March 2000

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Safe Drop Height for Fixed-Wing Airtankers

Greg Lovellette, Project Leader

The Aircraft Use Triangle addresses cost effectiveness, use of the correct aircraft, and safety. While cost effectiveness and use of the correct aircraft are important parameters in choosing the drop height for an airtanker, safety must be the most important factor.

Low-altitude retardant drops from fixed-wing airtankers may injure firefighters. Injuries are caused by flying debris (such as treetops or rocks) or when firefighters are knocked down. If the retardant has lost all of its forward momentum and is falling vertically as a heavy rain, the danger to firefighters is minimal. Safe drop height can be defined as the distance below the airtanker at which the retardant begins to fall vertically. Figure 1 shows a drop that was released from a safe drop height, has lost all forward motion, and is falling vertically. Figure 2 shows a drop that was released below the safe drop height and still had considerable forward motion when it reached the ground.

Photo of vertical falling drop with no forward motion from a safe drop height.
Figure 1— Retardant drop released
from a safe height.

Photo of vertical falling drop with forward motion from an unsafe drop height.
Figure 2—Retardant drop intentionally released
below the safe drop height during testing.

Video and films of drop tests for a variety of airtankers dropping full and partial loads have been examined. The distance below the aircraft where retardant began to fall vertically was recorded and compared to the flight and release characteristics. From these data, a relationship between safe drop height, peak flow rate, and load size was developed. The relationship can be expressed as:

S = 101 + 0.0112 * L + 0.0202 * P + 50
Where S = Safe drop height in feet
L = Load size in gallons
P = Peak flow rate in gallons per second.

The additional 50 feet are a safety factor. The formula does not accurately represent constant-flow tanks and single-engine airtankers. The data table uses actual measurements for these airtankers.

Table 1 lists each type of fixed-wing airtanker approved by the Interagency Airtanker Board and the safe drop height for full and partial loads. The safe drop height for trail drops depends on the interval between releases. To determine the safe drop height for trail drops treat the total volume being released as if it were a single release.

Airtankers are listed by the manufacturer of the tank and gating system rather than by the operator of the airtanker. Airtanker numbers are shown to help determine the type of tank on each airtanker.

Airtanker type	Airtanker numbers	Tank volume	Flow setting	Full Load	1/2 Load	1/3 Load	1/4 Load	1/6 Load	1/8 Load	1/16 Load
				Safe drop height (feet above impact)
ARDCO Aero Flite DC-4 ARDCO Aero Flite DC-4	65, 119, 151, 152, 160,161, 166 65, 119, 151, 152, 160,161, 166	2000 2000	High Low	319 231		192 170	192 170		172 161
Aero Union Corp. C-130 Aero Union Corp. C-130 Aero Union Corp. C-130 Aero Union Corp. C-130 Aero Union Corp. C-130 Aero Union Corp. C-130 Aero Union Corp. C-130 Aero Union Corp. C-130	30, 31, 63, 64, 67 30, 31, 63, 64, 67 30, 31, 63, 64, 67 30, 31, 63, 64, 67 30, 31, 63, 64, 67 30, 31, 63, 64, 67 30, 31, 63, 64, 67 30, 31, 63, 64, 67	3000 3000 3000 3000 3000 3000 3000 3000	1/2 1 2 3 4 6 8 9	130 137 145 150 169 185 190 249	130 137 145 150 169 185 190		130 137 145 150 169 185		130 137 145
Aero Union Corp. P3 Orion Aero Union Corp. P3 Orion Aero Union Corp. P3 Orion Aero Union Corp. P3 Orion Aero Union Corp. P3 Orion Aero Union Corp. P3 Orion Aero Union Corp. P3 Orion Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21 00, 22, 23, 25, 26, 27, 21 00, 22, 23, 25, 26, 27, 21 00, 22, 23, 25, 26, 27, 21 00, 22, 23, 25, 26, 27, 21 00, 22, 23, 25, 26, 27, 21 00, 22, 23, 25, 26, 27, 21 00, 22, 23, 25, 26, 27, 21	3000 3000 3000 3000 3000 3000 3000 3000	1/2 1 2 3 4 6 8 Max	140 149 166 175 204 223 253 310	140 149 166 173 200 215 242 244	140 149 166 173 194 204 196		140 149 166 166 192
Aero Union Corp. SP2H Aero Union Corp. SP2H Aero Union Corp. SP2H Aero Union Corp. SP2H Aero Union Corp. SP2H Aero Union Corp. SP2H Aero Union Corp. SP2H	01, 03, 16, 18 01, 03, 16, 18 01, 03, 16, 18 01, 03, 16, 18 01, 03, 16, 18 01, 03, 16, 18 01, 03, 16, 18 01, 03, 16, 18	2000 2000 2000 2000 2000 2000 2000 2000	1/2 1 2 3 4 5 6 ED	128 135 143 156 169 196 215 234	128 135 143 150 169 173 173 173		128 135 143 150		128 135 143
Aero Union Conventional C-54 - DC-4 Aero Union Conventional C-54 - DC-4	14, 15 14, 15	2000 2000	High Low	260 202	207 180		180 171		169 161
Aero Union Conventional DC-7 Aero Union Conventional DC-7	33, 60, 62, 66 33, 60, 62, 66	3000 3000	High Low	284 219	221 188		189 171		170 163
Hawkins & Powers C-130 Hawkins & Powers C-130	81, 83, 88, 130, 131, 133 81, 83, 88, 130, 131, 133	3000 3000	High Low	287 241	219 196		185 173		168 163
Hawkins & Powers KC-97 Hawkins & Powers KC-97 Hawkins & Powers KC-97	97 97 97	3200 3200 4000 4000	High Low High Low	323 290 348 298	237 219 257 228		196 186 204 190		174 169 178 171	162 160 164 160
Hawkins & Powers SPB4Y2 Hawkins & Powers SPB4Y2 Hawkins & Powers SPB4Y2 Hawkins & Powers SPB4Y2	121, 123, 124, 126, 127 121, 123, 124, 126, 127 121, 123, 124, 126, 127 121, 123, 124, 126, 127	2200 2200 2000 2000	High Low High Low	265 235 254 223	208 193 203 187		180 172 177 169		165 162 164 160
Marsh Turbo Thrush 60/40 Door Marsh Turbo Thrush 60/40 Door Marsh Turbo Thrush 60/40 Door		380 380 380	40% Door 60% Door Both Doors	80 90 100
Minden Air Inc. P2V-7 Minden Air Inc. P2V-7 Minden Air Inc. P2V-7 Minden Air Inc. P2V-7	55, 99 55, 99 55, 99 55, 99	2450 2450 2000 2000	High Low High Low	288 231 283 228	221 170 217 162	199 145 196 139		176 123 175 121
Neptune, Inc. P2V-7 H & P P2V-7 Neptune, Inc. P2V-7 H & P P2V-7	08, 09, 10, 11, 139, 140 08, 09, 10, 11, 139, 140	2450 2450	High Low	288 215	219 183	199 172		175 162
Neptune Inc. P2V-5 Neptune Inc. P2V-5	05, 06, 07 05, 06, 07	2450 2450	High Low	291 214	221 182	201 173		178 163
Pierce Turbo Thrush		450		80
Snow Airtractor 802 Snow Airtractor 802 Snow Airtractor 802 Snow Airtractor 802 Snow Airtractor 802 Snow Airtractor 802	181, 182 181, 182 181, 182 181, 182 181, 182 181, 182	800 800 800 800 800 800	1/2 1 2 3 4 Max	92 102 123 151 179 186	92 102 123 151 179 186		92 102 116 151 144 144
Western Pilot Services Dromader – Melex gate		500		140
Western Pilot Services Dromader – Transland		500		100

About the Author

Greg Lovellette is a Physical Scientist at MTDC. He received a Bachelor of Science degree in Chemistry at the University of Montana in 1980. Before joining the Forest Service in 1989, he worked as an oil well analyst throughout Montana, Wyoming, and North Dakota.

Additional single copies of this document may be ordered from:
USDA Forest Service Missoula Technology and Development Center
5785 Hwy. 10 West Missoula, MT 59808-9361
Phone: (406) 329-3978
Fax: (406) 329-3719
E-mail: wo_mtdc_pubs@fs.fed.us

For additional technical information, contact Greg Lovellette at the address above.
Phone: (406) 329-4815
Fax: (406) 329-3719
Lotus Notes: Greg Lovellette/WO/USDAFS
E-mail: 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

The Forest Service, United States Department of Agriculture (USDA), 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 names in this document 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.

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