Evolving Reliable Engines

In a twin-engine aircraft, if one engine fails, the plane can continue to fly with the other one operating. In three-engine planes, even if two engines fail, the aircraft can still fly to a safe landing on one engine. If all engines stop functioning at a cruising height of 35,000 feet, the aircraft can glide for 30 minutes and can be landed safely if there is a runway available within glide range. Under conditions of total loss of power, the aircraft can glide up to 200 kilometres. As for bird hits, nothing can stop a jet engine. It has been proved in testing phases of jet engines where whole chickens have been thrown through the jet engines to ensure they maintain full functionality.

These are just a few very obvious features of engine capabilities. The manufacture of aircraft engines, whether piston or jet, involves the conversion of raw materials into extremely reliable precision machines designed so as to never fail.

Over the past 84 years, from Whittle’s first turbojet engine to the current turbofans, reliability of engines has been the most critical element in their development. Modern turbofan engines are now capable of producing around 440 kN (1,00,000 lb) of thrust, compared with 7 kN (1,600 lb) for the first turbojet engines. Over this period, the specific fuel consumption has halved and the thrust-to-weight ratio has increased significantly. The performance and reliability of aero-engines have revolutionised both civil and military aeronautics all over the world.

January 16, 2000, marks the 70th anniversary of Whittle’s patent for “Improvements in Aircraft Propulsion”. This invention led to the start of the jet age in which the gas turbine replaced the piston engine in all but the smallest aircraft. The high thrustto-weight ratio, efficiency and reliability of modern engines has resulted in a spectacular increase in the number of people who now take flying for granted and improvements in performance have led to a decrease in the real cost of air transport.

The introduction of “bypass engines” in 1962 marked a significant development for aero-engines. The Rolls-Royce Conway, a lowbypass-ratio engine was used to power both military and civil aircraft. In the 1960s, the Pratt & Whitney JT-9 twin-spool High Bypass Ratio (HBR) engine dominated the civil and military transport market. In an attempt to catch up with its US competitors, Rolls-Royce began to develop the RB-211, a triple-spool HBR engine.

Composites in Play

Aircraft engines are primarily constructed of metallic components though recent years have seen the introduction of plastic composites for certain parts. Various aluminium and titanium alloys are used where strength and light weight are of primary importance especially in structural components, compressor sections and engine frames. Chromium, nickel and cobalt alloys are used where resistance to high temperature and corrosion are required such as in combustor and turbine sections. Numerous steel alloys are used in intermediate locations.

Since weight minimisation on aircraft is a critical factor in reducing life-cycle costs (maximising payload, minimising fuel consumption), advanced composite materials have recently been introduced as light-weight replacements for aluminium, titanium and some steel alloys in structural parts and ductwork where high temperatures are not experienced. These composites consist primarily of polyimide, epoxy and other resin systems, reinforced with woven fibreglass or graphite fibres.

In order to assure the reliability of aircraft engines, a number of inspection, testing and quality-control procedures are performed during the fabrication and on the final product. Common non-destructive inspection methods include radiographic, ultrasonic, magnetic particle and fluorescent penetrant. They are used to detect any cracks or internal flaws within the parts. Assembled engines are usually tested in instrumented test cells prior to delivery to a customer.

Continuous Evolution

In consideration of increasing demand in the avionics sector, particularly in modern military and civil aircraft, safety and reliability are the prime concerns to complete the mission successfully. Technocrats are made to rethink the safety of complete systems by adding redundancy to the critical activities. A Rotor Support System (RSS) is an integral part of a gas turbine engine used in any aircraft. As its name implies, the RSS shares the load of the rotating component of an engine, hence it plays a vital role in any aircraft engine. It shares the load of compressor rotor and stator, turbine rotor and stator, inter-casing and exhaust system of a gas turbine engine. A failure in such a system may affect the entire aircraft.

Therefore, all engine manufacturers carry out Failure Modes and Effects Analysis (FMEA) on such a critical system. FMEA is one of the most effective reliability assessment tools which evaluates systematically and documents the potential failure modes of a system or equipment and their causes. It helps in grading the severity of all potential failure modes and is useful in carrying out the changes in the early phase of design. The analysis starts with the potential failure of the smallest component at the final indenture and goes up to the initial indenture level. All engine manufacturers give utmost importance to reliability and here are some developments from a few of the original equipment manufacturers (OEMs).

CFM’s New Generation

A new generation of engines being developed by the world’s largest jet engine maker CFM, a partnership between GE and Snecma of France, will allow aircraft to use about 15 per cent less fuel, enough to save about $1 million per year per airplane and significantly reduce carbon emissions. The first of these new engines called LEAP will feature a technology that has never before been used in large-scale production of jet engines. This technology employs ceramic composite materials that weigh far less than the metal alloys they will replace and can endure far higher temperatures. The engine will also make use of parts produced through 3D printing, a new kind of manufacturing process that can produce complex shapes that would be difficult or impossible to make with conventional manufacturing techniques. “These technologies could eventually be used to make more parts of the engine, leading to further advances in efficiency”, says Gareth Richards, LEAP Program Manager, GE Aviation.

Having been selected to power nearly 60 per cent of the A318/A319/A320/A321 aircraft ordered, the CFM-56-5B is the engine of choice for the A320 family. One of the primary factors behind the CFM-56-5B’s broad-based market acceptance has been its simple, rugged architecture which gives it the highest reliability, durability and reparability in its class. The CFM-56-5B, a high-performance, low-risk derivative engine of the CFM-56 family was originally developed to power the Airbus A321. Today, it is the only engine that can power every model of the A320 family with one bill of materials. More than 5,000 CFM-56-5B engines have been delivered and this fleet has accumulated more than 80 million flight hours.

GE Development

GE Aviation is developing a revolutionary new jet engine that aims to combine the best traits of turbojet and turbofan engines, delivering supersonic speed capability and fuel efficiency in one package. The new engines are being developed under the ‘USAF Advent’ project, which is seeking 25 per cent fuel saving which will in turn lead to an increase in mission capability. GE’s Advent designs are based on new manufacturing technologies such as 3D printing of intricate cooling components and super-strong but lightweight ceramic matrix composites. These allow the manufacture of highly efficient jet engines operating at temperatures above the melting point of steel.

Engineers also designed the new engine to be easy to fly. “We want the engine to take care of itself and let the pilot focus on the mission,” says Abe Levatter, Project Manager, GE Aviation. “When the pilot says, ‘I’m out of danger, I want to cruise home’, the engine reconfigures itself. We take it upon ourselves to optimise the engine for whatever the pilot wants.”

PW Revolutionising Design

Since 1925, Pratt & Whitney has been a global leader in the design, manufacture and service of aircraft engines, auxiliary and ground power units, small turbojet propulsion products and industrial gas turbines. From its first 410 horsepower, air-cooled Wasp engine to its award-winning PurePower engine with patented Geared Turbofan technology, the company continues to revolutionise engine design to anticipate changing customer needs. Pratt & Whitney’s large commercial engines power more than 25 per cent of the world’s mainline passenger fleet. The company also provides high-performance military engines to 29 armed forces around the world.

For over three decades, Pratt & Whitney has leveraged the power of engineering simulation to launch its groundbreaking innovations with the incredibly high degree of confidence required in the aerospace and defence industry. Pratt & Whitney’s PurePower engine design represents one of the biggest advances in jet engines in the past 50 years. Pratt & Whitney engineers recognised that engine performance could be significantly improved if the fan and turbine that drive it could be operated at their own optimal speeds.

Thus, development of lightweight metal alloys, advanced aerodynamic designs for engines and fans as well as advanced gearing systems have all enabled the fuel economy advantages of higher bypass ratio engines. Other engine efficiency improvements include increased engine inlet temperature, high temperature materials, increased compressor pressure ratio and improved fan and nacelle performance. In addition, reduction of noise, emissions and improved reliability have led to the significant improvement in performance of modern jet engines.

The evolution of fan blades from the early (solid) RB211 design to the (hollow) Trent 800, the success of the turbofan engine has been helped by major improvements in the manufacture of the fan blades. After the disappointing performance of carbon-fibre fan blades in the 1960s, the development of the wide-chord hollow blades by Rolls-Royce in the 1980s was highly successful. Rolls-Royce has never had a service failure of conventional fan blades in over 40 million hours of operation, and there have been no service failures of wide-chord blades in over ten million hours of operation.

The economic and environmental drivers for modern engine designs are high power-to-weight ratio, high reliability, low cost, low specific fuel consumption and low noise/emission levels.