Aerial Application Operations.

4.3 Aerial Application Operations (crop dusting, fire fighting):

Special nature of aerial application flying (multiple take-offs and landings, low level flight, heavily loaded, tight maneuvers, protective equipment)
Flight profile
Thermal stress
Effects of exposure to aerial application chemicals
Crash survival and crash worthiness

Section II 4.3 AERIAL APPLICATION OPERATIONS (Crop Dusting, Fire Fighting)

This chapter is designed to provide you with a basic understanding of the activities of specialized aircraft that provide low level delivery of agricultural products. Their activities of crop dusting and fire fighting require specialized equipment and training. Emphasis is placed on understanding unique requirements for low level flight, the physiologic demands of high frequency, short flights in demanding conditions of high heat and repeated acrobatic activities. Toxic hazards are present due to the use of pesticides and herbicides by some aerial applicators. Due to the low altitude flying, collision with the ground, trees, wires and other objects is a constant risk and occur with greater frequency than in other aviation events. These aircraft are designed to provide protection and survivability to their pilots in the event of a crash. You will understand the process of creating and evaluating an aircraft for crashworthiness.

Aerial application operations consist of agricultural spraying and dispensing of materials from low-flying aircraft. This can include insecticides and pesticides, herbicides, fertilizers, defoliants, seed, fire control substances, and other potentially toxic materials. Dispensing materials from low-flying aircraft for agricultural or fire control purposes is hazardous, due to the proximity of ground, repetitive low level maneuvers, the potential of striking objects on the ground, and some of the chemicals being dispensed. Commonly referred to as crop dusting in the United States, this includes fire fighting aircraft and what is termed top dressing in New Zealand and Australia.

The initial testing of aircraft for dispensing airborne dusting equipment began in 1922 at Delta Laboratory in Tallulah, Louisiana as part of a United States Department of Agriculture experiment. As part of this program, the Starman C3B biplane became a primary dispenser and became widely available when the Army Air Service and the U.S. Navy adopted it as a primary trainer. Following World War II, thousands of these aircraft became available and were converted to duster/sprayer activities. Soon they became an essential contribution to American agriculture. A number of other aircraft were converted for agricultural application operations, but in 1957, Grumman Aircraft designed and built an aircraft specifically for crop dusting needs. This aircraft became the Grumman Ag-Cat. This biplane had a large capacity for carrying agricultural chemicals, as well as structural designs to make it highly maneuverable at low altitude and crashworthy.

Grumman Ag-Cat

Aerial application pilots are covered by FAR Part 137. These regulations govern agriculture aircraft in the United States and encompass the dispensing of any chemical designed to treat the soil or crops. Special certification rules for these pilots include knowledge and skill tests for the safe handling of poisons, agricultural chemicals, and basic medical knowledge of the symptoms of poisoning. There is also a special flight test and many of the rules of other aircraft operations are altered for Part 137 aircraft. These include a requirement for a safety harness, operating without positioning lights, and restrictions from operating over congested areas. Special record keeping must be maintained. If a private individual dispenses spray over their own property, they may operate as a private pilot, with a third class medical certificate, but those individuals flying for hire are required to have a commercial license and a second class medical certificate.

The flight profile of an agricultural applicator is typically a day of short flights from a small, uncontrolled airfield where the aircraft’s tanks are filled. The flight then proceeds to the target track and several passes are made at a low level of three to five feet above the crop, at a typical speed of 110 miles per hour. The agricultural applicator pilot must have inspected the field prior to flight and be aware of obstructions to flight, such as power lines and poles and trees, the relative flatness of the field, and adjoining fields, and must be extremely knowledgeable of the weather and wind conditions. Wind drift must be taken into account to avoid having the spray contaminate adjacent fields. A typical run will be straight downfield. At the end of the field a “P” turn is executed, using a bank angle of 45 to 60 and approximately 2-4 g of force on the aircraft. This increases the aircraft’s stalling speed by over 40%. The P turn completed, the aircraft flies back down the field and sprays the next lane.

One applicator estimated that he performed over 9,000 P turns in one summer. Since crop dusting is often done in summer under high temperature conditions, thermal stress may be an added problem. Repetitive turns at even these modest G loads coupled with dehydration can lead to g-induced loss of consciousness (G-LOC). Under these circumstances, the pilot will wake up in the wreckage of the aircraft.

Considerable care must be taken during the application of agricultural chemicals. In some parts of the country, application of chemicals is limited, but, in others, most notably the Sacramento River delta of California, virtually all crop care is provided from the air. Within this delta are a number of islands with dikes around them, making them resemble Dutch farmlands. However, unlike Holland, these delta islands are not reclaimed, but instead have been leveled, using laser surveying techniques so that an entire field will differ in altitude by only a few inches. At the beginning of the season, the island is flooded and aircraft will dispense rice seed, performing the planting. Subsequent flights will provide fertilizers and herbicide for weed suppression. Later flights will deliver pesticides to the area. The farmers need not actually enter their fields until the harvest is ready.

The nature of chemicals used in aerial application varies widely. The greatest concern is with pesticides, especially organophosphate insecticides, which include malathion and parathion. These are acetylcholinesterase enzyme inhibitors, which are, in general, slightly more toxic to mammals than insects. A special license from the United States Department of Agriculture is required to obtain and dispense these pesticides.

Related to organophosphate insecticides are the carbamate insecticides, which are also acetylcholinesterase inhibitors, but do not bind irreversibly to the enzyme. Both organophosphates and carbamate insecticide poisoning in humans can be treated by atropine, which provides a cholinergic blockade, preventing the symptoms of excess acetylcholine caused by the poisoning. In addition, pralidoxime can reactivate the acetylcholinesterase enzyme. Plasma cholinesterase enzyme measurements should be taken periodically of any individual involved in cholinesterase or carbamate insecticide applications.

Other pesticides include the chlorinated insecticides, which are being used less and less. The prototype of this group is dicholordiphenyltrichloroethane (DDT), which is now banned in the U.S. but still used elsewhere in the world. Others, such as aldrin and dieldrin are rarely used in the United States. There may be some central nervous system irritation due to overexposure to these chemicals, which can be treated by intermediate acting barbiturates.

Herbicides are commonly used, the most potent of which is paraquat. This is a powerful skin irritant and inhalation can cause pulmonary edema, which can only be treated symptomatically. Similar reactions occur with its chemical analog, diquat. Survivors typically have severe pulmonary fibrosis. Other herbicides, such as 2, 4D and 2, 4, 5T have a minimal toxicity in humans, although one combination of this, known as Agent Orange, was used in Vietnam and its contaminant 2, 3, 7, 8-TCDD (dioxin) was suspected of causing significant problems. Aerial applicators can also dispense insects directly used for fighting infestations, such as ladybird eggs and larvae or indirectly used such as sterile male screw worm flies.

Recent events have led operators of agricultural spray aircraft to take precautions to prevent their aircraft from being used for terrorist attacks. This includes greater security measures. However most applicators use fairly large nozzles, dispensing liquids and solids with a particle size on the order of 50 microns or larger: Large particles tend to settle out onto crops. Biological and chemical terrorism requires a much smaller particle size on the order of 1-5 microns in order to be inspirable and remain airborne for long periods.

Helicopters participate in aerial application operations as well beginning in the 1950’s. Experienced pilots are able to fly at low level at the more traditional altitudes for helicopters. Helicopters must be equipped with special wire cutting blades in the event of impact of power and telephone lines, but wire strikes still take a high toll. Crash worthiness is being designed into newer helicopters.

A variation on aerial application is employed by fire suppression aircraft. Water bombers, which include helicopters, will be loaded with water, either from a tanker on an airfield or, in the case of amphibious planes and helicopters, at a nearby lake. Aerial fire fighting began with surplus amphibious aircraft after World War II. Special use designed fire fighting aircraft did not appear until the Bombardier CL215 in the 1960s. Water bombers quickly became and remain the preferred method of fighting deadly crown fires, which can spread rapidly and jump fire breaks cut by ground personnel.

This aircraft and its successors can land in a lake, rapidly fill its tanks with water, and take off to dispense water on a nearby fire, using adjustable openings in its tanks for different water spread patterns.

In the U.S., many organizations operate firefighting aircraft, include state and federal programs (the Air National Guard converts some C-130 Hercules for firefighting in the summer) plus special fire districts and the U. S. Forrest Service. Flying through fires creates additional hazards for the water bomber pilot. Visibility is often reduced and there are significant updrafts created from the fires themselves, which may not be occurring on perfectly flat terrain, but are often in hilly or mountainous areas, where no other fire suppression can be brought to bear. Usually the water is mixed with foam to provide better penetration of the bolus though trees and a colored nitrogen-based fertilizer is mixed with the water to provide additional fire suppression, creating an orange cloud that marks the target and provides for later growth of plants that will fill in the burned out areas. The use of aging aircraft such as World War II vintage planes and 1950s era transports in water bomber missions led to two catastrophic crashes in the summer of 2002 when the wings failed on a 40-year-old C130 Hercules, followed shortly thereafter by a similar failure in a converted B-26 Intruder.

Water bombing with fire retardants added give the solution an orange color.

The risk of crashing an agricultural spray aircraft is very high and accident rates are many time higher than for general aviation. Risks include G-LOC syndrome, tree and wire strikes, spatial disorientation during turns, and low altitude stalls. Because of the dangers of low altitude flying, crashworthiness was first built into aerial application aircraft, beginning with the Ag-Cat and its many successors. Crashworthiness is an engineering design concept based on aeromedical principals, intending to “improve the odds of crash survival”. It was first used in agricultural application aircraft because the risk of crashing was so great that it made the benefits of building additional structural changes in the aircraft to be cost effective. Crash injury research, conducted in the 1940’s and 1950’s by Drs. Ross McFarland and John Paul Stapp led to understanding the physics of crash dynamics and how to protect the occupants of an aircraft, not to mention cars and any other form of motor vehicle. These are stated in the acronym of CREEP, which stands for:

C-container
R-restraint
E-environment
E-energy absorption
P-post crash hazards

In using this acronym to analyze aircraft accidents and crashworthiness, we begin with the Container, that is, the crew compartment. This structure must protect the occupants and remain intact. If it is crushed or collapses so that survivable space does not exist, the occupants will suffer injuries. In the case of agricultural aircraft, the pilot’s compartment has become a structure of reinforced steel rods, creating a roll cage. This compartment is structurally extremely strong and separate from the rest of the aircraft imbedded within it. Such designs for roll cages have been transferred to racing cars and, later, in a different form, to passenger cars, where the passenger compartment becomes reinforced with side bars and structural supports to the roof, in case of rollover. All aircraft crashworthiness research is transferable to any other form of motor vehicle.

Crashworthy design places the pilot’s compartment far behind the engine and hopper tank within a structurally reinforced cage. The pilot has a five point restraint system. Energy-absorbing wheel struts, cable cutters and self-sealing fuel lines all reduce risk of pilot injury.

The R represents the restraint system. Restraints were initially lap belts only. By 1914, most aircraft were equipped with lap belts for the pilots. In the 1930’s, shoulder harnesses were discovered to prevent the torso and head from moving forward and impacting other objects within the compartment, creating a “strike envelope”. Similarly, seatbelts have been installed in American cars since 1965 and shoulder harnesses since 1984. Additional restraint systems in aircraft have included a fifth point, consisting of a strap securing the lap belt and going between the legs. This prevents the occupant from submarining and going underneath the lap belt. Sometimes, a six point restraint is used where there are two crotch straps. Additional restraint systems for the head and neck have been necessary in racing cars, particularly on the NASCAR circuit. A restraint and container for the head alone, known as a crash helmet, was developed by the Stapp team in the 1940’s. This modified old aviation helmets of padded leather, which were basically modified football helmets. The crash helmet consists of an outer hard shell that prevents penetration of the container and an inner energy-absorbing material that restrains and reduces the energy applied to the wearer’s head. Crash helmets are now, of course, used by motor cyclists, bicyclists, and skiers.

The Environment includes the cockpit itself and the potential for loose objects to become missiles. Propelled unrestrained objects may break loose and fly around the occupant’s compartment, causing additional injuries. Likewise, there may be toxic chemicals carried by the aircraft, which provide additional hazards.

Energy absorption has to do with decelerating the occupant at levels below those capable of causing human injury. In conjunction with restraints, which should be broad and thick to distribute energy and positioned to cross bony structures, such as the pelvis and thorax, other structures of the aircraft itself crush and absorb energy. In an agricultural aircraft, this will include placing the engine at the far front of the aircraft, accounting for the somewhat unusual long-nosed appearance of modern agricultural aircraft. In helicopters, the engine may be mounted above the cabin. This way, during deceleration, the engine itself is separated from the cockpit. Structures built into the aircraft can absorb energy by collapsing slowly at a designed level of acceleration. For example, the belly of an aircraft may be built of crushable aluminum, designed to be collapsed at a level of 14 g’s acceleration. This is common in newer helicopters. Likewise, the seat structure of aircraft can be designed to bend and deform at a limit of 14 g’s. This is now applied to most commercial aircraft and helicopters. The 14 g level derives from research indicating that this level of acceleration will cause no permanent injury to a normal human being. Higher levels of acceleration are required to cause injuries, such as 50 g’s to break long bones and approximately 250 G’s to fracture the human skull. Agricultural aircraft will also have specially designed features, such as landing gear cut struts designed to collapse, wire cutters built in front of the windscreen to avoid decapitating the pilot, and full restraints and attachments for the seatbelts.

Fire is the greatest post crash hazard.

Finally, Post crash hazards include escape from the aircraft, fire, survival, and potential toxic exposures. Fire is the greatest hazard and 65% of all aircraft fatalities occur in the 15% of aircraft that burn. The chances of survival in a crash where there is no fire exceed 90%. A method of crash-proofing an aircraft is to use fuel lines that quick disconnect when too much stress is applied and self-seal. The result is a fuel line that struck by a metal post on impact will simply disconnect rather than rupture and only a small amount of fuel will be spilled, reducing the chances of fire. Some effort has been placed on finding fuels that are less likely to burn, but, so far, this research has been frustrated by the primary reason for having fuel, which is to burn it in the engine. Other items that can burn in a crash include the upholstery of an aircraft, which, if they are nylon or other synthetics, may produce toxic fumes, including cyanides and carbon monoxide. This has caused commercial aircraft to be refitted with safer material, such a Nomex. If there is a risk of landing in water, additional precautions, such as life vests or flotation devices, must be provided. Finally, an emergency locating transponder (ELT) must be installed in all new aircraft. This device, which is triggered by high acceleration levels, automatically broadcasts on 121.5 MHz and 243.0 MHz. An ELT will be immediately detected by Federal Aviation Administration air traffic controllers, military rescue centers, and orbiting satellites, which are equipped with special location devices. Within minutes of any activation of an ELT, the crash site can be localized by these means. Search and rescue will be activated and coordinated.

References

Aldman, B. and Chapon, A., editors. The Biomechanics of Impact Trauma, International Center for Transportation Studies, Washington, D.C. 1984.

Federal Aviation Administration, “Transport Airplane Cabin Interiors’ Crashworthiness Handbook”, U.S. Department of Transportation Advisory Circular No. AC25-17 July 15, 1991.

Gurdjian, E.S., Lange, W.A., Patrick, L.M., and Thomas, L.M. Impact Injury and Crash Protection. CC Thomas, New York 1970.

Mohler, S.R. Civil Aviation Medicine. Fundamentals of Aerospace Medicine, 2nd edition. R.L. de Hart, editor. Williams & Wilkins, Baltimore 1996.

Petersen, R. So You Want to Be a Spray Pilot? AgAir Update February 2002. The National Agricultural Aviation Association and the National Agricultural Aviation Museum.