/ DATE . N()Vld 1577 . 9/21/77 

LJBRAWY ADVI SORY 

CIRCULAR 

DEPARTMENT Of TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATIDN 

SUBJECT: GENERAL GUIDEZXNES FORME&XJRING FIRE-EXTINGUISRINGAGENT 

CONCENTRATIONS IN POWERPLANT COMPARTMENTS 

1. PURPOSE. The purpose of this Advisory Circular is to describe the 

installation and use of a model GA-24 fire extinguisher agent concentration recorder in determining the distribution and concentration of fire-extinguishing agents when discharged in an aircraft 

powerplant compartment. 

2. RFFEZREXCE. 

a. Report No. FAA-DS-70-3, "Criteria for Aircraft Installation and 

Utilization of an Extinguishing Agent Concentration Recorder," 

is available from the National Technical Information Service, 

5285 Port Royal Road, Springfield, Virginia 22151, under 

Accession No. AD 712 191. 

b. FAR Sections 25.1195(b), 29.1195(b), and 29.1195(c). 

C. FAR 21.21(b)(2) as'it affects powerplant fire-extinguisher 

installations accomplished on an optional basis. 

3. BACKGROUND. From 1959 to 1968, the Federal Aviation Administration 

-actively engaged in the evaluation of aircraft powerplant 

fire-extinguishing systems. During this period, the F%A provided, 

without charge, specialized fire-extinguishing agent concentration 

recorder equipment and knowledgeable personnel to organizations 

requesting assistance in the evaluation of aircraft fire-extinguishing systems. Testing was conducted and data was obtained for 

STOL, VTOL, helicopters, large transport category aircraft, military 

aircraft, executive aircraft, turbojet, turbopropellers, and reciprocating engine-powered aircraft, This evaluation program provided 

information helpful in establishing criteria for the installation 

and utilization of the fire-extinguishing agent concentration 

recorder. The FAA no longer makes evaluations. This type of work 

is now being performed by private companies for a fee. The FAA 

Initiated by: ~~~-140 
AC No. 20-100 9/21/77 

believes that some of the procedures developed during the evaluation 

period should be preserved and made available to others as advisory 

material. This document is intended to accomplish this objective. 

4. INTRODUCTION. Mkasuring fire extinguishing agent concentration and 

distributions in aircraft powerplant compartments could be a timeconsuming procedure if it were not for the fact that the fireextinguishing agent concentration recorder was designed and constructed 

for this purpose. Use of the recorder requires familiarity with both 

the recorder and the powerplant being evaluated. The following test 

will aid in gaining familiarity with the equipment and procedures. A 

more comprehensive familiarization may be obtained by reviewing Report 

No. FAA-DS-70-3. 

5* FUNCTIONAL DESCRIPTION OF EQUIPMENT. The agent concentration recorder 

equipment consists of a recording oscillograph, a vacuum pump, a control 

unit, gas analyzer units, and sampling probes. An event indicator is an 

integral part of the oscillograph. Schematic views of this equipment 

are shown in Figures 1 and 2. The equipment is supplied as a system to 

assure proper matching of components. 

a. The vacuum pump is used to draw gas samples from the area being 

evaluated through the sampling probes to the analyzer units. The 

sampling probes are equal lengths of tubing of uniform diameter 

&inch O.D. with .0304nch wall) designed to convey samples at 

a uniform flow rate from the sampling area to the analyzer units. 

For applications where the boiling point of the extinguishing agent 

is higher than the ambient air temperature, a special set of 

sampling probes equipped with heaters and insulating jackets is 

needed. These sampling probes must be fabricated. None of the 

commonly used agents, however, need heated sampling probes. 

b. The anaJyzer units are used to measure a pressure drop across a 

porous metal plug within the units. The drop is a function, of the 

viscosity and flow rate of the gas. The analyzer units consist of 

four analyzer cells each. A transducer assembly contained within 

each analyzer cell produces a signal which is a function of the 

pressure drop and transmits the signal to the recording oscillograph. The event indicator provides an input to the recording 

oscillograph which indicates the time span from initiation of agent 

discharge to termination of the test. 

6. INSTALLATION AND LOCATION OF EQUIMENT. 

a. Location of Sampling Probes. The proper installation and location of 

the agent sampling probesare essential for a meaningful test program. 

Since the existing concentration recorders use a maximum of 12 

probes, careful consideration must be given to the selection of 

areas to be sampled in order to provide an optimum amount of data 

per test. 

Page 2 Par 3 
g/21/77 

(1) 

(2) 

(3) 

(4) 

(5) 

AC No. 20-100 

Usually, no attempt is made to position the probes symmetrically or uniformly within the nacelle. Instead, the probes 

are located in areas where both a source of fuel and ignition 

exist, areas where component or accessory location or airflow 

might prohibit adequate extinguishing agent coverage, areas 

near hot air ducts, and the lower portion of the nacelle 

where flammables might collect. 

However, a preliminary survey can be made using either of two 

general methods to obtain overall pictures of agent concentration. Results from such a survey can then be used to determine 

the locations where further investigation is required, or if 

the agent discharge nozzles need repositioning. Figures 3 and 

4 are typical installation schematic diagrams for such preliminary surveys. 

For a relatively long nacelle, probes can be positioned in 

four rings of three probes each. The probes in each ring 

should be spaced at 12C" intervals radially about the engine. 

The probes in each succeeding ring should be rotated 600 with 

respect to the preceding ring. 

For a shorter nacelle, three rings of four probes will e&at+ 

lish a good sampling. The probes in each ring should be 900 

apart and each succeeding ring should be phased 45O. 

Many variations of these positions are possible. The positioning may start with any ring or any angular location. Also, by 

deviating from the ring station location (or the suggested 

angular positioning) or by varying the radial distance, many 

specific fire hazard areas (or areas of suspected low agent 

concentratio4 may be investigated without disrupting the overall coverage. 

b. Probe Installation. 

(1) All probes must be the same length to allow identical flow 

times from the sampling point to the analyzer cells. There 

fore, the length of the longest probe should be determined 

first, and then all other probes should be cut to this length. 

Coiling of the excess lengths of tubing will then be required 

for the majority of the probes. The excess tubing should be 

coiled such that the coiled portion is located where airflow 

and agent distribution will not be disrupted. The coiling 

preferably should be outside the area being sampled. The 

coils should be as large a diameter as practical to minimize 

line losses, and extreme care should be taken while coiling to 

assure that there will be no collapsing of the tubing. 

Par b Page f 
AC No. 20-100 g/21/77 

(21 

(31 

(41 

Within the nacelle, the probes should be clamped securely to 

structure or to the engine at a sufficient number of points to 

prevent shifting. It is extremely important to assure that 

shifting does not occur. Misinterpretation of the data, interference with engine controls, or chafing of electrical or fuel 

lines could result from such a situation. The probe should be 

firmly secured as closely as possible to the free end. The open 

ends of the probes should be several inches from any object 

that might cause blockage or flow obstruction. Probe clearances 

and position should be checked at intervals throughout the test 

program. 

The open ends of the probes should be positioned normal to the 

expected local airflow. Probes should not be located close to 

the agent discharge nozzles because the probes would pick up 

agent samples which may be greater than the actual concentration 

in these areas. 

When installing the probes, and throughout testing, line obstructions and contamination must be avoided. Prior to connecting 

the probes to the analyzer cells, the open ends of the probes 

must be deburred, and the entire line must be cleared with high 

pressure&y air or nitrogen. The clearing process must be 

repeated whenever there is a possibility that contaminants have 

entered the probes during the course of testing. THE PROBE 

LINES MUST BE DISCONNECTED FROM THE ANALYZER CEILS BEFORE CLEARINGTO~TDAMAGETOTHEANALYZERFROMTHEHIGHPRESSURE 

GAS. 

c. Installation of Analyzer Units. 

(1) The analyzer units may be mounted in any position, but they must 

be shock mounted to eliminate shock damage to the transducer 

assemblies. Each unit should be mounted outside the nacelle 

where samples from four of the locations selected can be drawn 

conveniently. The analyzer cell number and corresponding sam, 

plinglocation should be noted for each of the four cells of the 

analyzer units as the units are installed. 

(2) After the analyzer units are mounted, the vacuum manifold fitting may be located so that the vacuum lines from the manifold 

to the analyzer units are as short as possible. These lines 

may be seamless copper or aluminum tubing. 

d. Installation of Vacuum Pump. 

(1) The vacuum pump is usually located in the cabin. It is not 

necessary to shock mount the pump. The line connecting the 

vacuum pump to the vacuum manifold may be of seamless copper or 

aluminum tubing. 

Page 4 Par 6 
9/21/77 AC No. 20-100 

7. 

(2) If the ambient temperature of the region in which the pump is 

installed is expected to be below 4OoF, a means of heating the 

pump must be provided. Heating can be accomplished with a 

heater-blower system. 

(3) The pressure side of the pump may be vented overboard if 

desired. However, if the pump is vented overboard, care must 

be taken that the location of the venting system does not 

cause a flow to be induced through the sampling system with 

the pump off. 

e, Installation of Recording Oscillograph and Control Unit. The 

recording oscillograph and control unit should be located in the 

cabin where they will be accessible to the operator at all times 

during a test. The electrical cables must be fabricated during 

installation. Wiring diagrams are furnished with the equipment. 

For certain type oscillographs, the event indicator is connected 

through a relay to the aircraft fire-extinguishing discharge 

switch. 

OPERATING PROCEEURE. After the components of the agent concentration 

recorder have been installed in the aircraft, a thorough ground check 

of the operation of the equipment should be made. Specific operating 

instructions must be obtained from the instruction manual for the 

equipment being used. The general operating procedure will require 

that the analyzer units be stabilized at operating temperature. If 

the preheated sampling tubes are to be used, the sampling tube heater 

switch should not be turned on until operating temperature has been 

attained in all three analyzer units. Because of their small thermal 

mass, the sampling tubes should come up to operating temperature in a 

few minutes. 

temperature, 

After the instrument has been brought up to operating 

the galvanometers are zeroed by aligning all 12 galvaneter. traces manually so that they are spaced equally. This should be 

done in accordance with the procedure outlined in the operations manual 

which accompanies the oscillograph. The vacuum pump is turned on. 

While it is in operation, the traces again are examined and readjusted 

as necessary for correct spacing and to insure that none fall outside 

the chart range. The procedure should be repeated several times to 

insure that all galvanometer traces are aligned and identified properly. 

With the vacuum pump in operation, a check is made in each galvanometer 

in regard to the location of each sampling pickup in its circuit. This 

may be done by directing a small jet of carbon dioxide (CO2) from a 

small hand fire extinguisher into a plastic-,bag and then holding the 

bag over the open end of the sampling tube and noting the corresponding 

galvanometer deflection. This method avoids flooding the entire area 

and thus affecting other pickups in addition to the one being checked. 

Par 6 Page 5 
AC No. 20-100 9/21/77 

a. The event indicator also should be checked before an actual test is 

conducted. This is done by disconnecting the agent container discharge valve from the discharge switch. The discharge switch then 

is activated and sirmltaneously an interrupted light beam should be 

observed in the oscillograph. The interruption of the light beam 

is indicated by a broken line on the record chart. When conducting 

a test, the recorder chart speed should be adjusted to approximately 

1.50 inches per second. 

b. The flight conditions under which the test is conducted sre 

determined by the test requirements. 

c. Tests usually are conducted during takeoff and normal cruise conditions, although a Vne cruise condition is very often tested since it 

is a designated flight velocity; and if coverage is adewate at this 

condition, it will beadequateat normal cruise conditions. More 

detailed descriptions of test conditions are contained in Report No. 

FAA-DS-‘W3. It is possible in some cases to conduct tests on the 

ground, simulating flight conditions by introducing the appropriate 

airflow in the nacelle. If ground testing is feasible, a considerable 

savings in time and costs may be realized. 

d. Before the equipment is airborne, it should be warmed up on the 

ground; otherwise, it will be necessary to extend the flight time 

to allow for heating the equipment. After the desired altitude is 

reached, a final check is made on the alignment of the galvanometer 

traces. This should be done with the oscillograph on and the 

vacuum pump operating intermittently. 

e. In conducting the test, the emergency fire shutdown procedure 

specified for the aircraft is followed. At least 20 seconds before 

the agent discharge switch is activated, the oscillograph is turned 

on and the chart is started. Approximately 10 seconds before the 

agent is discharged, the vacuum pump is turned on. The equipment 

should remain in operation approldmately 30 seconds after the agent 

is discharged. At the time of the test, the ambient pressure, 

temperature, and humidity should be recorded. By following the 

above sequence of operation, the concentration recorder is calibrated for 100 percent air sample before each test run is made. A 

typical oscillogram is shown in figure 5. 

DATA REDUCTION. The oscillogram of the test shows the agent concentration buildup and decline at each of the 12 sampling locations on a time 

basis from the time of discharge until the agent is dissipated from the 

protected zone. A timing signal generated and printed on the oscillti 

gram provides the time reference. At this point it should be recognized 

that a time and pressure lag will exist in long runs of tubing and must 

be taken into consideration. 

Page 6 Par 7 
9/21/77 AC No. 20-100 

a. Since the galvanometer deflection for a given channel is propoe 

tional. to the pressure drop across the porous plug, a definite 

deflection will result for each pure gas. @ mixture of two 

gases,*such as air and an extinguishing agent, will produce a 

deflection somewhere between the respective values for the pure 

constituents. Subsequently, the relative concentration of an 

extinguishing agent in any mixture of the agent and air may be 

determined by the for&a shown below: 

Percent relative concentration = AD - MD 

AD- CD x100 

where 

AD = displacement for lOO$ air, in inches, from baseline 

MD = displacement for mixture, in inches, from baseline 

CD = displacement for pure agent, in inches, from baseline 

b. It is not necessary to calibrate the instrument for 100 percent car+ 

centration of the agent each time a test is conducted. By calibrating the instrument experimentally, the ratios for galvanometer 

deflection for 100 percent agent to galvanometer deflection for iO0 

percent air can be determined for various agents. These ratios are 

determined as'an average for all of the individual channels in 

operation within standard deviation limits. The calibration ratios 

for various extinguishing agents are as follows: 

Calibration 

Agent Ratio 

Carbon Dioxide (CO2) 0.717 

Methyl Bromide (CH3Br) 0.466 

Bromochloromethane (CH2BRCl) 0.395 

Dibromodifluoromethane (CBr2F2) 0.319 

Bromotrifluoromethane (CBrF3) 0.401 

c. The ratios are based upon the pressure drop obtained with dry air. 

If there is an appreciable amount of mo%ure in the air, a slight 

error will result from the use of these ratios. Figure 6 gives the 

correction factor by which the ratios rmst be multiplied to correct 

for the specific humidity and ambient temperature conditions 

existing at the time the test is conducted. 

d. The following illustrates how the relative concentration may be 

determined for any channel, for a specific time interval, after the 

agent has been discharged. Figure 5; shows that on channel 9 the 

deflection for air is 3.08 inches (AD = 3.08). The calibration ratio 

for CO2 agent is O/717. The deflection corresponding to 100 percent 

Par 8 Page 7 
AC No. 20-100 g/21/77 

9. 

concentration of the agent then is determined as 3.08 x 0.717 = 

2.21 inches. This means CD= 2.21. To compute the deflection from 

the 100 percent airline corresponding to 100 percent of the agent, 

subtract the deflection due to the agent from that due to air. For 

channel 9, Figure 5, (AD-CD) is given by (3.08 - 2.21) = 0.87. TO 

determine the CO2 concentration for channel 9 at 2 j!$ seconds after 

discharge, extend the 100 percent air deflection line to the right 

and beyond the vertical time interval line, which indicates a time 

lapse of 2 Q seconds from discharge. Determine by measurement the 

deflection for channel 9 from the 100 percent airline at this point. 

This is 0.46 inches and represents the value (A&MD). The ratio of 

this deflection to the deflection from the reference line from 100 

percent concentration of the agent, expressed as a percentage, will 

give the concentration of the mixture. In this example, the relative 

CO9 concentration 

AD-MD 

- x 100 is: AD-CD -+g-- x loo = 52.9 percent. 

. 

INTERl'RRTATION OF DATA. 

a. The percentage of relative concentration, based on the ratio of 

oscillograph galvanometer displacements, does not indicate the cocentration percentage on a volumetric or weight basis. To convert 

to a volumetric percentage or weight percentage, the curves shown 

in figures 7 through 11 may be used. Referring to figure 7, the 

concentration percentage corresponding 52.9 percent would be approximately 4.9 percent by volume and 60 percent by weight. 

b. In order to detertine by means of the recorder whether the extinguishing agent is to be effective, concentrations sufficient to 

extinguish fire must exist simultaneously at all sampling points 

long enough to provide initial extinguishment and assure protection 

while flammable vapors dissipate and hot surfaces cool. The minirmun 

time that the minimum concentration should exist is one-htif second. 

Page 8 Par 8 
g/21/77 AC No. 20-100 

Examples of minimum concentrations sufficient to extinguish 

fire and prevent its recurrence are as follows: 

AGIST CONCENTRATION IN PERCaJT REMTIVE CONCIWI'RATION 

BYWEXHT BYVOLUME PERCDJT 

co2 49 37 40 

CH3Br 30 11 15 

CH2BRCl 36 11 25 

CR@2 26.5 5 15 

CFF 22 6 15 

Acting.DiTector 

Flight Standards Service 

Par‘8 Page 9 
9/21/77 AC No. 20-100 

Appendix 1 

I’ 

L 

- 

4 

c 

Page 1 
AC No. 20-100 

Appendix 1 

1 -TEMPERATURE-REGULATING POROUS PLUG 

2 - PRESSURE-DROP POROUS PLUG 

3 - CRITICAL FLOW ORIFICE 

4 - VACUUM MANIFOLD 

5 - PRESSURE-SENSITIVE BELLOWS 

6 - STRAIN GAGE 1 

TRANSDUCER ASSEMBLY 

7 - HEATING ELEMENT 

8 - THERMOSTAT TEMPERATURE CONTROL 

9 - METAL BLOCK 

Figure 2. Typical Gas Analyzer Cell 

Page 2 
AC No. 20-100 

Appendix 1 

I 

I 

Page 3’ 
AC No. 20-100 

Appendix I 

g/21/77 

Page 4 
. 

AGENT CONTAINER DISCHARGE 

INdlCATOR TRACE DISCHARGED 

. . . ..---. ..U. .C.--U.*-.---- 

Figure 5.. Typical Flight Test Oscillograph Record 
.AC No. 20-100 

Appendix 1 

9/21/77 

Page 6 
g/21/77 AC No. 2Q:lOO 

Appendix 1 

CONCENTRATION (PERCENTAGE) 

Figure 7. Curve for Converting Relative Concentration to Volume and Weight 

Percentages of Carbon Dioxide in Air 

Page 7 
RECORDER INDICATION 

(PERCENT RELATIVE CONCENTRATION) 
CONCENTRATION (PERCENTAGE) 

Figure 9. Curve for *meting Rolldim Comcentmtion to Volume and Wemt 

Percentage of ,Bromochlaromothm in Air 
AC No. 20-100 

Appendix 1 

9121177 

20 40 80 80 100 120 

CONCENTRATION (PERCENTAGE) 

Figure 10. Curve for Converting Relative Concentration to Volume and Weight 

Percentages of Dibromodifluoromethane 

Page 10 
g/21/77 AC No. 20-100 

Appendix 1 

20 40 60 60 100 120 

CONCENTRATION (PERCENTAGE) 

Figure 11. Curve for Converting Relative Concentration to Volume and ieioht 

Percentages of Bromotrifluoromethane in Air 

* U. 8. GOVERNMENT PRINTING OFFRX : 19l’l 731-174/51 

Page 11