Aerial firefighting
Aerial firefighting is a method to combat wildfires using aircrafts. The types of aircraft used may be either fixed-wing or helicopters. Agents used to fight fires may be either water or specially-formulated fire retardants.
Equipment
A wide variety of helicopters and fixed-wing aircraft are used as aerial firefighters. Many (if not most) of these aircraft are privately owned and leased to government agencies. Several state National Guards and the U.S. Marines also maintain fleets of firefighting aircraft.
Helicopters may be fitted with tanks or carry buckets. Buckets are usually filled by submerging in lakes, reservoirs or even swimming pools. The most popular of the buckets is the flexible Bambi Bucket. Helicopter tanks may be filled with either water or retardant on the ground or water may be pumped from lakes or reservoirs through a hanging snorkel. Variants of the Bell UH-1 are popular for firefighting helicopters although many others are also used.
Fixed-wing aircraft are always fitted with tanks. The tanks may be filled on the ground (usually with fire retardant) or specially equipped aircraft may be able to land on lakes or reservoirs and pump or scoop water into the tanks.
Various fixed-wing aircraft have been used over the years for firefighting. World War II era bombers were once the mainstay of the aerial firefighting fleet but these are being phased-out in favor of newer ex-military and purpose-built firefighters. The smallest is the Thrush Single Engine Aerial Tanker (SEAT). This is a purpose-built agricultural sprayer modified to drop up to 450 gallons of water or fire retardant. The S-2 Tracker (retrofitted with turboprop engines) is used by the California Division of Forestry as a medium-sized aerial firefighter. The Douglas DC-4 and its variants, the Lockheed C-130 Hercules and the Lockheed P-3 Orion are used as heavy tankers. The largest aerial firefighters currently in use in North America are two converted US Navy Martin Mars flying boats in British Columbia. Each carries 7,200 U.S. gallons of water or fire retardant. The Russian government operates an IL-76 aerial firefighter that can carry 11,000 gallons of water. However, Evergreen International Aviation is marketing a Boeing 747 aerial firefighter that can carry 24,000 gallons.
The Canadair CL-215, CL-217 and Bombardier CL-415 are purpose-built firefighting aircraft. These amphibious aircraft, known as "Super Scoopers", can skim the surface of a lake or reservoir and fill their tanks without landing. The Canadian scoppers are generally spurned in the United States but Los Angeles County has leased two CL-415s from the Province of Québec during the fire season for about a decade.
The O-2 Skymaster and OV-10 Bronco are used as spotters and the Beechcraft Baron is used as a lead plane.
Fire retardant
Borate salts were used in the past to fight wildfires but were found to sterilize the soil and were toxic to animals. Newer retardants use ammonium sulfate or ammonium polyphosphate with attapulgite clay thickener or diammonium phosphate with a guar gum derivative thickener. These are not only less toxic but act as fertilizers to help the regrowth of plants after the fire (however, the retardants appear to be toxic to fish and heavy application can have an adverse effect on the environment). Fire retardants conatin preservatives and rust inhibitors and are is colored red with ferric oxide to mark where they has been dropped. Brand names of fire retardants for aerial application are Fire-Trol and Fire-Chek.
Some water-dropping aircraft carry tanks of a guar gum derivative to thicken the water and reduce runoff.
Tactics
Helicopters can hover over the fire and accurately drop water or retardant directly on the flames. Fixed-wing aircraft must make a pass and drop water or retardant like a bomber. Spotter aircraft will orbit the fire at a high altitude to coordinate the efforts of the retardant-dropping aircraft. Lead planes fly ahead of larger aircraft, such as the DC-4, to mark the trajectory for the retardant-dropping aircraft.
Water is usually dropped directly on flames because it's effect is short-lived. Fire retardants are usually dropped ahead of the moving fire and may remain effective for two or more days. This creates artificial firebreaks where the terrain is too rugged or remote for ground crews to cut firebreaks or battle flames.
Aerial firefighting is always used in conjunction with ground-based efforts as aircraft are only one weapon in the firefighting arsenal. However, there have been cases of large aircraft extinguishing fires long before ground crews were able to reach them.