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 Title: In-Flight Suppression Deployment Temperatures 

AD Number: ADA444883 Corporate Author: NAVAL AIR SYSTEMS 

COMMAND LAKEHURST NJ Personal Author: (Not Available). Report Date:

February 28, 2006 Media: 46 Pages(s) Distribution Code: 01 - 

APPROVED FOR PUBLIC RELEASE Report Classification: (Not Available). 

Source Code: 443169 From the collection: Technical Reports 

 

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1

In-Flight Suppressant Deployment Temperatures 

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 

BACKGROUND 

Top-level military aircraft specifications typically require operational performance capability 

over a broad temperature range. For example, V-22 requires such capability for any exterior 

ambient atmosphere temperature between -65°F (-54°C) and 125°F (52°C) and an interior 

ambient atmosphere temperature between -65°F (-54°C) and 160°F (71°C) [1]. In developing 

the temperature envelope for future aircraft specifications, acquisition officials and technical 

experts will derive requirements from guidance documents [2,3], which may likely be used to 

also derive development and testing requirements for aircraft systems, subsystems, and 

components (e.g., nacelle fire suppression systems, nacelle fire bottles, etc.). In previous aircraft 

acquisitions, a requirements document [4] was used to develop aircraft temperature envelope 

requirements. Therefore, what was once a requirement is now guidance, thus allowing future 

military acquisition program managers flexibility in specifying realistic requirements. 

The temperature data that is recommended to define the low and high temperature extremes for 

airborne systems are world-wide air environments (WWAE) that are described in [2] (and 

previously [4]) as values with a 1%, 5%, 10% and 20% frequency of occurrence. In commercial 

aviation, standard climates in Joint Aviation Regulation 1 (JAR-1) [5] define temperatures based 

on arctic, temperate, tropical, intercontinental, and standard-day conditions. Figure 1 plots these 

climates versus the WWAEs used for military acquisition programs. With regards to low 

temperature requirements Figure 1 shows that the JAR-1 arctic climate generally correlates with 

the low temperature WWAEs, with the 1%, 5%, and 10% WWAEs being lower than the JAR-1 

arctic climate and the 20% WWAE being slightly higher. Figure 2 depicts the worldwide land 

environments from [2], which categorizes these into four land environment types: basic, hot, 

cold, and severe cold. 

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

systems, i.e., engine nacelle and auxiliary power unit (APU) fire suppression systems, is that the 

low temperature extreme defined for the envelope has historically been applied to drive the 

boiling point requirement for the fire suppression agent in the aforementioned systems. Thus in 

the context of the WWAEs, this boiling point requirement appears to exist to address the need to 

provide for in-flight fire suppression under atmospheric conditions consistent with arctic-like 

temperature conditions. 
2

-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

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

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%

 Note: ICAO is the acronym for International Civil Aviation Organization. 

Figure 1. JAR-1 Standard Climates vs. MIL-HDBK-310 WWAEs 
3

Figure 2. MIL-HDBK-310 Land Environments 

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. Reference [2] indicates a low-temperature 1% frequency of occurrence of -50°F (-45.6°C) 

for cold land environments and a low-temperature 20% frequency of occurrence of -60°F 

 (-51°C) for severe-cold land environments. Designers of legacy aircraft would develop fire 

suppression systems whose requirements [6,7] were tailored to halon 1301 properties to likely 

assure fire suppression performance at such temperature conditions. 

During the engineering and development process for a military aircraft, trade-offs are inevitably 

made that take into consideration performance requirements versus operational contexts, weight, 

cost, schedule, and safety. Such trades are assessed and categorized by applying programmatic 

and safety risk matrices. Military aviation program managers then take into consideration such 

assessments during their decision making process to select a particular trade option. For 

example, the USAF C-103J and Navy KC-130J programs elected to use a high-boiling-point 

agent, halon 1211, for engine nacelle fire suppression to minimize developmental impact 

associated with system changes that would be needed to accommodate non-ozone depleting 

halon alternative suppression agents. (For years, many legacy C-130 aircraft have also used a 

high-boiling-point agent, halon 1011.) Even though neither a Department of Defense (DoD) nor 

a Federal Aviation Administration (FAA) certification requirement was identified for halon 

1211, a United Kingdom Ministry of Defense nacelle fire suppression requirement of 10.5% had 

been identified [8]. This requirement is indicated to include a safety factor over a minimum 

halon 1211 concentration for fire extinguishment (7.1%). However, technical rationale was 

compiled to support a lower certification requirement of 6.5%, which is recognized by the U.K. 
4

Civil Aviation Authority [9]. In the context of low temperature WWAEs and ceilings for these 

aircraft (legacy C-130s: 19,000-33,000 feet; C-130Js: 28,000 feet), use of such agents would be 

considered not technically feasible. Boiling points for these agents are 25°F (-9°C) for halon 

1211 and 151°F (66°C) for halon 1011 [10]. (It is interesting to note that existing commercial 

aircraft specifications [11,12] indicate that “safe limits” for “unwinterized” halon 1011 or halon 

1301 cylinders are -65°F to 200°F.) Justification for likely acceptable performance at low 

temperature (i.e., -40°F) asserted that in order to generate a vapor-phase extinguishing 

atmosphere, the vapor pressure of halon 1211 at -40°F (-40°C) must be greater than the partial 

pressure required for a halon 1211 extinguishing concentration [9], and data was provided that 

indicated this was the case.. 

Boiling point (or 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 DoD Next Generation Fire 

Suppression Technology Program (NGP) [13]. Currently, this criterion is -40°C (-40°F). It was 

also one of the parameters considered during research efforts that identified pentafluoroethane 

(HFC-125) as the best near-term alternative to halon 1301 for use in aircraft nacelle fire 

suppression system applications [14]. However, even during those efforts it was recognized that, 

when operational contexts were considered such as the likely temperature environment within an 

operational engine nacelle at the time of discharge of a fire suppression agent, the typical low 

temperature performance requirement could be a candidate for a performance trade. Discussions 

in this regard related to minimum nacelle operating temperatures were indicated to range from 

less than 0°F (-17.8°C) to 100°F (37.8°C) [15]. Review of nacelle compartment airflow 

temperature data for a variety of aircraft platforms indicates temperatures ranging from -1.3°F 

(-18.5°C) to 525°F (273.9°C) [16 through 20]. Though this data may not address every 

operating environment, they suggest that even at low outside air temperatures (OAT) it is 

probable that the typical operational engine nacelle compartment temperature will be greater than 

-40°F (-40°C). 

It must also be considered that for years, even decades, high Tb agents 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 F-111 aircraft [21]. Their successful implementation and history 

is likely attributable to several factors such as 1) the fact they are brominated halogens, 2) 

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 [22]. The obvious 

conclusion is that both low- and high-boiling-point agents are likely to realize higher probability 

of success as distribution is optimized.

With regards to the potential for nacelle fires at low temperatures on the ground (i.e., OAT less 

than an agent’s Tb), procedures can be written to mitigate the risk of fire during engine start. 

Guidelines for cold weather operation have long existed for commercial aviation [23]. For 

military aircraft, flight manuals will provide procedures for cold weather operation and typically 

require the presence of a “fire guard” or “fire watch” during engine start-up (i.e., a person 

standing by with a fire extinguisher in the event of a start-up-related fire). The flight manual for 
5

the Navy KC-130J aircraft [24], which utilizes halon 1211, a high boiling-point agent for nacelle 

fire suppression, requires that “If the ambient temperature is expected to be -40°C (-40°F) or 

below, ensure ground support equipment and personnel are available at destination or plan to 

provide needed support.” Specific procedures are also provided for engine starting at such 

temperatures taking into consideration whether or not the aircraft has been preheated. Thus the 

likelihood of a nacelle fire during engine start at OATs below an agent’s Tb, for which no fire 

guard is present and for which a catastrophic fire event occurs, becomes a hazard for which 

safety risk is accepted by the aviation program. Typically, such risk is accepted when it has been 

assessed as a low safety risk. 

There is previous analysis of military aircraft experience regarding release of nacelle fire 

suppression agents [25], the purpose of which was to quantify halon discharges at altitude for the 

purpose of evaluating discharge frequency and quantity of agent discharged below and within the 

ozone layer. Combining both fixed-wing and rotary aircraft discharges that analysis indicates: 

• Approximately 77% occurred between 0 and 10,000 feet, and approximately 92% 

occurred between 0 and 20,000 feet. 

• Over 60% of all discharged agent was accounted for by only three of the thirty military 

aircraft platforms covered by the study. These three platforms have altitude ranges that 

extend beyond 20,000 feet but are contributors to the frequency of discharges below 

10,000 feet. 

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

1202). 

A heretofore operational context that has not been investigated, and is thus the subject of this 

study, is whether temperature conditions at the time of agent release correlate with the typical 

boiling point temperature requirement. These include OATs, nacelle operating temperatures, 

cold-soak temperature conditions and cold climatic extremes. Based on review of nacelle 

compartment airflow temperature data for a variety of aircraft platforms, it is 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 20,000 feet, with over 75% occurring below 10,000 feet, 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 agent 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 

suppression agent 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 agents with higher boiling points are not likely 

to appreciably increase risk under such conditions.

TECHNICAL APPROACH 

The technical approach for this project consisted of four elements: 
6

1. 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 agent release has historically taken place. Based on this 

data it may be possible to assess probability of agent release under conditions that are 

likely to be well above an agent’s boiling point. 

2. 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 an agent’s boiling point. 

3. Evaluate implications of aircraft cold-soak conditions, particularly during aircraft takeoff. 

4. Assess safety risk, considering the findings in the preceding elements, of utilizing a fire 

suppression agent whose boiling point is much higher than of those agents commonly 

fielded today in military aircraft (i.e., halon 1301). 

SAFETY CENTER DATA 

Aviation fire incident data was obtained for the years 1980 through 2002 from the U.S. Army, 

Navy and Air Force Safety Centers. Each Safety Center was also asked to provide as part of that 

data information for altitude, outside air temperature, and location associated with each incident. 

In addition, data obtained for this effort was also correlated with previous efforts related to 

aircraft halon discharges [25] and aircraft halon fire extinguishing systems effectivity [26,27]. 

Table 1 summarizes the number of incidents provided by the Safety Centers. 

Table 1. Number of Incidents 

Service, 

Aircraft Type 

Army Navy Air Force 

Fixed Wing 88 1,212 3,932 

Rotary 465 834 98 

The fire incident data was reviewed to determine whether agent release occurred and to identify 

the altitude and OAT associated with each release. Only agent releases associated with 

discharge of systems protecting the following were considered: engine nacelle, auxiliary power 

unit (APU), auxiliary power plant (APP), and gas turbine compressor (GTC) compartment. For 

incidents that provided altitude data but did not include temperature data, the standard 

atmosphere model indicated in Figure 3 was applied to estimate OAT. Rationale for use of this 

model is provided later in this paper under discussion of results. For incidents without altitude 

and temperature information, the methodology for assuming flight altitude based on aircraft 

flight phase from previous work [25] was applied. This is summarized in Table 2. Military 

Internet sites were also used as needed to obtain aircraft ceiling altitudes as well as hover in 

ground effect and out of ground effect for various rotary aircraft platforms. Once an altitude was 

assigned, the OAT at release was then determined by applying the standard atmosphere model. 

Ground fire incident locations and in-flight fire nearest locations were also compared to the 

climatic land environments in Figure 2. 
7

Figure 3. Standard Atmosphere Model 

Table 2. Altitude Assumptions Applied from Previous Halon Discharge Analysis [25] 

(Used only when no altitude data was available.) 

Phase 

Altitude Range 

(km) 

Assumed Altitude 

(km) 

Assumed Altitude 

(ft) 

Low below 1.5 1.00 3280.84 

Range work below 1.5 1.00 3280.84 

Bomb run below 1.5 1.00 3280.84 

After takeoff below 1.5 1.00 3280.84 

Cruise-fighter 6.06 - 7.58 6.82 22375.33 

Cruise-cargo 9.14 - 10.67 9.90 32480.32 

Refueling 8.79 8.79 28838.58 

Pilot Response 

Fire incident data was also reviewed to ascertain pilot response time in effecting agent discharge. 

The purpose of this effort was to provide background in support of NGP efforts to model fire 

suppressant dynamics in cluttered weapon systems compartments, e.g., engine nacelle 

compartments. During planning efforts it was questioned how long does a fire burn before the 

pilot effects agent discharge. Response time was categorized qualitatively as follows: 
8

• Normal – the fire incident narrative indicated that the pilot response followed typical 

emergency procedures for effecting agent release without delay. After receipt of a fire 

warning, the pilot isolates the affected compartment, which removes fuel flow to that 

compartment, and arms the fire suppression system. The pilot then confirms that he still 

has the fire warning and has some secondary indications of the fire condition. Upon 

these confirmations the pilot then discharges the agent into the compartment. 

• Slow – the fire incident narrative indicated that the pilot response followed typical 

emergency procedures for effecting agent release but there was some delay prior to 

discharging agent. For example, after shutting off fuel flow and arming the suppression 

system, the pilot still is receiving the fire warning but can’t obtain any other secondary 

indication. The pilot then may ask crew members to investigate, and after a brief period 

of time the fire is confirmed and the pilot discharges the agent. 

• Long – the fire incident narrative clearly indicated that the pilot response was protracted. 

In some of these narratives response times in excess of 1 or 2 minutes are indicated. 

IN-FLIGHT NACELLE AIR TEMPERATURE MODEL 

An in-flight nacelle air temperature model was constructed to estimate nacelle air temperature 

during flight conditions. The model uses the U.S. Standard Atmosphere 1976 data on pressurealtitude, temperature, and viscosity [28]. The model treats the nacelle as an air heat exchanger, 

and it computes the terminal temperature difference based on average, bulk values. The inlet 

conditions at the ram scoop are computed to be the stagnation properties for the given flight 

conditions, and these are taken to be the same as those inside the nacelle, close to the inlet. The 

effect of conduction and radiation heat transfer is assumed negligible; i.e., heat losses from air 

through the nacelle wall to the ambient outside by convection and conduction. 

Inputs to the model are as follows: 

• dc Average nacelle clearance, feet 

• dn Average nacelle diameter, feet 

• vavg Average nacelle ventilation air velocity, feet/second 

• H Altitude, feet 

• L Nacelle length, feet 

• TS Average engine surface temperature, °F 

• V Aircraft true airspeed, knots 

Constants and conversion factors utilized within the model are as follows: 

• cp, specific heat for air at constant pressure, 0.24 BTU/lb-mol°F 

• g, acceleration due to gravity, 32.174 ft/second2

• M, molar mass of air, 28.966 lb/lb-mol 

• Prair, Prandtl Number for air, 0.73, assumed constant 

• R, universal gas constant, 1545 (ft-lb)/(lb-mol°R) 

• 1 BTU = 778 ft-lb 
9

• 1 knot = 1.689 feet/second 

• 1 hour = 3600 seconds 

• °F = °R – 460 

Computational relationships utilized within the model are as follows: 

Ambient outside air temperature, Ta, and ambient pressure, pa, were derived from [28] by linear 

regression analysis: 

 Ta = 518.7 – 0.003567(H), °R (1) 

 pa = 2094.8 – 0.07006(H) + 6.97x10-7(H2), lbs/ft2 (2) 

Ambient atmospheric air density, ρair, is derived for ambient temperature and pressure by the 

following: 

 ρair = (M/R)(pa/Ta), lbm/ft3

 (3) 

 

The nacelle inlet air temperature is determined by: 

 Tin = [(1.689V)2 – (vavg)

2

]/[2g(778)cp], °R (4) 

The thermal conductivity of air, k, was fitted by a linear interpolation between -148°F (-100°C) 

and the freezing point of water, resulting in the following: 

 k = 0.0091 + [0.0049(Tin + 148)/180], BTU/hr-ft-°F (5) 

The absolute viscosity of air within the nacelle, μ, is determined from Equation 51 from [28]: 

 μ = 2.2(10-8)( Tin

1.5)/(Tin + 198.72), lb-second/ft2

 (6) 

Nacelle stagnation pressure, p2, nacelle air density, ρnacelle, and nacelle surface area, S, are 

determined as follows: 

 p2 = pa + ρair(1.689V)2

/2g, lbs/ft2 (7) 

 ρnacelle = (M/R)(p2/Tin), lbm/ft3 (8) 

 S = πdnL, ft2 (9) 

The Reynolds number, Re, for the nacelle is estimated as: 

 Re = (ρnacelle)(vavg)(dn)/μg (10) 
10

The convective heat transfer coefficient, h, is determined using equation 9-10a from [29]: 

 h = 0.023(Re0.8)(Prair

0.4)k/dc, BTU/hr-ft-°F (11) 

The temperature rise within the nacelle, ΔT, is determined by: 

 ΔT = hS[TS + 460 – 0.5(Tin + Texit)] / [ρair(3600)(vavg)(π/4)((dn + dc)

2

 - dc

2

)] , °R (12) 

The exit temperature, Texit, is then simply: 

 Texit = Tin + ΔT, °R (13) 

Calculations for ΔT and Texit are iteratively calculated in incremental changes of 0.001. 

To evaluate in-flight conditions generally representative of likely flight and nacelle operating 

conditions, the model was evaluated against the high and low operational parameter settings 

listed in [30] for nacelle configuration (length), clearance, airflow, and surface temperature. 

Settings for these parameters from [30] are shown in Figure 4. An additional surface 

temperature condition of 400°F was also evaluated. Conditions were evaluated for altitudes of 0 

feet, 1,000 feet, 5,000 feet, and in subsequent increments of 5,000 feet up to 30,000 feet. 

Rationale for applying the model only up to 30,000 feet is provided later in this paper under 

discussion of results. 

COLD SOAK CONDITIONS 

A literature review was performed to identify existing work related to evaluation of aircraft cold 

soak conditions. Two Transport Canada reports were identified [31,32], which provide 

information relative to aircraft wing surface temperatures during ground operations during 

Canadian winter and aircraft cold soak conditions after flights at altitude. 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 (flights were conducted in North America: 

Canada, Alaska). The reports also provide data that indicate the following relative to non-deiced spot wing temperatures: 

• 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. 

• Above 0°C (32°F) OAT, wing temperatures were generally lower than OAT. 

• Radiative cooling on the ground (i.e., aircraft parked overnight in cold weather) is more 

likely than in-flight conditions to result in cold-soak conditions. Possible wing-to-OAT 

differential due to radiative cooling may range from -6°C at 0°C OAT and reducing to 

-2°C at -25°C OAT [33]. 
11

Figure 4. Nacelle Operational Parameter Settings [30] 

 

The Transport Canada data includes a flight profile for a flight at altitude in Alaska during which 

wing surface temperatures were recorded. The cruise altitude is not specified but is likely to be 

approximately 30,000 ft based on aircraft type. In review of the temperatures recorded it is 

interesting to note the following: 

• At the time lift off from the ground occurs (t = 0), measured wing temperatures are 

higher than both OAT and the initial wing temperatures. 

• During takeoff climb, there is one test point where there is a temperature increase of 

approximately 5°F over the first five minutes, before it begins to decrease. (All other 

measurements are noted to begin to immediately decrease.) 

• 15 minutes after takeoff, wing temperatures are generally -5°F (-20.5°C). 

• 60 minutes after takeoff, wing temperatures are generally -10°F (-23.3°C) 
12

• Wing temperatures are noted to warm to approximately 20°F (-6.67°C) in 15 minutes 

time during descent to final landing 

With regards to potential halon alternative candidates, an issue has been raised during the NGP 

and in previous research efforts that cold-soak conditions were of concern with regard to the 

boiling point of an agent and agent discharge under such conditions, especially during takeoff, 

hypothetically supporting the need for an agent with a boiling point of -40°F (-40°C) or lower. 

Except for the radiative cooling condition, the aforementioned data suggest that temperature 

conditions of an operational aircraft are likely to be higher than the cold soak temperature 

conditions, suggesting that aircraft component temperatures are also likely to be higher. It is 

interesting to note that the lowest OAT for which cold-soak data is recorded is -13°F (-25°C). 

To analyze the concern of fire suppression agent and system component temperatures under cold 

or extreme cold temperature conditions during takeoff, a model was constructed to estimate 

stagnation temperature conditions during takeoff. The premise is that for agent bottle(s), 

distribution lines and components not located near/within heated compartments but adjacent to 

exterior surfaces, the stagnation temperature should provide a reasonable estimate of likely 

temperature conditions of components adjacent to the exterior surfaces. Assumptions applied in 

constructing the model include: 

• If the aircraft has been deiced, it is assumed that there is minimal to no heat transfer 

effect to components adjacent to exterior surfaces by the time takeoff occurs, based on 

discussions with Transport Canada. 

• The radiative cooling condition is not considered because aircraft are likely to be started 

and warmed up prior to takeoff. Thus in cold or severe-cold environments, component 

temperatures are likely to be at least the same as OAT. 

• The first case assumes an initial OAT for cold/severe-cold environments based on the 

JAR-1 Arctic Standard Climate profile, and then the profile is applied during takeoff 

climb. This is assumed to estimate a lower bound profile. 

• A second case assumes an initial OAT of –40°F (–40°C) and maintains this temperature 

during takeoff climb until OAT changes in the JAR-1 Arctic profile. 

• The third case assumes an initial OAT of –40°F (–40°C) and applies a 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 takeoff climb 

with the bias condition continuously applied to estimate a potential upper bound profile. 

• Speed conditions are sub-sonic, less than Mach 1, and there are no aerodynamic heating 

affects. 

• In calculating stagnation temperature, equation 2.58 from [34] was applied: 

 

 T = Ta + KV2

/7592, °K, (14) 

where: 

• T is the stagnation temperature, °K 

• Ta is the ambient temperature, °K 

• K is the temperature recovery factor, which can range from 0.7 to 1.0 
13

• V is the true airspeed, knots 

• Conversion factors utilized were: 

o °K = °C + 273.15 

o °F = 1.8°C + 32 

RISK 

The Safety Center data reviewed under this effort along with previous work [26,27] were 

considered to assess likelihood of agent release and likelihood of aircraft loss due to not 

suppressing an engine nacelle fire. Assessment of risk is done considering guidelines in military 

system safety standards [35] and service-specific guidelines [36-38]. 

RESULTS 

SAFETY CENTER DATA 

Results presented are based on NAVAIR System Safety interpretation of aviation fire mishap 

data provided by the Army, Navy, and Air Force Safety Centers. 

Location 

Incident data was first reviewed for the geographic locations where fires (and releases) occurred. 

Table 3 summarizes the results of this review. This review was performed to: 

1. Assess occurrence of fire in cold or severe-cold environments, as defined in [2], for 

incidents occurring on the ground. 

2. Assess occurrence of fire for incidents occurring in flight or characterized as in-flight for 

aircraft operating nearest to locations in cold or severe-cold environments. 

Table 3. 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. 

Rationale for Estimating In-Flight Temperatures Using the Standard Atmosphere Model 

Generally, the fire 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 is typically in terms of pressure altitude, while the Standard Atmosphere 
14

is based on geopotential altitude. Figure 5 illustrates the variation of ambient pressure versus 

pressure altitude and geopotential altitude on standard-day and non-standard-day temperature 

conditions. The implication with regards to estimating OAT using the 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 

3 suggest strongly that applying the MIL-HDBK-310 cold WWAEs or the JAR-1 Arctic Climate 

profile as the basis for estimating temperature conditions would not reflect operational 

experience. 

Safety Center data that included both altitude and OAT for which agent release occurred is 

plotted in Figure 6. Distribution of those events indicates releases typically about or above the 

Standard Atmosphere profile. Below the profile, the lowest OAT was indicated is 26°F (-3.3°C), 

which occurred at ground level. For the highest altitude below the profile, 5,400 feet, the lowest 

OAT is indicated as 35.6°F (2°C). 

Figure 7 plots other fire incidents for which Safety Center data included both altitude and OAT

but in which there may have been no engine or APU fire or for which there was an engine or 

APU fire but no agent release. The majority of the incidents at altitude occur above the profile 

and below 20,000 feet. Only one incident is indicated that is beyond the profile at altitude and 

below 0°F (-17.8°C), which occurred at 40,000 feet, -77.8°F (-61°C). This only incident for 

which both altitude and OAT were provided that occurred below -40°F (-40°C). The highest 

altitude below the profile and above 0°F (-17.8°C) occurred at 6,400 feet with an OAT indicated 

as 14°F (-10°C). There are three incidents indicated at zero altitude (on the ground) and below 

0°F (-17.8°C). These occurred with OATs at -13°F (-25°C), -18°F (-27.8°C), and -27°F 

(-32.8°C). 

Figure 5. Variation of Ambient Pressure versus Pressure and Geopotential Altitudes on 

Standard-Day and Non-Standard-Day Temperature Conditions [39] 
15

-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

Notes: (1) Agent releases are black data points 

(2) Refer to Figure 1 for identification of profiles depicted. 

(3) The dark blue profile is the Standard Atmosphere profile. 

Figure 6. Plot of Standard Climate Profiles and WWAEs and Agent Releases for which Safety 

Center Data Provided both Altitude and OAT 
16

-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

Notes: (1) Black data points are other fire incidents for which Safety Center data provided both 

 altitude and OAT 

(2) Refer to Figure 1 for identification of profiles depicted. 

(3) The dark blue profile is the Standard Atmosphere profile. 

Figure 7. Plot of Standard Climate Profiles and WWAEs and Other Fire Incidents for which 

Safety Center Data Provided both Altitude and OAT 

An additional consideration for using the Standard Atmosphere Model for estimating OATs 

during suppressant release is fuel flammability limits. Figure 8 depicts flammability limits of 

Jet-A and Jet-B versus altitude and standard atmospheres (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 is 

dependant on many variables, Figure 8 suggests that for military aircraft using JP-8 and JP-5 
17

fuels, attaining the flammability limits is more likely at atmospheres above the Standard 

Atmosphere. 

Figure 8. Flammability Limits of Jet-A and Jet-B Fuels versus 

Altitude and Standard Atmospheres [21] 

Based on review of the findings presented in Table 3 and Figures 6 through 8 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. 

Summary of All Suppressant Releases (Includes Estimated OATs) 

Figures that follow summarize altitude and OAT for all suppressant releases identified in the fire 

incident data provided by the Safety Centers. This includes incidents for which Safety Center 

data included both altitude and OAT, incidents for which altitude data was provided and OAT 

was estimated using the Standard Atmosphere Model, and incidents for which both altitude and 

OAT were estimated. Figures 9 through 16 summarize fixed-wing aircraft suppressant release 

by altitude and OAT thresholds. Figures 17 through 21 summarize the same for rotary aircraft 

suppressant release. Table 4 summarizes suppressant release by Service aircraft category 

(fixed-wing and rotary). No occurrence was found in which currently fielded high-boiling point 

suppressants failed to discharge due to cold temperature conditions (i.e., frozen agent). 

 

°C: 96 -73 -51 -29 -18 -7 16 38 60 82 

°F: -140 -100 -60 -20 0 20 60 100 140 180 

103 ft (km)

 80 (24.4) 

 60 (18.3) 

 40 (12.2) 

 20 (6.1) 

 0 
18

 

1%

99%

> 40,000 feet

All Others

Figure 9. Percentage of Fixed-Wing Aircraft Suppressant Releases 

at or Greater than Altitude of 40,000 feet 

6%

94%

> 30,000 feet

All Others

Figure 10. Percentage of Fixed-Wing Aircraft Suppressant Releases 

at or Greater than Altitude of 30,000 feet 
19

28%

72%

> 20,000 feet

All Others

Figure 11. Percentage of Fixed-Wing Aircraft Suppressant Releases 

at or Greater than Altitude of 20,000 feet 

6%

94%

< -40F (-40C)

All Others

Figure 12. Percentage of Fixed-Wing Aircraft Suppressant Release 

at or Less than OAT Equal to -40°F (-40°C) 
20

9%

91%

< -30F (-34.4C)

All Others

Figure 13. Percentage of Fixed-Wing Aircraft Suppressant Release 

at or Less than OAT Equal to -30°F (-34.4°C) 

10%

90%

< -25F (-31.7C)

All Others

Figure 14. Percentage of Fixed-Wing Aircraft Suppressant Release 

at or Less than OAT Equal to -25°F (-31.7°C) 
21

23%

77%

< -20F (-28.9C)

All Others

Figure 15. Percentage of Fixed-Wing Aircraft Suppressant Release 

at or Less than OAT Equal to -20°F (-28.9°C) 

25%

75%

< -13F (-25C)

All Others

Figure 16. Percentage of Fixed-Wing Aircraft Suppressant Release 

at or Less than OAT Equal to -13°F (-25°C) 
22

3%

97%

> 15,000 feet

All Others

Figure 17. Percentage of Rotary Aircraft Suppressant Releases 

at or Greater than Altitude of 15,000 feet 

13%

87%

> 10,000 feet

All Others

Figure 18. Percentage of Rotary Aircraft Suppressant Releases 

at or Greater than Altitude of 10,000 feet 
23

3%

97%

< -5F (-20.6C)

All Others

Figure 19. Percentage of Rotary Aircraft Suppressant Release 

at or Less than OAT Equal to -5°F (-20.6°C) 

7%

93%

< 10F (-12.2C)

All Others

Figure 20. Percentage of Rotary Aircraft Suppressant Release 

at or Less than OAT Equal to 10°F (-12.2°C) 
24

13%

87%

< 20F (-6.7C)

All Others

Figure 21. Percentage of Rotary Aircraft Suppressant Release 

at or Less than OAT Equal to 20°F (-6.7°C) 
25

Table 4. Summary of Suppressant Release by Service Aircraft Category 

(Fixed Wing and Rotary) 

Army Navy Air Force 

Fixed-wing aircraft: 

• One release identified at 

20,000 ft, another at 

25,000 ft. Temperature 

conditions estimated as 

-12.2°F (-24.6°C) and 

-30°F (-34.4°C), 

respectively 

• One release identified at 

16,000 ft, another at 

18,000 ft. Temperature 

conditions estimated as 

2°F (-16.6°C) and -5.1°F 

(-20.6°C), respectively 

• Remainder of releases at 

10,000 ft or lower. 

Temperature conditions 

estimated to be >45°F 

(7.2°C) 

Fixed-wing aircraft: 

• No releases identified at 

>30,000 ft 

• Two releases identified 

at >20,000 ft, 

temperature conditions 

estimated as -16°F 

(-26.7°C) and -34°F 

(-36.7°C) 

• One release identified at 

18,000 ft, another at 

19,000 ft. Temperature 

conditions estimated as 

-9°F (-12.8°C) and -5°F 

(-20.6°C), respectively 

• Remainder of releases at 

12,000 ft and lower, 

temperature conditions 

estimated to range 

between 16°F (-8.9°C) 

and 93°F (33.9°C) 

Fixed-wing aircraft: 

• Overwhelming majority 

of releases <30,000 ft

• Two releases identified 

at >40,000 ft, 

temperature condition 

estimated as -70°F 

(-56.7°C) 

• 11 releases identified at 

>30,000 ft, temperature 

conditions estimated to 

range between -47.8°F 

(-44.3°C) and -70°F 

(-56.7°C) 

• Many releases (31%) at 

>20,000 ft, temperature 

conditions estimated to 

range between -33.6°F 

(-36.4°C) and -12.2°F 

(-24.6°C) 

• For releases at <20,000 

ft, temperature 

conditions estimated to 

range from 2°F (-16.7°C) 

to 59°F (15°C) 

Rotary aircraft: 

• Majority of releases 

< 15,000 ft. 

• Two releases identified 

at 19,150 ft, temperature 

condition estimated as 

-9.2°F (-22.9°C) 

• Remainder of releases at 

11,000 ft and lower, 

temperature conditions 

estimated to range 

between 19.8°F (-6.8°C) 

and 93.2°F (34°C) 

Rotary aircraft: 

• No releases identified 

above aircraft service 

ceilings, which range 

between 10,000 ft and 

19,150 ft for all but one 

aircraft type. 

Temperature conditions 

estimated as -3°F 

(-19.4°C) to 23°F 

(-5°C) for this ceiling 

range 

• One Navy rotary 

platform has a ceiling of 

27,900 ft , but no 

releases were identified 

for this type 

Rotary Aircraft: Lowest 

temperature condition estimated 

for release from a rotary aircraft 

discharge was 5.6°F (-14.7°C) 
26

Pilot Response 

Figure 22 summarizes pilot response time to initiate agent release based of review of Navy fixedwing fire incident data and application of the qualitative categorization of response time as 

discussed under Technical Approach. Review of previous work to assess effectivity of 

currently-fielded halon 1301 systems [26,27] indicates that there is not a one-for-one 

correspondence of pilot response to fire-out success (i.e., effectivity was noted to be much less 

than 100%). 

93%

2% 5%

normal

slow

long

Figure 22. Pilot Response Time to Initiate Agent Release During 

Navy Fixed-Wing Aircraft Engine Nacelle Fires 
27

IN-FLIGHT NACELLE AIR TEMPERATURE MODEL 

The nacelle air temperature model described under Technical Approach was applied to 1,025 

cases, bounded by altitude to 30,000 feet. Model runs were limited to this altitude for 2 reasons: 

1) only 6% of all fixed-wing aircraft fire suppressant releases were indicated to have occurred 

above 30,000 feet, and 2) though the Standard Atmosphere Model for the tropopause has a 

ceiling of 36,152 feet, only 1.7% of all fixed-wing aircraft fire suppressant releases were 

indicated to have occurred above this ceiling. The OAT ranged from -48.3°F (-44.6°C) to 58.7°F 

(14.8°C) based on the model, and input conditions for aircraft airspeed, altitude, nacelle physical 

dimensions, nacelle airflow velocity, and nacelle surface temperature were as described under 

Technical Approach. Results are indicated in Figures 23 and 24, which depict peak nacelle 

temperature versus altitude for the two airspeed conditions modeled: 50 knots and 400 knots. 

In review of the model output, 88% of the cases indicated nacelle air temperatures greater than 

0°F (-17.8°C). Closer review of the remaining 12% of the cases (those less than 0°F) were noted 

for input conditions at 20,000 ft or greater, and 89% of these cases (89% of the 12%) were noted 

for at airspeeds of 50 knots. The implication of these cases is that 1) they are not credible for 

typical military rotorcraft, which typically have operational ceilings less than 20,000 feet, and 2) 

they are not credible for typical military fixed-wing aircraft that have nacelle fire suppression 

capability (e.g., fighter/attack aircraft, cargo transports, patrol aircraft) as 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 30,000 feet and 400 knots and indicated nacelle air 

temperatures ranging between -10°F (-23.3°C) and -12°F (-24.4°C), which equated to 1.5% of all 

cases modeled. 

It must be noted that if the non-credible cases are then removed from consideration the actual 

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

greater than 88%. So modeling additional cases up to the ceiling of the Standard Atmosphere 

Model for the tropopause is likely to result in additional nacelle air temperatures less than 0°F 

(-17.8°C), but it is likely to not dramatically impact the percentages described. 

The results indicated appear counterintuitive, in that at higher airspeed the results would be 

expected to indicate lower compartment air temperature. This is due to the assumption in the 

model that the temperature at the nacelle inlet is based on the stagnation properties for the 

airspeeds chosen. Thus, at the higher airspeed the inlet temperature is greater. To investigate 

this further, results from the model were compared to data obtained previously during in-flight 

measurement of nacelle air temperatures for several different aircraft platforms [16-20]. 

Additionally, nacelle air temperature data was obtained from current in-flight rotary aircraft 

propulsion temperature survey testing. Results of the comparison are summarized in Table 5 and 

are the temperatures determined by equation (13). When applying the model 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 is not 

uniform, thus for each case this parameter was varied between the low and high values indicated 

in Figure 4, unless specific nacelle clearance information was obtained. 
28

-60

-40

-20

0

20

40

60

80

100

120

140

160

180

0 5000 10000 15000 20000 25000 30000 35000

Altitude, feet

Temperature, degrees F

-51

-46

-40

-35

-29

-24

-18

-13

-7

-2

4

9

15

20

26

31

37

42

48

53

59

64

70

75

81

0 1524 3048 4572 6096 7620 9144 10668

Altitude, meters

Temperature, degrees C

Figure 23. Peak Nacelle Temperature at 50 knots Airspeed versus Altitude 

-60

-40

-20

0

20

40

60

80

100

120

140

160

180

200

220

0 5000 10000 15000 20000 25000 30000 35000

Altitude, feet

Temperature, degrees F

-51

-46

-40

-35

-29

-24

-18

-13

-7

-2

4

9

15

20

26

31

37

42

48

53

59

64

70

75

81

86

92

97

103

0 1524 3048 4572 6096 7620 9144 10668

Altitude, meters

Temperature, degrees C

Figure 24. Peak Nacelle Temperature at 400 knots Airspeed versus Altitude 
29

Similarly, engine surface temperature will vary, typically lower at the forward end of the engine 

and increasing towards the aft end, thus for each case this parameter is varied between the low 

and high values indicated in Table 5. The model under-predicts the measured values, leading to 

a conclusion that the model is conservative. 

Table 5. Comparison of Modeled versus Predicted Nacelle Air Temperatures 

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. 410 Not modeled since engine 

surface temperature range 

not indicated. 

COLD SOAK CONDITIONS 

As indicated under Technical Approach, the premise is that for agent, agent bottle(s), distribution 

lines and components not located near/within heated compartments but adjacent to exterior 

surfaces, the stagnation temperature should provide a reasonable estimate of likely temperature 

conditions of agent and components adjacent to exterior surfaces. Thus estimation of these 

temperature conditions during takeoff focused on larger fixed-wing aircraft, i.e., jet and 

turboprop transport aircraft, thus takeoff climb speeds to cruise altitude for these type of aircraft 

were applied. Such aircraft require takeoff climb speeds starting at approximately 135 knots, 

increasing to the 150 to 320-knot range during climb, depending on type of aircraft (turboprop or 

turbofan), and then leveling off to cruise at speeds ranging between 190 and 340 knots, again 

depending on altitude and aircraft type. Conditions for a fixed-wing fighter aircraft were also 

considered for comparison. Due to aircraft location restrictions, fighter aircraft may be more 

likely to have components located adjacent to heated compartments (e.g., EA-6B aircraft have a 
30

fire bottle located within each nacelle; the F/A-18 aircraft fire bottle is located adjacent to an 

engine compartment). 

Three scenarios were evaluated as described below. Takeoff climb profiles and speeds for the 

fighter aircraft and the turboprop transport aircraft were obtained from [40]. The takeoff climb 

profile for the jet transport aircraft was derived from [41]. 

1. Scenario 1, Figure 25, is based on JAR-1 Arctic profile only, thus assumes starting OAT 

is -58°F (-50°C) based on the JAR-1 Arctic profile. It also assumes that this starting 

OAT represents agent and component temperature conditions at takeoff. The temperature 

recovery factor in equation (14) is set equal to 1. This is the worst-case scenario. 

However, each of the profiles depict a period of stagnation temperature above -25°F 

(-31.7°C), with this period of time being shortest for the jet fighter aircraft 

(approximately 3.7 minutes) and longest for the jet transport aircraft (approximately 14.2 

minutes). The -25°F (-31.7°C) threshold is achieved in approximately 1.25 minutes for 

the jet fighter aircraft and in approximately 3.8 minutes for the jet transport aircraft. 

2. Scenario 2, Figure 26, is based on JAR-1 Arctic profile only, but starting OAT is -40°F 

(-40°C) and OAT is held at this temperature during takeoff climb until OAT begins to 

change per the JAR-1 Arctic profile. It also assumes that this starting OAT represents 

agent and component temperature conditions at takeoff. The temperature recovery factor 

in equation (14) is set equal to 1. Again, each of the profiles depict a period of stagnation 

temperature above -25°F (-31.7°C), with this period of time being shortest for the jet 

fighter aircraft (approximately 2.7 minutes) and longest for the jet transport aircraft 

(approximately 14.2 minutes). Again, the -25°F (-31.7°C) threshold is achieved in 

approximately 1.25 minutes for the jet fighter aircraft and in approximately 3.8 minutes 

for the jet transport aircraft. 

3. Scenario 3, Figure 27, is based on JAR-1 Arctic profile only, but starting temperature is 

-40°F (-40°C) with a bias assuming a temperature differential based on the Transport 

Canada cold-soak temperature measurements, and the biased OAT is held at this level 

until OAT begins to change per the JAR-1 Arctic profile. The starting OAT is assumed 

to represent agent and component temperature conditions at takeoff. The temperature 

recovery factor in equation (14) is set equal to 1. The Transport Canada work indicated a 

+6°C differential of wing skin temperature above OAT at -25°C, the lowest OAT for 

which data was collected, thus this was the differential assumed for Scenario 3. This 

scenario likely provides an estimate of the upper bound for possible component 

temperature conditions during takeoff climb. Each of the profiles depict a period of 

stagnation temperature above -15°F (-26.1°C), with this period of time being shortest for 

the jet fighter aircraft (approximately 3 minutes) and longest for the jet transport aircraft 

(approximately 14.8 minutes). The -15°F (-26.1°C) threshold is achieved in 

approximately 1.2 minutes for the jet fighter aircraft and in approximately 3.6 minutes for 

the jet transport aircraft. Because of the duration of the climb for the jet transport 

aircraft, temperature is indicated to increase for a period of time to approximately 4.5°F (-

15.3°C) before beginning to decrease. 
31

Each of the preceding scenarios are modeled with the temperature recovery factor in equation 

(14) is set equal to 1. However, Reference [34] indicates the temperature recovery factor can 

vary between 0.7 and 1. Table 6 summarizes the difference in results when the temperature 

recovery factor is set to 0.7. In general, the affect of the recovery factor value set equal to 0.7 is 

to shorten the duration at or greater than the temperature thresholds indicated in the table and 

slightly lengthen the time to reach those thresholds. In the case of the turboprop aircraft in 

scenarios 1 and 2, a period of time is sustained above -26°F (-32.2°C), 1°F lower, and in all three 

scenarios the turboprop transport is indicated to take the longest time to reach the -25°F (-31.7°C) 

threshold (scenario 3) or the -26°F (-32.2°C) threshold (scenarios 1 and 2). The longer time can be 

attributed to the lower turboprop transport airspeed. 

Table 6. Comparison of Takeoff Scenarios for Temperature Recovery Factor (K) at 1 and 0.7 

K = 1 K = 0.7 

Aircraft Duration Time to Achieve Duration Time to Achieve

Scenario 1: 

• Jet Fighter 

• Jet Transport 

• Turboprop 

Transport 

At least -25°F 

(-31.7°C) for: 

• 3.7 minutes 

• 14.2 minutes 

• 6 minutes 

Achieved -25°F 

(-31.7°C) in: 

• 1.25 minutes 

• 3.8 minutes 

• 5.5 minutes 

At least -25°F 

(-31.7°C) for: 

• 2 minutes 

• 10.5 minutes 

• -26°F (-32.2°C) 

for 5.9 minutes 

Achieved -25°F 

(-31.7°C) in: 

• 1.4 minutes 

• 4.4 minutes 

• 5 minutes 

Scenario 2: 

• Jet Fighter 

• Jet Transport 

• Turboprop 

Transport 

At least -25°F 

(-31.7°C) for: 

• 2.7 minutes 

• 14.2 minutes 

• 5.9 minutes 

Achieved -25°F 

(-31.7°C) in: 

• 1.25 minutes 

• 3.8 minutes 

• 5.35 minutes 

At least -25°F 

(-31.7°C) for: 

• 1.95 minutes 

• 10.4 minutes 

• -26°F (-32.2°C) 

for 5.9 minutes 

Achieved -25°F 

(-31.7°C) in: 

• 1.4 minutes 

• 4.3 minutes 

• 5 minutes 

Scenario 3: 

• Jet Fighter 

• Jet Transport 

• Turboprop 

Transport 

At least -15°F 

(-26.1°C) for: 

• 3 minutes 

• 14.8 minutes 

• 7.1 minutes 

Achieved -15°F 

(-26.1°C) in: 

• 1.25 minutes 

• 3.8 minutes 

• 4.8 minutes 

At least -15°F 

(-26.1°C) for: 

• 2.2 minutes 

• 11 minutes 

• 3.2 minutes 

Achieved -15°F 

(-26.1°C) in: 

• 1.3 minutes 

• 4.2 minutes 

• 7.3 minutes 
32

-80.00

-75.00

-70.00

-65.00

-60.00

-55.00

-50.00

-45.00

-40.00

-35.00

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Time, seconds

Temperature, degrees F

Jet Transport Aircraft

Fighter Aircraft

Turboprop Transport Aircraft

Figure 25. Scenario 1, OAT at Takeoff is -50°C (-58°F) 
33

-80.00

-75.00

-70.00

-65.00

-60.00

-55.00

-50.00

-45.00

-40.00

-35.00

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Time, seconds

Temperature, degrees F

Jet Transport Aircraft

Fighter Aircraft

Turboprop Transport Aircraft

Figure 26. Scenario 2, OAT at Takeoff is -40°C (-40°F) 
34

-80.00

-75.00

-70.00

-65.00

-60.00

-55.00

-50.00

-45.00

-40.00

-35.00

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Time, seconds

Temperature, degrees F

Jet Transport Aircraft

Fighter Aircraft

Turboprop Transport Aircraft

Figure 27. Scenario 3, OAT at Takeoff is -40°C (-40°F) with Bias Applied to OAT 

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 such as those outlined in [35] are applied to assess worstcredible 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.) Figures 28 through 30 

illustrate the risk matrices used within DoD aviation organizations to categorize risk. Generally, 

assessment of fire hazards results in an assignment of a Catastrophic severity. 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 development of 

References [26,27] are considered, the aggregate rate of occurrence for an engine nacelle fire 

event during the period evaluated is approximately 8 per 106

 flight hours. Using data from 

[26,27], the potential hazard frequency of a catastrophic event due to an engine nacelle fire 

hazard at any time can be then assessed as indicated in Tables 7 and 8. Note that in each case, 
35

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 operation in a low (or high) 

temperature climatic extreme, the hazard frequency would also be assessed as improbable as 

indicated in Tables 9 and 10. 

Figure 28. Army Aviation Hazard Severity-Probability Risk Assessment Matrix [36] 
36

CATASTROPHIC (1) CRITICAL (2) MARGINAL (3) NEGLIGIBLE (4)

FREQUENT (A) 

= or > 100/100K flt hrs 1 3 7 13

PROBABLE (B) 

10-99/100K flt hrs 2 5 9 16

OCCASIONAL (C) 

1.0-9.9/100K flt hrs 4 6 11 18

REMOTE (D) 

0.1-0.99/100K flt hrs 8 10 14 19

IMPROBABLE (E) 

= or < 0.1/100K flt hrs 12 15 17 20

PMA Acceptance

11-17 LOW SAFETY RISK

IPT / FST / SSWG Acceptance

6-10 MEDIUM SAFETY RISK 18-20 VERY LOW SAFETY RISK

Severity is the worst credible consequence of a hazard in terms of degree of injury, property damage or effect on mission defined below:

Catastrophic - Class A ( damage > $1M / fatality / permanent total disability)

Critical - Class B ($200K < damage < $1M / permanent partial disability / hospitalization of 5 or more personnel)

Marginal - Class C ($10K < damage < $200K / injury results in 1 or more lost workdays)

Negligible - All other injury/damage less than Class C

Probability of occurrence for discreet events may replace Frequency based upon the chart below:

Frequent Probable Occasional Remote Improbable

1/103 1/104 1/105 1/106

UNDESIRABLE ACCEPTABLE 

WITHOUT REVIEW

1-5 HIGH SAFETY RISK

PEO / AIR-1.0 Acceptance

S E V E R I T Y

F

R

E

Q

U

E

N

C

Y

HAZARD 

CATEGORIZATION

UNACCEPTABLE ACCEPTABLE 

WITH REVIEW

CNO / TYCOM / Fleet Acceptance

Figure 29. Navy Aviation Hazard Severity-Probability Risk Assessment Matrix [37] 

Figure 30. Limits of Risk Acceptability, Air Force Systems and Equipment Design [38] 
37

Table 7. Estimate of Rate of Occurrence of Aircraft Lost Due to Failure 

to Extinguish a Nacelle Fire, Any Time (Does not Consider Multiple Engines) 

 Probability Probability Probability End Event 

Probability Occurrence Occurrence Occurrence Rate of MIL-STD-882 

Occurrence At Any Given Fire NOT Aircraft Occurrence Hazard 

Aircraft Category In Flight Time Extinguished Lost Per Flight Hour 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) 

 Probability Probability Probability End Event 

Probability Occurrence Occurrence Occurrence Rate of MIL-STD-882 

Occurrence At Any Given Fire NOT Aircraft Occurrence Hazard 

Aircraft Category on Ground Time Extinguished Lost Per Flight Hour 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) 

Notes: 

(1) Data in Table 1 based on References [26,27]. 

(2) 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 fires over all flight hours 

for the period evaluated in References [26,27]. 

(3) No rotary aircraft were indicated lost in ground fire events in Reference [27] but a 1% probability 

is assumed for discussion purposes. 

Table 8. Estimate of Rate of Occurrence of Aircraft Lost Due to Failure 

to Extinguish a Nacelle Fire, Any Time (Assumes 2 Engines per Aircraft) 

 Probability Probability Probability End Event 

Probability Occurrence Occurrence Occurrence Rate of MIL-STD-882 

Occurrence At Any Given Fire NOT Aircraft Occurrence Hazard 

Aircraft Category In Flight Time Extinguished Lost Per Flight Hour 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) 

 Probability Probability Probability End Event 

Probability Occurrence Occurrence Occurrence Rate of MIL-STD-882 

Occurrence At Any Given Fire NOT Aircraft Occurrence Hazard 

Aircraft Category on Ground Time Extinguished Lost Per Flight Hour 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) 

Notes: 

(1) Data in Table 1 based on References [26,27]. 

(2) 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 fires over all flight hours 

for the period evaluated in References [26,27]. 

(3) No rotary aircraft were indicated lost in ground fire events in Reference [27] but a 1% probability 

is assumed for discussion purposes. 
38

Table 9. Estimate of Rate of Occurrence of Aircraft Lost Due to Failure 

to Extinguish a Nacelle Fire in a Climatic Extreme (Does not Consider Multiple Engines) 

 Probability Probability Probability End Event 

Probability Occurrence Occurrence Occurrence Rate of MIL-STD-882 

Occurrence in Climatic Fire NOT Aircraft Occurrence Hazard 

Aircraft Category In Flight Extreme Extinguished Lost Per Flight Hour Frequency 

Fixed-wing 0.55 0.2 0.24 0.09 1.97E-08 Improbable (E) 

Rotary 0.35 0.2 0.53 0.27 8.30E-08 Improbable (E) 

 Probability Probability Probability End Event 

Probability Occurrence Occurrence Occurrence Rate of MIL-STD-882 

Occurrence in Climatic Fire NOT Aircraft Occurrence Hazard 

Aircraft Category On Ground Extreme Extinguished Lost Per Flight Hour Frequency 

Fixed-wing 0.45 0.2 0.38 0.03 8.50E-09 Improbable (E) 

Rotary 0.65 0.2 0.36 0.01 3.88E-09 Improbable (E) 

Notes: 

(1) Data in Table 1 based on References [26,27]. 

(2) 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 fires over all flight hours 

for the period evaluated in References [26,27]. 

(3) No rotary aircraft were indicated lost in ground fire events in Reference [27] but a 1% probability 

is assumed for discussion purposes. 

(4) Probability of operation in climatic extreme assumes either MIL-HDBK-310 low or high 

temperature 20% WWAE. 

Table 10. Estimate of Rate of Occurrence of Aircraft Lost Due to Failure 

to Extinguish a Nacelle Fire in a Climatic Extreme (Assumes 2 Engines per Aircraft) 

 Probability Probability Probability End Event 

Probability Occurrence Occurrence Occurrence Rate of MIL-STD-882 

Occurrence in Climatic Fire NOT Aircraft Occurrence Hazard 

Aircraft Category In Flight Extreme Extinguished Lost Per Flight Hour Frequency 

Fixed-wing 0.55 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) 

 Probability Probability Probability End Event 

Probability Occurrence Occurrence Occurrence Rate of MIL-STD-882 

Occurrence in Climatic Fire NOT Aircraft Occurrence Hazard 

Aircraft Category on Ground Extreme Extinguished Lost Per Flight Hour 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) 

Notes: 

(1) Data in Table 1 based on References [26,27]. 

(2) 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 fires over all flight hours 

for the period evaluated in References [26,27]. 

(3) No rotary aircraft were indicated lost in ground fire events in Reference [27] but a 1% probability 

is assumed for discussion purposes. 

(4) Probability of operation in climatic extreme assumes either MIL-HDBK-310 low or high 

temperature 20% WWAE. 
39

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 31 summarizes fixed-wing fire incidents by phase of operation. The takeoff-related 

categories total to 18.7% of all incidents. When this incident data is reviewed for the number of 

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

the takeoff phases. However, when these releases are reviewed further, only 4% of the takeoffrelated releases (and thus less than 1% of all releases) occurred in land environments categorized 

as cold or severe cold. This strongly suggests that risk is low (improbable hazard frequency) for 

an engine nacelle fire during takeoff on a cold-soaked aircraft and in which the fire suppression 

system fails to extinguish the fire and a catastrophic event occurs. 

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%

Figure 31. Fixed-Wing Aircraft Fire Incidents by Phase of Operation 
40

CONCLUSIONS 

The intent of this effort was to investigate whether temperature conditions at the time of release 

of an aircraft engine nacelle fire suppression agent correlate with the agent low boiling point 

temperature criterion being applied under the NGP, which is currently -40°C (-40°F). This effort 

focused on evaluation of OATs at the time of agent release, nacelle operating temperatures, and 

cold-soak conditions. The effort was accomplished by: analysis of aircraft fire incident data 

from the Army, Navy, and Air Force Safety Centers; development and correlation of an in-flight 

nacelle air temperature model to estimate nacelle operating temperature conditions at altitude; 

consideration of potential cold-soak conditions, including temperature conditions during takeoff; 

and discussion of risk associated with the occurrence of nacelle fires, including operation in a 

climatic extreme and during takeoff. Significant findings are as follows: 

1. Safety Center Data: 

• No rotary aircraft suppressant releases were identified at altitudes equal to or greater than 

20,000 feet, corresponding to a Standard Atmosphere OAT of -13°F (-25°C). The lowest 

release OAT indicated was temperature –9.2°F (-22.9°C). This is suggesting minimal 

risk if the low temperature boiling point requirement for rotary aircraft was comparable 

to -13°F (-25°C). Given that only 3% of all rotary aircraft releases occurred at altitudes 

greater than 15,000 feet, minimal risk is also indicated for agents with a boiling point 

corresponding to a Standard Atmosphere OAT of 5.6°F (-14.7°C). Only 7% all rotary 

aircraft releases occurred at or less than an OAT of 10°F (-12.2°C) 

• The lowest fixed-wing aircraft release at altitude was estimated to have occurred at an 

OAT of -70°F (-56.67°C), accounting for less than 1% of all fixed-wing aircraft releases. 

The overwhelming majority of fixed-wing aircraft suppressant releases were found to 

have occurred below 30,000 feet (94%), with 9% of all releases and at or less than an 

OAT of -30°F (-34.4°C) and 10% of all releases at or less than an OAT of -25°F 

(-31.7°C). Seventy-two percent (72%) of all releases were found to have occurred below 

20,000 feet, and 75% of all releases were found to have occurred at or less than an OAT 

of -13°F (-25°C). Consideration of a lower temperature threshold for fixed-wing aircraft 

would need to take into consideration findings described below related to nacelle air 

temperatures and cold soaking. 

• Review of fire incident data from the Safety Centers that provided both altitude and OAT 

indicated few incidents at low temperature, with only three indicating OAT at or below 

-13°F (-25°C). 

• The Standard Atmosphere Model provides a reasonable estimation of OAT at altitude, as 

the majority fire incident data from the Safety Centers that provided both altitude and 

OAT were indicated to fall above the Standard Atmosphere profile. 

• Even though qualitative assessment of pilot response indicates that response to nacelle 

fire conditions is timely 93% of the time, previous work to assess effectivity of currentlyfielded halon 1301 systems [26,27] indicates that there is not a one-for-one 

correspondence in fire-out success (i.e., effectivity was noted to be much less than 

100%). This suggests strongly that alternative suppression agents having improved 

effectiveness over halon 1301, methods to ensure better fire suppression system design, 
41

or a combination of both would enhance the benefit provided by on-board fire 

suppression systems. 

2. In-Flight Nacelle Air Temperature Model: 

• 88% of the cases indicated nacelle air temperatures greater than 0°F (-17.8°C) 

• Of the remaining 12% of the cases modeled, all were indicated for altitudes at 20,000 feet 

or greater, and 89% of these cases were for 50-knot airspeed conditions. The 

overwhelming majority of these remaining cases were considered not-credible in that: 

o Military rotorcraft typically have operational ceilings less than 20,000 feet. 

o A 50-knot airspeed for a military fixed-wing aircraft would be typically below

stall speed. 

• The remaining 11% (i.e., 11% of the 12%) were noted for input conditions at 30,000 feet 

and 400 knots and indicated nacelle air temperatures ranging between -10°F (-23.3°C) 

and -12°F (-24.4°C), which equated to 1.5% of all cases modeled. 

• Comparison of model output to limited flight test data suggests that the model is 

conservative. 

3. Cold-Soak Conditions: 

• A literature review identified previous work to assess aircraft cold-soak conditions. The 

lowest OAT for which aircraft wing surface temperatures are recorded in that literature is 

-25°C. This is suggesting that below -25°C aircraft operations on the ground in cold or 

extreme cold climates is infrequent, i.e., why endeavor to measure for such conditions 

unless such measurements are recorded during representative operations. Depending on 

the cold-soak mechanism, wing surface temperatures varied about this temperature by 

+6°C and -6°C (non-radiative cooling temperature differential range was 2°C to 6°C, 

whereas the likely radiative cooling temperature differential range is likely -6°C to -2°C. 

• Estimation of aircraft stagnation temperatures was performed to assess the potential for 

increase in temperature of agent and fire suppression system components that are located 

adjacent to surfaces, with the assumption that the stagnation temperature will 

approximate the temperature of these items. Larger fixed-wing aircraft were considered 

since these are the aircraft type that meet this criterion. Depending on the scenario and 

fixed-wing aircraft type: 

o In the cases where component temperatures are assumed the same as OAT, the 

resultant temperature profiles indicate a period above -25°F (-31.7°C) for as few 

as 3.7 minutes and as long as 14.2 minutes. The time to reach this threshold is 

indicated to occur approximately within 1.25 minutes or within 3.8 minutes. 

o In the cases where a temperature differential is applied in which the component 

temperatures are assumed to be slightly greater than OAT, the resultant 

temperature profiles indicate a period above -15°F (-26.1°C), for as few as 3 

minutes and as long as 14.8 minutes. The time to reach this threshold is indicated 

to occur approximately within 1.25 minutes or within 3.6 minutes. Because of the 

duration of the climb for the jet transport aircraft, temperature is indicated to 

increase for a period of time to approximately 4.5°F (-15.3°C) before beginning to 

decrease. 
42

4. Risk 

• For either the fixed-wing aircraft or rotary aircraft case, the hazard of catastrophic aircraft 

loss due to a nacelle fire in which the on-board nacelle fire suppression system fails to 

extinguish the fire was indicated to have an improbable hazard frequency, indicating a 

low-risk hazard. 

• For either the fixed-wing aircraft or rotary aircraft case, the hazard of catastrophic aircraft 

loss due to a nacelle fire in which the on-board nacelle fire suppression system fails to 

extinguish the fire while operating in a climatic extreme (i.e., cold environment) was 

indicated to have an improbable hazard frequency, indicating a low-risk hazard. 

• The probability of agent release during takeoff on a cold-soaked aircraft and in which the 

fire suppression system fails to extinguish the fire and a catastrophic event occurs is 

considered improbable. 

In terms of the -40°F (-40°C) boiling point criterion being applied by the NGP, the 

preponderance of the release data combined with the preceding analyses suggests: 

• For rotary aircraft, the criterion could be increased to 5.6°F (-14.7°C) or 10°F (-12.2°C) 

with minimal risk. 

• For fixed-wing aircraft the percentage of releases at or less than an OAT of -13°F (-25°C) 

is 75%, easily a majority of all releases. However, nacelle air temperature modeling 

discussed previously suggests that within the nacelle, this temperature threshold is likely 

the minority air temperature condition, i.e., temperature conditions within an engine 

nacelle are likely higher. On the ground, published data with regards to cold soaking 

provided no data for OATs below -13°F (-25°C), suggesting that aircraft operations on 

the ground in cold or extreme cold climates is infrequent. This also appears to be 

supported by the fire incident data. Additionally, it is also estimated that during takeoff 

climb within a standard arctic profile that the stagnation temperatures can increase to 

greater than -13°F (-25°C) for a period of time, taking into consideration the likelihood 

that aircraft surface temperature will be greater than OAT at takeoff. The relevance of 

this is that fire suppression system components adjacent to these surfaces are likely to be 

similar in temperature. Thus for fixed-wing aircraft it appears the criterion could be 

increased to -13°F (-25°C). Preceding risk analysis in terms of likelihood of a 

catastrophic fire event in cold or severe-cold conditions, in low-temperature worldwide 

air environments, and during takeoff after being cold-soaked suggests this would be low 

risk 

RECOMMENDATIONS 

NGP efforts to evaluate high-boiling point halon alternative agents should verify the assertion 

that in order to achieve acceptable performance at low temperature, the vapor pressure of the 

extinguishing agent must be higher than the partial pressure required for an extinguishing 

concentration. 
43

A criterion for certifying any fire suppression system with halon alternative agents identified 

should be established that is consistent with the application in which the agent is to be used. For 

example, worst-case conditions for certification may be for highest aircraft speed and lowest 

altitude, i.e., sea level. With engines running, nacelle compartment temperatures are likely to not 

be cold. 

Previous justification for implementing currently-fielded high-boiling-point agents needs to be 

better understood. These agents have boiling points far in excess of -40°F (-40°C): halon 1011, 

Tb = 151°F; halon 1202, Tb = 76°F; and most recently halon 1211, Tb = 25°F. Lessons learned 

from this experience in terms of fire suppression design (e.g., “winterization”) and system 

certification may serve to guide any verification testing of new halon alternatives identified by 

the NGP as well as provide additional rationale that a boiling point criterion even greater than 

-13°F (-25°C) is feasible. 

New halon alternatives identified by the NGP and systems to deliver them should provide 

military aviation program managers with at least the same level of risk as currently provided by 

legacy halon 1301 systems, i.e., they should not increase the probability of not extinguishing fire. 

Research, engineering and testing should provide the necessary confidence in this regard. 

 

ACKNOWLEDGEMENTS 

This research is part of the Department of Defense’s Next Generation Fire Suppression 

Technology Program (NGP), funded by the DoD Strategic Environmental Research and 

Development Program (SERDP). 

The author is grateful for the funding support provided by the NGP, which made this effort 

possible. Additionally, the author wishes to express his appreciation to Dr. David Keyser, INS 

Corporation, for development of the Nacelle Air Temperature Model, and Edward Gaborow, 

NAVAIR System Safety, for his assistance in researching the fire incident narratives provided by 

the Safety Centers. Additionally, this effort required support from various DoD Agencies and 

professionals and the author expresses his appreciation to them. They include: 

• U.S Army Safety Center 

• U.S. Air Force Safety Center 

• U.S. Navy Safety Center 

• Transport Canada 

• APS Aviation, Inc. 

• INS Corporation 
44

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