EXAMINING THE FEASIBILITY OF EXTINGUISHING ENGINE NACELLE FIRES 

BY THE STRATEGIC PLACEMENT OF INTUMESCENT MATERIALS 

Ginger Bennett 

9002-Allen & Hamilton. Inc. 

ABSTRACT 

The objective of this program was to explore strategic placement of intumescent materials within the aircraft engine 

nacelle (AEN) for fire protection. Since the funding ofthis program was limited, an experimental evaluation was 

not allowable and only a literature review and manufacturer survey were performed. Intumescent materials respond 

to impingement of a tire by swelling and forming a protective char, to physically and thermally protect the coated 

structure. They have been used (or investigated for use) in various military platforms for all three services. lradeoffs must be made between the various features and requirements most important to the platform. The system can 

be engineered and problems designed around. This study showed the feasibility of utilizing strategic placement of 

intumescent materials within the AEN. For full exploitation of this technology, an experimental program is recommended. Justification for utilization of intumescent materials could be supported using a cost analysis. 

BACKGROUND 

AIRCRAFT ENGINE NACELLES 

Aircraft engine nacelles (enclosures around the cngine core) have fluid lines that are routed within the 

cnclosure on the exterior of the machinery, to provide fuel, oil or hydraulicibrake fluid for the machinery 

(all of which are flammable). These enclosures/nacelles are typically ventilated with forced airflow, by a 

fan or by the free stream outside the aircraft, to prevent the accumulation of any flammable vapors, and to 

provide some cooling. In a typical fire scenario, one of the fluid lines leaks, and sprays or streams the 

flammable fluid onto the hot machinery, which results in a fire. The ventilation airflow continues to 

support the fire and directs the orientation of the resultant flame torch downstream. An automatic extinguishing system may be discharged from upstream to apply the extinguishing agent to the fire, but due to 

rapid dilution by the ventilation airflow and short residence time near the fire (and robustness of a bluffbody stabilized flame), many of these fires will not be extinguished with agent quantities permissible for 

many of these applications. Another concern is that the fire may reignite due to the fluid continuing to 

flow onto the hot surface with replenished airflow after the extinguishant has been drawn downstream. 

Because of these challenges, current engine nacelle applications have serious problems with fire events, 

and extinguishing techniques have only limited success, or require extinguishing quantities and hardware 

that are impractical because of size and weight. Other diverse techniques proposed to date to avoid the 

limitations of traditional extinguishing systems have been inadequate for many of these applications or 

have additional unacceptable disadvantages. 

Many aircraft currently use firewalls at some location adjacent to engine nacelles to prevent fire propagation away from the engine. Unfortunately, these locations are usually limited to areas like the engine pylon (if the engine is mounted away from the aircraft body or wing), because it is desired to avoid constriction of the ventilation airflow directly around the engine under normal operating conditions. Such firewalls can also be heavy, and are only needed when an actual fire occurs and is near the site of the fire. 

The intumescent material design described here could provide such protection in a lightweight form at the 

location of the fire, without impeding the normal flow of air. 

INTUMESCENT MATERIALS 

Intumescence may be defined as “thermally induced expansion of a material.” The popping of corn, 

expansion of perlite and vermiculite, and puffing of wheat, rice, and other grain cereals are common 

examples of intumescence. The pyrotechnic “snake” (fireworks) is another familiar example; the mixture 

llalon Options Technical Working Conference 24-26 April 2001 3 I 
of sugar, oxidizer, and certain fuels generates a carbon char of highly expansive, voluminous, and friable 

nature. The mechanism of intumescence may be described as the rapid release of gas or vapor from a 

matrix which, upon rapid heating, undergoes a plastic or viscoelastic transformation that permits it to be 

expanded, inflated, or dilated by the expanding vapor or gas [I]. Intumescent materials come in several 

different forms, which include coatinglpaint, tape, caulk/ sealant, and putty. The char thickness may 

range from between 2 and 80 times that of the original material and result in an expansion amount from 

1-30 in. The char thickness can be characterized by either high (>IS), moderate (3 to 15), or low (<3) 

volume expansion. Intumescent coatings activate in a temperature range of 270 to SO0 OF. 

POTENTIAL APPLICATION IN THE AIRCRAFT ENGINE NACELLE 

The intumescent coating can be applied as a very narrow and thin strip, in a form of one or more closed 

rings on the exterior of the machinery. These rings are positioned to swell against the enclosure at 

locations where clearance is minimal. If a fire occurred in an engine nacelle (such as due to a leak of 

flammable fluids onto hot exterior machinery components), the resulting flame would impinge onto a 

portion of the intumescent material, which would swell upon heating, normally several orders of magnitude beyond its original thickness. This swelling would block off the downstream airflow path in the 

vicinity of the fire, depriving it of a steady flow of oxygen and facilitating self-extinguishment. If the 

blockage is only partial, and the flame follows the re-directed airflow around the sealed-off area, the local 

intumescent-covered portion in that region would also swell, sealing off a perimeter of the machinery 

space and depriving oxygen flow until the fire self-extinguishes. In this manner, a series of “fire-walls” 

can be formed using a minimal quantity of intumescent. If an extinguishing system is also used, it can 

improve its effectiveness or permit smaller systems by weakening the fire and reducing the airflow dilution of the extinguishant. Previous analysis performed by the USAF suggested feasible application for 

machinery spaces (the concept having recently been submitted by the USAF for government patent 

protection, but not yet physically demonstrated). 

The intumescent coating may only be needed in a limited region of the compartment, where the origin of 

fires is most likely. The intumescent material could also be mounted on the enclosure interior side, if it is 

deemed beneficial. If the gap is relatively large between the machinery and the enclosure, then a strip of 

coating may be placed on both the enclosure and machinery surfaces, which upon expansion could meet 

in the middle. 

There is a need to find replacement extinguishing agents to the halons currently in use because production 

has been banned due to environmental concerns, but to date such replacement chemicals have shown 

reduced performance relative to halons. As a result, the Department of Defense is seeking additional 

design techniques that can assist in the performance of these replacement chemicals. This concept is 

ideal for this purpose because it weakens the flame due to oxygen starvation, and by restricting airflow 

increases its local concentration and residence time near the fire. This technique may be sufficient in 

many cases to permit the omission of an extinguishing system altogether. 

Figure 1 is a cross-sectional view of the region between the exterior of a piece of hot machinery and the 

enclosure (or nacelle) that surrounds the machinery. The machinery exterior is hot enough to ignite fluids 

in this region, or ignition may occur near a source of electrical energy, such as a wire bundle or connection. The outer enclosure or nacelle is some distance from the heated machinery. In this example, a 

region chosen in proximity to a structural rib is illustrated. The clearance between the rib and machinery 

is commonly no more than 1-3 in. Other components may also be present or used as substitutes to the 

ribs that also result in local small clearances. They may be general enclosure contours, conduits, or other 

components. In addition, some components may be present on the machinery surface itself, which may 

reduce the clearance with the enclosure. An intumescent coating could be applied in the form of a 

32 Halon Options Technical Working Conference 24-26 April 2001 
id Line 

Structural Ri 

Intumescent Material f 

Figure I. Cross-section view of region between nacelle and hot machinery. 

narrow strip onto the machinery surface, or the components attached on it, in the region of reduced clearance. This region is just downstream of a location where a fluid line is mounted. Intumescent materials 

expand greatly in size when impacted by a flame or extreme heat, forming a carbonaceous structure, possibly with an outer char layer, to form an effective thermal barrier against extreme fire conditions, and are 

widely used in many applications. Intumescent materials come in several different forms that include 

coating/paint, tape, caulkkealant, and putty. The char thickness may range from between 2 and 80 times 

that of the original material and result in an expansion amount of 1-30 in. The char thickness can be 

characterized by either high (>15), moderate (3 to IS), or low (13) volume expansion. 

As illustrated in Figure 2, the fluid line may have a leak, which would result in sprayed or leaking fluid 

that could ignite on or near the hot surface. The flame would thus orient itself downstream in the direction of ventilation airflow. Such a flame would impinge upon the strip of intumescent material placed 

downstream. Upon impingement, the intumescent material would expand, thereby forming around the rib 

or component above it and blocking off the flow of air locally. Such a local fire block would weaken the 

fire due to oxygen starvation (by preventing airflow in that direction) and possibly extinguish it. 

Flame 

I 

t-- 

Intumescent Ventilation Airflow 

Material 

Figure 2. Fluid leak and subsequent fire. 

CURRENT APPLICATIONS OF INTUMESCENT MATERIALS 

Intumescent materials have been used (or investigated for use) in various military platforms for all three 

Services. Literature sources were reviewed, and the following applications, protected areas, and uses 

were found (Table 1). Intumescent materials have been used (or investigated for use) in various 

commercial applications. Literature sources were reviewed, and the following applications, protected 

areas, and uses were found (Table 2). 

OBJECTIVE 

The objective of this program was to explore a novel approach to engine nacelle fire protection using 

strategic placement of intumescent inaterials within the ventilated aircraft engine nacelle. Since the 

funding of this program was limited, an experimental evaluation was not feasible. Therefore, the purpose 

ofthis study was to evaluate the potential application of intumescent materials in an aircraft engine 

nacelle environment by reviewing existing literature and surveying various manufacturers. 

Halon Options Technical Working Confercnce 24-26 April 2001 33 
TABLE 1. MILITARY PLATFORMS. 

Application Protected Areas Uses 

Aircraft Fuel tanks, fire zone bulkheads, military Thermal barriers, insulators, 

ordnance, stored munitions, dry bays, self-sealing 

fuel lines, cockpit 

Fire zone bulkheads, aircraft fuel tank spillage, 

prevent burnthrough, prevent toxic 

fume, smoke, and fire penetration 

Thermal barriers, insulators, 

fume, smoke, and fire penetration 

prevent bumthrough, prevent toxic 

fume, smoke, and fire penetration 

Ships 

(aircraft military ordnance prevent burnthrough, prevent toxic 

carriers) 

Submarines Fire zone bulkheads, military ordnance Thermal barriers, insulators, 

TABLE 2. COMMERCIAL APPLICATIONS. 

Application Protected Areas Uses 

Aircraft Fire zone bulkheads, military ordnance, 

stored munitions, dry bays, self-sealing fuel 

lines, cockpit, engine struts 

Residential and 

commercial 

buildings 

Automobiles 

Doorways, vents, openings, steel 

Body panels separating the passenger 

compartment from the engine compartment, 

underbody, and the trunk compartment 

Off-shore oil Metal structures and fuel valves 

platforms 

Thermal barriers, insulators, prevent 

burnthrough, prevent toxic fume, 

smoke, and fire penetration, crashworthiness, structural integrity 

Prevent toxic fume, smoke, and fire 

penetration, structural integrity 

Prevent toxic fume, smoke, and fire 

penetration, crashworthiness 

Protect flammable materials 

APPROACH 

Approximately 30 reports/papers were reviewed; the relevant data gained included the following: 

definition applications (military and commercial) 

activation temperature protected areas 

methods to increase char strength uses 

issues (toxicity, heat exposure, hazards protected against 

fragility of char, installation, humidity) 

Approximately 80 manufacturers were reviewed, downselected,and divided into three intumescent 

categories (tapes [9 manufacturers], coatings [I4 manufacturers], and caulks [9 manufacturers]). 

Relevant data gained from this survey included the following: 

expected expansion factor and resulting expansion amount based upon original thickness 

adhesiveness and vibration-resistance of the expanded char following activation by fire 

physical properties of the expanded char 

0 durability of the coating 

activation temperature 

intumescent forms 

34 Halon Options Technical Working Conference 24-26 April 2001 
ANALYSES CONDUCTED 

The following analyses were conducted using a notional aircraft: (1) weight impact due to addition of 

intumescent material; (2) resistance of intumescent material to airflow environment; and (3) reduction in 

suppressant required due to presence of intumescent material. 

SYSTEM CONFIGURATION DATA 

Current aircraft engine nacelle configuration data were obtained and used for the physical and functional 

limitations of these intumescent systems. These data included aircraft operating conditions, engine 

materials, and areas of minimal clearance and other dimensional data. These data were used to examine 

the application of intumescent materials to a notional tighter aircraft. 

OPERATING CONDITIONS 

The following sources (compiled in Table 3), were used to select operational conditions for this notional 

aircraft examination application. 

“Halon Replacement Program for Aviation, Aircraft Engine Nacelle Application, Phase IOperational Parameters Study” [2] 

Next-Generation Fire Suppression Technology Program (NGP) Element 1A (“Fires Experienced 

and Halon 1301 Fire Suppression Systems In Current Weapon Systems”) [3] 

“Halon Fire Protection Systems for Aviation-Existing System Configuration and Operational 

Environment Specifications” [4] 

“Fire Suppression System Performance of Alternative Agent in Aircraft Engine and Aircraft Dry 

Bay Simulations” [SI 

TABLE 3. RANGE OF OPERATING CONDITIONS IN AIRCRAFT ENGINE 

NACELLE ENVIRONMENT. 

Parameter Minimum Maximum 

Air Pressure (psi) 1 14.7 

Air Temperature (“F) -65 630 

Internal Air Flow (Ib/s) 1.25 2.75 

Surface Temperature (OF) 175 1300 

Fire Initiation Temperature (“F) 540 790 

Free Volume (ft’) 5 100 

MATERIALS USED IN AIRCRAFT ENGINES 

A brief review of structural and engine components was performed to identify potential material compatibility issues. These materials could be exposed to residual and combustion products in the post-deployment process. Some typical and future aircraft engine materials include titanium, aluminum, Inconel, 

stainless steel, titanium aluminide, silicon carbide fiber/silicon carbide ceramic matrix composite, and 

nickel aluminide. 

CLEARANCE AND DIMENSIONAL DATA 

A fighter and/or attack aircraft provides the most feasible platform to examine the implementation of 

intumescent materials due to the smaller clearance between the structural ribs and the engine core. The 

width of the (irregular) annular air passage is commonly no more than 2 to 6 in. 

Halon Options Technical Working Conference 24-26 April 2001 35 
WEIGHT IMPACT 

Many types of intumescent materials are currently in use. If one considers a strip 0.5 in wide, 0.12 in 

thick (to seal a clearance gap of 2 in or more) spread over an engine core of 36 in in diameter (which 

would represent an F-22 type engine), then a total volume of 0.00365 ft’ per ring would result. Accounting for the densities of the example product just mentioned, a weight 0.23 Ibs per ring would exist. Even 

if four rings were used at various regions of the nacelle, a total weight of only 0.93 Ibs would be added. 

This weight is minimal in comparison to the size of extinguisher systems currently in use, which can 

range from 10 to 20 Ibs total weight per engine. 

RESISTANCE TO AIRCRAFT OPERATING ENVIRONMENT 

Without experimental results, it is difficult to determine accurately the response of the intumescent 

material to the aircraft operating environment because it is configuration and flight condition dependant. 

However, the following methodology was employed to assess these characteristics: 

determine area of aircraft engine nacelle 

determine distributed load (pressure profile) using fluid mechanics (Ideal Gas Law, Continuity 

equation, and Bernoulli’s equation) 

determine shear and normal stresses using mechanics of materials 

compare shear and normal stresses with manufacturer information 

Determine Area of Aircraft Engine Nacelle 

The equation for the area of a cylinder was used to determine the values of A, and A, (Table 4) 

TABLE 4. PARAMETERS FOR AREA CALCULATION 

Parameters Value 

Nacelle Diameter (in) 

Core Diameter (in) 

Area rib - A, (ff) 

Area _rr - A, (ft2) 

A, = A,-A, (ft*) 

Area n.mIIC - AN (ft’) 

A, = AR-Ac-AM (ft2) 

52 

36 

14.75 

10.56 

5.94 

8.81 

GO 

Determine Distributed Load (Pressure Profile) 

Next, the Ideal Gas Law, the Continuity equation, and Bernoulli’s equation [6] along with the parameters 

in Table 5 were used to estimate the pressure profile on the expanded char. It is assumed that the dynamic pressures as a result of the airflow are the loading on the intumescent material. It is assumed that the 

velocity is the outside airflow velocity (approximately 700 mph). The resulting distributed load (pressure 

profile = p2-p,) is 8.70 Ibfhn’. 

Determine Normal and Shear Stresses 

Next, equations in mechanics of materials were used to estimate the normal and shear stresses on the 

expanded char [6]. The expanded intumescent material was represented as a cantilever beam. 

36 Halon Options Technical Working Conference 24-26 April 2001 

~~ ~ ~~__ 
TABLE 5. PARAMETERS FOR DISTRIBUTED LOAD CALCULATION. 

Parameters Symbol Value ~~ 

Pressure (Ib/ft2) 

Density (Ibift’) 

Gas constant (ft-lbf)/(lb-mole-R) 

Gas constant for air = R,,JMW (((ft-lbf)/(lb-mole-R))/ 

Molecular weight A,R (Ib/lb-mole) 

Temperature (“F) 

‘Temperature (R) 

Internal mass flow rate (Ib/sec) 

Volumetric flow rate (ft%nin) 

Cross-section of area of flow (ft’) 

Average velocity of flow (ftlsec) 

Specific weight of air (Ib/ft’) 

Average velocity of the fluid at section 1 (ft/sec) 

Average velocity of the fluid at section 2 

Acceleration due to gravity (ftisec’) 

Vertical distance from a datum (potential energy) (in) 

MW (Ib/lb-mole)) 

P 

P 

Rbar 

R 

MW 

T 

T 

Mdot 

Q 

A 

V 

Y 

VI 

v2 

g 

ZliZ2 

... 

0.04 

1545 

53.33 

28.97 

500 

959.67 

2.2 

3191.5 

.._ 

... 

0.077 

1026.67 

0 

32.17 

0 

Compare Shear and Normal Stresses with Manufacturer Information 

Finally, the values determined previously for normal and shear stresses were compared with the manufacturers’ information given for the expanded intumescent material. Physical properties (including the char 

strength) of the various intumescent materials (tapes, coatings, and caulks) investigated during this study 

were collected. It is assumed that the data provided by the manufacturers is accurate and verifiable. It is 

unclear whether some of these values (particularly the tensile strength values) are of the unexpanded or 

expanded material. Further investigation needs to be performed to determine this. 

REDUCTION IN SUPPRESSANT REQUIRED 

An estimate was made of the reduction in suppressant needed as a result of using this technology. To 

accomplish this, the following three scenarios were examined and the results are given in Table. 6: 

(1) inerting the full free volume (100 ft’); (2) two-thirds of the free volume (66 ft’); and (3)one-third of 

the free volume (33 ft’). The following assumptions were made to accomplish this estimate. 

Assumed the free volume has been sealed off by the expanded intumescent material and thus 

creating the three scenarios (full free volume [lo0 ft’], two-thirds of the free volume [66 ft’], and 

one-third of the free volume [33 ft’]). 

Treated the free volume as a total-flood application (no airflow) and determine the amount of 

agent needed to inert the fixed volume. 

Assumed that 6% by volume of Halon 1301 is required to inert the space. 

Utilized the following information: One mole of gas = 22.4 litersigram-mole. ’The molecular 

weight of Halon 1301 is 148.91 gradgram-mole. 

Also calculated was the estimate of Halon 1301 from MlL-E-22285 (Table 7) required [7]. As evidenced 

by this table, the values calculated to inert the fixed volume are less than both the values calculated using 

MIL-E-22285 and the values used in a certified system. 

Halon Options Technical Working Conference 24-26 April 2001 37 
TABLE 6. ESTIMATE OF HALON 1301 REQUIRED TO INERT FREE VOLUME. 

AEN, V, 6%V, Halon 1301, Halon 1301, Halon 1301, 

ft’ ft’ ft3 I gram Ib 

Free vol 100 6 169.88 1129.34 2.49 

2/3 free vol 66 3.96 112.12 745.36 1.64 

1/3 free vol 33 I .98 56.06 372.68 0.82 

APU, V, 6% V, Halon 1301, Halon 1301, Halon 1301, 

ft’ ft’ A’ I eram Ib 

Free vol 5 0.3 8.49 56.47 0.12 

2/3 free vol 3.3 0.198 5.61 31.27 0.08 

1/3 free vol 1.65 0.099 2.80 18.63 0.04 

TABLE 7. ESTIMATE OF HALON 1301 FROM MIL-E-22285. 

AEN, V, Halon Halon 1301, Halon 1301, Halon 1301, Halon 1301, 

ft’ ft’ 1301, Ib Ib Ib Ib Ib 

W=O.O5V W=O.O2V+O. W=3(0.02V+0.2 W=O. I6V+0.56 Calculated From 

25Wa 5Wa) Wa lnerting (Table 6) 

Rough Rough Rough nacelle/ Deep frame 

nacelle/ nacelle/ high airflow nacelle/ 

low airflow low airflow high airflow 

Free vol 100 5.00 2.55 7.65 17.232 

2/3 freevol 66 3.30 1.87 5.61 I I .792 1.64 

1/3 free vol 33 I .65 1.21 3.63 6.5 I2 0.82 

Free vol 5 0.25 0.65 1.95 2.032 0.12 

2/3 free vol 3.3 0.17 0.616 1.848 1.76 0.08 

1/3 free vol 1.65 0.08 0.583 1.749 1.496 0.04 

All of these engine nacelle systems are “certified” or approved in a given design configuration for a particular fire zone application and aircraft. The current specifications for Halon 1301 require a minimum of 

6% concentration by volume in air be present simultaneously at all points in the engine nacelle for a minimum of 500 ms [3]. Therefore, to obtain this simultaneous concentration at several points (typically 12), 

additional Halon 1301 is added to ensure the system fully meets this requirement. It was also evidenced 

that the intumescent material reduces the required amount of Halon 1301. However, experimentation is 

needed to verify these calculations. 

CONCERNS AND POTENTIAL RESOLUTIONS 

Application of intumescent materials in the aircraft engine nacelle causes some concerns that must be 

recognized and accommodated. However, these can be designed and engineered around. The main 

concerns uncovered in this investigation were the following: potential toxicity, fragility of char, response 

in a high humidity environment, installation in highly cluttered areas, early expansion due to low activation temperature. Each concern uncovered was addressed with a potential resolution developed during 

this investigation. 

38 Halon Options Technical Working Conference 24-26 April ZOO1