Machine Design

Airborne Fire Trucks

Engineering advances and ingenuity are making planes and helicopters even better frontline firefighting assets.

Stephen J. Mraz
Senior Editor

The Supertanker, a converted B-747 cargo hauler, can drop 24,000 gallons of water, foam, or in this case, fire retardant.

Bombardier's CL-415 airborne tanker has a maximum speed of 234 mph, a range of 1, 518 miles, a service ceiling of 14,700 ft, and can climb at 1,378 fpm. It drops about 1,600 gallons through four belly doors.

The CL-415 uses a glass cockpit to ease the workload on flight crews and let them maintain awareness of what's happening outside the cockpit.

The Evergreen Supertanker uses pressurized air to shoot water out its four nozzles.

Bambi Buckets come in 22 sizes, from 72 to 2,600 gallons, and weigh up to 650 lb (empty). They can be fitted to any helicopter, including this Sikorsky CH-54 Sky Crane, and are one of the primary aerial assets for fighting fires.

Though Bambi Buckets are used most of the time on forest fires, there is increasing use of them to fight suburban and even downtown tower fires. Helicopter pilots can even fill their buckets from swimming pools, a great resource that can be close to the fire.

A Bambi Bucket equipped with Powerfill, a pump system, can fill at 500 gpm even from shallow waters such as this mountain stream.

Forest fires have the nasty habit of breaking out in areas fire trucks can't get to, so agencies responsible for putting those fires out must rely on airborne assets to get massive amounts of water on the flames. These assets mostly consist of helicopters carrying large "buckets" of water and a motley fleet of leftover World War II and Cold War planes converted to carrying and dropping water. But there is at least one aircraft designed from the outset to fight fires, the Bombardier CL-415, and a 747 Jumbo jet is being converted into the world's largest airborne tanker. And don't dismiss those seemingly simple "buckets." They could be the most versatile and effective weapons in a fire chief's arsenal.

Bombardier's CL-415 is a follow-on to its CL-215, an amphibious cargo airplane that was commonly converted to a widely used firefighting tanker aircraft. The design goal for the CL-415 was to keep the simplicity and strength of the 215, but add modern powerplants and avionics, along with aerodynamic upgrades such as wingtip end plates and finlets on the horizontal stabilizers.

The new powerplants, two Pratt and Whitney (Canada) PW123AF turboprops, replace less-reliable, maintenance-intensive radial-piston engines. The jet engines also generate more power, up to 2,380 shp each through a four-bladed Hamilton Standard constant-speed, reversible prop. The engines give the plane a power-to-weight ratio of 0.29 (with a full water load), which is good by airborne-tanker standards. The engines are also fuel efficient, so the amount of water scooped up and carried can be maximized.

The basic airframe is sturdy, simple, and reliable, which becomes important when planes are operating out of backwoods airfields with no supply lines or replacements quickly available. For instance, there is only one hydraulic system. Most aircraft have three, giving them redundant backups for flight controls. If the main hydraulics fail on the CL-415, flight controls are operated manually. And there are emergency hydraulics for closing the water doors, extending the landing gear, and energizing the landing brakes.

The airframe is rugged enough to withstand 3.25 gs of acceleration in its water-bombing configuration. This lets it handle steep descents and climb outs, as well as tight turns. This is critical in both putting water on fires and getting down to lakes and rivers to refill the water tanks.

To ensure the airframe retains its strength despite operating in and around saltwater, Bombardier added several layers of corrosion control. The two water tanks installed above the plane's floor, for example, are made of fiberglass. The other two are below the floor and made of an aluminum alloy protected with primer and an enamel topcoat. The airframe it-self is made of several aluminum alloys with outside surfaces given a sulfuric-acid-anodize treatment, two coats of primer, and a hard-enamel topcoat. And systems such as the flight controls actuators use corrosion-resistant steels. As a result, the CL-415, which cost $25.6 million, has a design life of 10,000 flight hours, 50,000 scoops, 50,000 water drops, 10,000 land landings, and 10,000 water landings.

The avionics upgrade included a glass cockpit in which LCDs and display screens replace dials and gages. The crew-configurable displays show only what pilots need, which reduces potential confusion. It lets the fight crew look outside the cockpit more, staying aware of their situation in regards to the fire and their mission.

Bombardier added a forward-looking IR camera (FLIR) for the CL-415 that lets pilots pinpoint hotspots — targets for their water drops — despite thick clouds of blinding smoke. The designers also added more glass around the cockpit, giving pilots larger windows.

Bombardier designed the cockpit and aircraft systems so pilots need only complete a short checklist before starting engines and taking off for a fire. Aircrews can actually perform all necessary checks, start then shut down engines, and the plane can be kept in Alert status. In most cases, this means crews can then launch for a fire with only a 5-min notice.

Two additional water tanks give the CL-415 a total of four carrying about 400 gallons each. The four tanks, along with four drop doors, let crews gravity drop four individual loads, a pair of larger loads, or a single salvo, depending on whether they are fighting a major blaze, creating a fire line, or combating smaller, dispersed fires. Doors can be opened manually by the pilots or by a computer programmed for a specific approach to the fire.

To fill the water tanks, two scoops, each 5-in. wide by 3-in. high, open and extend from the aircraft's belly. (The left scoop feeds the left-hand tanks; the right scoop feeds the right-hand tanks.) The plane sets up for a normal landing, with flaps at 15° and a controlled descent speed. Still, the scoops hit the water at 90 mph. "But the hull meets Federal Aviation Regulations for amphibious aircraft, which sets standards for impact loads and are considered quite conservative," says Ciro Guida, an amphibious aircraft engineer at Bombardier. "And although the crew has to add power to compensate for the weight of the water being rammed in and the water-brake effect of the scoops, passengers might not even notice any deceleration or surge while the tanks are filled."

To make foam in the tanks, pilots select an option that injects foaming chemicals shortly after the scoops are retracted (or manually trigger injection themselves). The sloshing water settling effectively mixes the foam.

Firefighters almost always wish airborne tankers could deliver more water, foam or retardant, either to help put out or cool a fire, or to prevent it from spreading. If all goes well at Evergreen Supertanker Services Inc. in Marana, Ariz. (,those firefighters could get their wish in the form of a tanker based on a B-747 freighter. It carries 24,000 gallons, seven times the load of today's largest firefighting aircraft.

As part of a three-year. $40 million development project, engineers at Evergreen mounted 10 tanks to carry water and foaming or fire-retardant agents, along with eight air tanks on standard aircraft pallets. (Ground crew can quickly off-load the palletized tanks and return the plane to its freighter configuration.) The air tanks are pressurized to 165 psi. When there's a drop, pressurized air forces water out of the tanks and through four 12-in. nozzles on the belly of the plane. Flight engineers vary the pressure and how much water is released to tailor the response to the fire. But the jet can release almost 21,000 gallons in just 10 sec using full pressure.

It takes 20 min to refill the tanks, which must be done while the plane is on the ground, and about the same amount of time to repressurize the air tanks. Evergreen is working on a system that will let them pressurize the tanks in flight. The three-man crew, along with four to eight ground crew, can get the Supertanker launched in 30 min from an airfield, if it is at least 8,000-ft long. (80% of U.S. airports meet this criteria.) Then it takes 15 min or less cruising at 600 mph to be over a fire within 100 miles of the airfield.

The Jumbo Jet might seem too ungainly to be yanking and banking down low to drop water on flames. But Evergreen engineers, with agreement from Boeing, the plane's manufacturer, say that even fully loaded with 24,000 gallons of water, the plane is 150,000 lb below its maximum take-off weight and still below its maximum landing weight. So it can take off and land with a full load and still have a healthy safety margin.

Other aerial tankers take off at maximum weight and are straining their limits as they maneuver to drop water on or near the fire. The supertanker, with its much higher power-to-weight ratio, will make this a safer mission.

Because the Supertanker forcibly expels its water load straight down rather than relying on gravity, as do all other aerial tankers, its accuracy is better. This means it can fly at 400 to 800 ft for drops, a much safer altitude than the 200 ft needed by other aircraft to get the water on target. During a drop, the 747 will be in its landing configuration and flying at 140 knots, 30% over its stall speed. In tests, the Supertanker has flown 145 knots at 400 ft doing 30° banks without exceeding its well-established flight envelope.

There are rumors that when the tanker lets its load go at full pressurization, it generates 50,000 lb of thrust. "But even at full four-nozzle dispersal, the pilot feels only a minor surge," says Evergreen Supertanker President Bob McAndrew. "That's because we manage the flow so that is has minimum effect on the plane's center of gravity.

Evergreen already owns and leases a fleet of 54 light, medium, and heavy-lift helicopters, 30 of which can be used to fight fires, and a few 747s that it leases as cargo carriers. The company is working with the FAA and other agencies to get Supertanker certified and approved for aerial fire-fighting. It plans to convert two more older B-747s in its fleet to Supertankers, and lease all three, along with ground and flight crews, to firefighting agencies. Evergreen expects to get a 10-year life out of each tanker, but won't say what it will cost to lease a Supertanker, nor what operational costs might be.

Since their introduction in 1983, Bambi Buckets from SEI Industries, British Columbia, Canada (, have become a common sight hanging below helicopters of all shapes and sizes battling fires in California forests, the Canadian wilderness, the Australian Outback, and practically everywhere in between. The familiar red buckets, named to be memorable rather than for any association with wildlife or cartoon characters, are simple, relatively easy to use, and have been technologically evolving since their inception.

At first, the buckets were very low-tech: just collapsible urethane containers that hung beneath a helicopter and carried anywhere from 72 to 2,600 gallons of water. Pilots filled them by dip-ping and dragging them for about 5 sec in any body of water at least 3-ft deep. And when a pilot pushed the release button, all the water streamed out of a valve in the bottom of the bucket.

These simple devices are still used, but the bucket soon became a woven scrim coated with specially formulated urethane. Vertical reinforcements and fiberglass battens replaced circular, ringlike reinforcements on the bucket. (As a pilot lifts a bucket from a lake, water pressure bends the shell and its battens outward, increasing the bucket's volume, hence the rule of thumb: A slow lift gives minimal fill. A fast lift gives maximum fill.) And the machined drop head, the valve that lets the water out, became a cast-aluminum component, which lowered costs while boosting precision and strength.

During typical firefighting missions, a helicopter burns fuel as it repeatedly fills and drops water. The lighter fuel load means the pilot can carry more water, but a one-size bucket prevented this. So SEI Industries added sizing straps that a pilot could cinch up at the beginning of a mission so the bucket carries less than a full load. After burning enough fuel, the pilot can land, quickly loosen the straps, and the bag is again at full capacity.

Another more important addition is Powerfill, which comes in three versions. It lets pilots fill buckets in murky, muddy water as shallow as 12 in. Powerfill II has a 500-gpm pump and goes on standard buckets. A screen stops large debris from being sucked into the bucket. Powerfill I has two 500-gpm pumps, and Power-fill III carries four of the same pumps, and both are for the buckets equipped with the newer Torrentula valves.

"In one fire, a commercial operator flying a Chinook (Boeing CH-47) carrying the 2,600-gallon bucket on the end of a 150-ft cable filled the entire bucket in 90 sec from a stream no more than a foot deep," recalls Jens Sigvardt, manager of firefighting products at SEI. "They could fill and drop in one-seventh the time of other helicopters because Powerfill let the pilot use a small stream at the bottom of a narrow ravine relatively close to the fire."

Another common accessory, the Sacksafoam, lets helicopters mix and drop foam. (As you can see the company owner likes catchy names. He named another firefighting piece of equipment, a pump, the Litre Hosen.) Foam does two major things for fire-fighters. First, it creates bubbles, increasing the surface area of the water and making it a more effective firefighting agent. And second, it decreases the water's surface tension, letting water penetrate into wood, houses, and other tight spaces ordinary water might not. In the past, operators had to dip buckets in holding tanks of foam.

Sacksafoam I is a fabric bag of foaming chemicals that attaches inside the bucket. The pilot checks a chart to determine how long to turn on the Sacksafoam pump based on the needed concentration of chemicals in the water (anywhere from 0.1 to 1%), and the size of the bucket being carried. The bag carries enough chemicals for a 1.5-hr mission. Sacksafoam II and III are carried in larger helicopters and can be re-filled in flight. And Sacksafoam III, for example, is for helicopters large enough to have a second crewman, an engineer, who controls filling and dumping the Bambi Bucket and keeping it armed with the right mix of foaming agent.

The Firesock, another accessory, attaches to the dump valve. It's a mesh bag that creates turbulence and helps aerate the foaming agent/water mixture. And even if foam is not being used, Firesock breaks water being dumped into a finer mist, increasing its surface area and its coverage on the ground.

One of the biggest advances to the Bambi Bucket has been the computer-controlled Torrentula valve, which was mentioned earlier. It lets pilots start and stop water drops as often as they want. Pilots can also vary how fast water comes out by controlling the valve opening.

For even more control, the pilot inputs height above ground and airspeed for an approach to the fire into a computer, and asks for a specific water coverage (measured in gpc or gallons per 100 ft 2 , a firefighting standard set by the U.S. Forest Service). The computer determines when to drop and for how long to get that coverage.

Torrentula eliminates the need to cinch buckets to reduce loads. The valve can be set to pick up just 80% of a load in case the helo has a full fuel tank or the altitude or temperature is too high, two factors that affect lift. The valve also lets pilots manually bleed off water if they need more lift.

SEI engineers redesigned the valve on the Torrentula so that it doesn't use a plunger or butterfly valve, which are used on standard Bambi Buckets. Both these valves work against the water pressure. The new valve uses a sleeve design that doesn't fight the water's weight while it's opening, so it can get away with a lighter, 5A, 24 to 18-V actuator.

The Torrentula valve, and its little brother, Torrentula Lite, work on buckets down to 120 gallon. "There's little point in breaking up bucket loads smaller than that into three or four drops," says Sigvardt.

And with buckets costing anywhere from $5,500 to $43,000 and up, operators aren't happy when a pilot accidentally hits the wrong button and drops a bucket into a lake or river. But it does happen. So SEI provides the option of a marine-recovery device. It mounts on a bucket, senses when a bucket is submerged too long, then releases a marker buoy on a 400-ft line.

SEI is currently working on an updated computerized drop system that will include some form of GPS and a flight-data recorder. It will give pilots and fire managers hard copy of where the helicopter flew, where it picked up water, and where and how much water it dropped.

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