A REVIEW OF THE HISTORY OF FIRE SUPPRESSION ON U.S. DOD 

AIRCRAFT 

Donald Bein 

Naval Air Systems Command (NAVAIR) 

System Safety, Code 4.1.6.1 

Highway 547, B562-2 

Lakehurst, NJ 08733 

Telephone: (732) 323-1660 

Fax: (732) 323-5269 

E-mail: donald.bein@navy.mil 

ABSTRACT 

A review of the history of fire suppression on U.S. Department of Defense (DoD) aircraft is 

presented to provide a context against which the findings of the Next Generation Fire 

Suppression Technology Program (NGP) can be assessed. These findings are to be published 

later this year (2006) in the NGP final report. Aircraft fire suppression applications reviewed are 

powerplant compartments, which include engine nacelles and auxiliary power unit (APU) 

compartments, dry bay compartments, and fuel tank ullage (wet bays). The evolution of engine 

nacelle fire suppression system designs are presented, from “conventional” systems design to 

current high-rate discharge systems. Nacelle/APU fire occurrence and suppression system 

discharge is presented relative to altitude and temperature. Pilot response and system 

effectiveness are also discussed. The evolution of active dry bay fire suppression is also 

presented, though active systems dedicated purely to dry bay fire protection have not been 

fielded until the advent of the Live Fire Test legislation. Technologies and methods 

implemented previously and currently for fuel tank ullage fire suppression are then discussed. 

BACKGROUND 

As the Department of Defense’s (DoD) Next Generation Fire Suppression Technology Program 

(NGP) culminates its research efforts, it is prudent to capture the history of fire suppression on 

DoD aircraft. This is being done as part of the final report being generated for the NGP, since in 

the years to come, it is possible that it will become increasingly difficult to assemble such 

history. Such difficulty was apparent when attempting to research the basis for aircraft nacelle 

fire suppression systems during the conduct of the DoD’s Halon Alternative Technology 

Development Program (TDP), which was the program that preceded the NGP. This paper 

provides a synopsis of the history of fire suppression in DoD aircraft. For the more detailed 

account, the reader is referred to the NGP final report, which is to be entitled Advanced 

Technology for Fire Suppression in Aircraft and will be published later this year (2006). 

1
DISCUSSION 

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. An 

aircraft 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, such 

as a hot engine case, 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. 

SAFETY PERSPECTIVE 

From a safety perspective, fire is one of many events that can result in loss of an aircraft and/or 

fatalities, as illustrated in Figure 1. Previous studies [1,2] suggest that in commercial aviation 

fire accounts for less than 5% of commercial aircraft accidents and fatalities, as indicated in 

Figures 2 and 3. 

Catastrophic 

Mishap

Controlled 

Flight Into 

Terrain (CFIT)

Power Loss Stall Mid-Air 

Collision

Icing Fire

Figure 1. Safety Perspective - Example Breakout of Some Catastrophic Mishap Causal Factors 

Throughout aviation history the evidence is overwhelming that the predominant safety fire threat 

is within aircraft powerplant compartments, and of these compartments the greatest frequency of 

occurrence of fire occurs in the engine nacelle. Fire suppression systems are typically not 

provided on DoD single-engine aircraft, since the aircraft design approach to date in DoD and 

commercial aviation has been to first isolate flammable fluids from the nacelle prior to discharge 

of a fire suppression system. For a single-engine aircraft, this would effect immediate loss of 

thrust. For multi-engine aircraft the likelihood of a catastrophic event from a fire in one of the 

nacelles is a stack-up of several probabilities or likelihoods. If p(in-flight nacelle fire mishap) 

denotes the probability of a catastrophic event resulting from a nacelle fire (i.e., aircraft loss 

and/or fatalities), then the stack-up of events to realize the mishap are: 

2
10/226 or 4.4%

Figure 2. Study of Fatal Accidents in Worldwide Commercial Jet Fleet, 1987 – 2004 [1] 

(Fire-NI denotes non-impact fire events.) 

Veiligheidsstatistieken luchtvaart 1980-2001, Civil Aviation Safety Data

Figure 3. Study of Fatal Accidents in Worldwide Commercial Jet Fleet, 1980 – 2001 [2] 

3
p(fire occurs) AND [p(pilot isolates nacelle from flammable fluid AND fire persists) 

 OR p(failure to isolate nacelle from flammable fluid AND fire persists)] 

 AND [p(nacelle fire suppression system fails to suppress) 

OR p(nacelle fire suppression system fails to activate)] 

AND p(safe landing of aircraft is not executed) 

Summaries of DoD aircraft mishap causal factors similar to those shown in Figures 2 and 3 are 

not available readily nor are they constructed easily. However, during the Halon Alternative 

Technology Development Program (TDP), a review of the U.S. Navy (USN) mishap and fire 

incident data was performed [3,4] and showed that when fire occurred for either fixed-wing or 

rotary aircraft, only 9% of the events resulted in aircraft loss, as illustrated in Figure 4. When 

that data is reviewed further it can be shown that utilization of nacelle fire suppression systems 

represents an even smaller subset of the aircraft operational experience. The same review of 

Navy fire mishap and incident data also reaffirmed that safety-related fires occurred 

predominately in engine compartments [5], as illustrated in Figure 5. (Note that in Figure 5 

forty-nine percent (49%) of the fires represented in the electrical equipment bar 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 5, ECS is an acronym for Environmental Control System.) 

Aircraft 

Lost or 

Destroyed 

by Fire

9%

Aircraft Not 

Lost or 

Destroyed 

by Fire

91%

When fire occurred in rotary or fixed-wing aircraft:

Of this 91%, Engine/APU fire suppression accounted for:

• 13% of suppressed fixed-wing aircraft fires

• 15% of suppressed rotary aircraft fires

Of this 9%, failure of nacelle fire suppression

to suppress fire accounted for:

• 14% of fixed-wing aircraft losses

• 27% of rotary aircraft losses

Figure 4. USN Fire Mishaps and Incidents, Aircraft Destroyed 

or Not Destroyed Due to Fire, 1977-1993 [3,4] 

4
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

No significant occurrence of 

safety-related dry bay fires; no 

occurrence of safety-related 

ullage fire/explosion.

Figure 5. USN Fire Mishaps and Incidents by Aircraft Compartment, 1977-1993 [5] 

ENGINE NACELLE FIRE SUPPRESSION 

As early as 1922 there is National Advisory Committee on Aeronautics (NACA) reference [6] to 

implementation of engine compartment fire suppression, which consisted of a fire extinguisher 

within that compartment and was controlled from the pilot’s seat. Additionally, shutters were 

installed to eliminate external airflow into the compartment. No specific reference is provided 

as to the fire suppressant in this case, but one could speculate. The Naval Studies Board has 

reported that in the 1920s non-fluorinated halon agents were tried experimentally in engine 

nacelle extinguishers, but their use was abandoned by the U.S. military in favor of the noncorrosive carbon dioxide (CO2) [27]. Figure 6 shows examples of contemporary engine nacelles 

and nacelle fire suppression system installations. Generally, the system design philosophy is 

similar to that described back in 1922: the fire suppressant bottle may be external or internal to 

the compartment, doors and actuators for closing off compartment air flow may be installed, and 

discharge control is manual and is effected from the cockpit (pilot’s seat). In the right 

photograph in Figure 6 CAD is an acronym for cartridge actuated device, and the white box 

below it is an actuator for a door to close off airflow into the nacelle air inlet. 

Figure 7 shows examples of various nacelle/APU fire suppressant bottle installations. These 

may be located within compartments on aircraft like the nacelle, in which compartment airflow 

temperature can easily exceed 93.3°C (200°F), or like a wheel well compartment, in which the 

compartment temperature may be relatively similar to the outside air temperature (OAT). 

5
Halon 

1301 

Bottle 

Nacelle 

Air Inlet 

Distribution 

Line 

Distribution 

Outlet 

Fire 

Suppressant 

Bottle 

(Mounted in 

Compartment 

External to the 

Nacelle) 

Distribution 

Line within 

Nacelle 

Discharge 

Line from the 

Bottle

Distribution 

Line Outlet 

Nacelle 

Air Inlet 

Structure 

CAD 

Figure 6. Examples of Contemporary Engine Nacelles and Fire Suppression Systems 

(Photo of spherical bottles is from the “737 web site” and

used with permission by its author, C. Brady.)

Figure 7. Engine Nacelle/APU Fire Bottle Installations 

6
FIRE SUPPRESSION FOR AUXILIARY POWER 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). Figure 8 shows examples of 

APU aircraft installations. 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 in-flight 

engine shutdown. 

Light 

Switch 

APU Fuel 

Line 

Generator 

Oil Filler 

Fuel Filter 

Fuel 

Nozzles Upper 

Shroud

Bleed Air 

Valve Start Motor 

Oil Filter 

Oil Tank 

Bleed Air 

Manifold 

Exhaust 

Muffler APU 

(Photo of APU compartment above is from the “737 web 

site” and used with permission by its author, C. Brady.)

Figure 8. Examples of Contemporary Auxiliary Power Unit (APU) Installations 

ENGINE/APU FIRE SUPPRESSION SYSTEM DISCHARGE 

As indicated previously, discharge of engine nacelle and APU fire suppression systems is 

effected manually. Upon detection of fire in either compartment type, a fire warning is 

generated in the cockpit. The pilot will isolate fuel flow from the affected compartment, most 

often by pulling a T-handle-type control, as shown in Figure 9(a); rotation of the handle then 

arms the fire suppression system. On some aircraft, illumination of the handle is the visual fire 

warning indication in the cockpit. On other aircraft, there may be separate fire warning 

indicators. The pilot will then confirm that the fire condition persists through verification that 

the warning indication remains illuminated and may also check, if possible, for secondary 

7
indications, such as smoke of flammable fluid leaking from the affected compartment, or if 

correlating failure conditions are indicated in the cockpit, such as a failure indication within the 

fuel system. If fire persists the pilot will discharge the fire suppression system, which in Figure 

9(a) is accomplished by pressing the controls labeled 1 (for fire bottle number 1) and if needed 2 

(for fire bottled 2). This results in an electrical signal from the cockpit to the fire bottle, which 

initiates a pyrotechnic or cartridge actuated device (CAD) that ruptures a burst disc on the fire 

bottle allowing agent to be released. There also exists, but to a minimal extent, mechanical 

systems such as pulleys to effect burst disc rupture, as shown in Figure 9(b). 

(Photo of cockpit fire panel above is from the “737 web 

site” and used with permission by its author, C. Brady.)

(a) Electrical (b) Mechanical 

Figure 9. Engine/APU Fire Suppression System Discharge Control 

EVOLUTION OF NACELLE FIRE SUPPRESSION SYSTEMS 

The requirements for design 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 civil aeronautics administration (CAA) in 1943 [7] for use of 

methylbromide (halon 1001) and CO2 and was relative to mass airflow in the compartment and 

the number of cylinders in a radial cylinder engine installation. 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. 

During World War II the German Navy sponsored efforts by I.G. Farbonindustrie to develop an 

alternative to methyl bromide (halon 1001) due to its toxicity, which resulted in the development 

of chlorobromomethane (CB or halon 1011) in the 1939-1940 time period [8]. 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 

8
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 262, the first operational jet-powered 

fighter. Given the time the directive was issued, it is likely D-L did not come into widespread 

use before the end of the war. After the war extensive evaluation of halon 1011 was conducted 

within the U.S. and by 1950, the U.S. Air Force (USAF) required use of halon 1011 systems 

instead of CO2 systems in new aircraft and subsequently issued a specification for such systems 

[9]. Design guidance for use of halon 1011 in powerplant fire suppression system evolved as jet 

propulsion became more widespread; however, the conventional distribution system approach 

was still employed for halon-1011-based fire suppression systems. 

Techniques for effecting and assessing adequate distribution changed along with the evolution to 

jet propulsion powerplants. Testing conducted by he CAA compared fire suppression 

performance of conventional systems versus open-ended systems [10], 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 out 

of which the fire suppressant would discharge at a much higher rate. Rather than relying on 

plumbing to disperse the suppressant, dispersion would be effected by the turbulent mixing of 

the suppressant discharge jet and the nacelle mass air flow, with the suppressant stored under 

higher pressure conditions than had been done for conventional systems. Further testing by the 

CAA demonstrated that the HRD design required less halogenated fire suppression agent to 

suppress nacelle fires and simplified 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 [11], and 

that a “critical saturation value,” in percent concentration by volume, occurred between 50 and 

100 feet per second for the suppressants evaluated (halon 1011, halon 1301, and halon 1202). 

Today, HRD systems utilizing halon 1301 are the most prevalent nacelle fire suppression system 

implementation for engine nacelle and APU fire suppression on DoD and commercial aircraft. 

Figure 10 illustrates evolution of nacelle fire suppression systems. 

FIRE SUPRESSANT BOILING POINT 

The fire suppressant boiling point (Tb) of a fire suppression agent has been used as one of the 

criterion to guide the search for new halon alternative chemical fire suppressants under the NGP 

[12]. The Tb criterion was established as -40°C (-40°F). It was also one of the parameters 

considered during research efforts that identified pentafluoroethane (HFC-125) as the best nearterm alternative to halon 1301 for use in aircraft nacelle fire suppression system applications [13] 

and has since been implemented for nacelle/APU fire protection on newer model DoD aircraft 

(MV/CV-22, AH-1Z, UH-1Y, F/A-18E/F, and F-22). Previous research conducted by the NGP 

[14] identified the relationship between the climatic profiles that influence aircraft design and the 

relevance of the aircraft temperature envelope requirement for engine nacelle and APU fire 

suppression systems. The Tb of halon 1301, generally –58°C (-72°F) at atmospheric pressure, is 

consistent with severe-cold land environments and the low temperatures recorded at lower 

altitudes in the cold world-wide air environments [15], though other halon fire suppressants such 

as halons 1011, 1202 and 1211 are also currently utilized for nacelle fire suppression and have 

9
much higher boiling points. Boiling points for these suppressants are 66°C (151°F), 22.5°C 

(72.5°F), and -9°C (16°F), respectively. As shown in Figure 11, the challenge confronted by the 

NGP was that potential alternatives to halon 1301 other than HFC-125 were indicated to have 

higher boiling point characteristics. Given that higher-boiling-point halons have been fielded for 

years, it was questioned whether operational experience supported the 40°C (-40°F) criterion. 

This led the NGP to investigate DoD fire mishap and incident data to assess temperature 

conditions at which nacelle fire suppressant releases have occurred. 

Post WWII

Halon 1001, Turboprop

WWII

Proposed Dachlaurin System: 65% 

Halon 1011, 35% CO2

From: Hitchcock, E.B. and Scarlon, E.A., “Use of Monochlorobromomethane

by the German Navy and Air Force as a Fire Extinguishing Agent,”

U.S. Naval Technical Mission in Europe, Technical Report No. 276-45, 1945 

From: Civil Aeronautics Administration Technical 

Development Report 198 

Halon 1301 Today

No. 1 

Engine 

No. 2 

Engine 

APU 

Thermal 

Relief 

Outlet

Aft Bottle

Directional 

Control 

Valve

Forward 

Bottle 

Halon 1011, Turbojet

From: Fire Suppression Substitutes and Alternatives to Halon for 

U.S. Navy Applications, National Academy Press, 1997 

From: Civil Aeronautics Administration Technical 

Development Report 240 

Figure 10. Evolution of Engine Nacelle Fire Suppression Systems 

10
-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Vapor Pressure, psia (boiling point)

Temperature (°F)

Halon 1301 HFC-227 HFC-125 atm. press, 0 ft

alt agent 1 alt agent 2 alt agent 3 alt agent 4

alt agent 5 5K ft 10K ft 15K ft

20K ft 25K ft 30K ft 35K ft

NGP* 

BoilingPoint (Tb) 

Criterion

NGP* 

BoilingPoint (Tb) 

Criterion

Figure 11. Suppressant Boiling Point vs. Standard Atmosphere Pressure 

OUTSIDE AIR TEMPERATURE 

A review conducted within U.S. Army aviation of rotary aircraft fires between 1985 and 1995 

[16] had found that the lowest outside air temperature (OAT) reported was 0°C (32°F) and the 

highest reported was the 35°C (95°F). 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. In making this conclusion, Reference [16] did not specify 

the temperature scale. The use of term remotely has significance in that within DoD it relates to 

hazard rate of occurrence. During the time period of the U.S. Army study, aggregate rate of 

occurrence of in-flight rotary aircraft fires was 4.9 per million flight hours, a remote rate of 

occurrence per Reference [17]. Thus likelihood of a nacelle fire occurring at lower outside air 

temperatures or in an extremely cold environment and resulting in loss of aircraft would be an 

even lower (improbable) likelihood. 

Publicly-Available Data 

Prior to investigating the DoD operational experience, the NGP surveyed publicly available data 

related to aircraft operation and outside air temperature (OAT). This was done to establish 

confidence that conducting the review of the combined DoD data, that is U.S. Army, USN, and 

USAF, would likely provide useful information. Figure 12 plots the then-available data points 

from the National Transportation Safety Board (NTSB) database that included both OAT and 

altitude. (Note: a data point representing the TWA 800 ullage explosion event at 13,800 feet is 

11
not shown as OAT for that event was not available.) Without further investigating the details 

behind the fire events (i.e., which were actually nacelle or APU compartment fires, this data 

suggested clearly preponderance of fire events well above -20°C (-4°F) and at altitude below 2.7 

kilometers (9,000 feet). 

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

35

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Altitude, feet

Temperature, degrees C

Figure 12. NTSB Database Fire Events Including Both OAT and Altitude, 1988 - 2000 

Another issue related operational temperature experience and aircraft fire suppression is that of a 

cold-soaked aircraft and a fire occurring during engine or APU start-up when cold soaked. 

Figure 13 plots cold-soak aircraft wing temperatures versus OAT during aircraft ground 

operations in Canadian winter weather [18]. This data suggested aircraft operational experience 

in a cold climate predominantly above -20°C (-4°F), and also shows generally that for a given 

OAT the aircraft cold-soak temperature is several degrees higher. 

A detailed account of the NGP investigation into the history of temperature conditions at which 

nacelle fire suppressant releases have occurred is described in Reference [14]. What follows 

provides a brief summary of those findings. 

DoD Data Review 

Table 1 summarizes the number of DoD fire mishap and incident data that were reviewed 

covering the period 1980 through 2002. Table 2 summarizes the percentage of fire incidents 

determined to have occurred in geographic cold or severe-cold environments, as defined by 

Reference [15]. Figures 14 and 15 plot the geographic locations of ground fire events versus the 

Reference [15] land environments for rotary aircraft and fixed-wing aircraft, respectively. In 

12
these figures it can be seen that the clear majority of events occurred in geographic locations 

associated with Reference [15] basic land environments. 

Figure 13. Cold-Soak Wing Temperatures in Aircraft Ground Operations in Canadian Winter 

Weather 

Table 1. Number of Incidents (Number of Mishaps/Incidents Reviewed), 1980-2002 

Service, 

Aircraft Type 

Army Navy Air Force 

Fixed Wing 88 1,212 3,932 

Rotary 465 834 98 

Table 2. Percentage of Fire Incidents Occurring in Geographic 

Cold or Severe-Cold Environments 

Service, 

Aircraft Type 

Army Navy Air Force 

Ground 0 1.5% 1.1% (a)

In-Flight < 1% < 1% 2.7% (b)

(a) From data categorized as ground fire incidents only. 

(b) From data categorized as in-flight fires only but also 

includes incidents on ground characterized as flight fire 

incidents. 

13
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 14. Distribution of DoD Rotary Aircraft Ground Fire Locations 

(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 15. Distribution of DoD Fixed-Wing Aircraft Ground Fire Locations 

14
Figure 16 shows flammability profiles for Jet A and Jet B aviation fuels. It had been postulated 

previously [19] that if the temperature profiles shown in Figure 16(a) are encountered in flight, 

the formation of flammable equilibrium vapor-air is limited to a tropical atmosphere for Jet A, 

whose flammability properties are similar to JP-8, and the standard atmosphere for Jet B, whose 

flammability properties are similar to JP-4. JP-8 fuel is now used predominantly by the USAF 

whereas USN aviation predominantly uses JP-5, which has a more conservative flash point. 

Figure 16(b) was derived previously by British Aerospace [20] and indicates increasing ignition 

energy with increasing altitude as well as when approaching the boundaries of the flammability 

limits. 

(a) Atmospheric Profiles 

(b) Estimated Minimum Electrical Ignition Energies 

Figure 16. Jet A and Jet B Flammability Profiles, Fuel Temperature versus Altitude 

15
Figures 16(a) and (b) served as a basis to plot the DoD fire mishap and incident data for which 

both altitude and OAT were provided in the data. Figure 17 shows the data plotted versus the 

Reference [15] world-wide air environments, the air environments published in the Joint 

Aviation Regulation [21], and a previously published subarctic profile [22]. In Figure 18 the 

data is plotted versus the flammability profiles derived from Reference [22]; the blue line is the 

standard atmosphere profile and the orange line is the tropical atmosphere profile. Figures 19 

and 20 plot the nearest-to geographic locations of in-flight fire events versus the Reference [15] 

land environments for rotary aircraft and fixed-wing aircraft, respectively. 

Qualitatively, the plots of the DoD fire mishap and incident data in the Figures 17 through 20 

indicate: 

• As altitude increases, the number of fire events decreases 

• As altitude increases, occurrence of events trends above the standard atmosphere profile 

• Rotorcraft fire events occurred below 4,000 meters (13,123 feet) 

• The overwhelming majority of fixed-wing aircraft fire events occurred below 6,000 

meters (19,685 feet) 

• Similar to publicly-available data, the vast majority of fixed-wing aircraft fire events 

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 Reference [15] basic land environments. 

Figure 17. Plot of Standard Climate Profiles and WWAEs versus All DoD Aircraft Fire Events 

and Suppressant Releases where Both Altitude and OAT Were Provided 

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 NACA TN 3276

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 NACA TN 3276

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

-85

-80

-75

-70

-65

-60

-55

-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

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

Altitude, feet

Temperature, degrees F

-73

-70

-67

-63

-60

-57

-54

-50

-47

-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

World-Wide Air

Environments (WWAE)

16
0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

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Rotary Fire Events from DoD Data

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Rotary Fire Events from DoD Data

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Fixed-Wing Fire Events from DoD Data

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Temperature, degrees C

Altitude, meters

Fixed-Wing Fire Events from DoD Data

Figure 18 DoD Aircraft Fire Events vs. Flammability Profiles 

17
(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 19. Distribution of DoD In-Flight Rotary Aircraft Fire Locations 

(Nearest-to Locations) 

(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 20. Distribution of DoD In-Flight Fixed-Wing Aircraft Fire Locations 

(Nearest-to Locations) 

18
NACELLE AIR TEMPERATURE 

Having reviewed extensively DoD mishap and incident fire data, the NGP sought to characterize 

in-flight temperature conditions within an engine nacelle by developing an in-flight nacelle air 

temperature model, which is described in Reference [14]. The model was applied to 1,025 cases, 

bounded by altitude to 9.8 kilometers (30,000 feet). In 88% of the cases the model indicated 

nacelle air temperatures greater than -17.8°C (0°F). Closer review of the remaining 12% of the 

cases (those less than -17.8°C) were noted for input conditions at 6.6 kilometers (20,000 feet) or 

greater, and 89% of these cases (89% of the 12%) were noted for at airspeeds of 50 knots. These 

cases were deemed not credible as 1) typical military rotorcraft have operational ceilings less 

than 6.6 kilometers (20,000 feet), and 2) for typical military fixed-wing aircraft that have nacelle 

fire suppression capability (e.g., fighter/attack aircraft, cargo transports, patrol aircraft) a 50-knot 

airspeed would be typically below stall speed for these aircraft. The remaining 11% (i.e., 11% of 

the 12%) were noted for input conditions at 9.8 kilometers (30,000 feet) and 400 knots and 

indicated nacelle air temperatures ranging between -23.3°C (-10°F) and -24.4°C (-12°F), which 

equated to 1.5% of all cases modeled. 

It was obvious that if the non-credible cases were removed from consideration the actual 

percentage of total cases indicating nacelle air temperatures greater than -17.8°C (0°F) would be 

greater than 88%. Thus it was assessed that modeling additional cases up to the ceiling of the 

Standard Atmosphere Model for the tropopause of 11.9 kilometers (36,152 feet) would result in 

additional nacelle air temperatures less than -17.8°C (0°F) but was likely to not dramatically 

impact the percentages described. As indicated in Table 3 the model tended to predict 

(conservatively) lower temperature ranges as compared to measured temperature ranges. 

OAT CONDITION AT SUPPRESSANT RELEASE 

Figure 21 presents the mass of nacelle/APU compartment fire suppressant discharged for DoD 

aircraft over the period covered by the NGP review of DoD fire mishap and incident data (1980-

2002). Figure 22 presents a summary of the NGP findings to characterize in-flight nacelle fire 

suppressant OAT conditions. The figures comprise data for which altitude and OAT were 

included, plus data for which altitude was provided and OAT estimated using the standard 

atmosphere model, plus data for which both altitude and OAT were not provided but were 

estimated – altitude was estimated based on aircraft maneuver then was OAT estimated using the 

standard atmosphere model. From Figure 21 it is easily seen that a substantial amount of fire 

suppressant was discharged while aircraft were on the ground. During the 1980-2002 period the 

average amount of fire suppressant discharged from engine nacelle and APU compartment fire 

suppression systems at 10 kilometers (33,000 feet) and higher, the ozone layer, was determined 

to be 3.6 kg (8.0 lbs) per year, which compares favorably to 4.6 kg (10.1 lbs) determined in a 

previous study [25]. That study also incorporated discharges from on-board portable fire 

extinguishers as well as extrapolated discharges from USN aircraft, for which only three years of 

data were reviewed, to align with the time period of the data evaluated for U.S. Army and USAF 

discharges. As presented earlier in Figure 18 for occurrence of fire events, Figure 22 presents 

that the OAT condition at suppressant release has historically differed between rotorcraft and 

fixed-wing aircraft. Figures 23 and 24 are presented to examine this further. 

19
Rotary Aircraft 

In Figure 23 DoD the rotary aircraft fire data are plotted as in Figure 18 versus the standard 

atmosphere (blue line) and tropical atmosphere profiles (orange line) and the 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 Figure 22. Also shown for 

reference purposes is an artifact from previous fire testing to determine the flame spectral 

characteristics for optical fire detection at altitude: that testing at pressure conditions 

representative of altitude of 11.5 kilometers (35,000 feet) resulted in inability to maintain 

sustained combustion [23]. The preponderance of fire events and suppressant releases on 

rotorcraft is shown to occur well below the typical operational ceiling for rotorcraft. Figure 23 

indicates clearly trade space above the -40°C (-40°F) NGP boiling point criterion, with 97% of 

all rotorcraft suppressant releases occurring above -12.2°C (10°F). 

Table 3. Comparison of Modeled (Predicted) versus Measured Nacelle Air Temperature Ranges 

Pressure Altitude 

(feet) OAT (°F) 

Engine Surface 

Temperature 

Range (°F) 

Measured Nacelle 

Air Temperature 

Range (°F) 

Predicted Nacelle 

Air Temperature 

Range (°F) 

Sea level 83.8 Not indicated. 92.4 to 180.6 

17,000+ -6.3 Not indicated. -1.3 to 87.2 

Not modeled since 

engine surface 

temperature range 

not indicated. 

1,400 82.5 214.7 to 737.6 92 to 180 73.25 to 91.13 

2,000 Not indicated. 350 to <500 ≈200 to 325 69.34 to 238.19 

2,000 to 45,000 Not indicated. Not indicated. ≈225 at 2,000 feet 

to ≈50 at 45,000 

feet 

Not modeled since 

engine surface 

temperature range 

not indicated. 

9,887 27.2 166 to 1089.6 72.4 to 200.2 26.65 to 82.48 

Sea level 27.41 140.4 to 873 66.6 to 131.4 61.04 to 102.09 

10,046 34.5 177.5 to 1088.3 80.3 to 200.9 26.25 to 81.94 

10,000 Not indicated. Up to 500 50 to 200 41.96 to 53.4 

11,000 Not indicated Up to 1,380 <230 to <525 44.31 to 72.82 

Sea Level Not indicated. Not indicated. 4101 Not modeled since 

engine surface 

temperature range 

not indicated. 

 

1

 Though the engine surface temperature data was not available for use in during modeling, this temperature data point from 

Reference [24] is listed as it was the highest nacelle airflow temperature found in the literature during the nacelle air 

temperature modeling effort. 

20
0.0

40.0

80.0

Figure 21. In-Flight Suppressant Deployment vs. Altitude, DoD Data, 1980-2002 

Fixed-Wing Aircraft 

Figure 24 plots the fixed-wing aircraft in similar fashion and requires a more careful 

examination. The overwhelming majority of fire events from the data is indicated below 7,000 

kilometers (23,000 feet), with 94% of all suppressant releases occurring at an altitude just above 

9,000 kilometers (29,500 feet). 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 previously referenced testing that indicated inability to maintain sustained combustion 

above 11.5 kilometers (35,000 feet) [23]. Additionally, the overwhelming majority of credible 

nacelle air temperature modeling cases occurred for OAT conditions at or above -25°C (-13°F), 

which also correlates well with that very few fire events are indicated below this OAT as well as 

with the distribution of fire events depicted previously in Figures 19 and 20. In the Basic land 

environment depicted in those figures, the worst cold temperature exposure is 1% at -31.7°C (-

25°F), which also correlates well with the temperature boundary for 94% all suppressant releases 

in Figure 24. 

Figures 23 and 24 represent that DoD rotary and fixed-wing aircraft operational environmental 

conditions (altitude, temperature) will differ, with fixed-wing aircraft having some greater 

exposure to colder ground and altitude environments. 

120.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

13716

11887

10058

9754

7925

7315

6820

6401

5837

5486

4572

4267

3658

3048

2621

2438

2134

1829

1707

1646

1372

1000

792

610

457

335

244

212

183

152

76

56

18

5

0

Altitude, meters

Halon Discharged, kg 60

56

52

48

44

40

36

32

28

24

16

20

64

68

21
3% 10%

87%

> 4.57 km (> 15,000 ft)

3.05 to 4.57 km (10,000 to

14,999 ft)

< 3.05 km (< 10,000 ft)

3% 3% 7%

87%

< -20.6°C (< -5°F)

< -12.2°C (< 10°F)

< -6.7°C (< 20°F)

≥ -6.7°C (≥ 20°F)

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)

Rotary

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)

Fixed Wing

Figure 22. Summary of OAT and Altitude Conditions at Nacelle Fire Suppressant Release 

Fire Events and Suppressant Release Under Cold Soak Conditions 

Figure 25 summarizes fixed-wing fire incidents by phase of operation. When considering the 

safety 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., 

temperature conditions that would effect aircraft cold-soaking. The takeoff-related categories are 

indicated to total 18.7% of all incidents in Figure 25. When reviewed for the number of 

suppressant releases during takeoff, approximately 16% of suppressant releases occurred during 

the takeoff phases. However, when these releases were reviewed further, only 4% of the takeoffrelated suppressant releases (and thus less than 1% of all suppressant releases) were indicated to 

have occurred in land environments categorized as cold or severe cold per Reference [15]. This 

suggests strongly that safety risk is low (an improbable hazard frequency) for an engine nacelle 

fire during takeoff for a cold-soaked aircraft and in which the fire suppression system fails to 

extinguish the fire and a catastrophic event occurs. It can be seen in the distribution of fixedwing fire events in Figure 24 that the majority of events are indicated above -10°C (14°F). For 

rotary aircraft fire events and suppressant releases, Figure 23 suggests strongly that cold-soak 

conditions alone are a low likelihood. 

22
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-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Temperature, degrees C

Altitude, km

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.

AFAPL-TR-73-83 

testing: no 

sustained 

combustion above 

35K ft.

Rotorcraft 

ceilings 

typically 

below 20K ft.

Altitude, meters

Figure 23. DoD Rotary Aircraft Fire Data versus Atmospheric Profiles, Flammability Limit 

Profiles, Operational Ceiling, and Suppressant Release 

PILOT RESPONSE VERSUS EFFECTIVITY VERSUS OVER-DESIGN 

Figure 26 illustrates results of NGP review described in Reference [14] of USN fire mishap and 

incident data to categorize pilot response time in effecting suppressant discharge and provides a 

comparison to previous work that describes effectivity of nacelle fire suppression systems on 

USN aircraft [3,4]. Response time was categorized qualitatively as normal, slow and long. The 

need for a design optimization capability cannot be overstated when considering that even with 

halon 1301 as the fire suppressant and normal pilot reaction to effect agent discharge, overall 

effectivity of halon 1301 nacelle fire suppression systems was still less than 80%. It is this 

dichotomy that challenges an overarching assertion that halon 1301 nacelle fire suppression 

systems are over-designed. Traditional design practice usually entails using design guidance 

(i.e., equations) to estimate suppressant mass quantity needed, conducting analysis of nacelle 

airflow characteristics for the purposes of designing the suppressant distribution system and 

discharge location or locations within the nacelle, and then performing qualification tests. If 

found necessary during those tests, discharge locations and/or nozzles may be adjusted in order 

23
to pass qualification; i.e., try different nozzles on the ends of the distribution tubing or changing 

the orientation of the discharge locations. (This traditional practice has also been applied in the 

design and qualification of the MV/CV-22 HFC-125 nacelle fire suppression system and the 

HFC-125 nacelle/APU fire suppression system on AH-1Z and UH-1Y aircraft. In the 

development of the F/A-18E/F and F-22 HFC-125 nacelle fire suppression systems, fire test 

programs were conducted to support system sizing and distribution design.) This can result in 

non-optimized suppressant distribution, which is illustrated in Figure 2-27. Whether the 

suppressant concentration provided above the qualification concentration is truly a margin of 

safety will be determined subsequently through operational experience. It can be that what 

appears to be margin is either over-design or in fact inadequate design because of peculiarities of 

the nacelle compartment (e.g., reverse airflows or proprotor-induced back pressure). Whether a 

system is inadequate, over-designed or not optimized is more appropriately assessed on an 

aircraft-by-aircraft basis. 

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-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Temperature, degrees C

Figure 24. DoD Fixed-Wing Aircraft Fire Data versus Atmospheric Profiles, Flammability Limit 

Profiles, Potential Stall Condition and Suppressant Release 

Altitude, k

m

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 majority of nacelle air temp conditions fell at/below 30K ft and above -25°C.

Cases beyond these conditions were indicative of potential aircraft stall.

AFAPL-TR-73-83 

testing: no 

sustained 

combustion above 

35K ft.

Altitude, meters

24
6.3%

2.0%

9.5%

13.2%

19.6%

0.4%

0.1%

0.5%

3.5%

0.6%

3.2%

7.1%

0.1%

0.4%

4.5%

0.3%

0.7%

0.1%

1.2%

4.2%

5.9%

8.4%

1.4%

3.0%

1.7%

2.4%

0.03%

Cruise Climb, 6.3%

Cruise Descent, 2%

Cruise Low Level, 9.5%

Combat Maneuver, 13.2%

Cruise, 19.6%

Ground Maintenance, 0.4%

Ground Operations, 0.1%

Hover, 0.5%

Landing Final (Final Approach), 3.5%

Landing (Forw ard Landing Area, FLAR), 0.6%

Landing Pattern, 3.2%

Landing Roll-Out, 7.1%

Not Available, 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%

Parked, Engine Start, 4.2%

Takeoff Abort, 5.9%

Takeoff Climb, 8.4%

Takeoff Runup (Runup Before Takeoff), 1.4%

Takeoff Roll-out, 3%

Taxi Landing, 1.7%

Taxi Out (Prior to Takeoff Runup), 2.4%

Not Specified, 0.03%

18.7%

14.4%

• Agent release 

during takeoff: 

≈16% of all 

releases

gent release 

during takeoff in 

cold or severecold 

environments: 

≈4% of all 

releases during 

takeoff

Data suggests 

that risk of unsuppressed fire 

during takeoff in 

cold or severecold 

environments is 

an Improbable 

likelihood (Low 

Risk)

• A

•

Figure 25. Fixed-Wing Fire Incidents and Mishaps 

SURVIVABILITY PERSPECTIVE 

Distinguishing DoD aircraft from commercial transport aircraft are survivability requirements to 

mitigate effects of enemy fire, which includes ballistically-induced fires. Generally, the areas 

vulnerable to these fire threats are dry bay compartments and the fuel tank ullage, the vapor 

space above the fuel within a fuel tank. 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 requirement for CO2

discharge was to be within one second, similar to the HRD systems developed by the CAA over 

a decade later.) 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, and a pilot-activated system was to be implemented for discharge into 

the tank itself. There is evidence that a concept for implementing active dry bay fire protection 

was being pursued by the Imperial Japanese Navy at the later stage of World War II, which is 

shown in Figure 28 and was planned to be an automatic system utilizing CO2 as the fire 

suppression agent [26]. 

25
93%

2% 5%

normal

slow

long

Navy Fixed-Wing Aircraft Effectivity

1977-1993, Nacelle Fire Suppression:

– Overall: 72%

– In-Flight: 76%

– On the Ground: 65%

• Navy Rotary Aircraft Effectivity

1977-1993, Nacelle Fire Suppression:

– Overall: 57%

– In-Flight: 47% - 63%

– On the Ground: 64%

From report 

NAWCADLKE-MISC-05-SR-0132

From report 

NAWCADLKE-MISC-05-SR-0146

•

Figure 26. Pilot Response vs. Nacelle Fire Suppression Effectivity 

Halon 1301 

requirement 

(6% vol. conc.)

Margin?

HFC-125 design 

equation max.

Margin?

Figure 27. Example Nacelle Fire Suppression Systems Certification Concentrations 

26
Figure 28. Dry Bay Compartments - WWII Concept 

During the Vietnam War the United States suffered combat losses totaling 5,000 aircraft – 2,500 

fixed-wing jets and 2,500 helicopters. As losses mounted during the course of the conflict, 

studies were initiated to see what might be done to lower loss rates, an effort that continued after 

the war. The analysis revealed that fuel fires and explosions accounted for 50% of the losses and 

that half of these were attributable to fuel explosions in dry bay compartments [27]. Methods for 

providing dry bay fire protection on U.S. DoD aircraft were implemented, which consisted 

primarily of passive techniques as illustrated in Figure 29 [28]. Halon 1301 was given credit for 

providing active dry bay in a few applications, but closer review shows that halon 1301 dry bay 

fire protection systems have not really been fielded on DoD aircraft (i.e., certified by 

Congressionally-mandated live fire testing). In Figure 29 an APU compartment is categorized as 

a dry bay because of its location aft of a fuel cell compartment, but the design and certification of 

APU compartment fire suppression is similar to that for engine nacelle compartments. Similarly 

an accessory compartment, or what may be considered a quasi dry bay, was given credit for 

having halon 1301 dry bay fire protection. However, even the fire suppression system for this 

type of compartment would have been certified in a manner similar to that for engine nacelles. It 

should be noted that space and electrical wiring provisions are installed on delivered F-22 

aircraft in the event it is decided in the future to install dry bay fire suppression system 

components. Though the system was designed for use of halon 1301, no F-22 aircraft are fielded 

at this time with halon 1301, i.e., no dry bay fire suppression system components are installed. 

27
Figure 30 shows the halon 1301 dry bay fire suppression system installed on United Kingdom 

CH-47 Chinook helicopters. Like halon systems designed to protect engine nacelles, the fire 

bottles used for dry bay fire suppression on the CH-47D helicopters, though much smaller, also 

utilize a pyrotechnic device to rupture a burst disk on the bottle to effect suppressant release, but 

the bottles are automatically activated upon receipt of a signal from a pressure transducer, and an 

initiator on the bottle effects discharge [29]. 

Accessory Compartment 

(i.e., a quasi dry bay)

Example:

APU Compartment

Per AFWAL-TR-87-

3032, not designed for 

combat fire protection

Figure 29. Vulnerability Reduction, DoD Aircraft Dry Bay Compartments (and Ullage) 

Though aircraft vulnerable areas have decreased over the years, as depicted notionally in Figure 

31, fuel systems and fuel containment systems today remain a significant vulnerability issue and 

drive the need for vulnerability-reduction measures such as fire suppression for aircraft dry bay 

compartments, both active and passive. Figure 31 is based on live fire testing (LFT) 

vulnerability assessments for armor-piercing incendiary (API) threats. Figure 32 shows 

examples of non-halon active and passive fire suppression techniques that have been 

implemented on DoD aircraft. In Figure 32 the photograph to the left is a dry bay compartment 

adjacent to a fuel tank that is protected by an optical fire detector and an inert gas generator, an 

active system in which detection is required to occur within 5μs and discharge of the inert gas 

occurs within 100ms. These halon alternative systems have been installed on MV/CV-22 and 

F/A-18E/F aircraft. The photograph to the right is a wing leading edge dry bay protected by 

aluminum-oxide powder panels, a passive system in which the powder is released when the 

ballistic projectile pierces the panel. In contrast, the system shown in Figure 30 utilizes piezoelectric sensors to sense a pressure rise associated with a ballistic projectile entering a fuel tank, 

which in turn commands discharge of the halon fire suppressant bottles into the adjacent dry 

28
bay(s), all in less than 50ms. The DoD also has fielded a manually-activated nitrogen fire 

suppression system on C-5 aircraft, which is an extension of the C-5’s fuel tank liquid nitrogen 

(LN2) inerting system and provides dry bay fire protection, though not against combat threats 

per Reference [28]. The system is required to reduce the oxygen level within the compartment 

into which it is discharged, ullage or dry bay, to 10% or less “in the time duration necessary to 

cope with the particular fire hazard.” 

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 Volume, 

Configuration and Congestion. 

The Number of Sensors Required 

Will Relate to Fuel Capacity 

Configuration and Threat. 

Fire Extinguishers: Cartridge 

Operated to Provide 

Protection within Vulnerable 

Compartments 

Projectile Sensors: 

Self-Operating/Fuel 

Immersed, Produces an 

Output in Response to 

Decelerating Strike 

Projectile 

Multi-Extinguisher Initiator: 

Self-Operating, Enables 

Several Extinguishers to be 

Initiated by Sensor(s) Output 

Multi-Extinguisher Initiator 

Fire Extinguishers 

Figure 30. Non-DoD Aircraft Halon 1301 Dry Bay Fire Protection System [30, 31] 

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

1980’s

Rotorcraft

State of the Art

Rotorcraft

Crew

Propulsion

Drivetrain

Flight Controls

Fuel System

Structure

Others

System Vulnerability:

1980’s

Rotorcraft

State of the Art

Rotorcraft

Crew

Propulsion

Drivetrain

Flight Controls

Fuel System

Structure

Others

System Vulnerability:

1980’s

Rotorcraft

State of the Art

Rotorcraft

Crew

Propulsion

Drivetrain

Flight Controls

Fuel System

Structure

Others

System Vulnerability:

Figure 31. Notional Reduction in Aircraft Vulnerable Area 

29
Active Passive

Optical Fire Detection + Solid 

Propellant Inert Gas Generators

Aluminum Oxide 

Powder Panels

Figure 32. Examples of Dry Bay Fire Suppression on DoD Aircraft Today 

Even though fuel tanks (or cells) are a significant contributor to an aircraft’s ballistic vulnerable 

area, a component failure within the tank can also provide an ignition source for the fuel-air 

mixture in the ullage. In either case, the time between initiation and the time at which 

combustion results in a catastrophic overpressure condition that exceeds the structural capability 

of the aircraft is brief, as illustrated in Figure 33. A summary of military safety-related fuel-tank 

explosions in the National Transportation Safety Board (NTSB) TWA-800 report [32] indicates 

all occurred with JP-4, and that none occurred on aircraft having some type of fuel tank ullage 

fire/explosion protection. Though the DoD primarily uses JP-8 (USAF) and JP-5 (USN), aircraft 

may still be required to use JP-4. 

Examples of fielded or previously-fielded ullage fire/explosion protection systems are shown in 

Figure 34. These are categorized as passive, reactive, and active. Passive systems include 

explosion suppressant foam (ESF) and aluminum mesh, though the DoD has only fielded ESF. 

Previously fielded reactive systems include the scored canister system (SCS), which released 

pentane to create a fuel rich environment to preclude combustion, and cylindrical suppressors, 

which utilized halon 1011. Other reactive systems investigated by the DoD but never fielded for 

aircraft ullage fire/explosion protection were linear fire extinguishers (LFE), the Parker Reactive 

Explosion Suppression System (PRESS), and the nitrogen-inflated ballistic bladder (NIBB). 

Active systems have been fuel tank inerting systems and have been designed to provide 

protection either continuously or when commanded. Currently-fielded LN2 and onboard inert 

gas generation systems (OBIGGS) provide continuous protection, while halon 1301 inerting 

systems would be activated prior to entering a potential threat environment. Only three DoD 

aircraft have utilized halon 1301 to provide fuel tank inertion, the USAF F-16 and F-117 aircraft 

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

require that ullage inerting systems reduce oxygen concentrations to 9% or less.). The USN A6E ullage inerting system was designed to maintain a halon 1301 volumetric concentration of 

20% upon activation. Today the only DoD aircraft that utilizes halon 1301 for fuel tank inerting 

is the USAF F-16. 

30
t = 0 t = 50ms t = 140ms t = 375ms

Figure 33. Spark-Initiated Combustion 

ESF

Scored Cannister System, used Pentane

(Used with permission by Kidde Aerospace)

Cylindrical Suppressor, used Halon 1011

(Used with permission by Kidde Aerospace)

Figure 34. Examples of Fuel Tank Ullage Fire/Explosion Protection Techniques 

31
OBIGGS Schematic [33] 

Halon 1301 Inerting System [34] 

Figure 34. Examples of Fuel Tank Ullage Fire/Explosion Protection Techniques 

(continued) 

Continuous Conditioned Air 

Input from Aircraft 

Environmental Control 

ystem (ECS) or other Bleed 

Air Source 

Nitrogen-Enriched Air (NEA) 

Output to Fuel Tank Ullage 

S

32
SUMMARY 

An understanding of history can be beneficial to those involved in the development, 

implementation and sustainment of aircraft fire suppression systems. History shows, through the 

modern day, that engine compartments are the most significant safety-related fire threat 

compartments on aircraft. For nacelle fire suppression systems history also shows that, even 

with halon 1301, their effectiveness is sensitive to the nacelle installation and possibly to 

influences by other systems or aircraft effects. The modern history, i.e., 1980 and subsequent, 

also shows clearly and convincingly that operating environments at which the vast majority of 

nacelle fire events have occurred have been at altitudes below 6.1 kilometers (20,000 feet) and at 

OATs above -20°C (-4°F). In the case of rotorcraft, that altitude is shown to be below 4 

kilometers (13, 123 feet). 

A review of history as well as consideration of LFT vulnerability assessments through the 

modern day continues to point to fuel systems (and the fuel containment systems) as the greatest 

contributor to aircraft vulnerable area from a survivability perspective. The history shows that 

passive protection techniques have predominated within the DoD. Also, the use of higher flashpoint fuels has been key to mitigating fuel tank explosions due to safety-related events. 

Nacelle/APU 

Compartments

Dry Bay 

Compartments

Fuel Tank 

Ullage

• Predominant safety 

fire threat. 

• Catastrophic aircraft 

loss for multi-engine 

aircraft is a stack-up 

of probabilities. 

• Passing 

Certification ≠ 100% 

Effectivity (inherent 

risk). 

• Cold temperature 

environments have 

not been the 

predominant fire risk 

exposure. 

• NGP cost/benefit 

study: fire 

suppression benefit 

outweighs cost to 

implement and 

sustain [35]. 

• Predominant 

survivability fire 

threat. 

• Catastrophic aircraft 

loss due to fire can be 

significant contributor 

to aircraft attrition 

rate. 

• Passive techniques 

have been typically 

employed on legacy 

DoD aircraft, though 

halon 1301 has been 

used to protect quasi 

dry bays. 

• Active gas generator 

systems fielded on 

some newer 

production aircraft. 

• NGP cost/benefit 

study: fire suppression 

benefit outweighs cost 

to implement and 

sustain [35]. 

• Significant 

contributor to 

vulnerable area 

(obvious). 

• Safety-related 

hazards mitigated 

to large extent by 

use of JP-5 and JP8 fuels instead of 

JP-4. 

• Passive 

techniques have 

been typically 

employed. 

• Active techniques 

(inerting used on 

few platforms). 

• Reactive 

techniques 

previously 

employed but not 

in use today in 

DoD. 

 

33
There are several key performance parameters (KPP) that are required to be met during the 

acquisition phase of the life cycle of a DoD aircraft program. During this phase an enormous 

amount of attention is paid to weight, performance, cost, reliability/maintainability, safety and 

survivability - vulnerable area (Av) and probability of kill (Pk). For each except safety, metrics 

are established. In the case of safety, analyses are developed to support a level of risk after all 

hazard controls have been implemented, and the residual risk is required to be accepted by the 

appropriate risk acceptance authority within the procuring the DoD activity. Inevitably, trades 

are made by the program manager to ensure KPPs are met, and in the current and potentially 

future era of constrained resources (i.e., dollars), tools that provide the capability to optimize the 

weight of an aircraft system and its performance and that permit life cycle cost evaluation and 

optimization of alternatives, or technologies that provide enhanced performance over currentlyfielded technologies should benefit program managers and ultimately the end users. Examples of 

NGP products that can be employed to conduct such trades are: 

• Nacelle CFD tool to optimize suppressant weight and distribution system 

• Chemically active gas generators and hybrids, which have been shown to require less 

suppressant mass yet be more effective than recently-fielded inert gas generators 

• Enhanced powder panels (EPPs), which have been shown to be comparable in weight 

with legacy powder panels but are also more effective and provide enhanced dispersion 

• NGP cost modeling tool to compare relative life cycle benefits of fire suppression 

technologies 

ACKNOWLEDGEMENTS 

The author expresses his sincere appreciation to the following persons and organizations. 

Without their assistance and contributions, the compilation and publication of information herein 

would not have been possible. 

• Next Generation Fire Suppression Technology Program (NGP) 

• NAVAIR PMA 234, PMA 241, PMA 261, PMA-275, and the NAVAIR Public Affairs 

Office 

• Donna Egner, SURVIAC 

• Joe Manchor, NAVAIR Survivability 

• Dr. David Keyser, previously NAVAIR and now with SURVICE 

• Vendors (Kidde Aerospace, Airscrew Ltd) – for permission to use illustrations 

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