Data are presented on operational practices and maneuver accelerations experienced by two Douglas DC-6B airplanes converted to retardant bombers and used in firefighting operations. The data cover two fire seasons in the mountainous regions of the northwestern United States

Avery, D. E.

Jewel, J. W., Jr.

Morris, G. J.

English

NASA Technical Reports Server

NASA TECHNICAL NASATMX-72622
M iS M O R A N O U M COPYNO.
(BASA-T_-X-?2622) OPER&TT_G £XPEEIENCES t75- I093q
, OF RETEBDABT BORBERS DOEING FTREFIGHTIHG
OPERATZONS (BASk) 23 p ldC $3.25
t CSCL 01C Unclas
b-- G3/05 02779
" IZ OPERATINGEXPERIENCESOFRETARDANTBOMBERS
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DURINGFIREFIGHTINGOPERATIONS
JOSEPHW. JEWEL,JR.; GARLAND J. MORRIS;
AND DONALDE. AVERY
NOVEMBER1974
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)I, Rqlart No. J 2. OcMm_ment/Mmm_on No. 3. Reci:iimt's Catalng No.
INASATM 1-72622 m
4. Title w_l _Mitle 5. Report Oct,
Operating Experiences of Retardant Bombers November1974 !During Ftreftghttng Operations a.P.,ormlmo_..i._I=oco_
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7. AutlV_(I) 8. PerformingOrglnlzltlo_t Report No. _
Joseph W. Jewel, Jr.; GarlandJ, Morris; TM 1-72622 _)
and DonaldE. Avery 1o.WorkUn_,No. )
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NatlonalAeronauticsand Space Admlnistration 11.co_t,,_,o,G an,No
Langley ResearchCenter NA '
Hampton,Virginia 23665
13. TyPe of R_ and PWiod Covered
12.s,o_,__w_vN,_,__ NASA TechnicalMemorandum
NationalAeronautlcsand Space Administration
Washington, DC 20546 14.sa0or,,ori,_A ,:v coa, i
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16. Al_uu=t
Data are presented on operational practices and maneuver accelerations
experienced by two Douglas DC-6B airplanes converted to retardant bombers
and used in firefighting operations. The data cover two fire seasons in ii
the mountainous regions of the northwestern Unite¢ States.
I1, _dm (_ by Autl_i_) (ITAn_-nderlhwmd) III. OIMrllwtiGn 9wtemem
Aircraft, ftreftghttng operations,
maneuveraccelerations Unclasst fted-Unltmt ted _!
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OPERATINGEXPERIENCESOF RETARDANTB_MBERSi
_t DURINGFIREFIGHTINGOPERATIONS
i_ _ By JosephW. Jewel Jr.; GarlandJ Morris;l •
i- _,_ and Donald E. Ave_
,. ! LangleyResearchCenter
i_ SUMMARY
An analysishas been made from NASA VGH data taken from two DouglasDC-6
airplanesthat were convertedto retardantbombersand used in forest
firefightingoperations. The data were taken duringtwo fire seasonsin
the mountainousregionsof the northwestsectionof the United States.
The majorityof the retardantdrops occurredfrom 10 to 40 minutesafter
lift-off,with the averageelapsedtime being 25 minutes. For given time
intervalsrequiredto reach a fire, there was a ratherbroad range of dash
speeds used. Rates of descentin a retardantdrop run rangedfrom zero for
flat approachesup to 9,000 feet per minute for steep approachesneededto
reach fires in canyonsor along canyonwalls. The designmaneuverlimit
load factorwas equaledor exceededduring recoveryfrom 10 percentof the
retardantdrops. The ultimateload factorwas exceededonce. The most
frequentnumberof retardantdrops made per flightwas three. Althoughthe
time intervalbetweendrops rangedfrom I to 20 minutes,the majoritywas
between2 and 4 minutes. There were no exceedancesof the placardnever-exceed
speed during the firefightingoperations.
INTRODUCTION
Forest fireftghting methodsand techniques have undergone considerable change
in the past few decades. One of the most significant of these changes has
been the introductionof aircraftinto the flreflghtingsystem. The size i
of the aircraftrange from the small crop-dustertype initiallyused to the i
large four-englneaircraftin servicetoday. In the later category,the
majorityof the airplaneswere originallydesignedas bomberor cargo planes
for the militaryservice,or, as commercialtransportsin civilianservice.
Becausetheseaircraftare used in operationsnot consideredin the design
loadsanalysis,NASA has initiateda programto measurethe operational
characteristicsof flreflghtingmissionsand the maneuverand gust loads
experiencedin theseoperations. For a periodcoveringtwo summerfire i
seasons,data were obtainedfrom NASA VGH flightrecordersinstalledon i
two Douglas DC-6Bairplanes converted to retardant bombers and used in
flreflghtlng service. Thls reportprovidesinformationrelatedto !
!
aircraftfireflghtlngmissioncharacteristicsand givesdata on the
maneuverloadsexperiencedduring retardantdrops,
SYMBOLS
MLLF maneuverlimit load factor
MULF maneuverultimateload factor 1
VF design flap speed, knots
VNE placardnever-exceedspeed, knots
INSTRUMENTATIONAND SCOPE
The data were collectedwith NASA VGH recorders_lhichprovidetime-history
recordsof indicatedairspeed,norml acceleration,and pressurealtitude,
A detaileddescriptionof the VGH recorderis given in referencei. Normal
acceleration_were sensedby an accelerometermountedon the wing's
centerspar n.91n_ters (3 ft) to the right of the fuselagecenterline.
Dynamicsand staticpressurefor the recorderwere sensedfrom the dynamic
and staticsourcesleadingto the copilot'sinstrumentation.
Data from both airplaneswere obtainedduringfirefightingoperationsfor
two fire seasonsin mountainousterrain. The majorityof the operations
were conductedwithin the area boundedby Salt Lake City, Utah; Sacramento,
California;and the northernboundaryof Washingtonand Idaho. A small
portionof the data was recordedduring fireflghtingoperationsin the
mountainousareas of the southeasternsectionof the United States. The
scopeof the data for each airplaneis summarizedin table I.
EVALUATIONOF RECORDS
4
All flights were considered to extend from lift-off to the instant of
touchdown. Only fltghts with retardant drop runs were evaluated. All
data used tn the evaluation were obtained by handreadtng VGHrecords with
a calibration overlay.
Average speed to the first drop was estimated to the nearest knot by
visually tntegratin9 the Indicated airspeed trace on the VGHrecord. The
maximumIndicated airspeed for each fltght was also identified and measured
to the nearest knot for comparison with the aircraft's never-exceed speed.
2
Ratesof descentwere determinedby obtainingthe difference,to the nearest
100 feet, betweenthe altitudewhere the steepestportionof the run began
and the altitudeat pulloutand dividingthls differenceby the lapsedtime
In seconds. Since flapswere sometimesused to slow the descentrate during
the retardantruns, valuesof indicatedairspeedwere also read at the
start of the steepestrate of descentand at pullout,for comparisonwith
the design flap speed.
Flightduration,time from lift-offto first drop, and time from lastdrop
to touchdown,were read to the nearestminute. The numberof retardantdrops
per flightwas determinedand the time betweendrops for given flightswas
measuredto the nearestquarterof a minute.
Accelerationdata were read in O.lg intervals. Only the maximumpositive
accelerationrecordedduring pulloutfrom a retardantdrop and the maximum
negativeaccelerationthat occured,eitherat pushoverto begin the run,
or in roundingout after the pullout,were read. These accelerationdata
were cumulatedby numberof runs from each aircraftfor comparisonwith each
other and with the designmaneuverlimit and _Itimate load factors. The
accelerationdata from both aircraftwere also combinedand cumulatedby
the frequencyof occurrenceper hour of flight for comparisonwith maneuver
accelerationsexperiencedby the same type of airplaneflown in commercial 1
transportoperations. 1
RESULTSAND DISCUSSION
The variationin flightdurationof 415 flightsmade by two retardantbombers
used in forestfirefightingoperationsis shown in figure I. The shortest
flightwas 12 minutesin duration,and the longestwas 147 minutes. The
averageflight lasted49 minutesand 68.3 percentof the flights,or the !
one-sigmavalue,were between28 and 70 minutesin duration.
Elapsedtime from lift-offto the first retardantdrop for 191 flightsis
shown in figure 2. Drops were made from as soon as 4 minutesto as long as
65 minutesafter 11ft-off. The majorityof the drops occurredfrom 10 to 40
minutesafter llft-offand the averageelapsedtimewas 25 minutes.
The variationin dash speed to the first drop is given in figure3. Dash _ I
speeds measured during 191 fltghts ranged from 143 to 195 knots. The * S
averagedash speed was 168 knots. The relatlonshipof time and dash speed I!
to the first retardantdrop Is shown in table II. In flightsto fires that _
require less than 20 minutes to reach, dash speeds of up to 180 knots were _ i
used. For fltghts tn which more than 20 minutes were needed to reach the _ i
fire, dash speeds of up to 195 knots were used. In general, table II _ !
Indicates that for a gtven time interval required Lo reach a fire, a rather 1
broad range of dash speeds ts used.
i
3
q
The rates of descentrecordedin 401 runs to drop fire retardantare shown in
figure4. Approachesfor the drops variedfrom flat runs havingzero rates
of descentto extremelysteep runs In which descentrates of up to g,o00 feet
per minutewere recorded. The averagerate of descentwas 2,483 feet per
minute. Discussionswith personnelflyingthe retardantbombersindicate
thatdescentrateswere affectedto a large extentby the physical
characteristicsof the terrainnear the fire. Fires along hill topsor on
ridgescould be approachedfor the retardantdrop at relativelyshallow
angles,whereas,steep approacheswere necessaryto reach fires along sides
of ridge walls or on canyonbottoms, Based on the data of figure4, it was
calculatedthat 30 percentof the runs made by retardantbombersin
mountainousareas can be expectedto equal or exceed ratesof descentof !
3,000 feet per minute.
Figure 5 shows the airspeedsat the start of the highestdescentrate in the
retardantdrop run and at pulloutfor 401 runs in which retardantdrops were
made. The resultsare shown as the summationof the numberof drops made in
each 5-knot incrementof airspeed. The designflap speed,VF, is also noted
on the figurefor comparativepurposes,
Start speedsranged fromas low as 100 knots to as high as 211 knots. The
low-startspeedswere normallyrecordedduringthe secondand succeeding
retardantruns in a flightsince the aircraftat such timewas near the top
of a climbingrecoveryfrom a precedingrun and was, therefore,at a
relativelylow airspeed. The higherstart speedswere usuallyexperienced
on the first run.
Pulloutspeeds rangedfrom 116 knots to 236 knots. The speed at the instant
of pulloutwas dependentto a large extenton the type of approachrequired
to reach the fire, Lower pulloutspeedswere usuallyrecordedduringflat or
shallowapproachesand the higherpulloutspeedsduring steep approaches,
The pilots indicatedthat approachspeedswere sometimescontrolledby using
flaps as speed brakesin order to arriveover the drop area at the desired
drop speed of 155 to 160 knots. Figure5 shows that 105,or about 26 percent,
of the pulloutsfrom retardantdrop runswere made at airspeedsof 160 knots
or higher. Thirty-fiveof the pulloutspeedsand five of the start speeds
exceededthe designflap speed;however,since the VGH recorddoes not
indicate flap position, it cannot be assumed that flaps were used during
these runs.
i The frequencydlstrlbu_lonof the maximumpositiveand negativeload factors
recordeddurlng1,175 retardantdrops is given in table Ill.Figure6 shows a
plot of these maximumload factorstor each airplaneas a summationof the
i numberof runs for each 0.1 value of load factor. The aircraftdesign
i maneuver ltmtt and ultimate load factors are also shown. A comparison of the
load factors experienced by the two airplanes indicates that values up to 3.1
were experienced more often by atrplane 2 than by airplane 1. Since both
airplane_ were flown by each of four ptlots (ages 35 to 55) no reason for the
difference tn load factors could be attributed to differences in ptloting
techniques. Because the differences are relatively small, however, they are
not considered significant.
4
The mest severepositiveload factor,3.go, was recordedby airplanel and
the largestnegativeload factor,-0.50,was recordedby airplane2.
Approximately7 percentof the load factorsrecordedduring pulloutfrom a
retardantdrop by airplanel, and about 14 percentby airplane2, equaledor
exceededthe designmaneuverlimit load factor. The ultimateload factor
exceedanceby airplane1 was a mandatorymaneuverexecutedafter the
retardantdrop to avoid a collisionwith a canyonwall obscuredduring the
approachby a smoke pall. Althoughthe recordedload factorexceededthe
ultimateload factor,no visiblestructuraldamagewas detected. The
absenceof damagewas probablydue to the fact that the airplanewas well
below the maximumgross weight,since two retardantdrops had been made prior
to the incident.
The cumulativefrequencyof occurrenceper hour of flightof maneuverload
factorsexperiencedby aircraftused in firefightingoperationsis compared
in figure 7 with a similardistributionfrom the same aircrafttype flown
in commercialtransportoperations,Data for the commercialtransports,
obtainedfrom reference2, includedall _he operationaland check-flight •
maneuversrecordedin the periodthe data samplewas taken. Data from the
firefightingoperations,however,includedonly the one maximumpositive
and the one maximumnegativeaccelerationrecordedduringeach retardantrun.
The maneuverlimit load factor,MLLF, and the maneuverultimateload factor,
I MULF, for the aircraftare also shown on figure7 for comparativepurposes.
The severityof the maneuverloads experiencedby airplanesinvolvedin
firefightingoperationsis well illustratedby comparisonwith the
maneuverload experiencefor aircraftflown in commercialtransportservice.
Where the firefightingaircraftexceededbeth the maneuverlimit and
ultimateload factors,the aircraftflown in commercialtransportservice,
with almost six times as many flighthours, recordedno maneuversat, or
above, the maneuverlimit load factor. The rate that maneuverload factors
between2.0 and 2.4 were experiencedby firefightingaircraftwas almost
1,000 times that for aircraftflown as commercialtransports. Becausethe
maneuverloading,in both the repeatedand high-magnitudeapplications is' i
so severe relativeto the design loads,shorteningof the structurallife i
of the aircraftshould be expected.
The frequencyof occurrencewith which specificnumbersof retardantdrops
per f11ghtwere made is shown in figure8. Althoughthe aircraftwere
capableof making from one to six drops, the most frequentnumberreleased i
per flightwas three. Only lg percentof the flightssampledmade four or
more drops per flight.
The variationof elapsedtime betweenretardantdrops in a given flight is _
shown in figure9. The tlme intervalsrangedfrom 1 to 20 minutes,with the
most frequentnumberoccurringbetween2 and 3 minutes. Of the 361 time _
Intervalsexamined,only 11, or 3 percent,were more than 8 minutesin i_
duration.
1
5
The elapsedtime from the last retardantdrop on a flightto landing
touchdownis shown in figure I0. Data for the figurewere groupedinto
5-mlnutetime intervalsfrom 191 flights. One-hundredsixty-eight,or
! 88 percentof the flights,requiredfrom 5 to 30 minutesto returnto the
! base. The averagetime to returnwas Ig minutes The averagetime from
_' •
_ lift-offto the first retardantdrop for the same 191 flightswas 25 minutes
_ The 6-minutedifferencebetweenthe two suggestsa possibleholdingon
i! stationprior to the retardantdrop while the pro)erdrop area, run angle,
and turbulencelevels were determined.
The relationshipof the maximumindicatedairspeedrecordedduring a
I firefightingflightto the placardnever-exceedspeed is shown in figureII.
! The data, taken from Igl flights,indicatetherewere no exceedancesofthe never-exceedspeed. Maximumairspeedsrecordedduring the flightsranged
from a low of 165 knots to a high of 275 knots. Speed exceedancesduring
firefightingoperationsdo not appear to be significant.
CONCLUDINGREMARKS
An analysishas beenmade of 337 hours of flightdata obtainedfrom NASA VGH
recordersinstalledon two DouglasDC-6B airplanesconvertedto retardant
bombersand used in forestfirefightingoperations. The majorityof these
operationswere over mountainousterrainin the northwesternpart of the
United States. I
High rates of descent,rangingfrom 0 to 9,000 feet per minute and averaging i
J
over 2,400 feet per minute,occurredduring the run to drop the retardant, i
The maximumpositiveand negativeload factorsmeasuredduringthe retardant I
drop and recoverywere 3.9 and -0.50,respectively, i
J
The designmaneuver limit load factorwas equaledor exceededin IO percent
of the pulloutsfrom retardantdrop runs and the design ultimateload factor i
was exceededonce. The severityof maneuverload applications,in both
magnitudeand frequencyof occurrence,is such that significantshortening
.ofthe structurallife of the aircraftshouldbe expected.
Dash speeds to the first retardantdrop extendedfrom 142 knots to 195 knots.
There were no exceedancesof the placardnever-exceedspeed duringthe
firefightingoperations.
F11ght lengthsvaried from 12 to 147 minutes. Retardantdrops were made
from as soon as 4 minutes to as long as 65 ,.linutesafter lift-off.The
most frequentlyused numberof retardantdrops per flightwas threewith
97 percentof the drops being made less than 8 minutesapart.
! t
l REFERENCES
l 1. Richardson, Noman R,: NACAVGHRecorder, NACATN 2265, 1951.I
2. Copp, Martin R.; and Coleman, ThomasL,: An Ana]ysis of Acceleration,
Airspeed, and Gust-Velocity Data From One Type of Four-Engine
Transport Airp]ane Operated Over Two Domestic Routes. NACATN ;475,
1955.
I TABLE I. , SCOPE OF DATA
I Airplane
! Item
i I 2 l&2
Flighttime, hr 175.1 161.6 336.7
I Numberof flights 216 199 415
Numberof drops 601 577 1178
Recordingperiod June 1972 June 1972
_ _ to to
Sept. 1973 Aug, 1973

TABLEIII. - FREQUENCYDISTRIBUTIONOF M_NEUVERLOADFACTORS
EXPERIENCEDIN RETARDANTDROPS
Load Airplane Airplane Airplane
I factor I 2 I a 2
-0.5 to -0.6 1 0 1
-0.4 to -0.5 0 0 0
-0.3 to -0.4 0 0 0
-0.2 to -0.3 1 0 1
-0.1 to -0.2 1 1 2
0.0 to -0.1 2 0 2
0.0 to 0.1 3 6 9
0.1 to 0.2 22 4 26
i 0.2 to 0.3 39 10 49.3 t .4 48 24 72
I 0.4 to 0.5 103 55 1585 6 59 76 235
! 0.6 to O.7 109 157 266
0.7 to 0.8 61 152 213
F- 0.8 to 0.9 24 80 104l 0.9 to 1.0 4 36 40
1.0 to 1.1 0 1 1
1.1 to 1.2 0 3 $
1.2 to 1.3 2 9 11
1.3 to 1.4 6 16 22
1.4 to 1.5 33 18 51
1.5 to 1.6 86 18 104
1.b to 1.7 92 35 127
1.7 tO 1.8 73 51 124
1.8 to 1.9 61 63 124
1.9 tO 2.0 53 53 106
2.0 tO 2.1 51 50 101
2.1 tO 2.2 41 57 98
2.2 tO 2.3 33 56 89
2.3 to 2.4 19 42 61
2.4 tO 2.5 9 22 31
2.5 tO 2.6 10 25 35
2.6 to 2.7 9 18 27
2.7 tO 2.8 8 12 20
2.8 tO 2.9 2 10 12
"2.9 tO 3.0 3 4 7
3.0 to 3.1 3 9 _2
3.1 to 3.2 2 2 4
3.2 to 3.3 1 1 2
3.3 to 3.4 0 1 1
3.4 to 3.5 2 0 2
3.5 to 3.6 0 1 1
3.6 to 3.7 1 0 1
3.7 to 3.8 0 0 O
3.8 to 3.9 0 0 0
3.9 to 4.0 1 0 1
C_
ot I--
t._ .1-
_cl LL
_'- 0 .--
...j
LL
{1_ _-
ILi ]_
Y
'(_1

10001 o START 1
O PULL-OUT
VF
SU_ATION OF
NUMBER OF
CHEMICAL DROPS
10
00 120 140_ 160 1 '200 220 ;240
,INDICATED AIRSPEED, KNOTS
FIGURE 5, - RELATIONSHIP OF SPEED DURING RETARDANT DROP RUN TO THE
DESIGN FLAP SPEED,


2.50-
200-
150
NUMBER
OF
FLIGHTS
100-
50-
O _ i
1 2 3 4 5 6
NUMBEROF DROPSPER FLIGHT
FIGURE 8, - VARIATION OF THE NUMBER OF RETARDANT DROPS MADE
PER FLIGHT,
140- ,_
i
120
i00
i
l
80
FREQUENCY
OF
OCCURRENCE
60.
20
0
0 4 8 12 16 20 24
TIME, MINUTES
FIGURE 9, - VARIATION OF TIME BETWEEN RETARDANT DROPS ON A
GIVEN FLIGHT, "

_, 1,000-
I,
t.
: ' VN E
r
i i00 -
i
SUMMATIeN OFN BER OF
i FLIGHTS
10 --
i---_ _ _ l ......ee i
0 150 180 210 240 270 300 330
INDICATEDAIRSPEED, KNOTS
FIGUREif,- RELATIONSHIP OF THE MAXIMUr_ INDICATED AIRSPEED RECORDED
DURING A FIRE FIGHTING FLIGHT TO THE rLACARD NEVER-EXCEED
SPEED,


Operating experiences of retardant bombers during firefighting operations

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