes, ventilation 

quirements, and geom tries differ greatly from aircraft to aircraft, and thus turbulence within a dry bay 

compartme y or y not be more complex than within an engine nacelle. Figure 2-29 provides a 

ontrasting ple of this when compared to Figure 2-26. Both depict fires within a dry bay 

compartment. Where igure 2-26 suggests a relatively benign turbulent environment, Figure 2-29 

rovides an ample of a highly turbulent environment. 

T ous is ventilation airflow through the compartment. The geometry of the comp

influence whether there are localized regions of higher ventilation airflow velocity

s fram stem

co

h

w

fl

re

turbulence conditi

one

tu tima

be highl prior to consid

th ngine nacelle may

e

fr

T airflow through a nacelle

not degrade operation of other subsystem co

e of the F-111 aircraft the nacelle design imparted uni

a

e ges per unit time

m

to-aft nacelle ventilation 

Th

re e

nt

e

ex

ma

xam

ma

c

as F

p

Turbulent flame within a dry bay Turbulent flame about and pool fire below

compartment. ruptured fuel line within a dry bay.

Figure 2-29. Turbulent Fire within Aircraft Dry Bay Compartments. 
54 History of Fire Suppression in Aircraft 

The same physical and environmental characteristics that contribute to turbulence will also influence 

flame strain, suppressant distribution, and suppressant effectiveness (and dilution), as the time required 

faces within an engine nacelle, reignition, which is 

Figure 2-30. Variation in Halon 

1301 Peak Flammability Limits 

for Isobutane 

 

for an agent to entrain into the recirculation zone is a key parameter in the effectiveness of suppression 

with respect to baffle-stabilized flames. In computational fluids dynamics (CFD) modeling and model 

validation testing performed by the NGP, success or failure in extinguishing nacelle fires was largely 

correlated with the ratio of the rate of injection of suppressant to the total inflow rate into the nacelle (that 

is, ventilation airflow plus suppressant flow). For a suppression system that is not an optimized design (it 

is possible for an un-optimized system to pass a certification test), turbulence can effect inhomogeneities 

and failure to suppress a fire. If regions of the flow field exist where the suppressant concentration is 

locally below the value that leads to suppression, there can be failure to completely suppress a fire, since 

small pockets of fire can quickly propagate through the remaining premixed gases in the nacelle leading 

to accelerated burning under certain conditions and catastrophic results.64

2.3.6 Aircraft Operational Temperature Environment44

As discussed previously, temperature is the primary basis of current aviation requirements for fire 

detection and component tolerance to fire. Temperature is also a variable with regard to the potential for 

ignition of a fuel-air mixture, the amount of energy to ignite a flammable mixture, suppressant 

performance capability, and, in the case of hot sur

discussed separately in this chapter. Given a constant initial pressure, it has been established previously 

that a non-flammable fuel-air mixture may become flammable for some period of time if its temperature 

is elevated sufficiently.65 Additionally, as shown in Figure 2-30, it had been established previously for 

halon 1301 that the peak flammability limit, the suppressant inerting concentration in air at which no 

mixture of fuel and air is flammable, can increase with temperature.66, xviii Lab-scale testing during the 

TDP with halon 1301 and several halon alternative suppressants also indicated similar trending as shown 

in Figure 2-31.13

with Temperature. 

 xviii The certification requirement for halon-1301-based nacelle fire suppression systems is suggested to be founded, per 

Reference 132, on the 6 % inerting concentration determined by the Purdue Research Foundation, whereas halon-1301-

based fuel tank inerting systems have been designed to maintain even higher concentrations (e.g., 20 %). 

0

1

2

3

4

5

-78 27 145

Temperature (degrees C)

Volumetric Concentrat

6

7

8

ion (%)
Types of Fires Experienced 55

Figure 2-31 Variation in Halon 1301 and Three Halon Alternative Mass Fractions (Beta) 

Required to Suppress JP-8 Spray Flames with Temperature.13

(Beta is the ratio of the mass flow of the suppressant to the total flow of suppressant and air.) 

What were not established until the NGP was the correlation of historical fire experience with outside air 

temperature (OAT) and altitude and how that correlates with suppressant low-temperature requirements.

The temperat ents but is currently promulgated as 

es for DoD aircraft systems are world-

%, 5 %, 10 % and 20 % 

ate profiles from the Joint Aviation 

opical, intercontinental, and standardD

environments below 60 degrees south latitude, the Antarctic.) Figu vs. the

WWAEs used for DoD acquisition programs. Figure 2-33 depicts the worldwide land environments 

provided as design guidance for DoD aircraft systems, which categ nment types: 

basic, hot, cold, and severe cold.67

The relevance of the aircraft temperature envelope requirement to on-board fire suppression systems that

utilize a chemical suppressant, i.e., the engine nacelle and APU fire suppression systems on DoD aircraft,

is that the low temperature extreme defined for the envelope has historically been applied to substantiate 

the boiling point requirement for the fire suppressant, which is predominantly halon 1301. Thus in the

context of the WWAEs, a boiling point requirement based on halon 1301 would provide for in-flight fire

suppression under atmospheric conditions down to temperatures consistent with arctic-like temperature 

conditions. 

ure data that was previously indicated as design requirem

design guidance to define the low and high temperature extrem

wide air environments (WWAE), which represent conditions having a 1 

frequency of occurrence.67 In commercial aviation, standard clim

Regulation (JAR) define temperatures based on arctic, temperate, tr

day conditions.68 (Note: there is no design guidance or Do /JAR climate profile for altitude

re 2-32 plots these climates

orizes four land enviro
56 History of Fire Suppression in Aircraft 

Figure 2-32. Commercial Aviation Standard Climates vs. DoD WWAE Design Guidance. 

JAR-1 Tropical Maximum

JAR-1/ICAO Intercontinental Maximum

JAR-1 Temperate and Arctic Maximum

JAR-1/ICAO International Standard

JAR-1 Tropical and Temperate Minimum

JAR-1 Arctic Minimum

Subarctic Profile from Reference [84]

MIL-HDBK-310 High Temperature 1%

MIL-HDBK-310 High Temperature 5%

MIL-HDBK-310 High Temperature 10%

MIL-HDBK-310 High Temperature 20%

MIL-HDBK-310 Low Temperature 1%

MIL-HDBK-310 Low Temperature 5%

MIL-HDBK-310 Low Temperature 10%

MIL-HDBK-310 Low Temperature 20%

Legend

JAR-1 Tropical Maximum

JAR-1/ICAO Intercontinental Maximum

JAR-1 Temperate and Arctic Maximum

JAR-1/ICAO International Standard

JAR-1 Tropical and Temperate Minimum

JAR-1 Arctic Minimum

Subarctic Profile from Reference [84]

MIL-HDBK-310 High Temperature 1%

MIL-HDBK-310 High Temperature 5%

MIL-HDBK-310 High Temperature 10%

MIL-HDBK-310 High Temperature 20%

MIL-HDBK-310 Low Temperature 1%

MIL-HDBK-310 Low Temperature 5%

MIL-HDBK-310 Low Temperature 10%

MIL-HDBK-310 Low Temperature 20%

Legend

JAR-1 Tropical Maximum

JAR-1/ICAO Intercontinental Maximum

JAR-1 Temperate and Arctic Maximum

JAR-1/ICAO International Standard

JAR-1 Tropical and Temperate Minimum

JAR-1 Arctic Minimum

Subarctic Profile from Reference [84]

MIL-HDBK-310 High Temperature 1%

MIL-HDBK-310 High Temperature 5%

MIL-HDBK-310 High Temperature 10%

MIL-HDBK-310 High Temperature 20%

MIL-HDBK-310 Low Temperature 1%

MIL-HDBK-310 Low Temperature 5%

MIL-HDBK-310 Low Temperature 10%

MIL-HDBK-310 Low Temperature 20%

Legend

-100

-95

-90

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

Altitude, feet

-73

-70

-85

-70

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

Temperature, degrees F

-67

-44

-40

-37

-34

-30

-27

-24

-21

-17

-14

-11

-7

-4

-1

3

6

9

13

16

19

22

26

29

32

36

39

42

46

49

0 1524 3048 4572 6096 7620 9144 10668 12192 13716 15240

Altitude, meters

Temperature, degrees C

-80

-75

-63

-60

-57

-65

-60

-55

-54

-50

-47
Types of Fires Experienced 57

Figure 2-33. DoD Land Environment Design Guidance.67

With regard to ground fire suppression, land environments indicated as cold or severe-cold climates are 

the likely environments in which cold-soak conditions prior to aircraft startup would exist. The DoD

design guidance indicates a low-temperature 1 % frequency of occurrence of -46 °C (-50 °F) for cold land 

environments and a low-temperature 20 % frequency of occurrence of -51 °C (-60 °F) for severe-cold 

land environments. Designers of legacy aircraft would develop fire suppression systems whose

requirements were tailored to halon 1301 properties to likely assure fire suppression performance at such

temp

the likel

co

irflow temperature data for a variety of aircraft platforms, including a commercial transport aircraft,

indicated temperatures ran 71,72,73,74,75 a

m

(OAT) it is probable that the typical operational engine nacelle compartment temperature will be greater

than -40 °C (-40 °F). 

erature conditions. 

Boiling point (or Tb) of a fire suppression agent has been used as one of the criteria to guide the search for

new halon alternative chemical fire suppressants during the NGP.69 Currently, this criterion is -40 °C 

(-40 °F). It was also one of the parameters considered during the TDP, which identified HFC-125 as the 

best near-term alternative to halon 1301 for use in aircraft nacelle fire suppression system applications. 

However, even during the TDP it was recognized that, when operational contexts were considered such as 

y temperature environment within an operational engine nacelle at the time of suppressant 

discharge, the typical low temperature performance requirement could be a candidate for a performance 

trade. Minimum nacelle operating temperatures were indicated to range from below -18 °C (0 °F) in

mmercial aviation to 38 °C (100 °F) in DoD aviation.70 A subsequent review of nacelle compartment 

ging from -18.5 °C (-1.3 °F) to 274 °C (525 °F). Though this dat

ay not address every operating environment, they suggest that even at low outside air temperatures

a
58 History of Fire Suppression in Aircraft 

For decades, high Tb suppressants have been used in military aircraft nacelle fire suppression systems. In 

addition to halon 1011 on C-130 aircraft, this also includes halon 1202 on C-5 and previously on F-111 

aircraft, which are now retired.76 Since the signing of the Montreal Protocol, at least two high Tb halon 

alternative agents have been fielded outside the DoD.xix,77,78 Successful implementation and history of 

high Tb suppressants is likely attributable to several factors such as (1) the fact they are brominated 

halogens, (2) elevated nacelle operating temperatures, (3) applications that may benefit from the high Tb

characteristic, e.g., single-phase flow in long distribution runs, (4) freezing points well below 

temperatures likely to be experienced on ground and at altitude, and (5) distribution system design that 

ensures adequate distribution throughout the nacelle. This last factor was also emphasized and applied in 

the development of the F/A-18E/F HFC-125 nacelle fire suppression system.79 HFC-125 is a low boiling 

point fire suppressant. The obvious conclusion is that both low- and high-boiling-point suppressants are 

likely to realize higher probability of success when distribution is optimized. Chapter 8 discusses 

additional factors identified during NGP CFD experiments and modeling that need to be considered 

relating to suppressant delivery and evaporation. 

Analysis of military aircraft experience regarding release of nacelle fire suppressants was conducted 

during the TDP, the purpose of which was to quantify halon discharges at altitude and for evaluating 

discharge frequency and quantity of agent discharged below and within the ozone layer.80 Combining 

both fixed-wing and rotary aircraft discharges that analysis indicates: 

• Approximately 77 % occurred between 0 km and 3 km (0 ft and 10,000 ft), and about 92 % 

occurred between 0 km and 6 km (0 ft and 20,000 ft). 

• Over 60 % of all discharged suppressant was accounted for by only th he 30 military 

aircraft -15, and P-3) have 

altitude ceilings that extend above 6 km (20,000 ft) but were contributors to the frequency of 

celle operating temperatures, cold-soak temperature conditions and cold climatic 

ree of t

 platforms covered by the study. These three platforms (C-130, F

discharges below 3 km (10,000 ft).xx

• Over 25 % of all suppressant discharged was high-boiling point suppressant (i.e., halons 1011 

and 1202). 

Prior to the NGP, an operational context that had not been investigated was whether temperature 

conditions at the time of agent release correlated with the typical boiling point temperature requirement. 

These include OATs, na

extremes. Based on review of nacelle compartment airflow temperature data for a variety of aircraft 

platforms, it was reasonable to assume that nacelle compartment temperatures are well above boiling 

points of fielded nacelle fire suppression agents. When considering that historic release of nacelle fire 

suppression agents has typically occurred below 6 km (20,000 ft), with over 75 % occurring below 3 km 

 

xix These agents are phosphorous tribromide (PBr3), which has a Tb of 173 °C, and triflouroiodomethane (CF3I), which has a Tb 

of -22.5 °C. PBr3 is installed for nacelle fire protection on Eclipse 500 commercial aircraft, which are small commercial jet 

transport aircraft, and CF3I is installed for nacelle and APU fire protection on Royal Australian Navy SH-2G rotary aircraft. 

The CF3I system is similar to a halon 1301 system in its implementation in that the suppressant is distributed throughout the 

nacelle by means of remotely-contained storage bottles and plumbing. The PBr3 system implementation is different: 

suppressant discharge is targeted to the flame holding regions within the nacelle, which were identified through analysis and 

test, per Reference 77. Considering nacelle operating temperature environments, which are described later, and boiling points 

of each suppressant, CF3I will likely be vaporized where as for PBr3 the heat from a fire will effect vaporization. xx The aircraft platforms in the TDP study, Reference 80, that each accounted for at least 5 % of discharged suppressant were the 

P-3, C-130, F-15, F-111, C-5, C-141, and A-10 platforms, all fixed-wing aircraft. Review of the data utilized for the 

reference [44] study found similarly that the aircraft platforms each accounting for at least 5 % of discharged suppressant 

were the P-3, C-130, C-5, A-10, F-15, F/A-18, and F-111, and in general that fixed-wing aircraft accounted for 85% of all 

suppressant discharged. 
Types of Fires Experienced 59

(10,000 ft), and that the likely occurrence of fire while either cold soaked or while in cold climatic 

extremes is likely a low probability event, the likelihood of not extinguishing a nacelle fire after 

suppressant release and realizing a catastrophic event under such conditions suggests strongly that the 

combination of these events has a low probability. The implication of the preceding is that selection of a 

fire suppressant whose boiling point is compatible with cold soaking or a cold climatic extreme results in 

a protection capability against events whose likelihood of occurrence has a very low probability, and that 

halon alternative suppressants with higher boiling points are not likely to appreciably increase risk under 

such conditions. These preceding assertions provided the NGP with the impetus to perform the following 

efforts:

• Obtain and review aviation Safety Center fire incident data to extract, if possible, altitude 

and/or outside air temperature (OAT) information that would permit characterization of OAT 

conditions during which suppressant release has historically taken place. Based on this data, 

it may be possible to assess probability of suppressant release under conditions that are likely 

to be well above a suppressant’s boiling point. 

• preceding elements, of utilizing a fire 

DoD Safety Centers Fire Incident/Mishap Data 

A review conducted within U.S. Army aviation of rotary aircraft fires between 1985 and 1995 had found 

that the lowest outside air temperature (OAT) reported was 0 °C (32 °F) and the highest reported was 

Ai

Sa s 

mi r 

 

• Construct and validate an in-flight nacelle air temperature model to estimate likely nacelle 

compartment air temperature for given altitude, outside air temperature, general engine 

surface temperatures, and aircraft airspeed conditions. Such a model would be useful in 

allowing system designers to assess compartment temperatures at altitude relative to a 

suppressant’s boiling point. 

• Evaluate implications of aircraft cold-soak conditions, particularly during aircraft takeoff. 

Assess safety risk, considering the findings in the 

suppressant whose boiling point is much higher than of those agents commonly fielded today 

in military aircraft (i.e., halon 1301). 

35 °C (95 °F).81 This review concluded that the only time a -50 degree temperature would remotely be 

encountered is at extremely high altitude or in extremely remote northern/southern areas of the earth.xxi 

The use of the term “remotely” has significance in that within DoD it relates to the rate of hazard 

occurrence. During the time period of the U.S. Army study, the aggregate rate of occurrence of in-flight 

rotary aircraft fires was 4.9 per million flight hours, a remote rate of occurrence. Thus the likelihood of a 

nacelle fire occurring at lower outside air temperatures or in an extremely cold environment and resulting 

in loss of aircraft would likely be even lower (improbable).xxii

Aviation fire incident data was obtained for the years 1980 through 2002 from the U.S. Army, Navy and

r Force Safety Centers. Table 2-3 summarizes the number of mishaps and incidents provided by the

fety Centers. (Note: the counts in Table 2-3 reflect incidents categorized by the DoD services a

shaps as well as lesser severity events or incidents.) The data was reviewed to determine whethe

 xxi The final memorandum did not indicate whether the temperature threshold considered was -50 °C or -50 °F. i

 The terms remote and improbable are categories of hazard probability as described in DoD specification MIL-STD-88

Standard Practice for System Safety. These terms may be described either qualitatively or quantitatively. Assigning a haz

probability category based on aircraft flight hours is one method per MIL-STD-882D of describing a probability 

quantitatively. 

xxi 2D, 

ard 
60 History of Fire Suppression in Aircraft 

suppressant release occurred and to identify the altitude and OAT associated with each release. Only 

suppressant releases associated with discharge of systems protecting nacelle and APU compartments were 

considered. Combat-related events, or direct-enemy action events, are typically not provided by the DoD 

Safety Centers, and if they are provided they only contain limited information. For mishaps and incidents 

that provided altitude data but did not include temperature data, the International Civil Aviation 

Organization (ICAO) standard atmosphere model was applied to estimate OAT. For mishaps and 

incidents without altitude and temperature information, the methodology applied was that used during the 

TDP for assuming flight altitude based on aircraft flight phase from the previously-described dischargeat-altitude analysis. 

Table 2-3. Number of Mishaps and Incidents. 

Service 

Aircraft Type Army Navy Air Force 

Fixed Wing 88 1,212 3,932 

Rotary 465 834 98 

Mishap and

releases oc

occurrence old land environments of Figure 2-33, for incidents occurring on 

the grou , ght 

for aircraft operating nearest to locations 

evere-Cold Environments. 

m pressure altitude, while the Standard 

applying the MIL-HDBK-310 cold WWAEs or the JAR-1 Arctic Climate profile as the basis for 

 incident data was first reviewed for the geographic locations where fires and suppressant 

curred. Table 2-4 summarizes the results of this review, which was performed to assess 

of fire in the cold or severe-c

nd and to assess occurrence of fire for incidents occurring in flight or characterized as in-fli

in those cold or severe-cold environments. 

Table 2-4. Percentage of Fire Mishaps and Incidents Occurring 

in Geographic Cold or S

(a) From data categorized as ground fire mishaps and incidents only. 

(b) From data categorized as in-flight fires only but also includes 

mishaps and incidents on the ground characterized as flight fire 

mishaps and incidents (i.e., intent for flight existed). 

Generally, the fire mishap and incident data provided by the Safety Centers included altitude information 

in terms of mean sea level (MSL), above ground level (AGL), or flight level (FL). Altitude expressed in 

these terms in the Safety Center data is typically in ter s of

Atmosphere is based on geopotential altitude. Figure 2-34 illustrates the variation of ambient pressure vs. 

pressure altitude and geopotential altitude on standard-day and non-standard-day temperature conditions. 

The implication with regards to estimating OAT using the ICAO Standard Atmosphere Model is that cold 

temperature conditions may be lower than estimated on non-standard-days or if altitude is based on 

pressure altitude. However, the percentages indicated previously in Table 2-4 suggested strongly that 

estimating temperature conditions would not reflect operational experience. 

Service 

Event Phase Army Navy Air Force 

Ground 0 1.5 % 1.1 % (a) 

In-Flight < 1 % < 1 % 2.7 % (b) 
Types of Fires Experienced 61

An additional consideration for using the ICAO Standard Atmosphere Model for estimating OATs during 

suppressant release is fuel flammability limits. Figure 2-35 depicts flammability limits of Jet-A and Jet-B

aviation fuels vs. altitude and standard atmospheres, including a subarctic profile (JP-8 limits are similar 

to limits for Jet A, and JP-5 limits are slightly higher than those depicted for Jet-A.). Though ignition 

depends on many variables, Figure 2-35 suggests that for military aircraft using JP-8 and JP-5 fuels,

attaining the flammability limits is more likely at atmospheres above the Standard Atmosphere. 

Standard Day

Non-Stand emperature 

 Gradient dard Day

Figure 2-34 Variat l Altitudes on

Standar e Conditions.82

ve the Standard Atmosphere 

profile. The lowest OAT below the profile for which suppressant release occurred on the ground was 

indicated as -3.3 °C (26 °F). The highest altitude below the profile for which suppressant release was 

indicated to have occurred was 1.65 km (5,400 ft), and the lowest OAT was indicated as 2 °C (36 °F). 

ion of Ambient Pressure vs. Pressure and Geopotentia

d-Day and Non-Standard-Day Temperatur

Figure 2-36 plots the safety center data for nacelle and APU fires in which suppressant release occurred

and for which suppressant release did not occur. In addition, the figure also plots fire events that were not 

nacelle or APU fires. This is done to plot all aircraft fire mishaps and incidents for which Safety Center 

data included both altitude and OAT. It is interesting to note in Figure 2-36 that there are just two data 

points indicating a fire occurrence above 10.7 km (35,000 ft), three data points indicating fire above 9 km 

(30,000 ft), and only four data points indicating fire above 7.6 km (25,000 ft). In combustion experiments 

that established the spectral criteria for optical fire detection, it was established that flames could be 

maintained up to 10.7 km (35,000 ft); however, at higher altitudes, the flames would self extinguish.59 

Distribution of Safety Center data that included both altitude and OAT for which suppressant release 

occurred indicated that suppressant releases occurred generally about or abo

Non-Standard Day – Temperature 

 Gradient Below Standard Day

ard Day – T

Above Stan

in. Hg: 0 10 15 30 

k Pa 0 33.9 50.8 101.6 

km (ft)

15 (50) 

12 (40) 

9 (30) 

3 (1

0 

6 (20) 

Altitude 

0)

5 20 25 

16.9 67.7 84.7

Ambient Pressure, Pressure Altitude 
62 History of Fire Suppression in Aircraft 

103 ft (km)

 80 (24.4)

 60 (18.3)

r Jet-A.) 

may agent release,

the m below 6 km

one incident is indicated that is beyond the profile at altitude and below -18 °C (0 °F),

which occurred at 12 km (40, (-78 °F). This was the only incident for which both altitude 

and OAT were provided and occurr low -4 (-40 The hi t altitude below the profile 

and above -18 °C (0 °F) occurred at indicated as -10 °C (14 °F). There 

are three incidents indicated at zero altitude (on the ground) and below -18 °C (0 °F). These occurred

w

Commercial aircraft Board (NTSB) 

 40 (12.2

20 (6.

 0 

°C: -51 -29 -18 -7 16 38 60 82 

°F -140 -100 -60 -20 0 20 60 100 140 180 

-96 -73

)

1)

Figure 2-35. Flammability Limits of Jet-A and Jet-B Fuels vs. Altitude 

and Standard Atmospheres.76, 83, 84, xxiii

(JP-8 limits are similar to limits for Jet A; JP-5 limits are slightly higher than limits fo

For other fire incidents for which Safety Center data included both altitude and OAT but in which there 

 have been no engine or APU fire, or for which there was an engine or APU fire but no

ajority of the incidents at altitude occurred above the Standard Atmosphere profile and 

(20,000 ft). Only

000 ft), -61 °C

that ed be 0 °C °F). ghes

1.95 km (6,400 ft) with an OAT

ith OATs at -25 °C (-13 °F), -28 °C (-18 °F), and -33 °C (-27 °F). 

data was also reviewed from the National Transportation Safety

aviation accident database for occurrence of fire at altitude vs. temperature. Data then obtained spanned 

the years 1988 through 2000. The distribution of fire events in which both altitude and OAT were 

provided is shown in Figure 2-37. The lowest temperature indicated in the Figure is -28 °C at zero

altitude. There were several events at this condition. (Whether any were nacelle or APU fires was not 

further researched.) One event was identified in the data at -32 °C, but no corresponding altitude was 

provided so it is not shown in the figure. Additional fire occurrences at low temperature that were

identified with no corresponding altitude and are not indicated in the figure occurred at -27 °C and -26 °C 

(one each) and at -22 °C (two events). 

 

xxiii Reid vapor pressure (RVP) is a measure of fuel volatility. The higher the RVP, the more volatile the fuel is and the more 

readily it evaporates. RVP is measured at 37.8 °C (100 °F). 
Types of Fires Experienced 63

-100

-95

-90

-85

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

Altitude, feet

-73

-70

-67

-80

-65

-60

-55

-50

-45

-40

-35

-3

-2

-2

-1

0

5

10

15

35

40

45

50

70

75

80

85

105

110

115

120

Tem

p

erature,

d

e

grees F

-63

-54

-50

-47

-44

-40

-37

-24

-21

-17

-14

-11

-7

-4

-1

3

6

9

13

16

19

22

26

29

32

36

39

42

46

49

1524 3048 4572 6096 7620 9144 10668 12192 13716 15240

0

5 -34

-30 0

5 -27

-10

-5

20

25

30

55

60

65

90

95

100

0

-75

-70

-60

-57

Altitude, meters

erature, degrees C

Fire 

Events and Suppressant Releases where Both Altitude and OAT Were Provided. 

T

e

m

p

Figure 2-36. Plot of Standard Climate Profiles and WWAEs vs. All DoD Aircraft 

JAR-1 Tropical Maximum

JAR-1/ICAO Intercontinental Maximum

JAR-1 Temperate and Arctic Maximum

JAR-1/ICAO International Standard

JAR-1 Tropical and Temperate Minimum

JAR-1 Arctic Minimum

Subarctic Profile from Reference [84]

MIL-HDBK-310 High Temperature 1%

MIL-HDBK-310 High Temperature 5%

MIL-HDBK-310 High Temperature 10%

MIL-HDBK-310 High Temperature 20%

MIL-HDBK-310 Low Temperature 1%

MIL-HDBK-310 Low Temperature 5%

MIL-HDBK-310 Low Temperature 10%

MIL-HDBK-310 Low Temperature 20%

Legend

JAR-1 Tropical Maximum

JAR-1/ICAO Intercontinental Maximum

JAR-1 Temperate and Arctic Maximum

JAR-1/ICAO International Standard

JAR-1 Tropical and Temperate Minimum

JAR-1 Arctic Minimum

Subarctic Profile from Reference [84]

Legend

JAR-1 Tropical Maximum

Legend

MIL-HDBK-310 High Temperature 1%

MIL-HDBK-310 High Temperature 5%

MIL-HDBK-310 High Temperature 10%

MIL-HDBK-310 High Temperature 20%

MIL-HDBK-310 Low Temperature 1%

MIL-HDBK-310 Low Temperature 5%

MIL-HDBK-310 Low Temperature 10%

MIL-HDBK-310 Low Temperature 20%

JAR-1/ICAO Intercontinental Maximum

JAR-1 Temperate and Arctic Maximum

JAR-1/ICAO International Standard

JAR-1 Tropical and Temperate Minimum

JAR-1 Arctic Minimum

Subarctic Profile from Reference [84]

MIL-HDBK-310 High Temperature 5%

MIL-HDBK-310 High Temperature 10%

MIL-HDBK-310 High Temperature 20%

MIL-HDBK-310 Low Temperature 1%

MIL-HDBK-310 Low Temperature 5%

MIL-HDBK-310 Low Temperature 10%

MIL-HDBK-310 Low Temperature 20%
64 History of Fire Suppression in Aircraft 

Given the preceding, it was concluded that the Standard Atmosphere Model could be used to provide a

reasonable estimate for OATs at which suppressant releases at altitude have occurred. This model was 

applied to fire incidents for which there were no OAT data, which were then combined with incidents that

had included OAT data. The results are summarized in Figures 2-38 through 2-41, with the data from 

Reference 44. 

Figure 2-37. Plot of 1988-2000 Commercial Aircraft Fire Events vs. Altitude and OAT, 

Events for which NTSB Database Provided Both Altitude and OAT. 

3% 10%

> 4.57 km (> 15,000 ft)

3.05 to 4.57 km (10,000 to

14,999 ft)

< 3.05 km (< 10,000 ft)

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

35

0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750

Altitude, meters

Te

m

perature, degrees C

87%

Figure 2-38. Rotary Aircraft Nacelle/APU Compartment Fire Suppressant 

Releases by Altitude. 
Types of Fires Experienced 65

1% 5%

22%

72%

> 12.19 km (> 40,000 ft)

9.14 to 12.19 km (30,000 to 30,999 ft)

6.1 to 9.14 km (20,000 to 29,999 ft)

< 6.1 km (< 20,000 feet)

Figure 2-39. Rotary Aircraft Nacelle/APU Compartment Fire Suppressant 

Releases by OAT. 

Figure 2-40. Fixed-wing Aircraft Nacelle/APU Compartment Fire Suppressant 

Fi t

3% 3% 7%

87%

< -20.6°C (< -5°F)

Releases by Altitude. 

< -12.2°C (< 10°F)

< -6.7°C (< 20°F)

≥ -6.7°C (≥ 20°F)

6%

3%

1%

13%

2%

75%

< -40°C (< -40°F)

< -34.4°C (< -30°F)

< -31.7°C (< -25°F)

< -28.9°C (< -20°F)

< -25°C (< -13°F)

≥ -25°C (≥ -13°F)

gure 2-41. Fixed-wing Aircraft Nacelle/APU Compartment Fire Suppressan

Releases by OAT. 
66 History of Fire Suppression in Aircraft 

emperature during

data on pressure-altitude,

te putes the

ter ons at the ram scoop are 

co are taken to be the

radiation heat transfer is 

bient outside by

o evaluate in-flight conditions generally representative of likely flight and nacelle operating conditions, 

the mode settings

utilized during the TDP for nacelle config ance (net volume), nacelle mass airflow,

and engine hot surface temperature. Conditions were evaluated for altitudes up to 9 km (30,000 ft). 

ft fire

ranged fro

m

-18 ° m

ed cases was considered to be artificial since military rotorcraft typically have

operatio -wing

aircraft that have nacelle fire suppre hter/attack aircraft, cargo transports, 

maritime patrol aircraft). The remaining 11 % (i.e., 11 % of the 12 %), or 1.5 % of all the cases modeled,

eratures ranging 

consideration, the 

me greater

odel for the 

 it was not likely

eed the results would be

tion in the model that 

en. This is

l

s by heat rejected by the engine.63 Thus, at the higher airspeed, the inlet temperature is greater. To

investig -flight 

measur

Nacelle Air Temperature Modeling 

An in-flight nacelle air temperature model was constructed to estimate nacelle air t

flight conditions. The model uses the U.S. Standard Atmosphere 1976 

mperature, and viscosity.

85 The model treats the nacelle as an air heat exchanger, and it com

minal temperature difference based on average, bulk values. The inlet conditi

mputed to be the stagnation properties for the given flight conditions, and these 

same as those inside the nacelle, close to the inlet. The effect of conduction and 

assumed negligible; i.e., heat losses from air through the nacelle wall to the am

convection and conduction. The model is described in detail in Reference 44. 

T

l was evaluated for over 1,000 cases against the high and low operational parameter

uration (length), clear

Model runs were limited to this altitude for two reasons: (1) only 6 % of all fixed-wing aircra

suppressant releases (Figure 2-40) were indicated to have occurred above 9 km (30,000 ft), and (2) though

the Standard Atmosphere Model for the tropopause has a ceiling of 11 km (36,152 ft), only 1.7 % of all 

fixed-wing aircraft fire suppressant releases were indicated to have occurred above this ceiling. The OAT

m -45 °C (-48 °F) to 15 °C (58.7 °F) based on the model. The results are indicated in Figures 

2-42 and 2-43, which depict peak nacelle temperature vs. altitude for the two airspeed conditions

odeled: 50 knots and 400 knots. 

Review of the model output showed that 88 % of the cases indicated nacelle air temperatures greater than 

C (0 °F). The remaining 12 % of the cases (those less than -18 °C) were for input conditions at 6 k

(20,000 ft) or greater, and 89 % of these cases (89 % of the 12 %) were noted for airspeeds of 50 knots. 

This group of calculat

nal ceilings less than 6 km, and 50 knots is below the stall speed for typical military fixed

ssion capability (e.g., fig

were for input conditions at 9 km (30,000 ft) and 400 knots and indicated nacelle air temp

between -23 °C (-10 °F) and -24 °C (-12 °F). If the artificial cases were removed from

actual percentage of total cases indicating nacelle air temperatures greater than -18 °C beca

than 88 %. So modeling additional cases up to the ceiling of the Standard Atmosphere M

tropopause was likely to result in additional nacelle air temperatures less than -18 °C, but

to dramatically impact the percentages described. 

The results of the modeling appeared counterintuitive, in that at higher airsp

expected to indicate lower compartment air temperature. This is due to the assump

the temperature at the nacelle inlet is based on the stagnation properties for the airspeeds chos

an assumption that had been applied in previous work, with the rationale that the ventilation airflow 

temperature through the nacelle is influenced by the stagnation temperature at the nacelle air inlets as wel

a

ate this further, results from the model were compared to data obtained previously during in

ement of nacelle air temperatures for several different aircraft platforms. 
Types of Fires Experienced 67

Figure 2-42. Peak Nacelle Temperature at 50 knots Airspeed vs. Altitude. 

Figure 2-43. Peak Nacelle Temperature at 400 knots Airspeed vs. Altitude. 

-60

-40

-20

0

20

40

0 5000 10000 15000 20000 25000 30000 35000

Altitude, feet

Temperat

-51

-46

-40

-35

-29

-24

-18

-13

-7

-2

4

Temperat

60

80

100

120

140

160

ure, degrees F

20

26

31

37

42

48

53

59

64

70

75

81

 degrees C

180

9

15

4572 6096 7620 9144 10668

Altitude, meters

ure,

0 1524 3048

-60

-40

-20

0

0 5000 10000 15000 20000 25000 30000 35000

Altitude, feet

-51

-46

-40

-35

-29

-24

-18

20

40

60

80

100

120

140

160

180

emperature, degrees F

4

9

15

20

26

31

37

42

48

53

59

64

70

75

81

86

perature, degrees C

200

T

-13

-7

-2

92

Tem

220

97

103

0 1524 3048 4572 6096 7620 9144 10668

Altitude, meters
68 History of Fire Suppression in Aircraft 

Addi on

temp odel 

as applied for the purposes of making comparisons, several of the inputs were varied to accommodate 

differing nacelle characteristics. For example, the clearance between the engine and the nacelle structure

not uniform, thus for each case this parameter was varied between the low and high values that had 

ecific nacelle clearance information was obtained. In general, the

odel tended to predict (conservatively) lower temperature ranges as compared to measured temperature

nges.

Table 2-5. Comparison of Modeled vs. Measured Nacelle Air Temperatures. 

tionally, nacelle air temperature data were obtained from current in-flight rotary aircraft propulsi

erature survey testing. The results of the comparison are summarized in Table 2-5. When the m

w

is

been utilized during the TDP, unless sp

m

ra

Altitude 

(km) OAT (°C) 

Engine Surface

Temperature Range

(°C)

Measured Nacelle Air 

Temperature Range

(°C)

Predicted Nacelle Air 

Temperature Range

(°C)

Sea level 29 Not indicated 33 to 83

>5 -21 Not indicated -18 to 30 

Not modeled since 

engine surface 

temperature range not

indicated 

0.4 28 102 to 392 33 to 82 23 to 33

0.6 Not indicated 176 to <260 ≈ 90 to 160 20 to 115 

0.6 to 14 Not indicated Not indicated ≈ 100 at 0.6 km to

≈ 10 at 14 km

Not modeled since 

engine surface 

temperature range not

indicated

3 -3 74 to 588 22 to 93 -3 to 28

Sea level -3 60 to 467 19 to 55 16 to 39

3 1 81 to 587 27 to 94 -3 to 28

3 Not indicated Up to 260 10 to 93 6 to 12

3 Not indicated Up to 750 < 110 to < 275 7 to 23

Sea Level Not indicated Not indicated 210

xxiv Not modeled since 

engine surface 

temperature range n

indicated 

ot

Cold Soak Conditions 

The NGP also examined the effect of fire suppression effectiveness under cold-soak conditions, i.e., cases 

which the aircraft had been on the ground in a cold climate. Of issue was the relationship between the 

OAT, the boiling point of a fire suppressant, and suppressant discharge under such conditions, especially 

uring takeoff. Such conditions have been used to support the need for a suppressant with a boiling point 

0 °F) or lower. 

dentify existing work related to evaluation of aircraft cold soak 

onditions.

44 Work performed by Transport Canada was identified that characterized aircraft wing

in

d

of -40 °C (-4

Bein performed a literature review to i

c

xxiv

 Though int from 

Refere

temperature modeling effort. 

the engine surface temperature data was not available for use in during modeling, this temperature data po

nce 13 is listed as it was the highest nacelle airflow temperature found in the literature during the nacelle air
Types of Fires Experienced 69

surface temperatures during ground operations during Canadian winter and aircraft cold soak conditions 

after flights at altitude.86,87 For flights in Canada and Alaska, a general conclusion was that wing 

temperature surveys of aircraft returning from flights at altitude failed to find evidence of significantly 

cold-soaked wing conditions. Their surveys generated data that indicate the following relative to non-deiced spot wing temperatures for aircraft on the ground: 

• Below 0 °C (32 °F) OAT, wing temperatures were generally higher than OAT. The 

temperature difference generally ranged from 2 °C at 0 °C OAT to slightly greater than 6 °C 

at -25 °C OAT. 

• Radiative cooling on th rcraf ld y 

diti oak e wi al 

due to radiative cooling may from -6 °C at 0 °C OAT and reducing to -2 °C at -25 °C 

88

The lowest OAT for which cold-soak data was record t 

aircraft operations on the ground in cold or extreme cold climates is infrequent. (Note that 

this correlates well with the operational fire experie ated in Figures 2 -37.) 

Th erat Transp rveys in file for in 

Alaska during which wing surface temp rded. The cruise altitude t is 

likely to be approximately 9 km (30,000 ft) based on the aircraft t was in . 

At the time liftoff from the ground occurred, measured wing temperatures were higher than both OAT 

and the initial wing te b, there w point indication perature 

increase of approxim y 3 °C over the first 5 min before that test location tempe egan to 

decrea whereas all other measurement locations were noted to begin to decrease im ely. At 

15 min after takeoff, wing temperatu enerally -20 °C (-5 °F). At 60 min after takeoff, wing 

temperatures we 3 °C (-10 °F). Wing temper nd to warm t imately 

7 ° in 1 desce ding. Except for the radiative cooling condition, these 

data indicated that temperature conditions of an operational aircraft are likely to be higher than the cold 

soak temperature conditions, suggesting that aircraft component temperatures are a r. 

o analyze the concern of over too low fire suppressant and system component temperatures under cold 

r extreme cold temperature conditions during takeoff, a model was constructed to estimate stagnation 

ing takeoff. The premise was that for suppressant bottle(s), distribution lines 

ed near/within heated compartments but adjacent to exterior surfaces, the 

an initial OAT for cold/severe-cold environments was based on the JAR-1 

Arctic Standard Climate profile, and then the profile was applied during takeoff climb. This 

greater than -25 °C (-13 

aircraft surface temperature will be greater than OAT at takeoff. The relevance of this is that 

 °F) for 

• Above 0 °C (32 °F) OAT, wing temperatures were generally lower than OAT. 

e ground (i.e., ai

ons to result in cold-s

 range

t parked overnight in co

 conditions. Possibl

 weather) is more likel

than in-flight con ng-to-OAT differenti

OAT.

• ed was -13 °F (-25 °C), suggesting tha

nce indic -36 and 2

e data gen ed during the ort Canada su

eratures were reco

cludes a flight pro a flight at altitude

is not specified bu

 type tha strumented and flown

mperatures. During takeoff clim as a test of a tem

atel rature b

se, mediat

res were g

re generally -2 atures were fou o approx

C (20 °F) 5 min during nt to final lan

lso likely to be highe

T

o

temperature conditions dur

and components not locat

stagnation temperature should provide a reasonable estimate of likely temperature conditions of 

components adjacent to the exterior surfaces. Figures 2-44 and 2-45 depict graphically the results of the 

minimum and maximum temperature profiles for cases modeled for a jet transport aircraft, fighter aircraft, 

and a turboprop transport aircraft. 

• In Figure 2-44, 

scenario was assumed to estimate a lower bound profile. The modeling estimated that during 

takeoff climb within a standard arctic profile that the stagnation temperatures can increase to 

°F) for a period of time, taking into consideration the likelihood that 

fire suppression system components adjacent to these surfaces are likely to be similar in

temperature. (The resultant temperature profiles indicate a period above -32 °C (-25
70 History of Fire Suppression in Aircraft 

as few as 3.7 min and as long as 14.2 min. The time to reach this threshold was estimated to 

occur approximately within 1.25 min or within 3.8 min.) 

• In Figure 2-45, an initial OAT of -40 °C (-40 °F) was assumed but also applies a temperature 

bias condition based on difference of wing skin temperature to OAT described in the 

Transport Canada work. The JAR-1 Arctic Standard Climate profile is then applied during 

tely within 1.25 min or within 3.6 min. Because of the duration of the climb 

d of time to 

In each of t

could increase above -40 °C (-40 °F) during takeoff 

through the J

necessa

establishing

takeoff. 

s improbable as indicated in Tables 2-8 and 2-9. 

 

takeoff climb with the bias condition continuously applied to estimate a potential upper 

bound profile. The resultant temperature profiles indicated a period above -26 °C (-15 °F), 

for as few as 3 min and as long as 14.8 min. The time to reach this threshold was indicated to 

occur approxima

for the jet transport aircraft, temperature was indicated to increase for a perio

approximately -15 °C (4.5 °F) before beginning to decrease. 

he scenarios modeled it was clear that fire suppressant and system component temperatures 

profiles for some period of time but continued ascent 

AR-1 Arctic Standard Climate profile would effect temperature decrease. Thus it became 

ry to review the DoD Safety Center data and the studies conducted during the TDP for 

 halon 1301 system effectivity to gage the DoD’s historical engine nacelle fire risk during 

Risk 

Within DoD System Safety, organizations assess risk associated with hazards identified during 

development as well as during fielded operations of weapon systems, including aircraft systems and 

subsystems. Analytical processes are applied to assess worst-credible and most-probable severity and 

likely occurrence of identified hazards. Likely occurrence may be expressed as a rate of occurrence, 

typically per flight hour, or as a probability. The resulting assessment of severity and probability is then 

categorized as to the level of risk it presents (e.g., high, medium, low, unacceptable, etc.). Generally, 

assessment of fire hazards results in an assignment of a “Catastrophic” severity. xxv The issue becomes 

whether the rate of occurrence or probability of a fire hazard results in a risk that is deemed not low. 

For example, when the total number of engine nacelle fires evaluated during the TDP for establishing 

halon 1301 system effectivity are considered, the aggregate rate of occurrence for a nacelle fire event 

during the period evaluated is approximately 8 per 106

 flight hours. Those same data were reviewed 

during the NGP to estimate the potential hazard frequency of a catastrophic event due an unsuppressed 

engine nacelle fire hazard, as summarized in Tables 2-6 and 2-7. Note that in each case, the hazard 

frequency would be assessed as improbable. A catastrophic-improbable hazard is categorized as low risk, 

which is typically accepted by military aviation program managers. If the same rate of occurrence is 

considered in conjunction with operating in a low (or high) temperature climatic extreme, the hazard 

frequency would also be assessed a

 

phic hazard severity is defined in DoD specification MIL-STD-882D, Standard Practice for System Safety, as a 

can result in death, permanent total disability, loss exceeding $1M, or irreversible severe environmental damage 

s law or regulation. The risk from fire in an aircraft would be presented to an aircraft program manager as the 

xxv A Catastro

hazard that 

that violate

potential consequences of the different categories of possible fires and the likelihood of fire occurring for each category. 
Types of Fires Experienced 71

-70

-65

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

Time, seconds

erature, degrees C

-60

-55

-50

-45

-40

Temp

Jet Transport Aircraft

Fighter Aircraft

Turboprop Transport Aircraft

Figure 2-44. Initial OAT per JAR-1 Arctic Standard Climate. 

(OAT at Takeoff is -50 °C (-58 °F).)

Figure 2-45. OAT at Takeoff is -40 °C (-40 °F) with Bias Applied to OAT. 

-60

-55

-50

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

-45

-40

-35

-30

-25

-20

-15

-10

Temperature, degrees C

-5

0

5

10

Time, seconds

Jet Transport Aircraft

Fighter Aircraft

Turboprop Transport Aircraft
72 History of Fire Suppression in Aircraft 

Table 2-6. Estimate of Rate of Occurrence of Aircraft Lost Due to Failure to Extinguish a 

Nacelle Fire, Any Time (Does not Consider Multiple Engines).xxvi

Aircraft 

Category 

Probability

Occurrence 

In Flight 

Probability

Occurrence 

At Any Given

Time 

Probability

Occurrence 

Fire Not 

Extinguished

Probability

Occurrence

Aircraft 

Lost 

End Event 

Rate of 

Occurrence 

Per Flight Hour 

MIL-STD-882

Hazard 

Frequency 

Fixed-wing 0.55 1 0.24 0.09 9.84E-08 Improbable (E)

Rotary 0.35 1 0.53 0.27 4.15E-07 Improbable (E)

Aircraft 

Category 

Probability

Occurrence 

On Ground 

Probability

Occurrence 

At Any Given

Time 

Probability

Occurrence 

Fire Not 

Extinguished

Probability

Occurrence

Aircraft 

Lost 

End Event 

Rate of 

Occurrence 

Per Flight Hour 

MIL-STD-882

Hazard 

Frequency 

Fixed-wing 0.45 1 0.38 0.03 4.25E-08 Improbable (E)

Rotary 0.65 1 0.36 0.01 1.94E-08 Improbable (E)

Table 2-7. Estimate of Rate of Occurrence of Aircraft Lost Due to Failure to Extinguish a 

Nacelle Fire, Any Time (Assumes Two Engines per Aircraft).xxvi

Aircraft 

Category 

Probability

Occurrence 

I

Probability

Occurrence 

At Any Given

Probability

Occurrence 

Fire Not 

Probability

Occurrence

Aircraft 

End Event 

Rate of 

Occurrence 

n Flight Time Extinguished Lost Per Flight Hour 

MIL-STD-882

Hazard 

Frequency 

Fixed-wing 0.55 1 0.24 0.09 4.92E-08 Improbable (E)

Rotary 0.35 1 0.53 0.27 2.08E-07 Improbable (E)

Aircraft 

Category 

Probability

Occurrence 

On Ground 

Probability

Occurrence 

At Any Given

Time 

Probability

Occurrence 

Fire Not 

Extinguished

Probability

Occurrence

Aircraft 

Lost 

End Event 

Rate of 

Occurrence 

Per Flight Hour 

MIL-STD-882

Hazard 

Frequency 

Fixed-wing 0.45 1 0.38 0.03 2.13E-08 Improbable (E)

Rotary 0.65 1 0.36 0.01 9.70E-09 Improbable (E)

 

 Probabilities in Tables 2-6 and 2-7 are derived from References 45 and 46. End event rate of occurrences determined by

multiplying 8/106

 flight hours by probabilities indicated. This frequency is based on the aggregate number of nacelle fire

over all f

xxvi

s 

indicated ground fire 

events in Reference 46, but a 1 % probability is assumed for discussion purposes. 

light hours for the period and aircraft evaluated in those references and is higher than the rate of 4.9/106

 flight hours 

 in an evaluation of Army rotary aircraft fires per Reference 81. No rotary aircraft were indicated lost in
Types of Fires Experienced 73

Table 2-8. Estimate of Rate of Occurrence of Aircraft Lost Due to Failure to Extinguish a 

xxvii Nacelle Fire in a Climatic Extreme (Does not Consider Multiple Engines).

Aircraft 

Category 

Probability 

Occurrence 

In Flight 

Probability 

Occurrence

In Climatic 

Extreme 

Probability 

Occurrence 

Fire Not 

E uished xting

Probability 

Occurrence 

Aircraft 

Lost 

End Event 

Rate of 

Occurrence 

Per Flight 

Hour 

M 882 IL-STDHazard 

Frequency 

Fix ed-wing 0.55 0.2 24 0.09 08 ) 0. 1.97E- Improbable (E

Rotary 0.35 0.2 0.53 0.27 Improbable (E) 8.30E-08 

Aircraft 

Category 

Probability 

Occurrence 

On nd Grou

Probability

Occurrence 

In Climatic 

Extreme 

Probability 

Occurrence 

Fire Not 

E uished xting

Probability 

Occurrence 

Aircraft 

Lost 

End Event 

Rate of

Occurrence

Per Flight 

Hour 

MIL-STD-882

Hazard 

Frequency 

Fixed 0. -wing 45 0.2 38 0.03 09 0. 8.50E- Improbable (E)

Rotary 0.65 0.2 0.36 0.01 3.88E-09 Improbable (E)

Table 2-9. Estimate of R rr c ue to Extinguish a 

Nac a x ines raft

ate of Occu ence of Air raft Lost D to Failure

elle Fire in Climatic E treme (Assumes 2 Eng per Airc ).xxvii

Aircraft 

C ategory

Probability 

Occurrence 

In t Fligh

Probability 

Occurrence

In Climatic 

Extreme 

Probability 

Occurrence 

Fire Not 

Exting d uishe

Probability 

Occurrence

Aircraft 

Lost 

End Event 

Rate of 

Occurrence 

Per Flight Hour 

MIL-STD-882 

Hazard 

Frequency 

Fixed g 0. -win 55 0.2 0.24 0.09 9.84E-09 Improbable (E) 

Rotary 0.35 0.2 0.53 0.27 4.15E-08 Improbable (E) 

Aircraft 

C ategory

Probability 

Occurrence 

On Ground 

Probability 

Occurrence

In Climatic 

Extreme 

Probability 

Occurrence 

Fire Not 

Exting d uishe

Probability 

Occurrence

Aircraft 

Lost 

End Event 

Rate of 

Occurrence 

Per Flight Hour 

MIL-STD-882 

Hazard 

Frequency 

Fixed-wing 0.45 0.2 0.38 0.03 4.25E-09 Improbable (E) 

Rotary 0.65 0.2 0.36 0.01 1.94E-09 Improbable (E) 

The implication of the preceding is that when considering the risk of a catastrophic end event, the 

likelihood is driven primarily by whether fire occurs, and this likelihood is reduced by the likelihood of 

operating in a climatic extreme (e.g., cold temperature conditions). For example, Figure 2-46 summarizes 

fixed-wing fire mishaps and incidents by phase of operation. The takeoff-related categories total to 

18.7 % of all events, and approximately 16 % of suppressant releases occurred during the takeoff phases. 

However, only 4 % of the takeoff-related releases (and thus fewer than 1 % of all releases) occurred in 

land environments categorized as cold or severe cold. This strongly suggests that risk is low (i.e., an 

 

xxvii Probabilities in Tables 2-8 and 2-9 derived from References 45 and 46. End event rate of occurrences determined by 

s 

rs 

e WWAE. 

multiplying 8/106

 flight hours by probabilities indicated. This frequency is based on the aggregate number of nacelle fire

over all flight hours for the period and aircraft evaluated in those references and is higher than the rate of 4.9/106

 flight hou

indicated in an evaluation of Army rotary aircraft fires per Reference 81. No rotary aircraft were indicated lost in ground 

fire events in Reference 46 but a 1 % probability is assumed for discussion purposes. Probability of operation in climatic 

extreme assumes exposure to either MIL-HDBK-310 20% low or high temperatur
74 History of Fire Suppression in Aircraft 

i

which th

Figure 2-46. Fixed-Wing Aircraft Fire Mishaps and Incidents by Phase of Operation. 

ground fire events occurred in geographic locations associated with the basic land environment category. 

craft 

and

env

mprobable hazard frequency) for an engine nacelle fire during takeoff on a cold-soaked aircraft and in

e fire suppression system fails to extinguish the fire and a catastrophic event occurs. 

Summary – Severity of Aircraft Temperature Environment Criterion 

The conservatism of the -40 °C criterion can be seen when the historical data is plotted against the DoD 

climatic land environment design guidance and previously published aviation fuel flammability limit 

profiles in conjunction with various atmospheric profiles. Figures 2-47 and 2-48 plot the geographic 

locations of ground fire events for rotary aircraft and fixed-wing aircraft, respectively, vs. the DoD 

climatic land environment design guidance. In these figures, it can be seen that the clear majority of 

Figures 2-49 and 2-50 plot the nearest-to geographic locations of in-flight fire events for rotary air

 fixed-wing aircraft, respectively, to depict where these events occurred relative to the climatic land 

ironments. 

6.3%

2.0%

9.5%

4%

3.5%

0.6

7.1%

0.3%

0.7%

0.1%

5.9

1.4

3.0%

1.7%

13.2%

% 19.6%

3.2%

0.

0.1%

0.5%

0.1%

0.4%

4.5%

1.2%

4.2%

%

8.4%

%

2.4%

0.03%

Cruise Climb, 6.3%

Cruise Descent, 2%

Cruise Low Level, 9.5%

Combat Ma , 13.2% neuver

Cruise, 19.6%

Ground Ma e, 0.4 intenanc %

Ground Operations, 0.1%

Hover, 0.5%

Landing Final (Final Approach), 3.5%

Landing ( andi Forw ard L ng Area, FLAR), 0.6%

Landing Pa 2% ttern, 3.

Landing Ro 1% ll-Out, 7.

Not Availab % le, 0.1

Parked, Maintenance, 0.4%

Parked, Wheels Chocked, 4.5%

Parked, De-arm, 0.3%

Pre/Post Taxi, Engines Running, 0.7%

Parked, Servicing (Fuel), 0.1%

Parked, Found During Inspection, 1.2%

P ne St arked, Engi art, 4.2%

Ta bort, keoff A 5.9%

Tak limb, 8.4% eoff C

Tak unup (Run eoff R up Before Takeoff), 1.4%

Takeoff Roll-out, 3%

Taxi Landing, 1.7%

T or to axi Out (Pri Takeoff Runup), 2.4%

No ied, 0 t Specif .03%
Types of Fires Experienced 75

Also: two ground refueling fires in the Antarctic;

no MIL-HDBK-310 guidance for environments 

below 60° south latitude.

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the

coldest parts) Also: two ground refueling fires in the Antarctic;

no MIL-HDBK-310 guidance for environments 

below 60° south latitude.

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the

coldest parts)

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the

coldest parts)

Figure 2-47. Distribution of DoD Rotary Aircraft Ground Fire Locations.89

Figure 2-48. Distribution of DoD Fixed-Wing Aircraft Ground Fire Locations.89

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the coldest parts)

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the coldest parts)
76 History of Fire Suppression in Aircraft 

Figure 2-49. Distribution of DoD In-Flight Rotary Aircraft Fire Locations 

(Nearest-to Locations).89

Figure 2-50. Distribution of DoD In-Flight Fixed-Wing Aircraft Fire Locations 

(Nearest-to Locations).89

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the coldest parts)

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the coldest parts)

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the coldest parts)

(1% hot temp. of 49°C (120°F) in hottest parts)

(1% cold temp. of -45.6°C (-50°F) during the worst month)

(During worst month, 1% hot temp. of 43.3°C (110°F) & 1% cold temp. of -31.7°C (-25°F) )

(20% cold temp. of -51°C (-60°F) in the coldest parts)
Types of Fires Experienced 77

In Figure 2-51, the DoD rotary aircraft fire data are plotted vs. the standard atmosphere (blue line) and 

m 

cs

11.5 

 Limit 

Figure 2-52 p of fire 

events fro

relevant

acelle air temperature modeling cases occurred for OAT conditions at or above -25 °C (-13 °F), which

s well with that very few fire events are indicated below this OAT as well as with the 

distributio ironment 

depicted in those figures, the worst c 1% at -31.7 °C (-25 °F), which also 

correlates well with the temperature boundary for 94 % all suppressant releases in Figure 2-52. 

tropical atmosphere profiles (orange line) and Jet A (right) and Jet B (left) flammability limit profiles. 

Also indicated is the typical rotorcraft operational ceiling and the majority (97 %) suppressant release 

envelope derived from Figures 2-38 and 2-39. Also shown for reference purposes is an artifact fro

previous fire testing, described previously in Section 2.3.5, to determine the flame spectral characteristi

for optical fire detection at altitude: that testing at pressure conditions representative of altitude of 

km (35,000 ft) resulted in inability to maintain sustained combustion. The preponderance of fire 

events and suppressant releases on rotorcraft is shown to occur well below the typical operational ceiling 

for rotorcraft and well above the -40 °C (-40 °F) NGP boiling point criterion, with 97 % of all rotorcraft

suppressant releases occurring above -12.2 °C (10 °F). 

0

1000

2000

3000

4000

5000

6000

7000

8000

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

21000

22000

23000

24000

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Temperature, degrees C

Altitud

e, k

m

Figure 2-51. DoD Rotary Aircraft Fire Data vs. Atmospheric Profiles, Flammability

Profiles, Operational Ceiling, and Suppressant Release.89

lots the fixed-wing aircraft fire data in similar fashion. The overwhelming majority

m the data is indicated below 7 km (23,000 ft), with 94 % of all suppressant releases occurring 

at an altitude just above 9 km (29,500 ft). Qualitatively, this latter altitude as a ceiling correlates well 

with the results of the conclusions drawn from previously described nacelle air temperature modeling and 

the testing described previously in Section 2.3.5. Additionally, the overwhelming majority of 

n

also correlate

n of fire events depicted previously in Figures 2-48 and 2-50. In the basic land env

old temperature exposure is

9000

10000

Fire events for which both

altitude and OAT were

provided in the DoD fire

mishap/incident data.

97% of rotorcraft 

engine/APU fire

suppressant releases.

Rotorcraft 

ceilings

typically

A

below 6 km. ltitude, meters

Testing described 

in Section 2.3.5: no

sustained 

combustion above

10.7 km.

0

1000

2000

3000

4000

5000

6000

7000

8000

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

21000

22000

23000

24000

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Temperature, degrees C

Altitud

e, k

m

9000

10000

Fire events for which both

altitude and OAT were

provided in the DoD fire

mishap/incident data.

97% of rotorcraft 

engine/APU fire

suppressant releases.

Rotorcraft 

ceilings

typically

A

below 6 km. ltitude, meters

Testing described 

in Section 2.3.5: no

sustained 

combustion above

10.7 km.
78 History of Fire Suppression in Aircraft 

0

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Altitu

d

e, k

m

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

21000

22000

23000

24000

Temperature, degrees C

Fire events for which both 

altitude and OAT were

provided in the DoD fire 

mishap/incident data.

99% of fixed-wing engine/APU fire

suppressant releases.

94% of 

fixed-wing 

engine/APU

fire suppressant releases.

In modeling described in Section 2.3.6 majority of nacelle air temperature conditions fell at or 

Altitude, meters

Testing described 

in Section 2.3.5: no

sustained 

combustion above

10.7 km.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

21000

22000

23000

24000

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Altitu

d

e, k

m

Temperature, degrees C

Figure 2-52 DoD Fixed-Wing Aircraft Fire Data vs. Atmospheric Profiles, Flammability

Limit Profiles, Potential Stall Condition and Suppressant Release.89

In summary, the preceding figures indicate: 

• As altitude increases, the number of fire events decreases. 

• As altitude increases, occurrence of fire events trends above the standard atmosphere profile. 

• Rotorcraft fire events occurred below 4 km (13,000 ft), below the typical operational ceiling 

for those aircraft, with 97 % of all suppressant releases occurring for OAT above -12.2 °C

(10 °F). 

• A clear majority of fixed-wing aircraft fire events occurred below 6.1 km (20,000 ft). 

• Similar to publicly-available data for commercial aviation shown in Figure 2-37, the vast 

majority of fixed-wing aircraft fire events, ground and in flight, occurred with OAT above 

-20 °C (-4 °F). 

• In-flight rotary and fixed-wing aircraft fire events occurred predominantly near geographic 

locations associated with the basic land environments, for which DoD design guidance 

indicates the worst cold temperature exposure as 1 % at -31.7 °C (-25 °F). 

The -40 °C criterion used by the NGP is likely conservative. The plots of the DoD fire mishap 

incident data in the preceding figures suggest that qualification of nacelle fire suppression systems at

and

 an 

below 9.1 km and above -25°C. Cases beyond these conditions were for airspeeds 

indicative of potential aircraft stall.

Fire events for which both 

altitude and OAT were

provided in the DoD fire 

mishap/incident data.

99% of fixed-wing engine/APU fire

suppressant releases.

94% of 

fixed-wing 

engine/APU

fire suppressant releases.

In modeling described in Section 2.3.6 majority of nacelle air temperature conditions fell at or 

Altitude, meters

Testing described 

in Section 2.3.5: no

sustained 

combustion above

10.7 km.

below 9.1 km and above -25°C. Cases beyond these conditions were for airspeeds 

indicative of potential aircraft stall.
Types of Fires Experienced 79

OAT of 0 °C (32 °F) would respond to over 90 % of the expected fires, based on past experience. The 

qualification requirements are generally a DoD safety policy matter that rests ultimately with the DoD 

aircraft programs. This means that the safety and survivability risks associated with qualifying a system 

with a suppressant that has a boiling point greater than criterion of -40 °C would need to be assessed to 

determine whether the risk level is acceptable by the aircraft program manager, and the specific 

performance requirements for the fire suppression system would be reflected in the aircraft specification. 

2.3.7 Reign

e reactive 

fuel/air mixture with the hot metal surface.43 In full-scale nacelle fire suppression testing during the TDP, 

it was observed that when hot operating engine case temperatures were simulated, reignition could occur 

due to residual fuel adhering to the surface or fuel continuing to be sprayed before being shut off. This 

was seen typically at surface temperatures at or above 538 °C.90 That testing also indicated that when a 

hot metal surface at 704 °C (1300 °F) was present, greater quantities of fire suppressant were required 

when compared to no hot surface reignition condition. The NGP has also investigated how induction and 

convection may control the reignition process. This investigation found that reignition is controlled by 

cooling and mass transport towards the hot surface. A “worst-case” scenario for reignition was 

characterized by maximizing the fuel mass transfer while keeping the characteristic time for cooling of 

the surface shorter than the time to attain a flammable mixture.91

Reignition suppression requirements within a dynamic environment on an aircraft are very dependent on 

the specific scenario. Currently, there is no reliable method to predict the optimal agent requirements to 

prevent it, other than conduct of full-scale testing for the unique aircraft configuration for all possible 

reignition conditions. Strategies to prevent reignition include removing fuel vapor, reducing surface 

temperatures, either through design or active cooling, and inerting the fuel/air mixture with a suppressant. 

Fuel vapors can be removed intentionally by design or unintentionally as an affect of the ventilation 

airflow required by a compartment for cooling. Leaking flammable fluids can be removed by drain holes 

and systems or sump ejectors. Typically, before activation of the engine nacelle fire suppression system, 

the jet fuel to the particular engine and hydraulic fluid flow to engine compartment accessories is shut 

down. This limits the amount of fuel in the nacelle, but it could take a relatively long time to remove the 

combustibles from the nacelle, especially low vapor pressure liquids. 

In considering the issue of reignition, it is important to distinguish between the temperature at which 

autoignition occurs vs. hot surface ignition. The autoignition temperature (AIT) is also referred to as the 

spontaneous ignition temperature, self-ignition temperature or autogenous ignition temperature. It is the 

lowest temperature at which the substance will produce a hot-flame ignition in air without the aid of an 

ition 

Reignition within an engine nacelle or dry bay compartment is always a threat so long as fuel vapor and 

air can come in contact with sufficiently hot surfaces or if there is some other type of ignition source 

present, such as arcing from an electrical harness or sparks generated from a rotating component 

unintentionally in contact with another component. Within an engine nacelle, hot surfaces are the 

primary ignition threat, as engine case temperatures can easily exceed 538 °C (1000 °F). After 

suppressant discharge within a nacelle or APU compartment, hot fuel vapor may exist at levels which are 

flammable, leading to the possibility of reignition. A puddle of hydraulic fluid or jet fuel leaking from a 

cracked or failed line can vaporize as heat is transferred from a nearby hot metal surface. In addition, hot 

metal surfaces may occur due to heating by the fire itself, which could occur within a nacelle or APU 

compartment or even a dry bay compartment. Reignition may then arise from contact of th
80 History of Fire Suppression in Aircraft 

external spark or flame.92 This temperature is determined by heating a sample of a fluid in air in a

laboratory flask. The AITs for JP-type military aviation fuels range between 204 °C (400 °F) and 232 

(450 °F). For MIL-H-83282-type hydraulic fluid, the AIT is approximately 354 °C (670 °F). Aircraft

manufacturers will typically self-impose a safe design temperature (SDT) practice that is usually 10 

(50 °F) lower than the AIT of a fluid that may come in contact with a hot surface. This cannot be

accomplished for engine nacelles or APU compartments due to their case temperatures. 

Reignition on a hot surface within a dynamic environment such as within an aircraft engine nacelle i

distinct from autoignition. Extensive lab-scale and full-scale testing has been conducted over the past 

several decades that demonstrates, generally, hot surface ignition of aviation JP-type fuels and hydraulic

fluids used in military aircraft occurs at surface temperatures greater than the AIT for those fuels and 

fluids. Minimum hot metal surface ignition temperatures for JP-8 (or kerosene) for ambient temperatures 

and pressures and low air flows have been found to vary from 360 °C to 650 °C, depending on test

conditions. Hot surface ignition temperatures will generally decrease as the size or surface area increases 

or as fuel contact time increases, and they will generally increase with increasing air velocity.

93 A

exception is MIL-H-83282 hydraulic fluid, which during testing conducted in a nacelle simulator with a 

portion of a simulated F-16 engine was found to have the potential to ignite when exposed to a hot surface 

below the fluid’s AIT and when ambient air temperature was heated to at least 150 °C (300 °F). At a

ventilation air temperature of 316 °C (600 °F) the fluid would ignite without the hot surface.94,xxviii 

From work conducted during the TDP it was asserted that a reasonable target concentration for

suppressant in the fire zone (not the free stream) is the concentration which ensures that the mo

flammable fuel/air ratio cannot occur. Such a suppressant concentration should ensure both flame 

suppression and prevention of re-ignition for a period of time on the order of the suppressant injection 

(discharge) duration. After this period, however, it is likely that re-ignition would still be possible.13

°C 

°C 

s

n

st 

2.

lementation of engine compartment fire suppression, which 

partment that was controlled from the pilot’s seat.

Additionall the compartment.95 No specific 

referenc s

Board reported that in the 1920s, non

extingu

as aircraft f

the rationale for a specific requirement. An example of this is the unpublished technical data and various

publish s 01 fire

suppres n

efforts to i

compilation

4 FIRE SUPPRESSANTS USED ON AIRCRAFT 

As early as 1922, there is reference to imp

consisted of a fire extinguisher within that com

y, shutters were installed to eliminate external airflow into

e i provided as to the fire suppressant in this case, but one could speculate. The Naval Studies

-fluorinated halon agents were tried experimentally in engine nacelle 

ishers, but their use was abandoned by the U.S. military in favor of the non-corrosive CO2.

8

 Even

ire suppression matured, there are instances in which researchers and aircraft designers ponder 

ed tatements supporting the certification requirement for engine nacelle halon 13

sio systems. Regardless, since the identification of stratospheric ozone depletion and subsequent 

dentify alternatives to the halons for aircraft fire suppression, there have been numerous 

s of the history of fire suppressants used on aircraft. 

research indicated in Reference 149 also indicated for MIL-H-83282 an AIT of 354°C and a s xxviii Previous tream hot manifold

ignition temperature of 322°C. The later was determined by Federal Test Method Standard (FTMS) 7916, Method 6053. In

hot surface ignition studies surveyed in Reference 150, some of the studies showed MIL-H-5606 was more ignitable than 

MIL-H-83282, whereas other studies surveyed, which used different test configurations, indicated diametrically opposite 

results. 
Fire Suppressants Used on Aircraft 81

Perhaps the most comprehensive compilations are those in reports generated by the National Institute of 

Standards and Technology (NIST) under the TDP, by the Federal Aviation Administration (FAA) as part 

of work it has been performing related to development of minimum performance standards for halon 

alternative nacelle fire suppression systems, and by Kidde Aerospace and Defense, PLC, one of the 

primary suppliers of aircraft fire suppression system components today.13,96,97 Additional historical 

references are contained within the investigation performed by the NGP to document active suppression

technologies for ullage fire/explosion protection and work done by Boeing to document options for dry 

ft engine compartment fire suppression 

systems for the B-36 and XR60-1 aircraft employed halon 1001. Some of the first aircraft to deploy fixed 

2, the first operational jet-powered 

fighter. Given the time the directive was issued, it is likely D-L did not come into widespread use 

bay fire protection.31,98 

The following provides a brief summary of fire suppressants that have been used on DoD aircraft for 

powerplant and dry bay fire protection, excluding halon alternative suppressants identified during the 

TDP or developed under the NGP. The reader is referred back to Section 2.2.6 for discussion of 

suppressants that have been utilized for ullage fire suppression. Fielding of halon alternative suppressants 

subsequent to the TDP is discussed briefly in the next section. Throughout this section, the phrase 

“powerplant” may refer to an engine or APU compartment or other type of powerplant described earlier 

in this chapter. 

2.4.1 Powerplant Compartments 

The requirements for and implementation of aircraft fire suppression for powerplant compartments have 

evolved for a variety of reasons, primary among them being powerplant design and the fire suppressant. 

For example, powerplant fire suppression system design guidance published by the CAA in 1943 for use 

of methyl bromide (halon 1001) and CO2 was relative to mass airflow in the compartment and the number 

of cylinders in a radial cylinder engine installation. For potential fire zones with high airflow, agent 

quantity was to be based on 20 % of the mass airflow through the zone in two seconds.99, xxix Agent 

distribution was to be accomplished using spray nozzles or perforated tubes providing approximate equal 

distribution and a “sheet of agent spray” across the cross section of the protected zone orthogonal to the 

airflow. These systems were to become known as conventional distribution systems. Figure 2-53 

illustrates an example of this type of installation. The aircra

CO2 fire suppression systems for engine compartment protection included the C-46, C-47, B-17, B-26, 

and the B-45 aircraft. 

During World War II the German Navy sponsored efforts by I.G. Farbonindustrie to develop an 

alternative to halon 1001 due to its toxicity, which resulted in the development of chlorobromomethane 

(CB or halon 1011) in the 1939 to 1940 time period. Halon 1011 was determined to be as effective as 

halon 1001 and less toxic. Testing in 1942 by then Junkers/Dessau for the German Luftwaffe focused on 

developing a powerplant fire suppression system using Dachlaurin (D-L), a mixture of 65 % halon 1011 

and 35 % CO2. In early 1945 the Luftwaffe approved the principle of the D-L system and ordered its 

installation on all German military aircraft, subject to then not-yet-established priorities. It was planned 

that the D-L system was to be installed on the Messerschmitt ME 26

100

before the end of the war. After the war extensive evaluation of halon 1011 was conducted within the 

U.S. and by 1950, the USAF required use of halon 1011 systems instead of CO2 systems in new aircraft 

 

xxix As of the writing of this book, CAA reports are available from the Department of Transportation’s Online Digital Special 

Collections at http://dotlibrary.specialcollection.net/. 
82 History of Fire Suppression in Aircraft 

and subsequently issued a specification for such systems.101,102,103 Design guidance for use of halon 1011

in powerplant fire suppression system evolved as jet propulsion became more widespread and was

provided relative to compartment airflow and free volume. However, the conventional distribution 

system approach was still employed for halon-1011-based fire suppression systems. Examples of these 

arrangements are shown in Figures 2-54 and 2-55. Some aircraft known to have utilized halon 1011 for 

powerplant fire suppression include the C-97, C-119F, C-123, C-130 and B-57 aircraft. 

Figure 2-53. Example Conventional Halon 1001 Powerplant Fire Suppression System 

Distribution Installation, USN Turboprop Powerplant.104

Techniques for assessing adequate distribution, which are described later in this section, changed along 

with the evolution to jet propulsion powerplants. During testing conducted by the CAA, it was observed

during filming and time recording of discharge duration from a conventional distribution system that the 

apparent full-strength discharge time was 1 s.104

 (The certification requirement that had been established

for the conventional fire suppression systems was, and still is, a required concentration level maintained

at all measured locations throughout the compartment for a minimum of 2 s.) During these same tests,

comparisons were made of fire suppression performance of conventional systems vs. open-ended systems,

which later became known as high-rate-discharge (HRD) systems. The HRD systems presented a

simplified distribution approach in that perforated distribution lines were replaced with few open tubes 

to d

disc

desi d

out of which the fire suppressant would discharge at a much higher rate. Rather than relying on plumbing

isperse the suppressant, dispersion would be effected by the turbulent mixing of the suppressant

harge jet and the nacelle mass air flow. Further testing by the CAA demonstrated that the HRD

gn required less halogenated fire suppression agent to suppress nacelle fires and simplifie
Fire Suppressants Used on Aircraft 83

distribution system design. Testing conducted later by the Wright Air Development Center (WADC) 

promoted the conclusion that the efficiency of a fire suppression system would be improved with 

increasing suppressant discharge velocity, and that a “critical saturation value,” in percent by volume,

occurred between 15 m/s (50 ft/s) and 30 m/s (100 ft/s) for the suppressants evaluated (halon 1011, halon 

1301, and dibromodifluoromethane or halon 1202).105 The example fire suppression system installation

shown earlier in Figure 2-8 is a representative halon 1301 HRD system installation. 

Figure 2-54. Example Conventional Halon 1011 Fire Extinguishing System Distribution 

Installation, Turboprop Powerplant.

106

Halon 1301, which had been determined to be superior to halons 104 and 1001 in hand portable 

concentration. This testing was not related to engine nacelle fire suppression. 

extinguishers during testing conducted by the Purdue Research Foundation (PRF),107 was also found to be

well-suited for use in powerplant HRD systems. During CAA tests that were performed that resulted in 

the design guidance for halon 1301, in which a minimum discharge duration of 0.5 s is required, 

discharge durations varied between 0.5 s and 0.9 s.108,109,110 It is interesting to note that technical

intelligence gathered after the end of World War II suggested that design policy for engine compartment 

CO2 fire suppression systems followed by the Imperial Japanese Navy required discharge within 1 s.52 A

timeline of the evolution of the 0.5 s duration requirement for HRD systems can be hypothesized based on

year of publication of the reports and specifications as follows: 

• 1948: As mentioned previously in Chapter 1, the U.S. Army commissioned the PRF to search 

for a suppressant of high fire suppression efficiency but low toxicity. During flammability

limit testing a halon 1301 inerting concentration was determined as 6 % volumetric
84 History of Fire Suppression in Aircraft 

Figure 2-55. Example Conventional Halon 1011 Fire Extinguishing System Distribution 

• 1953: Fire testing is conducted by the CAA to determine the minimum amounts of 

Installation, USAF Turbojet Powerplant.111

suppressant required for varying airflows using CO2 and halons 1001, 1011, and 1202.112 

Statham Laboratories began manufacturing the Model GA-1 Gas Analyzer for the USAF for 

use in measuring suppressant concentrations based on an experimental gas analyzer 

developed by the USAF.113 This device is to later become known throughout the aviation fire 

suppression field as the Statham Analyzer. 

• 1955: Fire and suppressant concentration testing was conducted by the CAA using halon 

1011. A recommendation was developed that halon 1011 systems provide a minimum 15 % 

volumetric concentration for 1 s.106

• 1956: Fire testing of HRD systems was conducted by the CAA using CO2 and halons 1001, 

1011, 1202, and 1301, and design formulae for suppressant quantity are published. In 

successful fire extinguishment tests using the halons, discharge durations are indicated to 

have varied between 0.5 s and 0.9 s.108 For CO2 the duration was indicated to have varied 

between 1.25 s and 1.35 s. 
Fire Suppressants Used on Aircraft 85

• 1958: Fire testing of conventional and HRD systems was conducted by the CAA for the 

Northrop F-89 Scorpion using halons 1011, 1202 and, to a lesser extent, halon 1301.114 This 

effort included specific tests to evaluate discharge duration and distribution using halon 1011. 

An overlapping time period of 0.44 s was noted for the two compartment sections evaluated, 

as indicated in Figure 2-56. (In Figure 2-56 the time axis is in 0.125 increments, thus 

0.44 occurs between three and four increments.) Though the report indicates that by the time 

halon 1301 had been evaluated in the HRD system for fire testing some degradation in the 

test article had occurred, there is no indication in the report that this was a factor in the halon 

1011 discharge duration and distribution tests (i.e., they may have been completed prior to 

conduct of halon 1301 fire testing). 

• 1959: Fire and concentration measurement testing (using a then Statham Laboratories 

concentration analyzer) at WADC concluded for halon 1301 that roughly a 5.8 % “critical 

saturation value,” in percent concentration by volume, is required for fire extinguishment.105 

Later during the year, the required minimum relative concentration for halon 1301 was 

published as 15 %, which was specified to be maintained in all parts of the affected zone and 

persist in each part of the zone for at least 0.5 s at normal cruising condition. The relative 

value is that indicated by the Statham measurement device corresponding to a halon 1301 

6 % volumetric concentration. At the end of the year, the military specification for 

installation and test of HRD aircraft fire suppression systems, MIL-E-22285, was issued.109 

The specification includes the previously published halon 1301 design formulae for 

suppressant quantity. 

• 1960: The military specification for installation and test of HRD aircraft fire suppression 

systems was reissued, revising the concentration requirement from 15 % relative to 6 % 

volumetric. 

Figu 

The HRD d

aircraft app nd were based 

on testing conducted on a single piston-engine powerplant and one jet power plant. Like the design 

re 2-56. Minimum Discharge Duration from an HRD System Using Halon 1011.114

esign formulae that have since been applied for sizing halon-1301-based systems in DoD 

lications have essentially remained unchanged since they were first published a

0.44s 
86 History of Fire Suppression in Aircraft 

guidance for conventional halon 1011 systems, HRD system design guidance is also relative to

mpartment airflow and free volume. Additionally, a review of the CAA reports indicates that the 

number of tests conducted using halon 1301 preceding the issuance of MIL-E-22285 was limited. The 

CAA report that issued the design formulae indicates very few data had been obtained for halon 1301 b

that it appeared equal to halons 1001 and 1202 on a weight basis. Given that the HRD design form

published by the CAA were identical for halons 1001, 1202, and 1301, it is possible that in the case of

halon 1301 the design guidance for it was asserted qualitatively at that time. It is also possible that this is

the case for the 0.5 s discharge duration requirement, as discharge duration testing conducted by the CAA 

prior to the issuance of MIL-E-22285 was conducted using halons 1011 and 1202. 

As indicated in Table 2-10, halon 1301 is today by far the most widely implemented of the 

suppressants for powerplant fire protection (nacelles and APU compartments) on DoD aircraft. Although 

Table 2-10 is not necessarily meant to be totally comprehensive, it depicts the magnitude of halon 1301 

mplementation across the DoD. The distribution systems are predominantly HRD designs. For 

reference, the table also lists other fire suppressants used today excluding halon-alternative fire 

suppressants installed on DoD aircraft since the Montreal Protocol. These are various forms of nitrogenbased fire protection and aluminum oxide, which is used on some aircraft in powder panels for passive 

y bay protection. 

2.4.2 Dry Bay Compartments 

As discussed earlier in this chapter there are no active halon fire suppression systems installed currently

on DoD aircraft for the specific purpose of ballistic dry bay fire protection, though there are 

mpartments for which halon fire suppression is provided to protect against safety fire threats and for

which such compartments are also vulnerable to ballistically-induced fire. The C-5 nitrogen fire 

suppression system provides protection for various dry bay compartments as indicated in Figure 2-57.

stem discharge is automatic for fires detected in the wing and pylon leading edge dry bays. Alum

oxide powder (Al2O3) has also been fielded on several DoD aircraft. The powder is contained within 

co

ut

ulae 

halon

i

dr

co

Sy inum 

a

arasitic honeycomb panel to prevent ballistically-induced fires in dry bays adjacent to fuel cells and was

y the Royal Aircraft Establishment in England.115 If ruptured by a ballistic projectile, the 

pa l

indicated in the example w ure 2-18 is an example of a 

parasitic o

design te n

2.4.3

The current ire-suppression systems on aircraft that use halon 

1301 re r

locations i

1011, 1202 Advisory Circular 20-100. (Military specification 

MIL-E1301 po

previous m

aerology ba

total quantit uming a 10 % system loss. 

p

first developed b

nel releases a powder suppressant to quench incendiaries released by the projectile. The powder pane

ing leading edge dry bay shown earlier in Fig

p wder panel installation. Chapter 9 describes NGP work to develop advanced powder panel

ch iques. 

Certification 

requirement to certify other than dry-bay f

qui es demonstrating a minimum suppressant concentration for at least 0.5 s at all measurement

s multaneously. Minimum suppressant concentration requirements for CO2, and halons 1001,

, and 1301 are indicated in FAA 116

22285 provides these requirements for halon 1301 only.) By today’s method for certifying halon

werplant fire suppression systems, previous methods would seem somewhat subjective. One 

ethod was to discharge water through the distribution system, capture the discharge by large 

lloons, and determine the quantity discharged as being equal to the difference between the 

y and quantity captured, ass
Fire Suppressants Used on Aircraft 87

Table 2–10. Fire Suppressants on DoD Aircraft.xxx

Halon 1011

(CH2ClBr)

Halon 1202

(CF2Br2) 

Halon 1211

(CF2ClBr) 

Halon 1301

(CF3Br) 

Nitrogen 

(N2) 

Aluminum 

Oxide (Al2O3) 

USAF: 

C-130 

USN: 

DC-130A

LC-130F/R 

TC-130G nd center wing) 

AH/UH-1, SH-2

CH/MH-53 

HH/SH-60 

C, E-6A 

EA-6B, F-14 

F/A-18C/D

P-3, S-3 

T-39, T-44A 

C-130 (excluding 

C-130s with halon 

1011, 1202 or 1211)

US Army: 

AH-64, CH/MH-47 

EH/MH/UH-60 

C-7, C/RC-12, C-20

OBIGGS NEA 

US Army – 

MH-47E 

MH-60 

AH-1W/Z 

USAF: 

C-5, C-141 

Note: halon

1202 is an

alternate for 

USAF: 

C-130J 

USN: 

KC-130J 

USAF: 

MH-53, HH-60

A/OA-10, B-2 

B-1B (overwing 

fairing)

C-5 (cargo, avionics, 

a

USAF - LN

(ullage and dry 

bay): 

C-5

USAF – 

USAF 

(dry bay):

CV-22 

USN (dry bay): 

MV-22 

2

halon 1011 on

USAF and USN 

C-130 aircraft, 

except for 

C/KC-130J 

aircraft. 

C-9, KC-10, C-12

C-17, C-20, C-22B

C-32A, C-40

C-135, VC-25A

C-141, E-3A

E-4A, E-8C 

F-16 (ullage) 

F-15, F-117

T-34A, T-39, T-43 

USN: 

(ullage): 

F-22 

C-17

USN - 

OBIGGS NEA 

(ullage):

V-22 

AH-1Z 

UH-1Y 

UH-1N/Y 

HH/SH/UH-3 

CH/HH-46 

OBIGGS NEA 

(ullage): 

AH-64 

C-2A, C-9

C-12, C-20

E-2

 excludes halon alternative fire suppressants installed on DoD aircraft since the Montreal Protocol. These air xxx Table 2-10 craft 

are the MV/CV-22, F-22, and AH-1Z/UH-1Y, each of which utilizes HFC-125 for nacelle fire suppression and OBIGGS 

NEA for fuel tank inerting. Also, the F/A-18E/F utilizes HFC-125 for nacelle fire suppression, except for EMD and LRIP 1 

aircraft, which utilize halon 1301. The F/A-18E/F and MV/CV-22 use inert gas generators for dry bay fire suppression. The 

gas discharge is a gaseous mixture of carbon dioxide, nitrogen, and water vapor. Gas generator technology is also being 

evaluated for dry bay fire suppression on the Joint Strike Fighter (JSF) aircraft. 
88 History of Fire Suppression in Aircraft 

Distribution ratios were determined for each zone protected by the system, thus the required quantity of 

suppressant for each zone could be determined prior to testing. Another previous method was to measure 

discharge duration by means of motion pictures taken at the rate of 32 frames per second. Neither of 

these previous methods could be accomplished in flight. 

The Statham-type suppressant concentration analyzer device mentioned in the previous section permits 

in-flight measurement of fire suppressant concentration.xxxi,117 This type of analyzer operates based on a 

linearized viscosity mixing theory using the weighted viscosities of a binary gas mixture, i.e., air and the 

fire suppressant. Since the viscosity of pure air differs from that of pure fire suppressants, readings will 

show that a mixture of gases is present and the “relative” concentration of the suppressant will be 

med only after thermal equilibrium is achieved. The sonic flow orifice ensures a 

constant flow while the capillary tubes create the pressure drop measured by a transducer. The transducer 

its th hich performs the necessary calculations and then 

records and displays relative concentrations.118 The relative concentrations are then converted to 

s before they reach the field. The vast majority of DoD aircraft fielded today had 

ment of this legislation. Thus it is probable that compartments on 

ression is provided to protect against safety fire threats and for which 

indicated by the differential pressure reading obtained. An algorithm converts relative concentration 

values to volumetric concentration values based on the unique calibration of the analyzer and the fire 

suppressant. A vacuum pump draws the gas mixture samples through twelve sampling probes (copper 

tubes) into sensor assemblies. Sampling probe ends are oriented perpendicular to compartment airflow 

and are located throughout the compartment (Figure 2-58). In each sensor assembly, the gas passes 

through filter screens, a heat exchanger, a capillary tube differential pressure sensor section, and finally 

through a sonic flow orifice. The heat exchanger section ensures uniformity of monitoring conditions, 

and tests are perfor

transm e pressure signal to a processor unit w

volumetric concentrations for evaluation as to whether the fire suppression system meets a certification 

requirement (Figure 2-59). 

Until recently, non-dry-bay fire suppression systems qualification has been normally accomplished by 

concentration measurement. There are unique cases where a system such as a nacelle fire suppression 

system will be qualified by fire testing. In this case, performance requirements will be specified and 

agreed to by the acquisition or certifying agency and the aircraft manufacturer. Nacelle fire suppression 

systems that have been qualified through fire testing are those on the USN F/A-18E/F, which uses 

HFC-125 as the fire suppressant, and the commercial Eclipse 500 aircraft, which uses PBr3. 

The certification requirement to demonstrate compliance with survivability and vulnerability 

requirements is Congressionally legislated in Title X, Section 2366 of the United States Code of Federal 

Regulations, which was passed in 1987. This live fire test legislation requires that realistic testing be 

done on new system

initial operating capability prior to enact

aircraft for which halon fire supp

such compartments are also vulnerable to ballistically induced fire, no live fire testing had been 

performed, and that such halon systems were certified solely by suppressant concentration measurement. 

 

xxxi There are several versions of this type of analyzer that are certified by the FAA for use in performing fire suppression system 

concentration measurement for the purpose of qualifying or certifying aircraft fire suppression systems. One is the Statham 

Analyzer, owned and operated by Walter Kidde Aerospace. This is a modified version of the original Statham analyzer 

described earlier. Another analyzer is the Halonyzer, of which there are currently two versions, Halonyzer 2 and Halonyzer 

3. One Halonyzer 3 analyzer is owned and operated by its manufacturer, Pacific Scientific/HTL Kin-Tech. One Halonyzer 3 

analyzer is also owned and operated each by Boeing commercial and by the USAF at WPAFB. One Halonyzer 2 analyzer is 

owned and operated by Airbus Industries in France. Finally, a ‘modified’ Halonyzer 2 analyzer is owned and operated by 

the FAA Hughes Technical Center Fire Safety Branch. 
Fire Suppressants Used on Aircraft 89

Nitrogen Fire Sup nes and Co pression Zo ntrols 

Zone Spaces Included in Zone 

Flight E s ngineer’

Panel Discharge 

Pushbutton 

Nose Wheel Well 

Panel Discharge 

Pushbutton 

1 Left wing dry bay, left outboard leading edge, left 

outboard pylon leading edge 

Left outboard wing Left wing 

2 Left wing root dry bay, left inboard leading edge, 

left inboard pylon leading edge 

Left inboard wing Left wing 

3 Right wing root dry bay, right inboard leading edge, 

right inboard pylon leading edge 

Right inboard wing Right wing 

4 Right wing dry bay, right outboard leading edge, 

right outboard pylon leading edge 

Right outboard wing Right wing 

5 Nose wheel well Nose wheel well −

6 Cargo under floor, forward Under floor, forward Under floor, forward 

7 Cargo under floor, middle Under floor, forward Under floor, forward 

8 Left main wheel well Left main wheel well −

9 Right main wheel well Right main wheel well −

10 Cargo under floor, aft Under floor, aft Under floor, aft 

11 Left power turbine unit (PTU) compartment Left PTU Under floor, forward 

12 Right power turbine unit (PTU) compartment Right PTU Under floor, forward 

Figure 2-57. Zones Protected by the Nitrogen Fire Suppression System on C-5 Aircraft.119

(Reprinted with permission by the USAF Warner Robbins Air Logistics Center) 

1 2

3

4

5

6

7

8

9

10

11

12 1 2

3

4

5

6

7

8

9

10

11

12
90 History of Fire Suppression in Aircraft 

Figure 2-58. Fire Suppressant Concentration Analyzer Sampling Probe Installation. 

(Printed with permission of the Naval Air Systems Command.) 

Figure 2-59. Example Fire Suppression System Suppressant Concentration 

Measurement. 

0.0

5.0

10.0

15.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

V

olu

m

etric C

o

20.0

25.0

30.0

35.0

e

ntr

atio

n, %

Time, seconds

n

c

ch1

ch2

ch3

ch4

ch5

ch6

ch7

ch8

ch9

ch10

ch11

ch12

Concentration at all locations 

for 0.5 s minimum 
Fire Suppressants Used on Aircraft 91

Previous testing with halon 1301 for suppression of dry bay compartment fires with ventilation induced 

 damage from a ballistic projectile indicated that halon 1301 would suppress fires if discharged ver

fast, on the order of 10 ms, from a fire bottle type used on the United Kingdom CH-47 Chinook 

helicopters, or if a comparable halon 1301 quantity was discharged from a bottle design typically

nacelle or APU fire suppression, from which the suppressant will discharge in approximately 1 s. E

though this testing showed that both types of bottle configurations provided halon 1301 concentrations 

well above 6 % for greater than 0.5 s,21 no concentration-based dry bay fire suppression sy

certification requirement was ever established by the DoD. Likewise, even though there is historical 

reference to the fielding of reactive ullage fire suppression systems, there is no DoD concentration-based

qualification requirement for these systems as well. It should be noted that the fast discharge capability

the fire bottle type United Kingdom CH-47 Chinook helicopters is consistent with the design guidanc

indicated in Figure 2-60 for a confined space (i.e., constant volume), which has existed since 1965. 

by y

used for 

ven 

stem 

of

e 

danc ppres

In t ca ogen fire suppression system, which is an extension of the fuel tank LN2 inerting 

system a rotection, the system is r e 

com m arged, ullage or dry bay, to 10 % or less “in the ti y

to cope with the particular fire hazard.” (Typically, the D ullage ine g systems

red o to 9 % or less.). The USN system wa esigned to

ma in tric concentration of 20 % upon

OB S craft were typ ied by n

oncentration with a single oxygen sensor installed at the fuel vent interface in one or more fuel tanks or 

towards d has been 

developed by the FAA that is utilized and operates in a manner similar to the concentration measurement

analyzers utilized for certifying nacelle fire suppression systems. 

Figure 2-60. MIL-HDBK-221 Design Gui e for Explosion Su sion.

120

he se of the C-5 nitr

nd provides dry bay fire p equired to reduce the oxygen level within th

part ent into which it is disch me duration necessar

oD will require that rtin

uce xygen concentrations A-6E ullage inerting s d

inta a halon 1301 volume activation.121

IGG installations on currently-fielded DoD air ically certif measuring the oxyge

c

by installing an oxygen sensor on a fill port, as indicated in Figure 2-61. Today, as work continues

eveloping OBIGGS for commercial transport aircraft, an oxygen-gas sampling system

Typical Pressure Curve o

Suppressed Explosion in

 Space 

f

Confined

0 5 1

Tim

0 1

e - Millisecon

5 20 25

ds

0 5 10 15 20 25 30 35 40 45 50 

Time - Milliseconds

Typical Pressure Curve of 

Explosion in Confined Space 

Explosion 

Suppressed

Agent

Scatters 

Agent

Released

Safe 

Pressure

Pressure - PSI 

Pressure - PSI 

5

4 

3 

2 

1

Response Time

90 

80 

70 

60 

50 

40 

30 

20 

10 
92 History of Fire Suppression in Aircraft 

Figure 2-61. Typical Legacy OBIGGS Qualification Test Oxygen Concentration 

Measurement Location. 

(Printed with permission of the Naval Air Systems Command.) 

2.5 HALON ALTERNATIVE TECHNOLOGY DEVELOPMENT PROGRAM (TDP) 

Chapter 1 discussed why the DoD initiated the TDP and its goal for identifying near-term, 

environmentally friendly, and user-safe alternatives to halon for aircraft engine nacelle and dry bay 

applications, and why the limitations of what had been identified along with new/emerging environmental 

onstraints required continued research and development for these applications. However, the breadth 

nd technical approach of the TDP is historically significant. Like previous efforts related to aircraft fire 

suppress s from 

government agencies, industry, and academi

c

a

ion, the TDP spanned several years and was a collaborative effort involving participant

a. 

OBIGGS

Porduct Line

Gravity

Fill

Vent Bypass Valve

Vent Climb/Dive Valve

Pressure

-2 to 5 psig Pressure

-2 to 5 psig

ENG

1

ENG

2

APU

Engine/APU Bleed Air

Line to OBIGGS

Pressure

0 to 100 psia

Temp

50° to 300°C Pressure

0 to 50 psig

Temp

-30° to 65°C

Bleed Air Line

to Aux Fuel System

Temp, -30° to 60°C

Pressure,

0 to 50 psig

Instrumentation for

Aux Fuel System 

Test

OBIGGS

O2

Pressure

-2 to 5 psig

AFT FUEL CELLS

Example Oxygen Sensor 

Location for Qualification of 

an OBIGGS System 
Halon Alternative Technology Development Program (TDP) 93

Following the DoD’s 1989 delineation of its policy on halon replacement research, over 600 potential

halon replacement chemicals were assessed.57,122 This was followed by the USAF initiating the Halon 

Replacement Program for Aviation in 1992 to develop a non-ozone depleting solution for on-board fire

suppression within a timeline to support the F-22 aircraft acquisition program. In addition to the F-22, the 

V-22 and F/A-18E/F acquisition programs also implemented requirements for fire suppression systems 

having non-ozone depleting fire suppression agents. Thus, the scope of program was subsequently

expanded to address requirements of all DoD and civilian aircraft engine nacelle and dry bay applications 

and was jointly-sponsored by the USAF, USN, U.S. Army, and the FAA as the TDP. Additionally, an 

oversight group, the Halon Alternatives Steering Group (HASG), was established to coordinate efforts 

within the TDP as well as other government research and development (R&D) programs related to fire 

suppression. This included coordination with related efforts under the EPA’s SNAP program, which

addressed both environmental acceptability and personnel safety, e.g., toxicity, of candidate halon

alternative suppressants. The TDP was a three-phase program, each of which is discussed briefly below. 

The reader is encouraged to refer the publications referenced in this section for m cussion.

2.5.1 Phase 1 – Operational Parameters Study

Phas ermined parameters in aircraft engine nacelles and dry t ha reate on

the y of fire suppressant required to extinguish fire in those ty mp 3,124 

Cha t tics of each of these compartment types on then-fielded aircraft were acquired to support 

dev t of a test matrix used during this portion of the P. A statistical design of experi nts 

(DOX) hodology was employed to reduce the number of possible test configurations to 32 using a 

Plackett rman two-level fractional factorial d per study of the effects of 14 parameters 

related to dry bay eters related to engine nacelle compartments along with 

interaction o f compartm

bay compartment factors and two combinations of engine compartment fact easible. The 

DOX methodolog effects of the parameters ype within the

physical and ec le to the TDP as well as to permit concurrent acquisition 

programs to meet schedule requirements for implementing non-ODS-based fire suppression systems.

st aterials 

compatibility, thermodynamic properties, fluid dynamics associated with discharge, flame suppression 

ore detailed dis

e 1

qua

rac

elop

det bays tha ve the g

pes of co

st influence

artments 12 ntit .

eris

men TD

mit

me

met

-Bu esign to

 compartments and 16 param

s f actors with only 32 test runs for each 

16

ent. Evaluating two14 combinations of dry 

ors was not f

y permitted evaluating

onomic resources availab

for each compartment t

Table 2-11 lists, in order of significance, the factors found during Phase 1 testing at Wright-Patterson Air

Force Base (WPAFB) to influence fire suppression the most in each compartment type. Concurrent 

udies and testing of candidate halon alternative chemicals were conducted by NIST to evaluate m

effectiveness, flame inhibition chemistry, agent stability under storage, and affects to personnel and the 

environment.125 The combined outcome from the WPAFB and NIST efforts resulted in the

recommendation that the chemicals listed in Table 2-12 be evaluated during Phase 2. 

Table 2–11. Most Significant Fire Suppression Parameters Identified 

During Phase 1. 

Engine Nacellexxxii Dry Bay 

Surface Temperature (34 %) Compartment Volume (48 %)

Fire Suppression Agent (14 %) Fire Suppression Agent (28 %) 

Clearance, or Nacelle Free Volume (12 %) External Airflow Rate (13 %) 

 

xxxii

 Two additional non-confounded parameters identified were nacelle airflow te

during Phase 1 also highlighted the significance of hot surface reignition. 

mperature and fuel temperature. Testing 
94 History of Fire Suppression in Aircraft 

Table 2–12. Halon Alternative Fire Suppressants Identified for Phase 2 Evaluation. 

Engine Nacelle Dry Bay 

Pentafluoroethane (HFC-125) Octafluoropropane (FC-218)

Chlorotetrafluoroethane (HCFC-124) Pentafluoroethane (HFC-125) 

Trifluoroiodomethane (CF3I) Heptafluoropropane (HFC-227ea)

Heptafluoropropane (HFC-227ea) Trifluoroiodomethane (CF3I)

2.5.2 Phase 2 – Operational Comparison of Selected Extinguishants 

The fire suppressants identified during Phase 1 as the most promising halon alternatives for engine 

nacelle and dry bay fire suppression applications were subjected to additional testing during Phase 2.126

The DOX methodology was again employed to further evaluate effects of the most significant parameters 

identified during Phase 1 for each compartment type. Additional factors were also included based on 

discussion with aviation fire suppression experts. For dry bay fire testing, two additional factors 

evaluated were fire suppression agent container temperature and clutter (obstructions in the dry bay

inhibit suppressant distribution). For nacelle fire testing, four additional factors were evaluated: fire 

location in the nacelle, fuel type, nacelle mass airflow rate, and fire suppression agent container 

temperature. Table 2-13 lists, in order of significance, the factors found during Phase 2 testing at 

WPAFB to influence fire suppression the most in each compartment type. Additional testing was 

conducted by NIST to further evaluate fire suppression efficiency, stability during storage, safety

following discharge, agent discharge behavior and performance, and interaction with metal fires.

Techniques for real-time concentration measurement were also developed along with guidance for engin

nacelle fire suppression design and certification.128,13

 The combined outcome from the WPAFB and NIST 

efforts resulted in the down-selection to pentafluoroethane (HFC-125) as the optimal near-term halon

alternative fire suppression agent for both engin

,127 

 that

e

cations. This chemical was 

taken into Phase 3 for detailed evaluation. 

Table 2–13. Most Significant Fire Suppression Parameters Identified 

During Phase 2. 

e nacelle and dry bay appli

Engine Nacelle Dry Bay 

Surface Temperature (39 %) Compartment Volume (45.3 %) 

Fire Suppression Agent (15 %) Fire Suppression Agent (23.6 %) 

Nace ion

Interaction (14.3 %) 

lle Airflow Temperature (10 %) Compartment Volume and Locat

Surface Temperature and Nacelle Airflow 

Temperature Interaction (6 %) 

Fire Location (4 %) 

Fire Suppressant Bottle Location (5.5 %) 

2.5.3 Phase 3 – Establishment of Design Criteria Methodologies 

During Phase 3, extensive engine nacelle and dry bay fire testing was conducted using HFC-125. The

DOX methodology was again employed to evaluate effects of the most significant parameters identified 

during Phase 2 for each compartment. Extensive statistical analysis of the test data was conducted to

develop design models (equations) for aircraft engine nacelle, APU and dry bay applications.90 The
Halon Alternative Technology Development Program (TDP) 95

equations were developed to assist designers in sizing nacelle and dry fire suppression systems using 

HFC-125. Table 2-14 summarizes the variables that are to be taken into consideration when applying 

each of the equations. 

Table 2–14. Fire Suppression Parameters in Design Guidance Developed 

from Phase 3. 

Engine Nacelle Dry Bay 

Surface Temperature 

(Set at 454.5 °C) 

Location – Impact Angle of Ballistic 

Projectile Relative to the Horizon 

Nacelle Airflow Temperature Dry Bay Free Volume 

Nacelle Mass Airflow Rate 

Fuel Type 

External Airflow Rate 

Evaluation of data from engine nacelle fire testing highlighted the significance of engine surface

ne that would provide a best fit to the 

enta face temperature was 454.4 °C (850 °F) and 

portant variable and 

is one of the bay model. 

During desi rovides an 

estimate of suppressa to re. Pri evelopment of this model 

there had been no halon 1301 dr stems, though 

analysis of result indicated a halo en 3 % and 9 % 

for a protec inating fire detection sensor, and a range between 6 % 

and 12 % fo g sensor.1

temperature. Various models were developed to converge on o

experim l data. Curve fits from test data in which the sur

lower better modeled the overall test results than curve fits that included test data in which the surface 

temperature exceeded 454.4 °C. This was attributed to the variability induced from hot surface reignition. 

Applying the HFC-125 engine nacelle fire suppression design model is a two-step process. The first step 

is to calculate the HFC-125 concentration required to certify the system. The equation bounds the 

resultant concentration to either a minimum of 14.6 % and a maximum of 26 %. Embedded within the 

concentration equation is a factor for a nacelle hot surface temperature of 454.5 °C. The calculated 

concentration is then used an input variable in a sizing equation to estimate the quantity of HFC-125 

needed to provide the needed concentration for certification. Though the sizing equation is similar in 

structure to the current design guidance for halon 1301 in that it contains nacelle mass airflow and nacelle 

free volume as variables, the testing performed under Phase 3 resulted in this two-step design guidance 

that includes concentration based on values of nacelle-specific parameters. Previous analyses of the 

current halon 1301 design guidance had indicated that the concentration variable may have been 

considered, but it may have been embedded with the concentration value fixed at the halon 1301 

flammability limit of 6 %.63,129

During dry bay testing, all ballistic shots were initially horizontal with the projectile entering the dry bay 

compartment from the side. However, additional consideration was given to the fact that some aircraft 

have belly dry bay compartments. This resulted in testing of vertical shots entering from the bottom of a 

dry bay. This testing and subsequent data analysis proved that shot angle was an im

 variables that must be taken into consideration when applying the HFC-125 dry 

gn of an HFC-125 dry bay fire suppression system, the dry bay design model p

nt quantity needed suppress a dry bay fi or to the d

design guidance for the sizing of 

 from a previous test effort

y bay fire suppression sy

s n concentration range betwe

tion system incorporating a non-discrim

r a system incorporating a discriminatin 7
96 History of Fire Suppression in Aircraft 

2.5.

Nacelle/APU F ions 

As the DoD has r for aircraft fire suppression applications, the impact of what 

was learned du bsequent airc d testing efforts 

as become clear. Application of the outcomes from the TDP would result in halon alternative fire 

suppression systems that, relative to halon 1301 systems, would: 

• reased size of the 

bottle required to store the suppressant; 

fire bottles, primary and reserve, each containing 0.9 kg (2 lb) 

of halon 13 he AH-1Z/ 

UH-1Y, incl ification of 

a nacelle fire suppre s n partm erature and suppressant 

distribution can b se aircraft were FC-125 as the 

halon-alternative on of the ana needed HFC125 mass qua ed for th lb) of halon 

1301 would have been required to protect the nacelles, but these aircraft now have a single fire bottle 

containing e discharge system. This was the mass quantity 

determined to b centration requirement based on application of the TDP 

concentration equatio to a need to only discharge a single fire bottle on the legacy 

ircraft, the upgrade aircraft HFC-125 suppressant quantity represents an increase of over 2.7 times the 

potential halon 1301 quantity required and 4 times the halon 1301 installed on legacy aircraft. 

lon 1 for the 6 % volumetric 

4 Impact of Halon Alternative Fire Suppression to the Aircraft 

ire Suppression Applicat

elied heavily on halon 1301

ring the TDP as well as from su raft-specific development an

h

Weigh more, due to both the increased mass of suppressant and the inc

• Be more costly to optimize via enhanced distribution, which itself may lead to an incremental 

weight increase; or 

• Cost more to support over the lifetime of an aircraft, e.g., increased weight translates to 

increased materials required for fabrication as well as increased fuel consumption and thus 

increased cost. 

For example, on the V-22 fire door, actuators will close off two of three air inlets prior to discharge of the 

nacelle fire suppression system, which substantially reduces airflow through the nacelle. It was estimated 

that approximately 1.07 kg (2.35 lb) of halon 1301 would be required to protect each V-22 engine nacelle 

under this reduced airflow condition, whereas a fire bottle containing 2.72 kg (6 lb) of HFC-125 is 

installed for each nacelle for this same condition (2.55 times the estimated halon 1301 quantity).130 At the 

time of design and qualification of the V-22 nacelle fire suppression system, the TDP design equations 

had not yet been developed. However, the then-available mass equivalence ratio for HFC-125 mass 

quantity relative to halon 1301 and a flame suppression number (FSN) were applied in the design analysis 

for the V-22 system. 

Legacy AH-1W/UH-1N aircraft have two 

01 for engine nacelle fire protection. The upgraded versions of these aircraft, t

ude exhaust suppression systems. Simply adding such systems can require requal

ssion system a acelle airflow, com ent temp

e impacted. However, the also required to implement H

 fire suppressant. Examinati lysis approach to determine the

ntity, which was similar to that us e V-22, suggested that 1.36 kg (3

131 3.72 kg (8.2 lb) of HFC-125, a singl

e necessary to meet a con

n. Thus relative 

a

For ha 301, peak flammability limit is indicated by the FAA as the rationale 

concentration certification requirement. What the preceding discussion of halon alternative 

implementation indicates, and what Figure 2-62 illustrates, is that implementing a chemical halon 

alternative for nacelle fire suppression, in this case HFC-125, is likely to require well over twice the mass 

quantity of suppressant relative to halon 1301, if typical nacelle fire suppression system design practices 

are followed. (Figure 2-62 provides a graphical comparison of the current halon 1301 certification 
Halon Alternative Technology Development Program (TDP) 97

requirements, the minimum and maximum HFC-125 design equation concentrations and the FAA 

HFC-125 minimum performance standard vs. various published peak flammability limits for both halon 

1301 and HFC-125.) 

ly entail using design guidance (i.e., equations) to estimate suppressant 

mass quantity needed, conducting analysis of nacelle airflow characteristics for the purposes of designing 

Figure 2-62. Comparison of Halon 1301 Certification Requirement and HFC-125 Design 

Equation Limits vs. Published Flammability Limits.xxxiii

Typical design practices usual

the suppressant distribution system and discharge location or locations within the nacelle, and then 

performing qualification tests and, if necessary, adjusting the discharge locations in order to pass 

qualification; i.e., try different nozzles on the ends of the distribution tubing or changing the orientation of 

the discharge locations. Though the systems may then meet certification requirements, the entire process 

can result in non-optimized suppressant distribution, which is illustrated in Figures 2-63 and 2-64. 

 

xxxiii In Figure 2-62, the n-heptane flammability limit is from Reference 65. i-Butane, Methane, Propane (max) and Propane 

(min) flammability limits are from Standard for Clean Agent Fire Extinguishing Systems, NFPA 2001, 2000 Edition. 

Propane (TDP) flammability limit is from Reference 13. The WADC Critical Saturation Value is from Reference 105. 

TDP Design Equation Min and Max values are from Reference 90. It is interesting to note that the same halon 1301 

volumetric concentration determined by the WADC in 1959 was also indicated in later work described in Dyer, J.H., 

Marjoram, M.J., and Simmons, R.F., “The Extinction of Fires in Aircraft Jet Engines – Part III, Extinction of Fires at Low 

Airflows,” Fire Technology, 13, 126-138 (1977). 

0

2

4

6

n-Heptane

(PRF)

i-Butane Methane Propane

(Max.)

Propane

(Min.)

Propane

(TDP)

1958 WADC

"Critical

Saturation

Value"

Current

Aviation

Cert. Reqt.

Min.

TDP

Design

Equation

Min.

TDP

Design

Equation

Max.

FAA

Minimum

Performance

Standard

8

10

12

14

16

olumetric Concen

18

20

22

24

26

28

V tration (%)

Halon 1301

HFC-125
98 History of Fire Suppression in Aircraft 

tion. 

Figure 2-63. Example of Non-Optimized 

Halon 1301 Nacelle Concentration 

Distribu

Figure 2-64. Example of Non-Optimized HFC-125 Nacelle Concentration Distribution. 
Halon Alternative Technology Development Program (TDP) 99

In the case of the F/A-18E/F aircraft, optimizing the nacelle fire suppression system distribution resulted 

in the fielding of a nacelle fire suppression system with an HFC-125 mass quantity (3.17 kg, 7.0 lb) only

1.27 times of that required for the halon 1301 system (2.49 kg, 5.5 lb) on legacy F/A-18 aircraft. While it 

has been long promoted that the distribution system itself is probably the most important single factor in

ystem design,

132 s the distribution system design on the F/A-18E/F resurrects the concept of increased 

dwell time within the nacelle after discharge. This implementation parallels a recommendation following 

detailed review of the F-22 full-scale engine nacelle fire test program to maintain suppressant inerting

concentrations for an appropriate duration by optimizing delivery and distribution.133 The F-22 is

discussed later in this section. Though the F/A-18E/F system is an HRD system, the suppressant dwell

time throughout the entire nacelle is similar to the duration requirement for historical conventional system 

designs in that the dwell at or above the HFC-125 critical suppressant volume fraction (8.7 %) is 

maintained for at least 2 s. However, in the portion of the nacelle for which there is a hot-surface ignition 

threat the suppressant concentration was at or above the flammability limit for at least 0.5 s. An example 

of this concept is shown in Figure 2-65. Strict application of the HFC-125 concentration equation for the 

F/A-18E/F would have imposed a certification requirement of 18.5 % volumetric concentration

throughout the nacelle for 0.5 s minimum.

47 In the case of the F/A-18E/F aircraft, it was necessary to 

optimize suppressant distribution to utilize the physical location of the fire bottle, which was to remain 

unchanged from the legacy F/A-18C/D design unless significant and costly aircraft structural

modifications were pursued. It is interesting to note that even if an 18.5 % volumetric concentration 

requirement were imposed, the design guidance for the HFC-125 equations indicates that they are 

intended to provide protection for fire events not subject to hot surface reignition.90

The F/A-18E/F experience is an example of aircraft program managers trading the impacts associated

with various design options. These trades also take into consideration the amount of resources available

to evaluate trade options adequately. Thus, limitation of such resources for another aircraft program

manager may only permit traditional design practices to be applied. However, either scenario necessitates

that the fire suppression s

d

ystem design be optimized to the greatest extent practicable. Chapter 8 provides 

etailed discussion of the CFD analysis tool developed under the NGP, which is intended for use by

ai

Another issue that was investigated during the F/A-18E/F testing efforts relates to the use of the Statham 

luation

was that such equipment may be a good technique for suppressants that chemically interact with the 

ombustion process, such as halon 1301, but may be limited in use with suppressants such as HFC-125,

which physically interact with the combustion process. Thus following traditional design practices, 

han

is a iled

info sor

(DIR CS-2

can

rcraft nacelle fire suppression system designers to optimize suppressant distribution. 

and Halonyzer suppressant concentration gas analyzers used currently to certify nacelle fire suppression

systems. The issue of response time of such equipment was investigated during the TDP, and during the 

F/A-18E/F efforts, testing was conducted to show that such equipment may miss the concentration levels 

that actually result in fire suppression.134 That this could occur had been noted in early application of the

Statham analyzer, for which it was concluded that as the suppressant-air mixture was drawn into a

sampling tube, normal gas diffusion would tend to level peak concentrations if the peak is preceded and

proceeded by lower concentrations.105

 A qualitative conclusion derived from the F/A-18E/F eva

c

HFC-125-based nacelle fire suppression systems may then be designed to accommodate more mass t

ctually required. The NGP has transferred to the DoD test and evaluation community deta

rmation for the development and use of a Differential Infrared Rapid Agent Concentration Sen

RACS-2), which is described further in Chapter 5. Application of CFD modeling and DIRRA

be used to resolve these issues in nacelle fire suppression system design. 
100 History of Fire Suppression in Aircraft 

TDP HFC-125 

Critical 

Suppressant 

Volume 

(a) Concentration throughout the Entire Nacelle 

(b) Concentration at Locations of Hot Surface Reignition 

Figure 2-65. Examples of HFC-125 Nacelle Concentrations. 

TDP HFC-125 

Flammability 

Limit 
Halon Alternative Technology Development Program (TDP) 101

Another fire suppression methodology that was developed concurrent with the TDP and subject to HASG 

oversight was the solid propellant gas generator (SPGG), which disperses an inert gas mixture of 

nitrogen, carbon dioxide, and water vapor. Similar to gas generators developed for automobile airbags, 

these devices rapidly generate large quantities of this inert gas mixture, which results from internal 

combustion of multiple pellets (or grains) of the propellant. The mixture is very hot upon exit from the 

device but cools rapidly as it expands within the compartment into which it is discharged. Full-scale 

engine nacelle fire suppression testing for the USAF F-22 evaluated HFC-125 along with inert and 

chemically-active gas generator technology.56 The USN had previously evaluated the inert gas generator 

technology for nacelle fire suppression for the F/A-18E/F and demonstrated that similar mass quantities 

of halon 1301 and the inert solid propellant would suppress fire and prevent reignition.135 During testing 

on the F-22 program, distribution lines from the generators to the nacelle would become white hot during 

discharge and for a brief period thereafter. In testing performed by the USN using a “single grain” inert 

gas generator, the same effect of heating of the distribution line during discharge was also noted. Placing 

inert gas generators within a nacelle and thus eliminating the need for distribution lines had been 

previously considered impractical due the potential degrading effect of the nacelle operating temperature 

environment on the life of the solid propellant. Inconel distribution lines were demonstrated to not melt 

but would still become white hot. The NGP subsequently developed and performed evaluations of solid 

propellant formulations that generate cooler effluent that is also more efficient at fire suppression 

(Chapter 9). 

It was during the F-22 nacelle fire suppression evaluations that these newer, chemically-active gas 

generator propellant formulations were tested in a full-scale application for the first time. In general at 

maller than that of 

HFC-125. It was also noted that 8 s after suppressant discharge, the concentrations in this zone remained 

least 25 % to 50 % less chemically-active propellant mass was required to suppress nacelle fires relative 

to the amount of inert propellant mass. For one formulation over 70 % less mass relative to the inert 

propellant provided successful fire suppression. However, the biggest surprise from the F-22 evaluations 

was that the same mass quantity of halon 1301 and HFC-125, 6.4 kg (14.1 lb) each, suppressed the worstcase hot-surface reignition fire threat condition in the F-22 nacelle.133 Though the suppressant mass was 

similar, in-depth analysis of the suppressant concentrations at the fire zone revealed that the inerting 

concentration (flammability limit) of halon 1301 was approximately a factor of two s

above the flammability limit. This 8 s dwell correlated with the criterion established for successful fire 

suppression, which was no occurrence of reignition within 8 s after suppression.133 Based on this testing, 

the USAF implemented nacelle fire suppression for the F-22 aircraft using 14 lb of HFC-125. 

The U.S. Army had also conducted an extensive halon alternative test and evaluation effort for the 

Comanche aircraft program. That effort was still in progress when the Comanche program was cancelled 

by the DoD. Extensive work had been done to replicate the nacelle conditions during flight modes, create 

the nacelle fires for those conditions, evaluate the potential hot-surface-reignition threat, and determine 

the agent quantity necessary to ensure suppression. Requirements for the system included reserve fire 

suppression capability. HFC-125 and, to a lesser extent, inert gas generators were evaluated. Since the 

majority of the testing had been conducted using HFC-125, it was selected as the agent for use in system 

optimization testing that was to be performed. At the time of program cancellation, no HFC-125 mass 

quantity had been established that would suppress fire under all test conditions, and the program had 

begun to evaluate trades related to retaining the reserve fire suppression capability, certification metrics 

and testing, and “fire-out” success criteria in testing.136 Currently, the Halon Replacement Program of the 
102 History of Fire Suppression in Aircraft 

U.S. Army Program Executive Office, Aviation, is evaluating use of HFC-125 as the halon replacement 

on CH-47 Chinook helicopters. 

As of the writing of this book, halon alternative testing by the Fire Safety Branch at the FAA Hughes 

Technical Center had recently been completed. That testing evaluated HFC-125, CF3I and, more recently,

FK-5-1-12 or dodecafluoro-2-methylpentan-3-one (CF3CF2C(O)CF(CF3)2) for nacelle fire suppression 

applications in commercial aircraft. Findings to date of notable interest for DoD consideration are 

concentrations required to suppress spray and pool fires and an overpressure phenomenon. Contrasting 

results from previous testing, higher HFC-125 and CF3I concentrations have been required for 

suppression of nacelle spray fires than pool fires. Also, HFC-125 and another halon alternative tested,

bromotrifluoropropene (2-BTP), were observed to produce overpressure p equent to

reignition within the test fixture flow path. Initial conclusions were that each suppressant appeared to act 

as a fuel in some instances, and that the phenomenon was not 100 % relia erpressure

phenomenon had also been observed with HFC-125 and 2-BTP in cargo com fire suppression

testing. During simulated aerosol can explosion tests conducted to evaluate explosion suppression

performance for aircraft cargo compartments, tests were conducted to provide 2-BTP volumetric 

concentrations of 3 %, 4 %, 5 %, and 6 %. An overpressure occurred in each test – the associated 

overpressures were 530 kPa (63 psig), 530 kPa (63 psig), 780 KPa (100 psig), and 7.3 kPa (93 psig), 

respectively. 2-BTP enhanced the explosion event with as much as 4 times greater pressures than the

n

was elow 11.0 %. It doubled

the blast pressure pulse peak, and it produced explosion overpressures of 4460 kPa (53 psig) in tests that

provided concentrations of 8.9 % and 11 % by volume. No explosion event was observed when the 

s

mo I and 

Dry

r use 

in dr ors. The

F-22 testing also evaluated a pyrotechnically-augmented liquid agent system (PALAS) that utilized 

HFC-227ea. The F-22 testing was specific to the F-22 main landing gear dry ba

testing evaluated wing dry bays. Figure 2-66 shows a summary of results from the C-130J testing.

Results from the F-22 testing are summarized in Table 2-15. These test programs provided the first fullscale comparisons of gas generator fire suppression effectiveness relative to near-term chemical halon

alternatives identified during the TDP (HFC-125, HFC-227ea). Both of these test programs clearly

showed that on a mass basis, less solid propellant was required to suppress a dry bay fire relative to the 

hemical alternatives tested, particularly when a chemically-active propellant was used. During the F-22 

testing it was found s required to effect

suppression as compared to the inert propellant. 

 

henomena subs

ble.

137 The ov

partment

u suppressed event and 23 times greater than the halon 1301 benchmark concentration (2.5 %). HFC-125 

also observed to enhance the explosion event when its concentration was b

concentration of HFC-125 was 13.5 % by volume.138 Bromotrifluoropropene evaluation wa

discontinued in the nacelle fire test program. The FAA minimum performance standard (MPS) for

certifying use of HFC-125, CF3I or, FK-5-1-12 in nacelle fire suppression applications is now being 

developed. The reader is referred to the Fire Safety Branch at the FAA Hughes Technical Center to learn 

re, particularly in regard to flame attachment behavior observed during testing with CF3

FK-5-1-12.xxxiv

 Bay Fire Suppression Applications 

Both the USAF F-22 and C-130J LFT programs evaluated HFC-125 and gas generator technology fo

y bay fire suppression. Both programs evaluated inert and chemically-active gas generat

y whereas the C-130J

c

that less than half of the chemically-active propellant wa

xxxiv

 http://www.fire.tc.faa.gov/ 
Halon Alternative Technology Development Program (TDP) 103

Figure 2-66. Summary of HFC-125 and SPGG Fire Suppression Results from 

C-130J Live Fire Testing.139

Table 2-15. Minimum Fire Suppressant Quantities from F-22 Main Landing Gear Dry Bay 

Live Fire Testing, 150-grain Fragment Threat and Jet Fuel Except 

Where Noted.140

Suppressant 

(a) 

Delivery 

System 

5 ms 

Delay 

5 ms and 105 

ms Delay 

400 ms 

Delay 

400 ms and 

480 ms Delay 

500 ms 

Delay 

HFC-125 Nitrogen 

Pressurized 

Bottle 

0.68 kg (b) N/A N/A N/A N/A 

HFC-125 Nitrogen 

Pressurized 

Bottle 

0.91kg N/A N/A N/A 1.5 lb 

HFC-125 Nitrogen 

Pressurized 

Bottle 

N/A N/A N/A N/A 1.13 kg (c) 

HFC-227ea PALAS 1.11kg 

Fan 

N/A N/A N/A 

Nozzle 

N/A 

HFC-227ea PALAS 0.89 kg 

Radial 

Nozzle 

N/A N/A N/A N/A 

FS01-40 Inert SPGG N/A 2 x 0.52 kg 

(1.03 kg) 

N/A 2 x 0.21 kg 

(0.42 kg)

N/A 

FS01-40 Inert SPGG N/A N/A N/A 2 x 0.35 kg 

(0.70 kg) (c)

N/A 

PAC-3302 Chemically 

Active SPGG 

N/A 2 x 0.19 kg 

(0.38 kg)

0.19 lb N/A N/A 

(a) A total of 24 tests were conducted using HFC-125, 21 tests were conducted using HFC-227ea, 19 tests were 

conducted using inert SPGGs, and 10 tests were conducted using chemically-active SPGGs. 

(b) Flammable fluid tested was hydraulic fluid. 

(c) Ballistic threat was 23 mm HEI. Only HFC-125 and inert SPGGs were evaluated against this threat. 
104 History of Fire Suppression in Aircraft 

Figure 2-67 illustrates the rapidity with which fire suppression is achieved using gas generators for 

suppression of ballistically-induced fires within an aircraft dry bay compartment. 

Figure 2-68. V-22 Inert Gas Generator Fire Suppression System.12

Figure 2-67. Time Sequence of Dry Bay Compartment Fire Suppression by Inert Gas 

Generators, Complete Suppression within 600 ms.139

The inert gas generator technology has been implemented for dry bay fire suppression on the USN 

F/A-18E/F and MV-22 and on the USAF CV-22; the chemically-active technology was not mature at the 

time that these systems were required to be developed. Figure 2-68 provides an illustration of the system 

implementation on the MV-22, without auxiliary wing fuel tanks. On the V-22, this system is referred to 

as the wing fire protection system (WFPS). The gas generators provide ballistic dry bay fire protection 

for the various wing dry bays and both ballistic and safety fire protection in the mid-wing compartment, 

which contains an APU, gearbox, shaft driven compressor, hot air ducting, electrical generators, and other 

equipment. Discharge of the generators in the mid-wing compartment is sequenced to preclude 

overpressurization of the compartment. During qualification testing of the WFPS for the mid-wing 

compartment it was observed that, even though the fires were quickly extinguished, it was noted upon 

review of the test videotapes from each of the tests that the fire location shifted position around the 

compartment, i.e., the fire would be pushed to different locations within the compartment as each of the 

gas generators would discharge until combustion could no longer be sustained.141 

Forward D

GG

D

GG

D

GG

D

GG - Gas Generator

GG D D GG GG

D

GG

D

GG

D

GG

D

GG

D

GG

D

GG

GG

GG

GG

GG

D D

D

D

D - Detector

GG
Halon Alternative Technology Development Program (TDP) 105

The V-22 WFPS development effort included a requirement for the capability to measure gas 

concentration levels within the midwing compartment to demonstrate adequate distribution of the inert

gases within the compartment, though no specific gas concentration levels and duration time were

pass-fail criteria. A gas sampling system was assembled using fast-response CO2 and oxygen sensors, 

and gases were drawn from the compartment through 5 m long, 1 mm inside-diameter Tygon tubing. 

Response time of the concentration measurement sensors was on the order of 120 ms for the CO2 senso

and 50 ms for the oxygen sensors. In general, lower CO2 was measured at locations closest to airflow

inlets and was greatest at locations near the inert gas generators and where air was drawn out 

partment, though peak CO2 concentrations between 13 % and 26 % were recorded.141

concentration measurement technique was also utilized during the F-22 engine nacelle test series in an 

mpt to correlate fire suppression with oxygen and CO2 concentrations within the nacelle. Analys

the data could not support any correlation.56 Detailed analysis of these measurements suggested that th

instrumentation could have been contaminated by particulates, water vapor, condensed water vapor, or 

drocarbon vapor. The need to better characterize the mechanisms by which gas generator as 

 set as 

rs

of the

com This 

atte is of 

e 

hy well as 

ther fire suppression technology provides fire suppression in a real-world environment such as an engine 

Application, Recirculating Pool Fire (TARPF), agent

effectiv

I

e

could require e protection,

t lo pact (i. it visi ver t of a ft

beco ified wn in F -69, ion t f new -active solid

propellant formulations under the NGP has demonst at least a 5 reduction i pellant m

ass.

142

o

nacelle led to NGP efforts to develop the Transient 

eness screen, which is described in Chapter 6. 

nert gas generators have thus far been proven to be effective though they currently require replacement

very five years. On the V-22 there are 17 generators. Given that similarly sized or even larger aircraft

similar or even larger numbers of these devices, if implemented for dry bay fir

he weight and co

m agn

gistics im e., un sts, spares pro

fire suppress

oning) o

e

he lifetime n aircra

es m . As sho igure 2 sting o er chemically

rated 0 % n pro ass. 

Figure 2-69 NGP Testing Results Comparing Solid Propellant Inert Gas Generator 

ly Active Gas Generator Propellant M

C

Inert Gas Propellant 

urrent lded

Propellant 

Formulations 

Evaluated by

the NGP 

ly Fie

Propellant Mass vs. Chemical
106 History of Fire Suppression in Aircraft 

Aircraft dry bay fire testing conducted by the USN using one of the newer propellant formulations has 

approaches are also being investigated. Chapter 9 describes the inert gas gener

demonstrated on-aircraft fire suppression with such reduction in propellant mass.143,144 However, 

application of single passive extinguisher (SPEX) concepts has received increased attention. SPEX 

ator technology and the

technologies and techniques developed under the NGP related to improving their performance. 

Ullagexxxv

HFC-227ea, HFC-125, and pentane using the LFE. The greatest concentrations of each were as follows: 

FC-218, 38. .5 %

(0.64 mas nts of each 

the simu

Exam out FC-218 

as having the arison of 

required to a ount 

As of the wri e 

suppression t

concepts focus on system simplification, eliminating the subsystems normally associated with active fire

suppression systems (i.e., no fire detection system, no crew alerting and controls, and no system health 

monitoring). These concepts are a recent emergence within the DoD and testing of several concepts has

been sponsored by the Joint Aircraft Survivability Program Office (JASPO). Concepts evaluated to date 

have been for dry bay fire suppression,145 and one concept using a plastic, heat sensing tube connected to

a low pressure container with monnex dry chemical extinguishant is planned for implementation on the 

V-22 for wheel well dry bay fire protection. Initially, chemically-active gas generators were one of

several options being considered for dry bay fire suppression on the USN P-8 MMA,34 but SPEX

Explosion suppression experiments were conducted to determine the ability of the SCS and LFE reactive 

ullage protection technologies to suppress ballistically-induced ullage fire/explosion events, with FC-218,

HFC-227ea, HFC-125, and pentane tested using the LFE and just pentane using the SCS. Ballistic threats 

tested were single 110-grain fragment, 12.7 mm API, and 23 mm HEI. Detection systems were not 

integral to the tests and thus the results do not take into consideration detector-related delays. The tests 

were sponsored by the Joint Technical Coordinating Group for Aircraft Survivability (JCTG/AS), now 

known as the Joint Aircraft Survivability Program (JASPO), and were conducted by the USN. 

SCSs filled with pentane to provide a 47 % concentration (approximately 0.54 mass fraction) suppressed

explosions in a 0.85 m3

 (30 ft3) volatile ullage simulator when initiated by both a single 110-grain 

fragment and a 12.7 mm API. Partial suppression only was realized against the 23 mm HEI. In the same

ullage simulator partial suppression only was realized against the 12.7 mm API in all tests of FC-218, 

8 % (0.67 mass fraction); HFC-227ea, 42 %, (0.67 mass fraction); HFC-125, 52

s fraction); and pentane, 37.5 % (0.48 mass fraction). Testing of varying amou

suppressant allowed definition of the effect of agent concentration and mass fraction on peak pressure in 

lator, as shown in Figure 2-70. 

ination of the concentration data revealed that, of the suppressants tested, pentane edged 

best suppression efficiency, followed by HFC-227ea, and then by HFC-125. Comp

peak explosive pressure as a function of suppressant mass fraction revealed that the amount of pentane

chieve the suppression level of FC-218, HFC-227ea, and HFC-125 was 70 % of the am

needed for those three suppressants (i.e., 30 % less pentane was required). 

ting of this book, none of the DoD services has pursued additional evaluation of the reactiv

echnologies using halon alternative suppressants in aircraft fire suppression applications. 

 is section is based on Reference 33. Summary and Conclusions from that report are presented with minor editing for 

tion within this chapter.

xxxv Th

presenta
Halon Alternative Technology Development Program (TDP) 107 Average Peak Pressure - PSIG 

Agent Concentration, %

Forw

Army Blackhawk/Seahawk Program. UAVs are being utilized with greater frequency during military 

conflicts. A review of potential cost effective survivability enhancements identified that passive fire 

suppression for large category UAVs and active fire suppression for Unmanned Combat Aerial Vehicles 

Agent Mass Fraction, %

Average Peak Pressure - PSIG 

Figure 2-70. Effect of Suppressant Concentration and Mass Fraction on Peak Explosive 

Pressure for LFEs Pressurized to 1,000 psi, 12.7 mm API Threat and 3 % JP-4S.33

ard-Looking Considerations 

Major aircraft acquisition programs today include those for unmanned air vehicles (UAV), the USN P-8 

MMA program, the USAF-USN-USMC F-35 Joint Strike Fighter (JSF), and the joint USN and U.S. 
108 History of Fire Suppression in Aircraft 

(UCAV) are potential cost-effective survivability enhancements for these aircraft.146 Dry bay fire 

suppression is planned for the JSF. Howe aircraft, which typically is not provided 

with nacelle fire suppression. This is be ach for nacelle fire suppression first 

involves isolating the nacelle compartment, meaning the pilot shuts off fuel flow, and that the fuel for a 

nacelle fire is likely that provided to the engine to generate propulsion power for flight. Reduction in or 

loss of that flow itself can or will lead to loss of thrust and thus lift and flight control. However, given the 

lessons learned from the Southeast Asia conflict and that a few extra minutes of flight may reduce 

likelihood of pilot fatality or enemy capture, the DoD’s JASPO is currently sponsoring investigation and 

testing of automatic nacelle fire suppression concepts as a survivability enhancement, particularly for 

single engine aircraft applications.147

As long as the DoD components (Army, Navy, and Air Force) operate current and procure new manned 

multi-engine aircraft, there will be a requirement for engine nacelle fire suppression. Like

aircraft procurements, the Live Fire Test law will drive requirements for prevention and s

ballistically-induced fires. Aircraft fire suppression systems are categorized as mi

applications, for which continued use of halon by the DoD has been permitted by law. New production 

aircraft are required to implement a non-ODS alternative or have had executed an ODS waiver certifying 

that no viable alternative exists for the application presented by that aircraft. While currently-fielded 

aircraft would most certainly continue to operate using the fire suppressant (or suppressants) installed 

curr ks) 

that could craft. With 

arameters 

(KPPs) are met. These parameters include weight, performance (range, drag), reliability, and probability 

). When weight and/or cost are considered too high, the program manger will trade risk 

ver, it is a single-engine

cause that the typical appro

wise, for new 

uppression of 

ssion critical 

ently, there are potential uncertainties (e.g., global warming, depletion or destruction of halon ban

 impact the type of suppressant implemented in fire suppression systems on these air

respect to global warming, parties to the Kyoto Protocol, of which the U.S. is currently not a signatory, 

have developed amendments to reduce emissions of fielded HFC systems that are to become effective in 

2007.148, xxxvi As of the writing of this book, these requirements would not apply to currently-fielded 

HFC-based fire suppression systems on DoD aircraft, but DoD environmental activities are likely to 

monitor the effect of these requirements and whether it is beneficial for DoD to implement similar 

requirements. The commercial aviation sector is more sensitive to such influences, and it is conceivable 

that a commercial-derivative aircraft procurement could include a non-halon non-HFC based fire 

suppression system. 

Regardless of potential external regulatory influences, the aircraft program manager is constantly 

managing risks (i.e., weight, performance, cost, safety and survivability, to name a few). As for any 

subsystem on an aircraft, there are weight, performance, and cost trades associated with implementing a 

fire suppression system, and the program manger desires to implement the lightest weight, best 

performing, lowest cost system. He is concerned with ensuring that aircraft key performance p

of kill (Pk

associated with various options, including safety and survivability risks. Such trades receive increased 

attention and scrutiny when resources (funds) are constrained. For example, if a specific aircraft 

maneuver drives a worst-case nacelle airflow condition, which in turn drives high the suppressant 

 

xxxvi The Kyoto Protocol is an agreement made under the United Nations Framework Convention on Climate Change 

(UNFCCC). Countries that ratify this protocol commit to reduce their emissions of carbon dioxide and five other 

greenhouse gases, or engage in emissions trading if they maintain or increase emissions of these gases. The five other gases 

are methane, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons or HFCs, and perfluorocarbons or PFCs. The USAF 

CV-22 and F-22, the USMC MV-22, and the USN F/A-18E/F use HFC-125 in their nacelle fire suppression systems. HFC125 is also approved for use by DoD, as well as by the FAA, as a halon 1301 simulant for certification testing of halon 1301 

nacelle fire suppression systems. 
References 109

quantity for a fire suppression system, yet performing such a maneuver is a small exposure over the 

projected lifetime of the aircraft, the program manager may accept the safety risk associated with fire

occurrence during that exposure in order to proceed with a lighter weight fire suppression system.

eviously unavailable in making such trades. 

3 m Powerplant

Fires in Aircr 1943. 

4. Ball, R.E., nd Edition, 

Ameri

5. Preliminar Aeronautics 

Research Mem

6. Pesm

Comm

7. Hughes, D. , 

8 December 

8. y

Press, Washi

9. Tedeschi, M. and Leach, W., “Halon 1301 Fire Suppression System Effectivity Aboard U.S. Naval

Aircraft,” 1995, in Gann, R.G., Burgess, S.R., Whisner, K.C., and Reneke, P.A., eds., Papers from 1991-

2006 Halon Options Technical Working Conferences (HOTWC), CD-ROM, NIST Special Publication

984-4, National Institute of Standards and Technology, Gaithersburg, MD, 2006.

10. Electronic Communication (email) between Donald Bein, NAVAIR - Naval Air Warfare Center

Aircraft Division, Lakehurst, and Joseph Manchor, NAVAIR - Naval Air Warfare Center Weapons 

Division, China Lake, 15 August 2005, 15 September 2005, and 3 October 2005. 

11. Bennett, J.M. and Kolleck, M.L., “Assessing the Cost Impact of Halon Alternatives on Aviation,”

1997, in Gann, R.G., Burgess, S.R., Whisner, K.C., and Reneke, P.A., eds., Papers from 1991-2006 Halon 

Options Technical Working Conferences (HOTWC), CD-ROM, NIST Special Publication 984-4, 

National Institute of Standards and Technology, Gaithersburg, MD, 2006. 

Likewise, from a survivability perspective, the program manager may trade passive protection techniques 

against active techniques for dry bay fire protection. He may find that the aircraft vulnerable area 

requirement can be met by implementing passive protection techniques at the risk of not protecting

another compartment that requires an active system and yet still meet the aircraft Pk KPP. That the NGP

has developed tools such as CFD modeling for design, development and optimization of nacelle fire 

suppression systems and cost modeling for assessing the life cycle cost benefit of providing nacelle and 

dry bay fire suppression systems should benefit the program in conducting risk trades. Better performing 

enhanced powder panels, chemically-active gas generators, and hybrid fire suppression techniques should 

provide the program manager options pr

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