# Allison T56
The **Allison T56** is a single-shaft military turboprop engine developed by the Allison Division of General Motors for the [Lockheed C-130 Hercules](https://www.edgepedia.org/lockheed-c-130-hercules), and still in production more than 70 years after its first flight. Its commercial counterpart is the Allison 501 (later the T56 501-D), and since Allison's acquisition by Rolls-Royce in 1995 the engine has been built by Rolls-Royce in [Indianapolis](https://www.edgepedia.org/indianapolis).[1](https://www.dst.defence.gov.au/sites/default/files/publications/documents/RAAF_100YRS-A4L-104p%20Text_WEB.pdf) • [2](https://www.rolls-royce.com/media/press-releases/2024/16-09-2024-rolls-royce-celebrates-t56-ae-2100-engine-milestones.aspx) More than 18,000 engines have been produced, operating in nearly 70 countries with more than 220 million accumulated operating hours, and the commercial 501-D is described by Rolls-Royce as the leading large turboprop engine in the world by number of units sold.[3](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx)
| Key fact | Detail |
|---|---|
| First flight | Flight-tested in 1954 in the nose of a Boeing B-17 testbed; first installed on the Lockheed YC-130, which first flew on August 23, 1954[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000) |
| Architecture | Single-shaft turboprop: 14-stage axial-flow compressor, cannular combustor with six flame tubes, four-stage axial turbine, two-stage reduction gearbox with propeller brake and torquemeter[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000) • [3](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx) |
| Power ratings | T56-A-1: 3,750 eshp at 13,820 rpm, weight 794 kg; T56-A-15: 4,910 eshp takeoff, 4,365 eshp normal, pressure ratio 10, airflow 33 lb/s[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000) • [5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91000EYQ.TXT) |
| Fleet size | Over 18,000 engines produced; operated in nearly 70 countries; more than 220 million service hours as of 2024[3](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx) • [2](https://www.rolls-royce.com/media/press-releases/2024/16-09-2024-rolls-royce-celebrates-t56-ae-2100-engine-milestones.aspx) |
| Main applications | C-130 Hercules (Air Force, Navy, Marine Corps), P-3 Orion, E-2 Hawkeye, C-2 Greyhound, Convair 580, Lockheed L-188 Electra, Super Guppy[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000) |
| Series 3.5 upgrade | Specification 7.9% fuel reduction; NOAA flight tests showed more than 13%; turbine temperatures more than 100 °C lower; reliability improved 22%[3](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx) |
| Successor | The C-130J replaced the T56 with the Rolls-Royce AE 2100D3, which passed 10 million service hours in 2024[6](https://www.marines.mil/News/Messages/Messages-Display/Article/164230/c-130-hercules/) • [2](https://www.rolls-royce.com/media/press-releases/2024/16-09-2024-rolls-royce-celebrates-t56-ae-2100-engine-milestones.aspx) |
## Design
The T56 is a constant-speed, single-shaft turboprop: the compressor, turbine, and propeller gearbox are all connected through one shaft, so the engine runs at a fixed design speed and power is changed by fuel flow rather than by engine speed.[3](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx) • [5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91000EYQ.TXT) Air enters through the inlet housing and is compressed by a 14-stage axial-flow compressor directly coupled to a four-stage air-cooled turbine. Fuel burns continuously in six through-flow combustion chambers assembled within a single annular chamber (a cannular, or can-annular, arrangement). Gases exiting the turbine retain enough residual energy to produce a small amount of jet thrust in addition to shaft power.[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91000EYQ.TXT)
The power section connects through extension shafting to a single reduction gear assembly with a two-stage gear reduction, a propeller brake, and a torquemeter assembly; in the T56-A-15 the propeller shaft is offset above the power section centerline. The gearbox converts the turbine's high-rpm, low-torque output into the lower speeds and higher torque needed for efficient propeller operation.[3](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx) • [5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91000EYQ.TXT) In flight the propeller governor holds the engine at 100% rated speed, 13,820 rpm, the design speed at which most power and best overall efficiency are obtained; an increase in fuel flow raises turbine inlet temperature, the turbine absorbs more energy, and the propeller increases blade angle to absorb the added torque while speed stays constant.[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91000EYQ.TXT)
The Smithsonian's example of the first-generation T56-A-1 (Series I) is rated at 3,750 equivalent shaft horsepower (eshp) at 13,820 rpm and weighs 794 kg; the engine is 368 cm long.[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000)
## Development and variants
A 1951 U.S. Air Force call for a mid-sized aircraft to handle both logistics and tactical transport duties led to the C-130. With Air Force support Allison designed the T56 for it and first flight-tested the engine in 1954 in the nose of a Boeing B-17 testbed; the first production installation was on the YC-130 prototype, which flew on August 23, 1954.[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000)
The commercial version, the Model 501, powered the [Lockheed L-188 Electra](https://www.edgepedia.org/lockheed-l-188-electra), which entered service with Eastern Airlines in 1959.[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000) The 501-D13 series powered the Electra and the Convair 580, and the 501-D22 series powered the commercial Hercules variants L-382 and L-100.[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91000EYQ.TXT)
Military variants track C-130 development. The initial C-130A used four T56-A-1A engines of 3,750 hp each driving three-bladed [Curtiss-Wright](https://www.edgepedia.org/curtiss-wright) electric-reversible propellers.[6](https://www.marines.mil/News/Messages/Messages-Display/Article/164230/c-130-hercules/) • [7](https://www.lockheedmartin.com/en-us/products/c130/history.html) The C-130B and C-130E introduced the T56-A-7, and June 1974 brought the first of 308 C-130Hs with the more powerful T56-A-15, a Series III engine whose higher power brought major performance improvements to the aircraft.[6](https://www.marines.mil/News/Messages/Messages-Display/Article/164230/c-130-hercules/) • [5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91000EYQ.TXT) Navy variants followed their own designations: the P-3C Orion uses four T56-A-14 engines of 4,600 hp each, the Navy and Marine Corps C/KC-130T uses four T56-A-16 engines rated at 4,910 standard horsepower, and the E-2C Hawkeye uses two 5,100-horsepower T56-A427 engines.[8](https://www.navy.mil/DesktopModules/ArticleCS/Print.aspx?Article=2166367&ModuleId=724&PortalId=1) • [9](https://www.navair.navy.mil/product/CKC-130-Hercules-Super-Hercules) • [10](https://www.history.navy.mil/content/history/museums/nnam/explore/collections/aircraft/e/e-2c-hawkeye0.html/1000) Some later models are rated in excess of 5,000 horsepower.[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000)
## Applications
Beyond the C-130 family, the T56 has powered the [Lockheed P-3 Orion](https://www.edgepedia.org/lockheed-p-3-orion), the [Grumman C-2 Greyhound](https://www.edgepedia.org/grumman-c-2-greyhound) and [E-2 Hawkeye](https://www.edgepedia.org/e-2-hawkeye) carrier aircraft, the Convair 580 airliner, and the Aero Spacelines Super Guppy.[4](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000) The engine's long production life is tied to the C-130H, which Rolls-Royce describes as part of the longest continuous military aircraft production run in history; the T56 itself made its first flight in 1954 and remains in production.[2](https://www.rolls-royce.com/media/press-releases/2024/16-09-2024-rolls-royce-celebrates-t56-ae-2100-engine-milestones.aspx)
## Series 3.5 upgrade
The **T56 Series 3.5** is a Rolls-Royce enhancement package for legacy T56 engines on P-3 and C-130 aircraft. Its specification calls for a 7.9% reduction in fuel use, but flight results exceeded the requirement: ground and flight testing of the first enhanced engine for NOAA's WP-3D demonstrated fuel use reduction over 13%, and the first four-engine Series 3.5 configuration flew on June 3, 2015 at [MacDill Air Force Base](https://www.edgepedia.org/macdill-air-force-base), showing approximately 12.8% lower specific fuel consumption over a three-hour flight.[3](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx) • [11](https://www.rolls-royce.com/~/media/Files/R/Rolls-Royce/documents/customers/defence-aerospace/series-3-5-status-update-august-2015.pdf) The package also lets the engine run at significantly lower turbine temperatures, more than 100 °C lower on the first upgraded U.S. Air Force C-130H, which flew in December 2016 with the Wyoming Air National Guard; lower temperatures extend parts life, and Rolls-Royce credits the package with a 22% reliability improvement.[3](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx)
The upgrade entered fleet service through the Air Force Reserve Command's T-56-15A engine Centralized Repair Facility at Little Rock Air Force Base, which since acquiring the maintenance contract in 2015 has upgraded more than 190 C-130H engines with the 3.5 modifications, each engine passing a performance check in a test cell before delivery.[12](https://www.arpc.afrc.af.mil/News/Article-Display/Article/2464708/reserve-centralized-repair-facility-provides-strategic-depth/) Fuel-efficiency work on the T56 dates back further: a 1980 Society of Automotive Engineers paper describes an Air Force Engine Model Derivative Program targeting a 10.5% fuel consumption improvement and a 24% power increase for the large T56-powered C-130 fleet.[13](https://saemobilus.sae.org/papers/fuel-efficiency-improvements-t56-turboprop-engine-801158)
## Re-engining the C-130
The T56's longevity set up a comparison with newer engines when the C-130 fleet's efficiency was studied. A 2007 National Research Council analysis of C-130 re-engining options, using Rolls-Royce data, found that upgrading the existing T56 with Series 3.5 was the most affordable option by maximum negative cash flow and payback time, ahead of a T56 derivative (T56-A427) and a new AE 2100 installation.[14](https://www.nationalacademies.org/read/11837/chapter/17) The newer engines save more fuel per aircraft: the study's assumptions put fuel savings at 8% for the T56-S3.5, 13% for the T56-A427, and 28% for the AE 2100, with maintenance savings of 30%, 40%, and 50% respectively, but the AE 2100 required the largest investment.[14](https://www.nationalacademies.org/read/11837/chapter/17) In the event, new-production C-130J aircraft adopted the AE 2100D3 with a six-bladed composite propeller, while the large existing C-130H fleet retained and upgraded its T56-A-15 engines.[6](https://www.marines.mil/News/Messages/Messages-Display/Article/164230/c-130-hercules/)
## References
1. [DSTG's support to the T56 engine, Australian Department of Defence](https://www.dst.defence.gov.au/sites/default/files/publications/documents/RAAF_100YRS-A4L-104p%20Text_WEB.pdf)
2. [Decades of service and more than two hundred million hours: Rolls-Royce celebrates T56, AE 2100 engine milestones (16 September 2024)](https://www.rolls-royce.com/media/press-releases/2024/16-09-2024-rolls-royce-celebrates-t56-ae-2100-engine-milestones.aspx)
3. [T56, Rolls-Royce product page](https://www.rolls-royce.com/products-and-services/defence/aerospace/transport-tanker-patrol-and-tactical/t56.aspx)
4. [Allison T56-A-1 (501-D13) Turboprop Engine, Cutaway, Motorized, National Air and Space Museum](https://airandspace.si.edu/collection-objects/allison-t56-1-501-d13-turboprop-engine-cutaway-motorized/nasm_A19620071000)
5. [Collection and Assessment of Aircraft Emissions Base-Line Data: Turboprop Engines (Allison T56-A-15), Detroit Diesel Allison for EPA, EDR 7200, September 1971](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91000EYQ.TXT)
6. [C-130 Hercules, United States Marine Corps message](https://www.marines.mil/News/Messages/Messages-Display/Article/164230/c-130-hercules/)
7. [C-130 History, Lockheed Martin](https://www.lockheedmartin.com/en-us/products/c130/history.html)
8. [P-3C Orion, U.S. Navy fact file](https://www.navy.mil/DesktopModules/ArticleCS/Print.aspx?Article=2166367&ModuleId=724&PortalId=1)
9. [C/KC-130 Hercules / Super Hercules, NAVAIR](https://www.navair.navy.mil/product/CKC-130-Hercules-Super-Hercules)
10. [E-2C Hawkeye, National Naval Aviation Museum](https://www.history.navy.mil/content/history/museums/nnam/explore/collections/aircraft/e/e-2c-hawkeye0.html/1000)
11. [T56 Series 3.5 Engine Enhancement Program Status Update, Rolls-Royce, August 2015](https://www.rolls-royce.com/~/media/Files/R/Rolls-Royce/documents/customers/defence-aerospace/series-3-5-status-update-august-2015.pdf)
12. [Reserve Centralized Repair Facility provides strategic depth, Air Reserve Personnel Center, 7 January 2021](https://www.arpc.afrc.af.mil/News/Article-Display/Article/2464708/reserve-centralized-repair-facility-provides-strategic-depth/)
13. [Munevar, H. and Verkamp, F., "Fuel Efficiency Improvements to the T56 Turboprop Engine," SAE 801158, 1980](https://saemobilus.sae.org/papers/fuel-efficiency-improvements-t56-turboprop-engine-801158)
14. [Improving the Efficiency of Engines for Large Nonfighter Aircraft, Appendix F, National Research Council, 2007](https://www.nationalacademies.org/read/11837/chapter/17)
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*Topic: Encyclopedia › Technology and the built world › Transport and spaceflight › Aviation › Aircraft › Aircraft technology: engines, components, configurations › Aircraft engines and propulsion systems › Turboprop and turboshaft engines*
*Initially written Sep 17, 2026 · Reviewed: Sep 17, 2026; Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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