CFM International CFM56

CFM International

Company type	Joint venture
Industry	Aerospace, aircraft engines
Founded	1974
Founder	General Electric, Safran Aircraft Engines (formerly Snecma)
Headquarters	Cincinnati, Ohio , United States
Area served	Worldwide
Key people	President (usually appointed from Safran)
Products	CFM56 turbofan engine family
Owner	Safran Aircraft Engines (50%), GE Aerospace (50%)

The CFM International CFM56—known in U.S. military service as the F108—represents a family of high-bypass turbofan engines jointly developed by France and the United States under CFM International (CFMI). These engines deliver thrust outputs ranging from . CFMI is a balanced partnership, equally owned by France's Safran Aircraft Engines (formerly Snecma) and the American company GE Aerospace. General Electric is responsible for manufacturing the high-pressure compressor, combustor, and turbine components, while Safran produces the fan, gearbox, exhaust system, and low-pressure turbine; additional parts are supplied by Italy’s Avio and U.S.-based Honeywell. Final assembly takes place at GE’s facility located in Evendale, Ohio and Safran’s plant near Villaroche, France. Although initial sales were sluggish, it eventually became the most widely adopted turbofan engine globally.

After its inaugural run in 1974, CFMI struggled to secure orders for five years until 1979 when it was on the verge of closure. The program was revitalized when airlines such as Delta Air Lines, United Airlines, and Flying Tigers chose the CFM56 to re-engine their fleets of Douglas DC-8 aircraft as part of the Super 70 initiative; these engines entered operational service in 1982. Following this success, Boeing selected the CFM56 for its re-engined Boeing 737 Classic models. The engine’s reduced noise emissions and enhanced fuel economy compared to earlier 737 powerplants led to sales that exceeded original projections.

In 1987, difficulties encountered with the rival IAE V2500 engine intended for the A320 caused numerous customers to switch allegiance to the CFM56. Early operational challenges included fan blade failures—one notably contributing to the Kegworth air disaster—and performance degradation during rain or hail conditions; all these issues were subsequently resolved through engineering improvements.

Origins

During the late 1960s, research efforts focused on developing next-generation commercial jet engines featuring high-bypass turbofan technology within the "10-ton" thrust category (approximately 20,000 lbf or 89 kN). Snecma (now Safran), primarily experienced in military propulsion systems, sought a commercial partner to collaborate on designing an engine in this class. Potential partners considered included Pratt & Whitney, Rolls-Royce, and GE Aviation. Initial discussions took place between GE’s Gerhard Neumann and Snecma’s René Ravaud at the 1971 Paris Air Show, culminating in an agreement to jointly pursue the project.

At that period, Pratt & Whitney dominated the commercial engine market. GE required an engine in this thrust segment, while Snecma had prior collaboration experience with GE on the CF6-50 turbofan used for the Airbus A300. Pratt & Whitney was independently upgrading their JT8D model, and Rolls-Royce faced financial constraints that limited their participation, positioning GE as Snecma’s optimal partner.

GE’s interest was driven by Snecma’s project being one of the few sources of development funding available for a 10-ton class engine at that time. Initially, GE intended to contribute technology derived from its CF6 engine rather than its more advanced F101 core developed for the B-1 Lancer bomber. However, when the U.S. Air Force announced its Advanced Medium STOL Transport (AMST) program in 1972—which included funding for a 10-ton engine—GE confronted a strategic decision: either develop a "limited" technology engine with Snecma or pursue an "advanced" design independently. To remain competitive and secure export rights for F101 core technology, GE applied for an export license aiming to balance collaboration with cutting-edge development efforts.

Challenges in Export Approval

In 1972, GE submitted an application for an export license as its primary contribution to the 10-ton engine initiative. However, the Office of Munitions Control within the U.S. Department of State advised against granting the license, citing national security concerns. These concerns stemmed from the engine's core technology being closely tied to the strategic B-1 bomber program, developed with Department of Defense funding, and fears that exporting to France would diminish American workforce participation. The refusal was formalized in a National Security Decision Memorandum signed by National Security Advisor Henry Kissinger on September 19, 1972.

Political considerations also played a role in the denial. French President Georges Pompidou made a direct appeal to U.S. President Richard Nixon in 1971, and Kissinger addressed the matter with Pompidou again in 1972. GE maintained that collaboration would secure access to half of the market, whereas exclusion risked losing it entirely if Snecma pursued development independently. Officials from the Nixon administration worried this could erode U.S. dominance in aerospace.

There was also speculation that the refusal served as a response to French attempts to discourage Switzerland from acquiring American LTV A-7 Corsair II aircraft, which competed with the French Dassault Milan. Ultimately, Switzerland selected the Northrop F-5E Tiger II instead.

The 1973 Nixon–Pompidou Summit

Although the export license was initially denied, both France and GE persisted in lobbying for approval. This matter became a significant topic during the 1973 Nixon–Pompidou summit held in Reykjavík. The discussions resulted in an agreement permitting the development of the CFM56 engine to move forward, contingent upon assurances that the engine core—originating from GE's military F101—would be produced within the United States and then shipped to France to safeguard sensitive technology. The joint venture consented to pay $80 million in royalties (amounting to $20,000 per projected engine) as reimbursement for government-funded F101 development expenses. Declassified records revealed that a crucial element of this deal included France’s commitment not to impose tariffs on American aircraft imports into Europe.

Formation of CFM International

Following resolution of export restrictions, GE and Snecma completed their agreement establishing CFM International (CFMI), a joint venture equally owned by both companies tasked with manufacturing and marketing the 10-ton CFM56 engine. Founded in 1974, CFMI managed coordination between GE and Snecma and acted as a unified customer interface. All major decisions required mutual approval from both partners, while CFMI oversaw daily operations.

CFMI’s board consists of ten members evenly divided between Snecma and GE (five each), supported by two vice presidents representing each company. The president—usually appointed from Snecma—is stationed at CFMI’s headquarters located near GE’s facilities in Cincinnati, Ohio.

The division of responsibilities designated GE to develop the high-pressure compressor, combustor, and high-pressure turbine sections; Snecma took charge of the fan, low-pressure compressor, and low-pressure turbine components. Snecma also led initial engineering for airframe integration, particularly nacelle design. Although initially responsible for the gearbox, this component’s production was later transferred to GE to enhance assembly efficiency.

Overview

The development of the CFM56 engine commenced prior to the formal establishment of CFMI. This international collaboration introduced distinctive logistical challenges: engine assembly and testing were conducted in both France and the United States. Due to export control regulations, GE manufactured the engine core in the U.S., which was then transported to Snecma’s facility in France. There, it was stored securely in a locked room that remained off-limits even to Snecma’s top executives. GE personnel installed Snecma-produced components before completing the final assembly process.

The inaugural CFM56 engine was successfully operated at GE’s Evendale facility on 20 June 1974, followed by a second unit tested in France on 13 December 1974 after its initial run in October of that year. These early production models were classified as the CFM56-2, marking the first variant of the series.

The engine’s maiden flight took place in February 1977 when it replaced one of four Pratt & Whitney JT8D engines on the McDonnell Douglas YC-15, which was part of the Air Force’s Advanced Medium STOL Transport competition. Shortly thereafter, a second CFM56 was mounted on a Sud Aviation Caravelle at Snecma’s flight testing center. This version incorporated a long bypass duct with mixed exhaust flow, contrasting with earlier models that featured short bypass ducts with unmixed exhaust flow. It also introduced the pioneering "Thrust Management System."

First customers

Following extensive testing over several years, CFMI focused on securing re-engining contracts for aircraft such as the Douglas DC-8 and Boeing 707 series, including military variants like the KC-135 Stratotanker. Initial market response was modest; however, Boeing recognized the CFM56 as a potential solution to meet forthcoming noise regulations. In 1977, Boeing announced plans to equip a 707 for flight testing with the CFM56 and subsequently offered a 707-320 model powered by this engine in 1978, labeled as the 707-700. Despite these efforts, low demand from airlines led Boeing to discontinue the 707-700 program by 1980 without any sales. Nonetheless, having a commercial 707 variant utilizing the CFM56 enhanced its appeal for military contracts such as the KC-135 re-engining program.

KC-135R

Winning the USAF contract to re-engine over 600 KC-135 tankers was crucial for CFMI’s success. After the Request for Proposal in 1977, support from the French government materialized through upgrading their fleet of 11 KC-135s in 1978, marking one of the earliest orders for the engine. By January 1980, the USAF chose the CFM56 over competing engines like Pratt & Whitney’s TF33 and JT8D variants. The upgraded KC-135R offered significant benefits including up to 3,500 feet (1,100 meters) shorter takeoff distances, fuel savings of approximately 25%, noise reduction by 24 decibels, and decreased life cycle expenses. These performance improvements influenced the United States Navy to select the CFM56-2 for their Boeing 707 variant known as the E-6 Mercury in 1982. Additionally, in 1984, the Royal Saudi Air Force opted for the CFM56-2 to power their E-3 Sentry aircraft; this engine also became standard equipment for British and French operators.

DC-8

During the late 1970s, airlines preferred to modernize their existing Douglas DC-8 fleets instead of purchasing new, quieter, and more fuel-efficient aircraft. After the French KC-135 order in 1978, a significant milestone occurred in April 1979 when United Airlines chose to retrofit 30 DC-8-61 aircraft with the CFM56-2 engine. This decision was crucial for the engine's advancement, as GE and Snecma were on the verge of discontinuing development without this commercial endorsement. This transaction represented the inaugural commercial acquisition of the CFM56, soon followed by orders from Delta Air Lines and Flying Tiger Line, thereby establishing the engine's footprint in both military and civilian sectors.

Boeing 737

In the early 1980s, Boeing selected the CFM56-3 as the exclusive powerplant for the 737-300. To accommodate the aircraft’s reduced ground clearance, engineers reduced the fan diameter and repositioned the accessory gearbox from the 6 o’clock to the 9 o’clock position, resulting in the characteristic flattened nacelle shape. These adaptations decreased thrust output due primarily to a lowered bypass ratio.

Beginning with an initial order of twenty 737-300s divided between two carriers, by April 2010, over 5,000 Boeing 737 models had been delivered equipped with CFM56 engines.

Tech56 and Tech Insertion

In 1998, CFMI initiated the "Tech56" program aimed at incorporating advanced technologies intended for next-generation single-aisle aircraft engines being considered by Airbus and Boeing. Instead of designing an entirely new engine, this program concentrated on integrating multiple innovative features. When Boeing and Airbus decided against developing completely new replacements for the 737 and A320 models, CFMI adapted Tech56 advancements into existing CFM56 engines through a "Tech Insertion" program launched in 2004. This effort focused on enhancing fuel efficiency, lowering maintenance expenses, and decreasing emissions by redesigning high-pressure compressor blades, upgrading combustor technology, and refining turbine components. The upgrades resulted in improved fuel consumption, reduced nitrogen oxides (NO_x) emissions, and approximately a 5% reduction in maintenance costs. Tech Insertion components entered operational service in 2007 and were incorporated into all new CFM56-5B and CFM56-7B engines thereafter. CFMI also provides retrofit kits for existing engines.

CFM56-7B "Evolution"

In 2009, CFMI unveiled the "CFM56-7B Evolution" (CFM56-7BE) upgrade coinciding with Boeing’s enhanced 737 Next Generation model. This iteration featured aerodynamic improvements to both high- and low-pressure turbines along with enhanced cooling efficiency while reducing component count. The objective was to achieve a 4% reduction in maintenance expenses alongside a fuel consumption improvement of approximately 1% (rising to around 2% when combined with airframe modifications). Flight testing completed by May 2010 demonstrated a better-than-projected fuel burn reduction of 1.6%. Following over 450 hours of testing, FAA and EASA certifications were granted on July 30, 2010, with deliveries commencing mid-2011.

The CFM56-5B/3 Performance Improvement Package (PIP) incorporates comparable technological enhancements aimed at reducing fuel consumption and maintenance costs; Airbus A320 aircraft began utilizing this upgraded engine variant from late 2011 onward.

LEAP

The LEAP engine, launched in 2016, was developed to succeed the CFM56 series, delivering a 16% boost in fuel efficiency by incorporating more composite materials and achieving bypass ratios greater than 10:1.

Operational history

As of June 2016, the CFM56 stood as the most extensively utilized high-bypass turbofan, having accumulated over 800 million flight hours and maintaining an average of one million flight hours every eight days. Forecasts anticipated reaching one billion hours by 2020. The engine supported more than 550 operators globally, with over 2,400 aircraft powered by CFM56 engines simultaneously airborne. Its reputation for dependability is highlighted by an average time on wing of 30,000 hours before the initial overhaul and a fleet record of 50,000 hours.

By July 2016, production totaled 30,000 CFM56 engines: roughly 9,860 units of the CFM56-5 variant for the Airbus A320ceo and A340-200/300 models, alongside more than 17,300 CFM56-3 and -7B engines for the Boeing 737 Classic and 737NG families. The outstanding order backlog was about 3,000 units. Lufthansa, the first customer for the CFM56-5C on the A340, operated an engine that surpassed 100,000 flight hours since its 1993 introduction, undergoing four major overhauls. In 2016 alone, CFM delivered 1,665 engines from the CFM56 line and secured orders for 876 additional units; spare parts manufacturing is planned through to 2045.

By October 2017, total deliveries exceeded 31,000 engines with approximately 24,000 active among 560 operators. The fleet had accumulated over 500 million flight cycles and nearly 900 million flight hours—300 million hours recorded since 1998 for the B737NG’s -7B model and 180 million hours since 1996 for the A320ceo’s -5B variant. By mid-2018, deliveries reached a total of 32,645 units. Robust demand sustained production schedules into 2020.

Maintenance expenditures include performance restoration shop visits costing between $300,000 and $600,000 for -5 series engines prior to removal from the wing; these procedures restore up to 80% of exhaust gas temperature margin. Replacement intervals for life-limited components range from 20,000 to 30,000 cycles with associated costs between $500,000 and $700,000 per part. Comprehensive engine overhaul expenses exceed $3 million plus labor charges.

By June 2019, the CFM56 fleet had surpassed one billion flight hours while transporting over 35 billion passengers worldwide.

Production has been gradually reduced following delivery of the final Boeing 737NG engine in 2019 and is expected to conclude with the last Airbus A320ceo engine scheduled for May 2020. Limited production will continue for military uses and spare parts supply until approximately 2024.

Unit price: roughly US$10 million (list price).

Summary

The CFM56 is a high-bypass turbofan engine family featuring bypass ratios ranging from approximately 5:1 to 6:1 and thrust outputs between 18,500 and 34,000 pounds-force (80 kN to 150 kN). All variants utilize a common two-shaft architecture with distinct high- and low-pressure turbines driving their respective shafts. The fan, booster stages, compressor sections, combustor design, and turbine components have undergone iterative development across different versions.

Combustor

Most variants of the CFM56 employ a single-annular combustor design—a continuous combustion chamber where fuel is injected and ignited to elevate airflow pressure and temperature. Fuel delivery is managed by a Honeywell hydromechanical unit (HMU) integrated with a full authority digital engine controller (FADEC).

In 1989, CFMI introduced a double-annular combustor featuring two separate combustion zones to support higher thrust levels while reducing nitrogen oxides (NO_x) and carbon dioxide (CO₂) emissions. This technology was first implemented in the CFM56-5B and -7B "/2" models starting in 1995.

Additionally, GE developed the Twin Annular Premixing Swirler (TAPS) combustor during its Tech 56 initiative. TAPS achieves an improved fuel–air mixture by swirling flow through dual combustion zones, resulting in a NO_x emissions reduction of approximately 46% compared to single-annular combustors and about 22% relative to double-annular designs. Analytical techniques derived from TAPS have also contributed enhancements to select combustors in certain CFM56-5B and -7B engines.

Compressor

At the heart of the initial export dispute lies the high-pressure compressor (HPC), present in all CFM56 models with a configuration of nine stages. This component traces its origins to GE's "GE^1/9 core"—a compact design featuring a single turbine and nine compressor stages. Its reduced compressor diameter allowed for a lighter and more compact engine by integrating accessory components such as bearings and lubrication systems within the primary aviation fuel circuit. Efforts to refine HPC performance concentrated on boosting airfoil efficiency. Under the Tech-56 initiative, CFMI engineered a six-stage HPC that achieved pressure ratios (~30) comparable to the original nine-stage model. This enhancement, part of the 2007 "Tech Insertion" program, improved blade dynamics without completely replacing the existing HPC design.

Exhaust

During early development, CFMI assessed both mixed and unmixed exhaust configurations; the majority of CFM56 engines employ an unmixed exhaust nozzle. The high-thrust CFM56-5C variant, designed for the Airbus A340, uniquely features a mixed-flow exhaust nozzle.

GE and Snecma also experimented with chevrons—serrated edges on exhaust nozzles intended to minimize jet noise. Following wind tunnel evaluations, CFMI conducted flight tests with chevrons installed on the core exhaust nozzle, achieving a noise reduction of approximately 1.3 perceived loudness decibels during takeoff. Chevron nozzles are currently available as an option on CFM56 engines installed on the Airbus A321.

Fan and booster

The CFM56 is equipped with a single-stage fan accompanied by a booster (low-pressure compressor) typically comprising three stages on the low-pressure shaft, except for the -5B and -5C variants which have four stages. The booster further compresses air after it passes through the fan before entering the high-pressure compressor. The initial CFM56-2 featured 44 fan blades with tip shrouds; however, subsequent versions reduced blade count in line with advances in wide-chord blade technology, reaching just 22 blades in the CFM56-7.

The fan blades are dovetailed, allowing them to be replaced without removing the entire engine—a capability first claimed by GE/Snecma that facilitates maintenance following events such as bird strikes.

Fan diameter varies among different models, directly influencing engine performance. For example, compared to the -2 model, the smaller fan diameter of the CFM56-3 reduces blade tip speed, resulting in a 5.5% increase in fan efficiency and nearly 3% improvement in specific fuel consumption, thereby enhancing overall fuel economy.

Thrust reverser

To assist aircraft deceleration upon landing, the CFM56 incorporates various thrust reverser systems. Boeing 737 variants powered by CFM56-3 and CFM56-7 engines utilize cascade-type thrust reversers. These systems retract sleeves to reveal mesh-like cascades and blocker doors that redirect bypass airflow through these cascades, diminishing engine thrust and slowing the aircraft.

Conversely, Airbus A320 models equipped with the CFM56-5 employ pivoting-door thrust reversers. This mechanism swings a door into the bypass duct to obstruct airflow and deflect it outward, generating reverse thrust.

Turbine

Every CFM56 variant includes a single-stage high-pressure turbine (HPT). Certain versions incorporate blades fabricated from single-crystal superalloys to enhance strength and resistance to creep. The low-pressure turbine (LPT) generally consists of four stages; however, the CFM56-5C features five stages to accommodate its larger fan.

The Tech56 program introduced aerodynamic refinements to LPT blades that reduced their number by 20%, resulting in weight savings; these enhancements were integrated into later Tech Insertion and Evolution upgrade packages.

The high-pressure turbine receives internal cooling air sourced from the high-pressure compressor. This cooling air passes through internal channels within the blades and exits at both leading and trailing edges to sustain performance and durability.

CFM56-2 series

The initial version, the CFM56-2, was mainly developed for military use under the designation F108, powering aircraft such as the KC-135, E-6 Mercury, and selected E-3 Sentry models. This variant incorporates a single-stage fan with 44 blades, a three-stage low-pressure compressor driven by a four-stage low-pressure turbine, and a nine-stage high-pressure compressor powered by a single-stage high-pressure turbine. Its combustion chamber is of the annular type.

Model	Thrust	BPR	OPR	Dry weight	Applications
CFM56-2A2 (A3)	24,000 lbf (110 kN)	5.9	31.8	4,820 lb (2,190 kg)	Boeing 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	4,635 lb (2,100 kg)	Douglas DC-8-70

CFM56-3 series

The CFM56-3 was engineered specifically for the Boeing 737 Classic lineup (737-300/-400/-500), providing static thrust ranging from . It utilizes a reduced diameter 60 in (1.5 m) "cropped fan," which is primarily derived from GE's CF6-80 turbofan, coupled with a revised booster stage. To maintain sufficient ground clearance on wing-mounted installations, the fan diameter was decreased and engine accessories were repositioned from underneath to the sides, resulting in the characteristic flattened nacelle underside and intake lip seen on 737s equipped with CFM56 engines.

Model	Thrust	BPR	OPR	Dry weight	Applications
CFM56-3B1	20,000 lbf (89 kN)	6.0	27.5	4,276 lb (1,940 kg)	Boeing 737-300, Boeing 737-500
CFM56-3B2	22,000 lbf (98 kN)	5.9	28.8	4,301 lb (1,950 kg)	Boeing 737-300, Boeing 737-400
CFM56-3C1	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

The CFM56-4 was an intended enhancement of the CFM56-2 tailored for the Airbus A320 series, aiming to deliver approximately 25,000 lbf (110 kN) of thrust through a new 68 in (1.73 m) fan, an upgraded low-pressure compressor, and FADEC integration. Nevertheless, following International Aero Engines’ introduction of the V2500 engine for the A320 in 1984, CFMI ceased development on the CFM56-4 to concentrate efforts on advancing the more competitive CFM56-5 family.

CFM56-5 series

Specifically developed for Airbus aircraft, the CFM56-5 family spans thrust ratings between 22,000 and 34,000 lbf (98 to 151 kN) and includes three variants: 5A, 5B, and 5C. This series incorporates FADEC along with aerodynamic improvements that differentiate it from engines used on the Boeing 737 Classic models.

CFM56-5A series

The original 5A version is designed to power Airbus A320 family aircraft intended for short to medium-range flights. Evolving from preceding CFM56 models, it produces thrust between 22,000 and 26,500 lbf (98 to 118 kN) and incorporates aerodynamic enhancements that boost fuel economy by approximately 10–11%.

Model	Thrust	BPR	OPR	Dry weight	Applications
CFM56-5A1	25,000 pounds-force (110 kN)	6.0	31.3	4,995 pounds (2,266 kg)	Airbus A320
CFM56-5A3	26,500 pounds-force (118 kN)	6.0	31.3	4,995 pounds (2,266 kg)	Airbus A320
CFM56-5A4	22,000 pounds-force (98 kN)	6.2	31.3	4,995 pounds (2,266 kg)	Airbus A319
CFM56-5A5	23,500 pounds-force (105 kN)	6.2	31.3	4,995 pounds (2,266 kg)	Airbus A319

CFM56-5B series

Building upon the 5A design, the 5B variant was originally created for the A321 and now serves all models within the A320 family (A318/A319/A320/A321), effectively superseding the 5A. Its thrust output ranges from 22,000 to 33,000 lbf (98 to 147 kN). Notable enhancements include an optional double-annular combustor that lowers emissions, especially nitrogen oxides (NO_x), a redesigned fan housed in an extended casing, and an upgraded four-stage low-pressure compressor replacing the previous three-stage design. This variant stands as the most extensively supplied engine for Airbus aircraft.

Model	Thrust	BPR	OPR	Dry weight	Applications
CFM56-5B1	30,000 pounds-force (130 kN)	5.5	35.4	5,250 pounds (2,380 kg)	Airbus A321
CFM56-5B2	31,000 pounds-force (140 kN)	5.5	35.4	5,250 pounds (2,380 kg)	Airbus A321
CFM56-5B3	33,000 pounds-force (150 kN)	5.4	35.5	5,250 pounds (2,380 kg)	Airbus A321
CFM56-5B4	27,000 pounds-force (120 kN)	5.7	32.6	5,250 pounds (2,380 kg)	Airbus A320
CFM56-5B5	22,000 pounds-force (98 kN)	6.0	32.6	5,250 pounds (2,380 kg)	Airbus A319
CFM56-5B6	23,500 pounds-force (105 kN)	5.9	32.6	5,250 pounds (2,380 kg)	Airbus A319, A320
CFM56-5B7	27,000 pounds-force (120 kN)	5.7	35.5	5,250 pounds (2,380 kg)	Airbus A319, A319CJ
CFM56-5B8	21,600 pounds-force (96 kN)	6.0	32.6	5,250 pounds (2,380 kg)	Airbus A318, A318CJ

CFM56-5C series

The CFM56-5C family represents the highest thrust range within the CFM56 lineup, delivering between 31,200 and 34,000 lbf (139–151 kN). It powers the long-haul Airbus A340-200 and -300 series aircraft, entering operational service in 1993. Among its significant advancements are a larger diameter fan, a fifth stage added to the low-pressure turbine, and the four-stage low-pressure compressor technology adapted from the -5B version.

A distinctive feature of the -5C is its mixed-exhaust nozzle design, which improves operational efficiency relative to other variants.

Model	Thrust	BPR	OPR	Dry weight	Applications
CFM56-5C2	31,200 lbf (139 kN)	6.6	37.4	8,796 lb (3,990 kg)	Airbus A340-211/-311

CFM56-7 series

The CFM56-7, which first ran on 21 April 1995, provides takeoff thrust ranging from and is the powerplant for the Boeing 737 Next Generation series (-600/-700/-800/-900). It delivers an 8% improvement in fuel consumption and reduces maintenance expenses by 15% compared to the CFM56-3.

Enhancements include a 61-inch titanium wide chord fan, cutting-edge 3D aerodynamic designs, a redesigned core, and a low-pressure turbine featuring single crystal high-pressure turbine blades, along with Full Authority Digital Engine Control (FADEC) technology. The number of fan blades was decreased from 36 to 24, incorporating elements such as an optional double-annular combustor derived from the -5B model.

Within just two years of operation, the Next-Generation 737 achieved FAA certification for 180-minute ETOPS. This engine also powers military platforms including the Boeing 737 AEW&C, C-40 Clipper, and P-8 Poseidon maritime aircraft.

In response to the Southwest Airlines Flight 1380 event, the FAA required Boeing to redesign the nacelle and inlet structure of the -7B engine variant to meet Part 25 regulatory standards. The agency has opened a comment period until January 26, 2024, with mandatory compliance scheduled by July 31, 2028.

CFM56-7B specifications

Model	Thrust	BPR	OPR	Dry weight	Applications
CFM56-7B18	19,500 pounds-force (87 kN)	5.5	32.7	5,216 pounds (2,366 kg)	Boeing 737-600
CFM56-7B20	20,600 pounds-force (92 kN)	5.4	32.7	5,216 pounds (2,366 kg)	Boeing 737-600, Boeing 737-700
CFM56-7B22	22,700 pounds-force (101 kN)	5.3	32.7	5,216 pounds (2,366 kg)	Boeing 737-600, Boeing 737-700
CFM56-7B24	24,200 pounds-force (108 kN)	5.3	32.7	5,216 pounds (2,366 kg)	Boeing 737-700, Boeing 737-800, Boeing 737-900
CFM56-7B26	26,300 pounds-force (117 kN)	5.1	32.7	5,216 pounds (2,366 kg)	Boeing 737-700, Boeing 737-800, Boeing 737-900, Boeing Business Jet

Reliability

The CFM56 engine family is recognized for an in-flight shutdown frequency of one event per 333,333 flight hours. The duration between overhauls extended from an initial benchmark of 30,000 hours in 1996 to over 40,729 hours by 2003 and eventually reached approximately 50,000 hours by 2016.

During its early operational phase, the CFM56 encountered several critical engine failures that resulted in temporary fleet groundings or necessitated design revisions. Occasional thrust instability was also traced back to problems within Honeywell’s hydromechanical control unit.

Rain and hail ingestion

Since its introduction, the CFM56 has experienced multiple flameouts caused by severe rain and hail ingestion. A notable incident in 1987 involved a double flameout due to hail ingestion; in 1988, TACA Flight 110 lost power in both engines after flying through hail and heavy rain but managed to land safely on a nearby grassy levee near New Orleans. In response, CFMI implemented a sensor designed to sustain continuous combustor ignition under such adverse weather conditions.

In another significant event in 2002, Garuda Indonesia Flight 421 ditched into a river following hail-induced flameouts that resulted in the death of a flight attendant and injuries among passengers. Previous incidents had led CFMI to redesign the fan/booster splitter and replace the intake spinner with an elliptical shape aimed at minimizing hail ingestion. Despite these modifications, they did not prevent the accident in 2002 where pilot errors during engine restart were a major factor. The subsequent investigation recommended enhanced pilot training and reexamination of FAA protocols for rain and hail testing but did not call for additional engine design changes.

Fan blade fracture incidents

Fan blade fractures represented a significant challenge for the CFM56-3C engine, most notably contributing to the 1989 Kegworth air disaster, which claimed 47 lives and caused injuries to 74 individuals. This tragedy stemmed from a fan blade breaking, followed by the flight crew mistakenly shutting down the healthy engine, resulting in complete engine failure during the final approach phase. Comparable fan blade failures later occurred on Dan-Air and British Midland 737-400 aircraft equipped with CFM56 engines; although these events did not lead to crashes or injuries, they prompted a temporary suspension of the entire 737-400 fleet.

During certification testing at that time, vibration modes encountered during high-altitude power climbs were not detected. Investigations revealed that fan blades were subjected to unexpectedly severe high-cycle fatigue stresses, which caused the fractures. Within one month of grounding, service was restored after replacing the affected fan blades and discs, alongside modifying the electronic engine control systems to reduce peak thrust from 23,500 lbf (105 kN) to 22,000 lbf (98 kN). Over 1,800 delivered CFM56-3C1 and CFM56-3B2 engines were retrofitted with redesigned fan blades.

More recently, in August 2016, Southwest Airlines Flight 3472 encountered a fan blade failure but managed to land safely without any injuries despite extensive aircraft damage. On 17 April 2018, Southwest Airlines Flight 1380 experienced a similar failure; debris from the engine damaged a cabin window, causing one fatality and multiple injuries. The Boeing 737-700 involved successfully completed an emergency landing following the incident.

Fuel flow irregularities

There have been 32 reported instances of sudden thrust fluctuations during flight operations, including cases occurring at high thrust levels during climbs. This persistent issue has been documented since 1998 when two pilots operating 737s observed their engine throttles unexpectedly surge to maximum power. Recent analyses indicate that the root cause is associated with the Hydromechanical Unit (HMU), potentially due to fuel contamination by water or particulate matter or excessive use of biocides leading to solid deposits forming within the fuel system. Boeing has indicated that CFM International implemented updates to its FADEC software designed to mitigate these thrust instability occurrences by periodically cycling the fuel monitoring valve (FMV) and electrohydraulic servo valve (EHSV) to clear deposits from the EHSV spool. Nonetheless, this software revision is not considered a permanent solution; since its introduction, CFM reports no further related incidents.

Unauthorized parts supply

In 2023, Bloomberg News disclosed that European aviation authorities uncovered that London-based AOG Technics—majority owned by Jose Zamora Yrala—had supplied components of uncertain origin accompanied by falsified paperwork for repairs on CFM56 engines. This revelation raises serious concerns regarding supply chain security and quality control in sourcing replacement parts.

Applications

• Airbus A320 family
  • Airbus A318
• Airbus A340
• Boeing 707-700 (prototype only)
• Boeing 737 Classic
• Boeing 737 Next Generation
  • Boeing E-7 Wedgetail
  • Boeing C-40 Clipper
  • Boeing P-8 Poseidon
• Boeing Business Jet
• Boeing E-3D Sentry
• Boeing E-6 Mercury
• Boeing KC-135R Stratotanker
  • Boeing RC-135
  • Boeing WC-135
• Douglas DC-8 Super 70

Specifications

Variant	-2	-3	-5	-5B	-5C	-7B
Type	High bypass ratio turbofan with dual rotors and axial flow design
Compressor	Single fan, 3 low-pressure stages, 9 high-pressure stages			Single fan, 4 low-pressure stages, 9 high-pressure stages		Single fan, 3 low-pressure stages, 9 high-pressure stages
Combustor	Annular type (double annular for -5B/2 and -7B/2 "DAC" variants)
Turbine	One high-pressure stage, four low-pressure stages				One high-pressure stage, five low-pressure stages	One high-pressure stage, four low-pressure stages
Control	Hydromechanical with limited electronic assistance		Dual Full Authority Digital Engine Control (FADEC)
Length	243 cm (96 in)	236.4 cm (93.1 in)	242.2 cm (95.4 in)	259.97 cm (102.35 in)	262.2 cm (103.2 in)	250.8 cm (98.7 in)
Width	183–200 cm (72–79 in)	201.8 cm (79.4 in)	190.8 cm (75.1 in)	190.8 cm (75.1 in)	194.6 cm (76.6 in)	211.8 cm (83.4 in)
Height	214–216 cm (84–85 in)	181.7 cm (71.5 in)	210.1 cm (82.7 in)	210.5 cm (82.9 in)	225 cm (89 in)	182.9 cm (72.0 in)
Dry weight	2,139–2,200 kg (4,716–4,850 lb)	1,954–1,966 kg (4,308–4,334 lb)	2,331 kg (5,139 lb)	2,454.8–2,500.6 kg (5,412–5,513 lb)	2,644.4 kg (5,830 lb)	2,386–2,431 kg (5,260–5,359 lb)
Takeoff thrust
Thrust-to-weight ratio	Approximately 4.49 to 4.9	Approximately 4.49 to 5.22	Approximately 4.2 to 5.06	Approximately 5.44 to 5.69	Approximately 5.25 to 5.72	Approximately 3.84 to 5
100% RPM	LP ~5176, HP ~14460	LP ~5179, HP ~14460	LP ~5000, HP ~14460	LP ~5000, HP ~14460	LP ~4784, HP ~14460	LP ~5175, HP ~14460
Airflow per second at sea level
Bypass ratio	Ranges from 5.9 to 6.0		Between 6.0 and 6.2	From 5.4 up to 6.0	Between 6.4 and 6.5	From 5.1 to 5.5
Maximum overall pressure ratio (OPR)	Between 30.5 and 31.8	Range of 27.5 to 30.6	Fixed at 31.3	Between 32.6 and 35.5	Range of 37.4 to 38.3	Fixed at 32.8
Fan diameter
Applications	Boeing KC-135 Boeing 707 Douglas DC-8 Super 70 Series models	Boeing 737 Classic	Airbus A319 Airbus A320	Airbus A320 family	Airbus A340-200 and -300 variants	Boeing 737 Next Generation
Takeoff TSFC (Thrust Specific Fuel Consumption)	Between 0.366 and 0..376 tsfc	Ranging from 0..386 to 0..396 tsfc	Fixed at approximately 0..3316 tsfc	Between approximately 0..3266 and 0..3536 tsfc	Ranging from about 0..326 to 0..336 tsfc	Between roughly 0..356 and 0..386 tsfc
Cruise TSFC	Approximately 0..65 tsfc for (-2B1)	Around 0..667 tsfc for (-3C1)	Near 0..596 tsfc for (-5A1)	About 0..545 tsfc for (-5B4)	Approximately 0..545 tsfc for (-5C2)

See also

• CFM International LEAP
• General Electric F101
• General Electric Affinity
• PowerJet SaM146
• IAE V2500
• Pratt & Whitney PW6000
• List of aircraft engines

External links

• Official page for the CFM56
• CFM56 information on GE Aerospace website
• Flight International, 6 June 1981, "CFM56 Revitalizes the DC-8"
• Flight International, 19 May 1999, "CFM56: Strength and Prestige"
• Flight Global, 2006, Cutaway illustration of CFM56-5C2
• DeviantArt, 2 April 2024, Half-cutaway depiction of CFM56-7B