0057-2852-MTDC
September 2000
5100/5700
Airtanker
United States Department of Agriculture
Forest Service
Drop Guides
Technology & Development Program
Ground Pattern Performance of the Snow Air Tractor
with Constant Flow Tank
Paul Solarz, Project Leader, and Cammie Jordan, Project Assistant
T
he Wildland Fire Chemical
Systems (WFCS) program tests a
variety of fixed- and rotary-wing
tankers to determine the parameters for
optimal ground pattern coverage over a wide
range of fuel and fire conditions. The Snow
Air Tractor AT-802F, owned and operated by
Queen Bee Air Specialties, is one of a family
of smaller airtankers used for initial attack fire
suppression.
The Snow Air Tractor fire retardant door
system contains a constant flow tank made of
stainless steel. The 800-gallon single tank is
divided into two hoppers with a connecting
lower section. Two opposing doors that hinge
on the airtanker’s longitudinal axis are used to
control the flow. The drop system control
panel allows the pilot to select coverage level
settings of 0.5 to 4 (77 to 450 gal/sec) or a
salvo drop, which opens the doors
completely. A hydraulic system actuates the
door opening. The control system specifies
the amount of retardant to be dropped,
coverage level, and ground speed. Tests
included airspeeds from 78 to 87 knots (90 to
100 mph) and drop heights from 40 to 113
feet (measured from the bottom of the tank to
ground). The drops were made with three
different materials: water, foam, and gumthickened retardant.
The Missoula Technology and Development
Center tested the Snow Air Tractor (Figure 1)
with a series of drops over an array of plastic
bowls much like Cool Whip containers. The
quantity of material in each bowl was
measured and the data were used to
determine the drop pattern.
Figure 1
100%
4.875 x 2.488
Original Photo 5x3.5
Print to Outside Edge of Borders
No Not Print Borders
Figure 1 —The constant flow tank for the AeroUnion SP-2H.
Flow rate, drop height, and airspeed affect the
drop pattern. Increasing the drop height
gradually widens the drop while decreasing
the coverage levels. This effect is modified by
the ambient wind. Increasing windspeed
widens the drop and decreases coverage
levels. Increasing airspeed increases the line
length while reducing the coverage level.
Because this airtanker has variable
flow rates, it can produce specific
coverage levels needed for accurate
drops using all of the liquid in the
tank. Figures 2, 3, 4, 5 and 6 show the
effect of increasing the coverage level
setting from 0.5 to 4.
The proper amount of fire-retarding material
(expressed as coverage levels in gallons per
100 square feet) differs depending on the fuel
model. Table 1 shows the coverage needed
for specific fuel models using both the
National Fire Danger Rating System
(NFDRS) and the Fire Behavior Fuel Model.
For additional Information contact: Greg Lovellette, Project Leader; Missoula Technology & Development Center; 5785 Hwy 10 W; Missoula, MT 59808 Phone: 406-329-4815;
Fax: 406-329-4811; E-mail: glovellette@fs.fed.us; Lotus Notes: Greg Lovelletteß/WO/USDAFS
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Table 1—Retardant coverage levels needed for specific fuel models.
Fuel Model
National Fire
Danger Rating System
(NFDRS)
Fire Behavior
Coverage Level
(gal/100 sq. ft)
A,L,S
1
C
2
H,R
8
E,P,U
9
Longneedle conifer; fall
hardwood
T
2
Sagebrush with grass
N
3
Sawgrass
F
5
K
11
G
10
O
4
F,Q
6
B,O
4
J
12
I
13
The results of drop tests allow
managers to estimate the length of
line a specific airtanker produces at
various coverage levels. Table 2 or
1
Description
Annual and perennial
western grasses, tundra
Conifer with grass
2
3
Shortneedle closed
conifer; summer
hardwood
Intermediate brush (green)
Light slash
4
Shortneedle conifer
(heavy dead litter)
Southern rough
6
Intermediate brush
(cured), Alaska black
spruce
California mixed
chaparral, high pocosin
Greater than 6
Medium slash
Heavy slash
Figure 7 can be used to estimate the
flow rate for a water drop to obtain
the longest line of the desired
coverage level. Table 3 or Figure 8
Table 2–Water tests producing the longest line at various flow rate settings using a
constant flow tank.
Coverage Level
(gal/100 sq. ft)
Flow Rate
(gal/sec)
Coverage Level
(setting)
Drop Length
(feet)
0.5
1.0
2.0
3.0
4.0
6.0
8.0
10.0
77
77
277
277
405
405
450
450
0.5
0.5
2
2
3
4
4
4
1604
1075
484
339
262
102
28
10
Table 3–Foam tests producing the longest line at various coverage level settings using
a constant flow tank.
Coverage Level
(gal/100 sq. ft)
Flow Rate
(gal/sec)
Coverage Level
(setting)
Drop Length
(feet)
0.5
1.0
2.0
3.0
4.0
6.0
8.0
10.0
77
140
277
277
405
405
405
-
0.5
1
2
2
3
3
3
-
1602
940
437
319
171
31
2
0
can be used to estimate the flow rate
for a foam drop to obtain the longest
line of the desired coverage level.
Table 4 or Figure 9 can be used to
estimate the flow rate for a gumthickened retardant drop to obtain
the longest line of the desired
coverage level.
2
Table 4–Gum-thickened retardant test producing the longest line at various flow rate
settings using a constant flow tank.
Coverage Level
(gal/100 sq. ft)
Flow Rate
(gal/sec)
Coverage Level
(setting)
Drop Length
(feet)
0.5
1.0
2.0
3.0
4.0
6.0
8.0
10.0
77
77
140
277
405
450
450
450
0.5
0.5
1
2
3
4
4
4
1710
1536
608
423
274
156
82
68
Snow Air Tractor With Constant Flow Tank
120
120
2.0
60
1.0
1.0
0.5
0
0
200
400
600
800
1.0
1.0
Width (feet)
Coverage Level Setting 0.5
1.0
60
0
1000
1200
1400
1600
1800
2000
Length (feet)
Figure 2–Drop pattern characteristics for the Snow Air Tractor with a coverage level setting of 0.5, an airspeed of 85 knots (98 mph), and a drop height of 46 feet. The contour lines are at coverage
levels of 0.5, 1, 2, 3, 4, 6, 8, and 10 gallons per 100 square feet.
The graphs predict line length (in feet)
as a function of flow rate (in gal/sec).
The tables are constructed by selecting
the drop producing the longest line (on
the ground) at each coverage level.
Either the graphs or tables may be used
to estimate the flow rate required to
produce the longest line for a given
coverage level. The tables show an ideal
case, while the graphs represent an
average.
To select the proper flow rate, first use
Table 1 to determine the coverage level
required by the NFDRS or Fire Behavior
Fuel Model. The coverage levels in
Table 1 represent the coverage level
required for average fire intensity for
each fuel model. The required coverage
level can be adjusted up or down
depending on the actual fire intensity.
Once the required coverage level is
determined, the flow rate can be found.
Use the graph for the material dropped
(water, foam, or gum-thickened retardant)
to find the flow rate that produces the
longest line for the desired coverage level.
The same information can be found in the
appropriate drop table.
3
Snow Air Tractor With Constant Flow Tank
Width (feet)
Coverage Level Setting 1
120
60
2.0
2.0
1.0 0
.5
120
60
0
0
0
200
400
600
800
1000
Length (feet)
1200
1400
1600
1800
2000
Figure 3–Drop pattern characteristics for the Snow Air Tractor with a coverage level setting of 1, an airspeed of 85 knots (98 mph), and a drop height of 90 feet. The contour lines are at coverage levels of 0.5, 1, 2, 3, 4, 6, 8,
and 10 gallons per 100 square feet.
Coverage Level Setting 2
0
120
0
1.
3.
Width (feet)
Snow Air Tractor With Constant Flow Tank
60
3.0
120
3.0
.0
2
.5
60
0
0
0
0
200
400
600
800
1000
Length (feet)
1200
1400
1600
1800
2000
Figure 4–Drop pattern characteristics for the Snow Air Tractor with a coverage level setting of 2, an airspeed of 87 knots (100 mph), and a drop height of 99 feet. The contour lines are at coverage
levels of 0.5, 1, 2, 3, 4, 6, 8, and 10 gallons per 100 square feet.
For example, if a fire is burning in
NFDRS Fuel Model C (Fire
Behavior Model 2), represented by
conifer with grass, a coverage level
of 2 is required (Table 1).
The graph for gum-thickened
retardant shows that for coverage
level 2, a coverage level setting of 1
produces the longest line (608 feet).
The ground drop characteristics for the Snow
Air Tractor were derived through controlled
drop test procedures on flat ground (Figure
10). This information is to serve only as a
guide to help field personnel determine the
proper drop height, airspeed, and door
opening for delivering water, foam, or gumthickened retardant. Actual coverage may
vary depending on terrain, wind, weather, a
nd pilot proficiency.
4
Snow Air Tractor With Constant Flow Tank
Width (feet)
Coverage Level Setting 3
4.
2.0 3.0.0 0
1
0.5
120
60
120
60
0
0
0
200
400
600
800
1000
Length (feet)
1200
1400
1600
1800
2000
Figure 5–Drop pattern characteristics for the Snow Air Tractor with a coverage level setting of 3, an airspeed of 81 knots (93 mph), and a drop height of 110 feet. The contour lines are at coverage
levels of 0.5, 1, 2, 3, 4, 6, 8, and 10 gallons per 100 square feet.
Snow Air Tractor With Constant Flow Tank
Width (feet)
Coverage Level Setting 4
2.0 3.0 4.0
1.0
0.5
120
60
120
60
0
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Length (feet)
Figure 6–Drop pattern characteristics for the Snow Air Tractor with a coverage level setting of 4, an airspeed of 78 knots (90 mph), and a drop height of 113 feet. The contour lines are at coverage
levels of 0.5, 1, 2, 3, 4, 6, 8, and 10 gallons per 100 square feet.
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Effect of Flow Rate on Length of Line at Various Coverage Levels
Effect of Flow Rate on Length of Line at Various Coverage Levels
1750
Snow Air Tractor With Constant Flow Tank Using Foam
Snow Air Tractor With Constant Flow Tank Using Water
1750
1575
CL = 0.5
CL = 1
CL = 2
CL = 3
CL = 4
CL = 6
CL = 8
1400
1050
1400
CL = 0.5
CL = 1
CL = 2
CL = 3
CL = 4
CL = 6
1225
CL = Coverage Level
Line Length (feet)
Line Length (feet)
1225
1575
875
700
1050
CL = Coverage Level
875
700
525
525
350
350
175
175
0
75
150
225
300
375
450
0
75
Flow Rate (gal/sec)
Figure 7–Use this graph to estimate the flow rate needed to produce the longest line of water at
various coverage
150
225
300
375
450
Flow Rate (gal/sec)
Figure 8–Use this graph to estimate the flow rate needed to produce the longest line of foam at
various coverage levels.
6
Effect of Flow Rate on Length of Line at Various Coverage Levels
Snow Air Tractor With Constant Flow Tank Using Gum-Thickened Retardant
1750
CL = 0.5
CL = 1
CL = 2
CL = 3
CL = 4
CL = 6
CL = 8
CL = 10
1575
1400
Line Length (feet)
1225
1050
CL = Coverage Level
875
700
525
350
175
0
75
150
225
300
375
450
Flow Rate (gal/sec)
Figure 9–Use this graph to estimate the flow rate needed to produce the longest line of gumthickened retardant at various coverage levels.
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About the Authors
Figure 10
83%
4.875 x 5.348
Original Photo 8 1/2 x 11
Print to Outside Edge of Borders
No Not Print Borders
Paul Solarz is Program Leader for the
Wildland Fire Chemical Systems Group.
He received his bachelor’s degree from
Eastern Oregon State College in 1986.
Paul has worked in Aviation and Fire
Management since 1973, serving at
seven Ranger Districts and in two Forest
Supervisor’s offices. He has an extensive
operational background in fire, fuels, and
aviation.
Additional single copies of this
document may be ordered from:
USDA Forest Service
Missoula Technology and
Development Center
5785 Hwy 10 W
Missoula, MT 59808
Phone: (406) 329-3978
Fax: (406) 329-4811
Internet: wo_mtdc_pubs@fs.fed.us
Figure 10–Drop test of the Aero Union SP-2H.
The Forest Service, United States Department of Agriculture, has
developed this information for the guidance of its employees, its
contractors, and its cooperating Federal and State agencies, and is
not responsible for the interpretation or use of this information by
anyone except its own employees. The use of trade, firm, or corporation
Cammie Jordan is a Project Assistant
for the Wildland Fire Chemical Systems
Program at MTDC. She is currently
enrolled at the University pursuing a
bachelorís degree in Elementary
Education with a Special Education
Endorsement. She has worked for
MTDC in Missoula since 1998.
names in this publication is for the information and convenience of
the reader, and does not constitute an endorsement by the Department
of any product or service to the exclusion of others that may be
suitable. The United States Department of Agriculture (USDA), prohibits
discrimination in all its programs and activities on the basis of race,
color, national origin, sex, religion, age, disability, political beliefs,
sexual orientation, and marital or family status. (Not all prohibited bases
apply to all programs.) Persons with disabilities who require alternative
means for communication of program information (Braille, large print,
audiotape, etc.) should phone USDA’s TARGET Center at (202) 720-
For additional technical information,
contact Greg Lovellette at the address
above.
Phone: (406) 329-4815
Fax: (406) 329-4811
Lotus Notes: Greg Lovellette/WO/
USDAFS
Internet: glovellette@fs.fed.us
An electronic copy of this document is
available on the Forest Service s
FSWeb Intranet at:
http://fsweb.mtdc.wo.fs.fed.us
For additional Information contact:
Greg Lovellette, Project Leader
Missoula Technology & Development
Center
5785 Hwy 10 W
Missoula, MT 59808
Phone: 406-329-4815
Fax: 406-329-4811
E-mail: glovellette@fs.fed.us
Lotus Notes: Greg Lovellette/WO/
USDAFS
2600 (voice and TDD). To file a complaint of discrimination, write: USDA,
Director, Office of Civil Rights, Room 326-W, Whitten Building, 1400
Independence Avenue, SW, Washington, DC 20250-9410, or call (202)
720-5964 (voice and TDD). USDA is an equal opportunity provider and
employer.
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