Summary
The CFM56 is a high-bypass turbofan engine (most of the air accelerated by the fan bypasses the core of the engine and is exhausted out of the fan case) with several variants having bypass ratios ranging from 5:1 to 6:1, generating 18,500 to 34,000 lbf (80 kN to 150 kN) of thrust. The variants share a common design, but the details differ. The CFM56 is a two-shaft (or two-spool) engine, meaning that there are two rotating shafts, one high-pressure and one low-pressure. Each is powered by its own turbine section (the high-pressure and low-pressure turbines, respectively). The fan and booster (low pressure compressor) evolved over the different iterations of the engine, as did the compressor, combustor and turbine sections.
Combustor
Most variants of the CFM56 feature a single-annular combustor. An annular combustor is a continuous ring where fuel is injected into the airflow and ignited, raising the pressure and temperature of the flow. Other types of combustors include can combustors, where each combustion chamber is separate, and canannularwhich is a hybrid of the two. Fuel injection is regulated by a Hydromechanical Unit (HMU), built by Honeywell. The HMU regulates the amount of fuel delivered to the engine by means of an electrohydraulic servo valve that, in turn, drives a fuel metering valve, that provides information to the full authority digital engine controller (FADEC).
In 1989, CFMI began work on a new, double-annular combustor. Instead of having just one combustion zone, the double-annular combustor has a second combustion zone that is used at high thrust levels. This design lowers the emissions of both nitrogen oxides (NOx) and carbon dioxide (CO2). The first CFM56 engine with the double-annular combustor entered service in 1995, and the combustor is used on "Tech Insertion" CFM56-5B and CFM56-7B variants.
GE started developing and testing a new type of combustor called the Twin Annular Premixing Swirlercombustor, or "TAPS", during the Tech 56 program. This design is similar to the double-annular combustor in that it has two combustion zones; however, this combustor "swirls" the flow, creating an ideal fuel–air mixture. This difference allows the combustor to generate much less NOx than other combustors. Tests on a CFM56-7B engine demonstrated an improvement of 46% over single-annular combustors and 22% over double-annular combustors. The analytical tools developed for TAPS have also been used to improve other combustors, notably the single-annular combustors in some CFM56-5B and -7B engines.
Compressor
The high-pressure compressor-HPC, that was at the center of the original export controversy, features nine stages in each variant of the CFM56. The compressor stages have been developed fromGE`s "GE core" (namely a single-turbine, nine-compressor stage design) which was designed in a compact core rotor. The small span of the compressor radius meant that the entire engine could be lighter and smaller,as the accessory units in the system (bearings,oiling systems) could be merged to the main fueling system running on aviation fuel. As design evolved HPC design improved through better airfoil design. As part of the Tech-56 improvement program CFMI has tested the new CFM-56 model with six-stage high-pressure compressor stages (discs that make up the compressor system) that was designed to deliver same pressure ratios (pressure gain 30) similar to the old nine-stages compressor design. While the new one was not fully replacing the old one, it offered an upgrade in HPC, thanks to improved blade dynamics, as a part of their "Tech Insertion" management plan from 2007.
Exhaust
Although CFMI tested both a mixed and unmixed exhaust design at the beginning of development, most variants of the engine feature an unmixed exhaust nozzle. Only the high-power CFM56-5C, designed for the Airbus A340, has a mixed-flow exhaust nozzle.
Additionally, GE and SNECMA tested the effectiveness of chevrons on reducing jet noise. After examining configurations in the wind tunnel, CFMI chose to flight-test chevrons built into the core exhaust nozzle. The chevrons reduced jet noise by 1.3 perceived loudness decibels during takeoff conditions, and are now offered as an option with the CFM56 for the Airbus A321.
Fan and booster[edit]
The CFM56 features a single-stage fan, and most variants have a three-stage booster on the low-pressure shaft, with four stages in the -5B and -5C variants. The booster is also commonly called the "low-pressure compressor" (LPC) as it sits on the low-pressure shaft and compresses the flow initially before reaching the high-pressure compressor. The original CFM56-2 variant featured 44 tip-shrouded fan blades, although the number of fan blades was reduced in later variants as wide-chord blade technology developed, down to 22 blades in the latest variant, the CFM56-7.
The CFM56 fan features dovetailed fan blades which allows them to be replaced without removing the entire engine, and GE/SNECMA claim that the CFM56 was the first engine to have that capability. This attachment method is useful for circumstances where only a few fan blades need to be repaired or replaced, such as following bird strikes.
The fan diameter varies with the different models of the CFM56, and that change has a direct impact on the engine performance. For example, the low-pressure shaft rotates at the same speed for both the CFM56-2 and the CFM56-3 models; however, the fan diameter is smaller on the -3, which lowers the tip speed of the fan blades. The lower speed allows the fan blades to operate more efficiently (5.5% more in this case), which increases the overall fuel efficiency of the engine (improving specific fuel consumption nearly 3%).
Reverse thrust[edit]
The CFM56 is designed to support several reverse thrust systems which help slow and stop the aircraft after landing. The variants built for the Boeing 737, the CFM56-3 and the CFM56-7, use a cascade type of thrust reverser. This type of thrust reverse consists of sleeves that slide back to expose mesh-like cascades and blocker doors that block the bypass air flow. The blocked bypass air is forced through the cascades, reducing the thrust of the engine and slowing the aircraft down.
The CFM56 also supports pivoting-door type thrust reversers. This type is used on the CFM56-5 engines that power many Airbus aircraft. They work by actuating a door that pivots down into the bypass duct, both blocking the bypass air and deflecting the flow outward, creating the reverse thrust.
Turbine
All variants of the CFM56 feature a single-stage high-pressure turbine (HPT). In some variants, the HPT blades are "grown" from a single crystal superalloy, giving them high strength and creepresistance. The low-pressure turbine (LPT) features four stages in most variants of the engine, but the CFM56-5C has a five-stage LPT. This change was implemented to drive the larger fan on this variant. Improvements to the turbine section were examined during the Tech56 program, and one development was anaerodynamically optimized low-pressure turbine blade design, which would have used 20% fewer blades for the whole low-pressure turbine, saving weight. Some of those Tech56 improvements made their way into the Tech Insertion package, where the turbine section was updated. The turbine section was updated again in the "Evolution" upgrade.
The high-pressure turbine stages in the CFM56 are internally cooled by air from the high-pressure compressor. The air passes through internal channels in each blade and ejects at the leading and trailing edges.
Variants
CFM56-2 series
The CFM56-2 series is the original variant of the CFM56. It is most widely used in military applications where it is known as the F108; specifically in the KC-135, the E-6 Mercury and some E-3 Sentryaircraft. The CFM56-2 comprises a single-stage fan with 44 blades, with a three-stage LP compressor driven by a four-stage LP turbine, and a nine-stage HP compressor driven by a single-stage HP turbine. The combustor is annular.
| Model | Thrust | Bypass ratio | Pressure ratio | Dry weight | Applications |
|---|---|---|---|---|---|
| CFM56-2A-2 (-3) | 24,000 lbf (110 kN) | 5.9 | 31.8 | 4,820 lb (2,190 kg) | E-3 Sentry, E-6 Mercury |
| CFM56-2B1 | 22,000 lbf (98 kN) | 6.0 | 30.5 | 4,671 lb (2,120 kg) | KC-135R Stratotanker, Boeing RC-135 |
| CFM56-2C1 | 22,000 lbf (98 kN) | 6.0 | 31.3 | 4,653 lb (2,110 kg) | Douglas DC-8-70 |
CFM56-3 series
The first derivative of the CFM56 series, the CFM56-3 is designed for Boeing 737-300/-400/-500 series aircraft, with static thrust ratings from 18,500 to 23,500 lbf (82.3 to 105 kN). A "cropped fan" derivative of the -2, the -3 engine has a smaller fan diameter at 60 in (1.5 m) but retains the original basic engine layout. The new fan is primarily derived from GE's CF6-80turbofan rather than the CFM56-2, and the booster was redesigned to match the new fan.
A significant challenge for this series was achieving ground clearance for the wing-mounted engine. This was overcome by reducing the intake fan diameter and relocating the gearbox and other accessories from beneath the engine to the sides. The resulting flattened nacelle bottom and intake lip yielded the distinctive appearance of the Boeing 737 with CFM56 engines.
| Model | Thrust | Bypass ratio | Pressure ratio | Dry weight | Applications |
|---|---|---|---|---|---|
| CFM56-3B-1 | 20,000 lbf (89 kN) | 6.0 | 27.5 | 4,276 lb (1,940 kg) | Boeing 737-300, Boeing 737-500 |
| CFM56-3B-2 | 22,000 lbf (98 kN) | 5.9 | 28.8 | 4,301 lb (1,950 kg) | Boeing 737-300, Boeing 737-400 |
| CFM56-3C-1 | 23,500 lbf (100 kN) | 6.0 | 30.6 | 4,301 lb (1,950 kg) | Boeing 737-300, Boeing 737-400, Boeing 737-500 |
CFM56-4 series[edit]
The CFM56-4 series was a proposed improved version of the CFM56-2 designed for the Airbus A320family of aircraft. Competing with the RJ500 engine being developed by Rolls-Royce, the -4 series was designed to produce 25,000 lbf (110 kN) and was to feature a new 68 in (1.73 m) fan, a new low-pressure compressor and a full authority digital engine controller (FADEC). However, soon after the upgrade project was launched in 1984, International Aero Engines offered their new V2500 engine for the A320. CFMI realized that the CFM56-4 did not compare favorably with the new engine and scrapped the project to begin working on the CFM56-5 series.
CFM56-5 series[edit]
The CFM56-5 series is designed for the Airbus aircraft and has a very wide thrust rating of between 22,000 and 34,000 lbf (97.9 and 151 kN). It has three distinct sub-variants; the CFM56-5A, CFM56-5B and CFM56-5C, and differs from its Boeing-fitted cousins by featuring a FADEC and incorporating further aerodynamic design improvements.
CFM56-5A series[edit]
The CFM56-5A series is the initial CFM56-5 series, designed to power the short-to-medium range Airbus A320 family. Derived from the CFM56-2 and CFM56-3 families, the -5A series produces thrusts between 22,000 and 26,500 lbf (98 kN and 118 kN). Aerodynamic improvements such as an updated fan, low-pressure compressor, high-pressure compressor and combustor make this variant 10–11% more fuel efficient than its predecessors.
| Model | Thrust | Bypass ratio | Pressure ratio | Dry weight | Applications |
|---|---|---|---|---|---|
| CFM56-5A1 | 25,000 lbf (111 kN) | 6.0 | 31.3 | 4,995 lb (2,270 kg) | Airbus A320 |
| CFM56-5A3 | 26,500 lbf (118 kN) | 6.0 | 31.3 | 4,995 lb (2,270 kg) | Airbus A320 |
| CFM56-5A4 | 22,000 lbf (97.9 kN) | 6.2 | 31.3 | 4,995 lb (2,270 kg) | Airbus A319 |
| CFM56-5A5 | 23,500 lbf (105 kN) | 6.2 | 31.3 | 4,995 lb (2,270 kg) | Airbus A319 |
CFM56-5B series[edit]
An improvement of the CFM56-5A series, it was originally designed to power the A321. With a thrust range between 22,000 and 33,000 lbf (98 kN and 147 kN) it can power every model in the A320 family (A318/A319/A320/A321) and has superseded the CFM56-5A series. Among the changes from the CFM56-5A is the option of a double-annular combustor that reduces emissions (particularly NOx), a new fan in a longer fan case, and a new low-pressure compressor with a fourth stage (up from three in earlier variants). It is the most numerous engine supplied to Airbus.
| Model | Thrust | Bypass ratio | Pressure ratio | Dry weight | Applications |
|---|---|---|---|---|---|
| CFM56-5B1 | 30,000 lbf (130 kN) | 5.5 | 35.4 | 5,250 lb (2,380 kg) | Airbus A321 |
| CFM56-5B2 | 31,000 lbf (140 kN) | 5.5 | 35.4 | 5,250 lb (2,380 kg) | Airbus A321 |
| CFM56-5B3 | 33,000 lbf (150 kN) | 5.4 | 35.5 | 5,250 lb (2,380 kg) | Airbus A321 |
| CFM56-5B4 | 27,000 lbf (120 kN) | 5.7 | 32.6 | 5,250 lb (2,380 kg) | Airbus A320 |
| CFM56-5B5 | 22,000 lbf (98 kN) | 6.0 | 32.6 | 5,250 lb (2,380 kg) | Airbus A319 |
| CFM56-5B6 | 23,500 lbf (100 kN) | 5.9 | 32.6 | 5,250 lb (2,380 kg) | Airbus A319 |
| CFM56-5B7 | 27,000 lbf (120 kN) | 5.7 | 35.5 | 5,250 lb (2,380 kg) | Airbus A319, A319CJ |
| CFM56-5B8 | 21,600 lbf (96 kN) | 6.0 | 32.6 | 5,250 lb (2,380 kg) | Airbus A318 |
| CFM56-5B9 | 23,300 lbf (100 kN) | 5.9 | 32.6 | 5,250 lb (2,380 kg) | Airbus A318 |
CFM56-5C series[edit]
With a thrust rating of between 31,200 and 34,000 lbf (139 kN and 151 kN), the CFM56-5C series is the most powerful of the CFM56 family. It powers Airbus' long-range A340-200 and -300airliners, and entered service in 1993. The major changes are a larger fan, a fifth low-pressure turbine stage, and the same four-stage low-pressure compressor found in the -5B variant.
Unlike every other variant of the CFM56, the -5C features a mixed-exhaust nozzle, which offers slightly higher efficiency.
| Model | Thrust | Bypass ratio | Pressure ratio | Dry weight | Applications |
|---|---|---|---|---|---|
| CFM56-5C2 | 31,200 lbf (139 kN) | 6.6 | 37.4 | 8,796 lb (3,990 kg) | Airbus A340-200/-300 |
| CFM56-5C3 | 32,500 lbf (145 kN) | 6.5 | 37.4 | 8,796 lb (3,990 kg) | Airbus A340-200/-300 |
| CFM56-5C4 | 34,000 lbf (151 kN) | 6.4 | 38.3 | 8,796 lb (3,990 kg) | Airbus A340-200/-300 |
CFM56-7 series[edit]
The CFM56-7 powers the Boeing 737 Next Generation series (737-600/-700/-800/-900). The CFM56-7 first ran in 1994 is rated with takeoff thrust from 19,500 to 27,300 lbf (86.7 kN to 121 kN). It has higher thrust ranges, improved efficiency, and lower maintenance costs than its predecessor, the CFM56-3 series. It incorporates features from the CFM56-5 series such asFADEC, double-annular combustor (as an option), and improved internal design. The basic mechanical arrangement is as the -3 series, but all aspects were aerodynamically improved from that model. For example, the improved wide-chord fan blades allowed the total number of fan blades to be reduced from 38 to 24. Other improvements came from material advances, such as the use of single-crystal turbine blades in the high-pressure turbine.
The CFM56-7-powered 737 is granted 180-minute Extended-Range, Twin-Engine Operations (ETOPS) approval by the U.S. Federal Aviation Administration. It also powers the military versions of the Next-Generation 737, the C-40 Clipper, the P-8 Poseidon, and Boeing 737 AEW&C.
| Model | Thrust | Bypass ratio | Pressure ratio | Dry weight | Applications |
|---|---|---|---|---|---|
| CFM56-7B18 | 19,500 lbf (86.7 kN) | 5.5 | 32.8 | 5,216 lb (2,370 kg) | Boeing 737-600 |
| CFM56-7B20 | 20,600 lbf (91.6 kN) | 5.5 | 32.8 | 5,216 lb (2,370 kg) | Boeing 737-600, Boeing 737-700 |
| CFM56-7B22 | 22,700 lbf (101 kN) | 5.3 | 32.8 | 5,216 lb (2,370 kg) | Boeing 737-600, Boeing 737-700 |
| CFM56-7B24 | 24,200 lbf (108 kN) | 5.3 | 32.8 | 5,216 lb (2,370 kg) | Boeing 737-700, Boeing 737-800, Boeing 737-900 |
| CFM56-7B26 | 26,300 lbf (117 kN) | 5.1 | 32.8 | 5,216 lb (2,370 kg) | Boeing 737-700, Boeing 737-800, Boeing 737-900 |
| CFM56-7B27 | 27,300 lbf (121 kN) | 5.1 | 32.8 | 5,216 lb (2,370 kg) | Boeing 737-800, Boeing 737-900, Boeing Business Jet |
Engine failures and fuel flow problems
Although the CFM56 is a very reliable engine (CFMI state that there is only one in-flight shutdown every 333,333 hours), there have been several engine failures throughout the life of the CFM56 family which were serious enough to either ground the fleet or require aspects of the engine to be redesigned. The engines have also suffered, periodically, from thrust instability events tentatively traced to Honeywell's hydromechanical unit.
Rain/hail ingestion
There are several recorded incidents of CFM56 engines flaming out in heavy rain and/or hail conditions, beginning early in the CFM56's career. In 1987, a double flameout occurred in hail conditions (the pilots managed to relight the engines), followed by the TACA Flight 110 incident in 1988. Both CFM56 engines on the TACA 737 flamed out while passing through hail and heavy rain, and the crew was forced to land without engines on a grassy levee near New Orleans, Louisiana. CFMI modified the engines by adding a sensor to force the combustor to continuously ignite under those conditions.
In 2002, Garuda Indonesia Flight 421 had to ditch into a river because of hail-induced engine flameouts, killing a flight attendant and injuring dozens of passengers. Prior to this accident, there were several other incidents of single or dual flameouts due to these weather conditions. After three incidents through 1998, CFMI made modifications to the engine to improve the way in which the engine handled hail ingestion. The major changes included a modification to the fan/booster splitter (making it more difficult for hail to be ingested by the core of the engine) and the use of an elliptical, rather than conical, spinner at the intake. While these changes did not prevent the 2002 accident, the investigation board found that the pilots did not follow the proper procedures for attempting to restart the engine, which contributed to the final result. Recommendations were made to better educate pilots on how to handle these conditions, as well as to revisit FAA rain and hail testing procedures. No further engine modifications were recommended.
Fan blade failure
One issue that led to accidents with the CFM56-3C engine was the failure of fan blades. This mode of failure led to the Kegworth air disaster in 1989, which killed 47 people and injured 74 more. After the fan blade failed, the pilots mistakenly shut down the wrong engine, resulting in the damaged engine failing completely when powered up after descent. Following the Kegworth accident, CFM56 engines fitted to a Dan-Air 737-400 and a British Midland 737-400 suffered fan blade failures under similar conditions, although neither incident resulted in a crash or injuries. After the second incident, the 737-400 fleet was grounded.
At the time it was not mandatory to flight test new variants of existing engines, and certification testing failed to reveal vibration modes that the fan experienced during the regularly performed power climbs at high altitude. Analysis revealed that the fan was being subjected to high-cycle fatiguestresses worse than expected and also more severe than tested for certification; these higher stresses caused the blade to fracture. Less than a month after grounding, the fleet was allowed to resume operations once the fan blades and fan disc were replaced and the electronic engine controls were modified to reduce maximum engine thrust to 22,000 lbf (98 kN) from 23,500 lbf (105 kN). The redesigned fan blades were installed on all CFM56-3C1 and CFM56-3B2 engines, including over 1,800 engines that had already been delivered to customers.
Fuel flow problems
Airlines have reported 32 events involving sudden instability of thrust, at various points during flight, including high thrust settings during climb to altitude. The problem has been long-standing. In 1998, two 737 pilots reported that their engine throttles suddenly increased to full thrust during flight. A very recent investigation has led to the tentative conclusion that the problem originates in the Hydromechanical unit, and may involve an unacceptable level of fuel contamination (with water, or particulate matter, including biodegradables that create solid chunks in the fuel), or overuse of biocides to reduce bacterial growth. Boeing told Aviation Week and Space Technology that CFM International had revised its FADEC software. The new software "'reduces the duration and degree of thrust-instability events' by cycling the FMV {fuel monitoring valve} and the EHSV (electrohydraulic servo unit) to clean the EHSV spool." This software fix is not intended to be a definitive solution to the problem; however CFM claimed that no further reports have reached it after this change was made.
