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The Allison YT-56-A-3 Turboprop Engine


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 Allison T-56-A-9D Turboprop

 The T-56, a jet engine that uses a propeller to produce most of its thrust, was originally designed to power the Lockheed C-130 Hercules. Because the T-56 compressor and turbine rotate at a high speed (13,820 rpm), a reduction gearbox is used to allow the propeller to turn at a much slower, more efficient speed. The production T-56 engine delivers approximately 4,000 horsepower to the propeller, while an additional 800 lbs. of thrust is produced by the jet exhaust. Its maximum operating altitude is 55,000 ft.

The T-56 evolved from Allison's T-38 engine and was first flown in the nose of a B-17 test-bed aircraft in 1954. The engine on display is a prototype of later engines in the T-56 series.

14-stage axial
Turbine: 4-stage axial
Gear Reduction Ratio: 12.5 to 1
Weight (without propeller): 1,610 lbs.
Max. Operating Altitude: 55,000 ft.
Cost: $150,000

The T56 is a turboprop, a jet engine that uses a propeller to produce most of its thrust. Because the T-56 compressor and turbine rotate at a high speed (13,820 rpm), a reduction gearbox is used to allow the propeller to turn at a much slower, more efficient speed. The production T-56 engine delivers approximately 4,000 horsepower to the propeller, while an additional 800 lbs. of thrust is produced by the jet exhaust. Its maximum operating altitude is 55,000 ft. The T56 turboprop, in continuous production since 1954, evolved from Allison's T-38 engine and was first flown in the nose of a B-17 test-bed aircraft in 1954. Originally designed to power the Lockheed C-130 Hercules, the T56 engine is installed in the P-3, C-130, and E-2/C-2 aircraft. The Navy also used a marine equivalent of the T56, the 501K engine, to generate electrical power for its destroyer-class ships.

The T56 and 501K engines are manufactured by Allison. As of 1997 DOD had 50 percent of the 14,130 engine population. Prior to the 1993 Base Realignment and Closure Commission (BRAC) decision, the Navy repaired its T56 and 501K engines at the Alameda Naval Aviation Depot.

The commercial equivalent of T56 is the 501D, which powers the Lockheed Electra and Convair 580 commercial aircraft. The 501K is a commercial engine used in several applications in the oil and gas industry. There is a worldwide network of commercial support for this engine group. During fiscal years 1993 through 1995, 27 percent of DOD’s T56 and 501K workload was commercially repaired.

In the early 1980s the focus of the T56 Series IV turboprop engine development program was to improve power and fuel consumption through incorporation of demonstrated technology improvements while retaining the long term durability and cost effective design of the T56 family. The T56-A-427, the Navy Series IV derivative of the 5000 shp (3728.5 kW) class T56 turboprop engine, resulted from over ten years of technology development via Advanced Turbine Engine Gas Generator (ATEGG), Joint Technology Demonstrator Engine (JTDE), and advanced component programs at Allison Gas Turbine Operations. An example of government and industry cooperation to transfer advanced gas turbine technology is the Air Force Engine Model Derivative Program (EMDP). The initial full-scale demonstration in this program confirmed a 10-1/2% reduction in specific fuel consumption (sfc) and a power growth of 21% in the basic T56 frame.^Continued early demonstrations and development by IR and D, Navy funds, and Allison discretionary funds showed a further sfc reduction to 13% and power increase of 28%. The full-scale development program was underway in 1984 to provide production engines in late 1986. Engines were available for the Grumman E-2 and C-2 aircraft, with follow-on adaptions for Lockheed C-130/L100 and P-3 aircraft, and generator sets for DD 963, DDG 993, CG 47 and DDG 51 warships.

Four decades have elapsed since the Air Force issued its original design specification, yet the remarkable C-130 remains in production. The initial production model was the C-130A, with four Allison T56-A-11 or -9 turboprops. A total of 219 were ordered and deliveries began in December 1956. The C-130B introduced Allison T56-A-7 turboprops and the first of 134 entered Air Force service in May 1959. Introduced in August of 1962, the 389 C-130E's that were ordered used the same Allison T56-A-7 engine, but added two 1,290 gallon external fuel tanks and an increased maximum takeoff weight capability. June 1974 introduced the first of 308 C-130H's with the more powerful Allison T56-A-15 turboprop engine. Nearly identical to the C-130E externally, the new engine brought major performance improvements to the aircraft.

Quick engine change (QEC) kits for the T56 engines are used in Air Force Special Operations Command (AFSOC) MC-130E, AC-130H, or AM-130P aircraft. AFSOC units face the difficulty of using and maintaining five different versions of T56 QECs across the AFSOC C-130 fleet. These versions are neither compatible nor interchangeable, greatly complicating the required logistics. These differences could be eliminated if the MC-130E and AC-130H aircraft received an oil cooler augmentation (OCA) and if the MC-130P aircraft received both the OCA and a generator modification.

The Hawkeye entered service in 1961 as the E-2A and was updated in 1969 to the E-2B. The E-2C was introduced in 1973 and has continued to incorporate improvements to keep pace with technology advances and the changing operational environment. The first E-2C was powered by two T56-A-425 turboprop engines and utilized an AN/APS-120 radar. Called "The Eyes of the Fleet," it was the world's premier AEW platform. In 1988, the Group I version was introduced featuring an upgraded T56-A-427 engine, which eliminated operating restrictions imposed by growth in the aircraft's gross weight due to incorporation of new systems. The new engine provides: much improved single-engine rate of climb; 23% more horsepower at high ambient temperatures; 13% lower specific fuel consumption which increases both the range and the time on station; 12% higher service ceiling at 37,000 feet; and 6% higher cruising speed at 260 knots.

The E-2C fatigue test and inspection of aircraft have identified fatigue stress cracks in Outer Wing Panels (OWP) which would cause the loss of aircraft and resulting in injury or loss of personnel. The OWP's installed on the E-2C aircraft are flight hour limited as follows: OWP's installed on T56-A-425 configured aircraft are limited to 6,000 flight hours and OWP's installed on T56-A-427 configured aircraft are limited to 7,500 flight hours. Teardowns of fleet OWP's showed that overhaul of the OWP is neither technically practical nor cost effective. This modification develops and incorporates enhancements to the OWP which extends the aircraft service life thru FY 2015. There are seventy-five (75) aircraft in the inventory. Thirty-four (34) aircraft will be enhanced with the AYC-1222 OWP (ECP 91145/C2A/859-97 Rev. FY04 funding of $1.5 million is a Congressional plus-up.

The C-2A Greyhound is a derivative of the E-2 Hawkeye and replaced the piston-engine powered C-1 Trader in the Carrier On-board Delivery role. The C-2A Greyhound provides critical logistics support to Carrier Strike Groups. Its primary mission is the transport of high-priority cargo, mail and passengers between carriers and shore bases. Powered by twin Allison T56-A-425 turboprop engines and Hamilton-Standard constant speed propellers, the C-2A can deliver a combined payload of 10,000 pounds over a distance in excess of 1,000 nm.


The Base Closure and Realignment Commission’s July 1993 recommendations for base closures and realignments included closing three of the Navy’ six aviation depots. One of these was the Alameda Naval Aviation Depot, California. Accordingly, the maintenance workloads performed at those facilities were redistributed to remaining depots operated by the Navy and other services. The Alameda depot performed maintenance on the TF34 turbine engine, used by the Navy on the S-3 aircraft and by the Air Force on the A-10 aircraft, and the Navy’s version of the T56 turbine engine used on C-130, P-3, and E-2 aircraft. The Alameda workload for the TF34 engines was transferred to the Jacksonville Naval Aviation Depot, Florida, and the T56 engine workload was added to the existing Air Force T56 workload at the San Antonio Air Logistics Center, Texas. The transition of maintenance capability to these facilities began in June 1994 and was completed by May 1996.

The transition of the TF34 and T56 engine workloads from the Alameda Naval Aviation Depot to the Jacksonville Naval Aviation Depot and the San Antonio Air Logistics Center, respectively, took longer than originally anticipated and was not executed in the most efficient manner. During the transfer, significant productivity and quality problems occurred at the gaining depots and the costs associated with establishing production capability were higher than expected. Neither the Air Force nor the Navy could quantify the increased costs to establish maintenance capability or the delays that the problems caused. They did, however, provide examples demonstrating the extent to which the identified problems affected cost and/or the time required to achieve production capability.

Limited access to the Alameda production lines resulted in some equipment needed to establish production capability not being identified. For example, the Navy had developed customized equipment to perform specific processes on the Navy T56 engines. However, San Antonio officials said that they did not become aware of the special equipment requirements until after the Navy T56 production line was established at their facility. They had to then obtain the needed equipment or develop alternate procedures to accomplish the required tasks.

Military and commercial gas turbine engines used for aircraft, ships, and utility power generation require more durable and more reliable hot-section components in order to achieve their “designed for” life. Less durable parts lead to increases in unscheduled (and costly) inspections, engine repairs, and major engine overhauls. For aircraft applications, this equates to less time “on-wing,” a severe reduction in flight safety, and a significant reduction in operational readiness. Typical in-service Naval aircraft that are affected include the AV-8B with the F402 engine and the E-2/P-3/C-2/C-130 aircraft with T56 engines and Global Hawk Unmanned Aerial Vehicle (UAV). A high temperature protective coating on components is key to enable longer life durability on the engine during these adverse conditions. In addition, T56-derivative engines (501K) are used for power generation on many Naval ships, and are experiencing thermal corrosion issues on turbine airfoils and have similar turbine durability issues.

Standard Navy maintenance practices require that aircraft engine gas pathways be flushed regularly to ensure optimal engine performance. This is accomplished by using the corrosion control cart—a piece of support equipment that dispenses detergent and fresh water into the aircraft engine gas pathway. Previously, the wash water from this practice was allowed to drain onto the tarmac uncontained. Although the detergent is not hazardous, it was recently determined that this wash water may contain harmful contaminants. The characteristics and quantity of contaminants in the wash water depend primarily on the type of aircraft engine. The Navy has conducted studies to identify which engines generate or have the potential to generate hazardous wastewater, as well as measures to eliminate the source of these contaminants. The Navy has also mandated that all engine wash water be contained and disposed of appropriately. From a Navy-wide perspective, it has been difficult to assess the scope of the problem due to varying operating procedures within the aircraft community and, in turn, different sample results. Other factors affecting sample characteristics include climate and environmental conditions, salt encrustation, engine type, age, and flight hours.

One engine in particular, the T56, has been proven to yield cadmium-contaminated wash water. The compressor blades within this engine are nickel-cadmium coated; since this coating is a sacrificial anode to control corrosion, the coating leaches into the wash water. The Navy's long-term plan to prevent this source of pollution involves replacing all cadmium-coated compressor blades in existing T56 engines with aluminum blades over a ten-year attrition schedule.

The WaterSmart ion adsorption pretreatment system consistently brought contaminant levels to near nondetectable concentrations with little intervention from site personnel. The cost savings are apparent compared to hazardous waste disposal, and the environmental benefit is a substantial reduction in hazardous waste production. The system has provided a reliable method of handling, storing, treating, and disposing of aircraft engine wash water and ensures compliance with all local, state, federal, Navy and NOTW requirements.




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