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Aviation has revolutionized firefighting. The DC-10 Air Tanker has unmatched access and capacity for dispersing fire retardant compounds.
The DC-10 Air Tanker exceeds firefighting performance in categories that other air tankers cannot compare. The Tanker is a modified version of the aviation marvel, the DC-10.
The Air Tanker is the only widebody jetliner currently used in the firefighting service. The aircraft brings the exceptional performance of the DC-10 to an operational environment it has never seen before. The Tanker delivers speed, capacity, agility, and reliability to the firefighting world.
Even today, in 2022, the Air Tanker is battling fires across the globe. The Air Tanker can deliver a high volume of fire retardant in record times. Its sophisticated dispersion system allows the aircraft to drop its payload at varying areas and densities.
Many pilots and engineers have praised the DC-10 for its exceptional performance. The scale at which the DC-10 Air Tanker can operate pales in comparison to anything I have ever seen.
History of the DC-10
The DC-10 was a product of its era. The aircraft was developed and manufactured by Mcdonnell Douglas. The aircraft was designed to fulfill demand in a segment where a suitable candidate didn’t exist. Innovation fueled by competition gave birth to the DC-10, the world's first wide-body trijet.
With fuel costs becoming more important to airlines, efficient routes using efficient aircraft were a high priority.Twin Jets were limited to flying within one hour of a diversion airport. On longer routes, airlines would have to fly a twinjet on an inefficient route or fly an inefficient quadjet, like the Boeing 747.
Airlines challenged manufacturers to deliver an aircraft that had the range of the jumbos with a per passenger operational cost comparable to a twinjet. And a design that was limited by the twin-engine regulations. McDonnell Douglas and Lockheed raced to produce the first widebody trijet.
The DC-10 made it to the finish line first, entering into service in 1971. The TriStar, the Lockheed entry, was delayed by engineering hurdles and budget overruns. This gave the DC-10 the opportunity to dominate the market in this segment. And it did. The DC-10 family outsold the TriStar by almost double.
Later on, the TriStar did make it to the market but its design was expensive and complex. The developers packed the aircraft with complex systems and features. This made the aircraft costly, hard to maintain, and harder for pilots to adapt. Meanwhile, the DC-10 was thriving with its simple, reliable design.
The DC-10 entered the market as an efficient, high-performance, and reliable jet. The aircraft featured an advanced design utilizing the latest technology available at the time. The result was an aircraft that could cross the Atlantic safer and cheaper.
Although the DC-10 family was plagued with criticism after multiple incidents and accidents. The cause of these incidents was found to be maintenance issues and not a flawed design. Over the next few decades, the DC-10 went on to obtain a safety record on par with its rivals.
Pilots flying the type noted how well the aircraft handled and performed. Its performance was unlike anything they had encountered before. On the other hand, operators and engineers praised the design for its reliability and ease of maintenance. Not forgetting to mention it was a passenger favorite delivering comfort in style.
The DC-10 and its Variants
The aircraft was adapted to serve in many categories. The design was stretched and furnished with more powerful engines extending its capabilities. The aircraft was available in a dedicated cargo variant and even went on to serve in the military as the KC-10.
The different variants all capitalized on its reliable design and adapted it for a unique purpose. The initial passenger variant was updated in 1981. The DC-10-30 featured more powerful engines and wider wings with a higher fuel capacity allowing its range to be extended by almost 70 percent.
After competing with Boeing and Lockheed, the DC-10 was selected as the newest military aircraft. The KC-10 is the military variant and allowed the military to conduct mid-air refueling in conditions only possible with the design of the DC-10. Its ability to carry large payloads and short takeoff and landing distances made it the ideal candidate.
The capabilities of the DC-10 aircraft and its history with modifications and adaptations makes it ideal for the conversation into the Air Tanker. The aircraft's performance and payload capacity affords it the title of an Air Tanker.
What is an Air Tanker?
Wildfires are known for barreling across densely forested areas. The problem is, that these areas are largely inaccessible by traditional fire fighting methods such as humans and fire fighting trucks. These fires are also on a scale too large for firemen and water cannons to handle. The answer was the adaptation of aircraft for the job. Since an aircraft can offer a unique reach, range, and capacity.
An air tanker is a dedicated aircraft used for the dumping of water or fire fighting compounds from the aircraft directly to the hazard. These aircraft can, in their payload tanks, load the fluid, takeoff, fly to the designated area and disperse the solution with precision.
The coloured mess left behind by some of these fire fighting missions may seem like a burden, but for some it is the mark of success. Those impacted are happy to see their property distressed by the colors since it's either this or the ruins of a fire.
These aircraft can vary in size, capacity, capability, and overall flight performance. From the smaller Canadair CL-215 and CL-415 to the now retired 747 Supertanker. Some of these aircraft take directly to the blaze while others aid in containment by creating a fireline.
With temperatures rising and droughts becoming more frequent, forest fires are becoming more prevalent. Luckily, the firefighting services have a few tricks up their sleeve and the DC-10 Air Tanker is the main act.
The Conversion of the DC-10 into the Air Tanker
There is currently no purpose-built Air Tanker that can operate on the scale and capacity of the Air Tanker. The alternative was an aircraft conversion. After their commercial service, many planes are given a second life when repurposed.
The Air Tanker is based on a stripped-down version of the DC-10 with storage tanks mounted to the belly of the aircraft. The idea was to remove as much weight from the aircraft as possible. This allows for the carrying capacity of the fluids used in fire fighting. Making way for the three massive storage tanks.
The three storage tanks are mounted under the aircraft fuselage. The aircraft fuselage was reinforced to accommodate the added load that the mounting points would put on the airframe structure. These tanks have a combined capacity of 12,000 gallons or (45,000 Liters).
These tanks were mounted outside of the aircraft instead of utilizing the inside volume. This decision allowed little modification to be made to the structure of the airframe. This configuration was the cheapest and placed the tanks in the ideal position for dispersion. The tanks feature a streamlined design most optimal for minimizing drag on the aircraft.
The aircraft was granted a supplemental type certificate by the Federal Aviation Administration (FAA) in early 2006. This gave the green light to modify the DC-10s for the aerial dispersion of fluids.
The operator, 10 Tanker, acquired a 14 Code of Federal Regulations Part 137 Agricultural Aircraft Certificate. This allowed them to obtain contracts and perform the required fire fighting missions.
The first Air Tanker, a DC-1-10 registered as N450AX, began its journey in 1975 with National Airlines (NA) as a commercial passenger jet. The aircraft was then retrofitted with the external tanks and the dispersion system. The aircraft was given the callsign Tanker 910. This aircraft was the first of 5 jets to be converted.
The DC-10 Air Tanker Specifications
With the exception of its firefighting addons, the Tanker shares its configuration with the DC-10-10 variant or DC-10-30 variant. However, only converted -30s are currently in operations. The design featured three turbofan engines, two wing-mounted and the number 2 engine furnished to the base of the horizontal stabilizer.
The tanker can use its 12,000-gallon tank to disperse water or a fire retardant compound. The under-mounted tanks offer easy dumping with their computer-controlled dispersion doors.Even with the tanks mounted the aircraft still has a ground clearance of 15 inches.
Aircraft like the DC-10 Air Tanker are most suitable for its purpose. Comparing its implementation cost to a newly constructed aircraft dedicated to fire fighting is not financially practical. Unlike commercial passenger jets, these aircraft will fly much fewer hours per month. This makes cost an even bigger concern.
The proven passenger design of the DC-10and its adaptation for cargo use and for the military paved the way for the development of the Air Tanker.
The Air Tanker’s Performance
The DC-10 family and its variants were built for performance. The passenger and cargo variants were designed to shoot across the sky at altitudes of up to 42,000ft at speeds of up to 473 kts. While the military version, the KC-10, could refuel fighter jets mid-air and transport a wide range of military assets such as ground vehicles and personnel. The Air Tanker, however, had its own purpose and mission.
The requirement for an aircraft used in fire-fighting is a bit unconventional. These Air Tankers, while on their mission, do not reach these bizarre altitudes or speeds but have demands of their own. The Air Tanker must fill its three tanks within minutes, climb out of its station and deliver a payload with precision, mid-flight.
The Air Tanker isn't called upon to fly across the Atlantic, so it can carry less fuel, much less. The Tanker only needs about 30 to 40 percent of the fuel needed for a commercial mission or military mission. This allows the aircraft to take off with a payload much lower than the MTOW.
The aircraft achieves this reduction in weight with its three external tanks full. This reduced weight allows the aircraft to climb at much faster rates since it still has the same three powerful engines delivering 52,000 lbs of thrust, each.
The power produced by the engines is the same as the DC-10. However, it operates at a much higher power-to-weight ratio resulting in more available power. This extra power availability allows the Tanker to reach its destination easier.
In addition to more power available due to the weight savings, the aircraft is much easier to maneuver and allows for tighter turns and more agile movements. This is ideal for situations where the dispersion pattern and location may be a bit more complex. Especially in areas with varying topography.
This extra available thrust and agility also allow the Tanker to operate at much safer margins. With the power of its three engines at its disposal in case of an emergency requiring a steep escape. This was demonstrated when the aircraft struck trees on a mission and went on to land without any injuries for significant airframe damage.
The Air Tanker allows the deposition of a high volume with precision. Due to the design of the DC-10 aircraft, the Air Tanker is able to take off and land at short airports compared to its size. It was one of the main reasons it was selected by the military. This allows the Tanker to serve a wider range of airports.
The Tanker does not directly put out the fire with its dispersion, but rather it uses a more passive approach to fire fighting. The Tanker is designed to create a continuous region of fire-resistant land curbing the spread of the fire. This is known as a fire line.
The dimensions of the fire line are dependent on the fire and the configuration of the drop for the specific mission. The sophisticated dispersion and tank system can accurately deposit the fluid in a controlled manner.
The DC-10 Air Tanker has a capacity, unlike any traditional tanker. It can deliver the same volume of liquid that would take a traditional tanker like the S-2T Airtanker twelve round trips. Helicopters simply lack the range and capacity to compete. This highlights how versatile and unique the Air tanker is and how the DC-10 aircraft is ideal for this task.
The Three Tanks
During the modification, the aircraft was fitted with three fluid tanks mounted under the belly of the aircraft. These three tanks can hold a maximum of 12,000 gallons of water or fire retardant fluid.
The three tanks can be filled simultaneously within 15 minutes via three-inch cam-lock couplings. This allows the aircraft to accomplish its mission in much less time. These tanks can hold over 100,000 lbs of fluid. Allowing for a much greater area to be serviced.
The thanks are prone to the development of a differential pressure which can lead to structural failure. During filling the tanks can experience a surge in pressure leading to expansion and during deposition, the tanks can experience a vacuum. Both conditions are mitigated by the inclusion of vents at the top of each tank.
One major concern with the transportation of fluids in such high volumes is their effect on the stability of the aircraft. Fluids are naturally affected by movement and gravity. As the aircraft climbs or descends this can cause the fluid to slosh around moving from one end of the aircraft to another.
This is critical as shifting of the fluid will move the center of gravity of the aircraft, potentially putting the aircraft into a tail-heavy condition or a nose-heavy condition if this shift is along the longitudinal axis of the aircraft. This can lead to an aircraft stall or an uncontrollable flight attitude which can be catastrophic.
The design of the tank system solves this potential issue. The three tanks create a physical separation between the volume of liquid, limiting the movement of the fluid. Additionally, each of the tanks is fitted with tank baffles to reduce the sloshing effect.
Tank baffles are basically perforated plates positioned within the tank. These plates create resistance in the tank for fluid moving around. The baffles act like a damper that dissipates the kinetic energy of the liquid. This allows the aircraft to maintain stability during operations. This is done without affecting the rate at which the fluid can be ejected.
The Deposition System
The aircraft can vary the width, length, and density of the fluid dispensed. This is achieved by a combination of regulating the flow rate, aircraft speed, and height. The entire load can be dropped in little as eight seconds over a wide area.
The rate at which the fluid can be dropped is determined by the opening of the electronically controlled tank doors. The crew can manage this by a computer on the flight deck which manipulates the distance the doors open and thus the flow rate.
The height of the aircraft is carefully selected as this can determine the width of the drop. As the aircraft flies higher the span of the dispersion is wider and this creates a wider drop zone. The opposite is true for flying at lower heights. This results in a narrower band of dispersion.
The speed of the aircraft can also determine the dimension of the covered area. The faster the aircraft flies the longer distance the aircraft can cover with the deposit.
The Tanker can slow to a flight speed of just 140 kts and drop to an altitude of just 200 feet. These flying conditions are unheard of for a jet this size but are normal operating conditions for the Air Tanker.
This allows the fluids to be dropped within the specified area and reduces the impact of relative wind. This not only keeps the drop on target but it minimizes the wasted compound and maximizes the fire fighting impacts of the drop.
The density, height, and speed are carefully controlled to give the desired concentration and dispersion area depending on the aircraft's mission. The Air Tanker is capable of creating a fire line that is 300 feet wide and over 5,200 feet long or about 1.6 miles.
Past Missions of the DC-10 Air Tanker
The US Forest Service was the first major client that the Tanker served. The aircraft earned an average of US$26,500 per flight hour. Upon approval for use in California, the first tanker, the 910, was utilized in the Sawtooth Complex fire. The impact of the Tanker exceeded what could be achieved with other aircraft in over 10 trips.
The year 2007 saw the first incident with the tanker. The aircraft struck some trees causing damage to the structure of the aircraft. The aircraft encountered an unexpected sinking pocket of air and had a dip in altitude that exceeded the minimum planned altitude.
The aircraft managed to climb out of the low altitude condition, dump its payload of fire retardant, and made an emergency landing at its Victorville, California base. There were no injuries and the damage sustained by the aircraft was repaired and the aircraft re-entered service.
Over the next few years, the Tanker went on to service numerous fires across the US. In the year 2010, the Tanker conducted its first mission in Australia leased by the National Aerial Firefighting Center.
The Air Tankers went on to serve for a number of years. The Tanker was credited by pilots for its maneuverability and handling. Its ability to navigate the mountainous terrain of the Catalina Foothills was exceptional.
As recently as May 2022, the Air tanker is still conducting missions. This not only demonstrates the reliability of the Air Tanker but the reliability and capabilities of the DC-10 aircraft as a whole.