		United States Department of Agriculture Forest Service Technology & Development Program

		March 2000	0057-2317-MTDC
		5700

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.

Figure 1-Retardant drop released from a safe drop height.

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.

Figure 2-Retardant drop released below the safe
drop height.

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.

A printable version of the table below

Safe drop height (feet above impact)
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

ARDCO Aero Flite DC-4 65, 119, 151, 152, 160, 161, 166 2000 High 319 235 192 172

ARDCO Aero Flite DC-4 65 119, 151, 152, 160, 161, 166 2000 Low 231 191 170 161

Aero Union Corp. C-130 30, 31, 63, 64, 67 3000 1/2 130 130 130 130

Aero Union Corp. C-130 30, 31, 63, 64, 67 3000 1 137 137 137 137

Aero Union Corp. C-130 30, 31, 63, 64, 67 3000 2 145 145 145 145

Aero Union Corp. C-130 30, 31, 63, 64, 67 3000 3 150 150 150

Aero Union Corp. C-130 30, 31, 63, 64, 67 3000 4 169 169 169

Aero Union Corp. C-130 30, 31, 63, 64, 67 3000 6 185 185 185

Aero Union Corp. C-130 30, 31, 63, 64, 67 3000 8 190 190

Aero Union Corp. C-130 30, 31, 63, 64, 67 3000 9 249

Aero Union Corp. P3 Orion 00, 22, 23, 25, 26, 27, 21 3000 1/2 140 140 140 140

Aero Union Corp. P3 Orion 00, 22, 23, 25, 26, 27, 21 3000 1 149 149 149 149

Aero Union Corp. P3 Orion 00, 22, 23, 25, 26, 27, 21 3000 2 166 166 166 166

Aero Union Corp. P3 Orion 00, 22, 23, 25, 26, 27, 21 3000 3 175 173 173 166

Aero Union Corp. P3 Orion 00, 22, 23, 25, 26, 27, 21 3000 4 204 200 194

Aero Union Corp. P3 Orion 00, 22, 23, 25, 26, 27, 21 3000 6 223 215 204

Aero Union Corp. P3 Orion 00, 22, 23, 25, 26, 27, 21 3000 8 253 242

Aero Union Corp. P3 Orion 00, 22, 23, 25, 26, 27, 21 3000 Max 310 244 196 192

Aero Union Corp. SP2H 01, 03, 16, 18 2000 1/2 128 128 128 128

Aero Union Corp. SP2H 01, 03, 16, 18 2000 1 135 135 135 135

Aero Union Corp. SP2h 01, 03, 16, 18 2000 2 143 143 143 143

Aero Union Corp. SP2h 01, 03, 16, 18 2000 3 156 150 150

Aero Union Corp. SP2h 01, 03, 16, 18 2000 4 169 169

Aero Union Corp. SP2h 01, 03, 16, 18 2000 5 196 173

Aero Union Corp. SP2h 01, 03, 16, 18 2000 6 215 173

Aero Union Corp. SP2h 01, 03, 16, 18 2000 ED 234 173

Aero Union Conventional C-54-DC-4 14, 15 2000 High 260 207 180 169

Aero Union Conventional C-54-DC-4< 14, 15 2000 Low 202 180 171 161

Aero Union Conventional DC-7 33, 60, 62, 66 3000 High 284 221 189 170

Aero Union Conventional DC-7 33, 60, 62, 66 3000 Low 219 188 171 163

Hawkins & Powers C-130 81, 83, 88, 130, 131, 133 3000 High 287 219 185 168

Hawkins & Powers C-130 81, 83, 88, 130, 131, 133 3000 Low 241 196 173 163

Hawkins & Powers KC-97 97 3200 High 323 237 196 174 162

Hawkins & Powers KC-97 97 3200 Low 290 219 186 169 160

Hawkins & Powers KC-97 97 4000 High 348 257 204 178 164

Hawkins & Powers KC-97 97 4000 Low 298 228 190 171 160

Hawkins & Powers SPB4Y2 121, 123, 124, 126, 127 2200 High 265 208 180 165

Hawkins & Powers SPB4Y2 121, 123, 124, 126, 127 2200 Low 235 193 172 162

Hawkins & Powers SPB4Y2 121, 123, 124, 126, 127 2000 High 254 203 177 164

Hawkins & Powers SPB4Y2 121, 123, 124, 126, 127 2000 Low 223 187 169 160

Marsh Turbo Thrush 60/40 Door 380 40% Door 80

Marsh Turbo Thrush 60/40 Door 380 60% Door 90

Marsh Turbo Thrush 60/40 Door 380 Both Doors 100

Minden Air Inc. P2V-7 55, 99 2450 High 288 221 199 176

Minden Air Inc. P2V-7 55, 99 2450 Low 231 170 145 123

Minden Air Inc. P2V-7 55, 99 2000 High 283 217 196 175

Minden Air Inc. P2V-7 55, 99 2000 Low 228 162 139 121

Neptune, Inc. P2V-7 H & P P2V-7 08, 09, 10, 11, 139, 140 2450 High 288 219 199 175

Neptune, Inc. P2V-7 H & P P2V-7 08, 09, 10, 11, 139, 140 2450 Low 215 183 172 162

Neptune, Inc. P2V-5 05, 06, 07 2450 High 291 221 201 178

Neptune, Inc. P2V-5 05, 06, 07 2450 Low 214 182 173 163

Pierce Turbo Thrush 450 80

Snow Airtractor 802 181, 182 800 1/2 92 92 92

Snow Airtractor 802 181, 182 800 1 102 102 102

Snow Airtractor 802 181, 182 800 2 123 123 116

Snow Airtractor 802 181, 182 800 3 151 151 151

Snow Airtractor 802 181, 182 800 4 179 179 144

Snow Airtractor 802 181, 182 800 Max 186 186 144

Western Pilot Services 500 140

Dromader-Melex gate

Western Pilot Services 500 100

Dromader-Transland

				Safe drop height (feet above impact)
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
ARDCO Aero Flite DC-4	65, 119, 151, 152, 160, 161, 166	2000	High	319	235		192		172
ARDCO Aero Flite DC-4	65 119, 151, 152, 160, 161, 166	2000	Low	231	191		170		161
Aero Union Corp. C-130	30, 31, 63, 64, 67	3000	1/2	130	130		130		130
Aero Union Corp. C-130	30, 31, 63, 64, 67	3000	1	137	137		137		137
Aero Union Corp. C-130	30, 31, 63, 64, 67	3000	2	145	145		145		145
Aero Union Corp. C-130	30, 31, 63, 64, 67	3000	3	150	150		150
Aero Union Corp. C-130	30, 31, 63, 64, 67	3000	4	169	169		169
Aero Union Corp. C-130	30, 31, 63, 64, 67	3000	6	185	185		185
Aero Union Corp. C-130	30, 31, 63, 64, 67	3000	8	190	190
Aero Union Corp. C-130	30, 31, 63, 64, 67	3000	9	249
Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21	3000	1/2	140	140	140		140
Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21	3000	1	149	149	149		149
Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21	3000	2	166	166	166		166
Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21	3000	3	175	173	173		166
Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21	3000	4	204	200	194
Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21	3000	6	223	215	204
Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21	3000	8	253	242
Aero Union Corp. P3 Orion	00, 22, 23, 25, 26, 27, 21	3000	Max	310	244	196		192
Aero Union Corp. SP2H	01, 03, 16, 18	2000	1/2	128	128		128		128
Aero Union Corp. SP2H	01, 03, 16, 18	2000	1	135	135		135		135
Aero Union Corp. SP2h	01, 03, 16, 18	2000	2	143	143		143		143
Aero Union Corp. SP2h	01, 03, 16, 18	2000	3	156	150		150
Aero Union Corp. SP2h	01, 03, 16, 18	2000	4	169	169
Aero Union Corp. SP2h	01, 03, 16, 18	2000	5	196	173
Aero Union Corp. SP2h	01, 03, 16, 18	2000	6	215	173
Aero Union Corp. SP2h	01, 03, 16, 18	2000	ED	234	173
Aero Union Conventional C-54-DC-4	14, 15	2000	High	260	207		180		169
Aero Union Conventional C-54-DC-4<	14, 15	2000	Low	202	180		171		161
Aero Union Conventional DC-7	33, 60, 62, 66	3000	High	284	221		189		170
Aero Union Conventional DC-7	33, 60, 62, 66	3000	Low	219	188		171		163
Hawkins & Powers C-130	81, 83, 88, 130, 131, 133	3000	High	287	219		185		168
Hawkins & Powers C-130	81, 83, 88, 130, 131, 133	3000	Low	241	196		173		163
Hawkins & Powers KC-97	97	3200	High	323	237		196		174	162
Hawkins & Powers KC-97	97	3200	Low	290	219		186		169	160
Hawkins & Powers KC-97	97	4000	High	348	257		204		178	164
Hawkins & Powers KC-97	97	4000	Low	298	228		190		171	160
Hawkins & Powers SPB4Y2	121, 123, 124, 126, 127	2200	High	265	208		180		165
Hawkins & Powers SPB4Y2	121, 123, 124, 126, 127	2200	Low	235	193		172		162
Hawkins & Powers SPB4Y2	121, 123, 124, 126, 127	2000	High	254	203		177		164
Hawkins & Powers SPB4Y2	121, 123, 124, 126, 127	2000	Low	223	187		169		160
Marsh Turbo Thrush 60/40 Door		380	40% Door	80
Marsh Turbo Thrush 60/40 Door		380	60% Door	90
Marsh Turbo Thrush 60/40 Door		380	Both Doors	100
Minden Air Inc. P2V-7	55, 99	2450	High	288	221	199		176
Minden Air Inc. P2V-7	55, 99	2450	Low	231	170	145		123
Minden Air Inc. P2V-7	55, 99	2000	High	283	217	196		175
Minden Air Inc. P2V-7	55, 99	2000	Low	228	162	139		121
Neptune, Inc. P2V-7 H & P P2V-7	08, 09, 10, 11, 139, 140	2450	High	288	219	199		175
Neptune, Inc. P2V-7 H & P P2V-7	08, 09, 10, 11, 139, 140	2450	Low	215	183	172		162
Neptune, Inc. P2V-5	05, 06, 07	2450	High	291	221	201		178
Neptune, Inc. P2V-5	05, 06, 07	2450	Low	214	182	173		163
Pierce Turbo Thrush		450		80
Snow Airtractor 802	181, 182	800	1/2	92	92		92
Snow Airtractor 802	181, 182	800	1	102	102		102
Snow Airtractor 802	181, 182	800	2	123	123		116
Snow Airtractor 802	181, 182	800	3	151	151		151
Snow Airtractor 802	181, 182	800	4	179	179		144
Snow Airtractor 802	181, 182	800	Max	186	186		144
Western Pilot Services		500		140
Dromader-Melex gate
Western Pilot Services		500		100
Dromader-Transland

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.

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
Building 1, Fort Missoula
Missoula, MT 59804-7294
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
E-mail: glovellette@fs.fed.us
Lotus Notes: Greg Lovellette/WO/USDAFS

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This page last modified January 8, 2001

Visitor since July 20, 2000