After a century of evolution, air travel has successfully established itself as one of the safest modes of transportation. This success was not attained without the reliability of commercial jet engines. Jet engines are a complex system of interacting subsystems with high demands for reliability. The strict safety and reliability requirements involved in the manufacturing and the development of aircraft engines takes a lot of years. But, with all the complexities associated with the making of jet engines, how has it also managed to be reliable?
The main reason jet engines are so reliable is that they have far fewer moving parts that rub against other things than any other type of engine. In addition, they are reliable because a lot of high-level expertise and resources are invested in their production to ensure the overall safety of the aircraft and passengers.
The important goal of jet engines is to keep very expensive planes safe in the air. The reliability of jet engines helps to prevent the loss of millions of dollars, legal trouble, and most importantly human lives.
Therefore, airplane engines tend to be designed at an 8 Sigma quality standard and not at Six Sigma quality standards.
The 8 Sigma standard in essence allows 1 mistake every 100,000,000,000,000 attempts.
To achieve this level of engine reliability, very intelligent and advanced-level engineering, management and equipment are required.
This further explains the reason there are few competitive players in the engine industry owing to the sophisticated level of infrastructure and culture required to reach those standards.
Table of Contents
What Is the Reliability of an Engine?
The reliability of an engine is the ability of the engine to function optimally without breaking down for a required duration of time, known as the function period or mission time, under specified performance conditions and limits when used for the intended purpose.
A reliable engine is a well-designed engine. It must be designed in such a way that there is a proper flow of fuel and lubricants. Also, its geometry should allow the efficient movement of the parts to minimize wear and tear owing to the effect of friction.
The measure of an engine’s reliability depends on the probability of the number of components working without failure for the required duration and the intended purpose.
The random failure, wear-out failure, fabrication failure, and design failure of any component amounts to the probability of failure of an engine.
Hence, it is important to note that reliability is a probability, not a rate, or lifespan. As such, can be used to predict mission success, availability, life cycle cost, lifespan, and to improve manufacturing efficiency and maintenance operations.
In simple language, an engine that hardly breaks down or requires repair only after a substantial period of time is said to be reliable.
Types of Reliability
High-cycle: This covers risks whose occurrence is often more than one operating cycle. This includes the use of engine components, shocks, vibration, and so on.
Low-cycle: This varies with the environment at a particular airport but aligns with the operating cycles, perhaps even longer.
Why Jet Engines Are Very Reliable
Safety
The most important consideration for aircraft and jet engines is safety. The reliability of jet engines has made air travel about the safest mode of transportation.
An airline doesn’t just buy any kind of engine, they buy engines with warranty. That is, they buy reliable engines. Nonetheless, major airlines have their engines pre-tested and maintained appropriately by an Engine Representative at their facility all the time.
The jet engine which is a more advanced and high-performance engine than the piston engine has more parts. These many parts, however, need more frequent inspection and maintenance for the jet engine to remain reliable in operation. While flying, the speed, temperature, and oil pressure of an operating engine is often monitored by the airline crew.
There is also a maintenance control center that does a more technical check from its base. For example, General Electric has a “Power by the Hour” center where it monitors the operation of aircraft engines for its respective clients. Whenever a problem is detected about an engine, the airline is alerted to conduct further inspection on the aircraft at a convenient overnight layover.
ETOPS: How Jet Engine Reliability Affects Long Range Flights
To emphasize how crucial the idea of safety is, there is a program called ETOPS (Extended-range Twin-engine Operational Performance Standards).
ETOPS is a certification that permits twin-engine aircraft to fly routes which are located X-minutes flying time from the nearest airport that is suitable for an emergency landing.
Different airplane and engine types have varying ETOPS ratings, depending on their perceived reliability. A few decades ago, the ETOPS rating never exceeded 60 minutes, meaning that all twin-engine planes at all times had to be within 60 minutes of flying time from an airport.
With the advancements in jet engine design leading to much fewer engine failures, the 60 minutes minimum flying time was soon extended to 90 and 120 minutes. At the moment, some airplanes have an ETOPS rating as high as 400 minutes!
Today twin-engine aircraft frequently cross large oceans, which requires a higher ETOPS rating. However, a few decades ago, these longer flights were only operated by four-engine planes, such as the Boeing 747.
One of the stipulations from the ETOPS program in a bid to ensure safety states that different ETOPS qualified mechanics would be required to change the oil on the two ETOPS certified engines. This is done to prevent the repetition of any error made by any of the mechanics on both engines.
Cost
A jet engine costs approximately between 12 to 35 million US Dollars. No airline will invest such an amount of money into an engine that has no guaranteed reliability. The huge cost of a jet engine arises from:
- The very rare and expensive materials used in manufacturing the thousands of parts of the engine.
- The high degree of optimization needed to make the engine as light as possible to maximize the aircraft’s range and make it very fuel-efficient.
- The enormous funding that goes into research and development of new and more reliable engines.
- The use of intelligent and highly-proficient human personnel.
- The use of highly specialized and expensive tools.
The reliability of jet engines is achieved through efficient design, precise manufacturing process, and regular maintenance.
Design: Few Moving Parts
Jet engines experience a different type of stress than a piston engine.
For instance, a 6-cylinder piston engine with valves moving to and fro running at 2000 RPM experiences about 25,000 changes per minute!
Those continuous pounding oscillating movements cause a lot of wear and tear on the engine.
On the other hand, the parts of a jet engine simply rotate, causing a lot less of oscillating stress.
However, rotation generates forces that impart continuous stress on the parts. Metals in jet engines have a better tolerance for continuous stress than the oscillating stress and accompanying vibrations in piston engines.
Despite the tremendous engineering and tight tolerances involved in the design, a turbine/jet engine is very reliable.
Turbines do not break down easily, and if they do, it is usually a supporting system that fails. A broken pump and a clogged pipe are much more likely than the turbine to malfunction.
The main reason jet engines are so reliable is that they have far fewer moving parts that rub against other things than any other type of engine.
Sliding friction involving main crank bearings, connecting rod bearings, and at the cylinder wall/piston ring interface present is absent in jet engines.
Carefully Balanced To Avoid Vibrations!
Jet engine parts, especially the turbine blades are carefully balanced so there is almost no vibration which reduces engine life and the surrounding accessories.
Besides, jet engines are built from materials with very high thermal capacity and remain strong almost up to the melting point of the material.
These improved properties of the jet engine result in the reduced probability of failure due to fatigue, leakage, jamming, and therefore increased reliability.
The precision of the jet engine components, especially the complexity involved in the manufacturing of turbine blades is remarkable. And a lot of work is involved in the design of such finely balanced components for an engine that generates so much thrust for its large size.
The Manufacturing Process
A jet engine takes decades to be developed, manufactured, and put into operation for reliability and safety concerns.
The initial phase of the manufacturing process is influenced by the aircraft type for which the engine is intended. Every component is carefully tested throughout the design process which ultimately leads to the first tests of the fully assembled engine in a special test cell.
Here, the engine is subjected to torturous testing of extreme forces and operational scenarios. Afterward, the engine is tested on a specially designed test aircraft. For instance, the GE90 has been subjected to 127,000 pounds or 100,000 horsepower of thrust!
It often takes about two years to successfully test a jet engine.
One of the tests includes the water-ingestion test where 4.5 tons of water is injected per minute into the engine and it must be kept running.
Afterward, 0.75 ton of hail is fired at the engine in 0.5 minutes. Meat, bones, and other items are also introduced to ensure that the engine does not sputter out.
After the engine has finally passed all these stages, the aircraft then undergoes certification to ensure its airworthiness.
The Turbine Blades
It is worthy to note that the design and manufacturing of turbine blades is one of the most advanced technologies involved in the manufacture of jet engines.
Turbine blades are made of single-crystalline metal, low emission combustors, composite fan blades, and alloy disks free from the tiniest imperfections.
The materials used (single crystal) in the manufacture of turbine blades and the cooling mechanism demand the highest level of tolerance, sophistication, and precision.
Turbine blades are forged from a strong material that can resist the incredible centrifugal forces they are subjected to. These blades are also highly corrosion-resistant and very heat resistant — can withstand extreme temperatures up to about 2500F.
Maintenance
In fielded systems, another problem arises as one problem is corrected. This is the reason jet engines need to undergo regular and meticulous maintenance.
Many modern aircraft turbine engines deliver long and reliable operation because they are maintained by repeated and satisfactory inspection programs.
Jet engines run smoothly within specifications like a kitchen oven compared to piston engines that sound more like machine guns.
Unlike most internal combustion aircraft engines that require a complete overhaul every 1,000 hours, jet engines can run 30 times more hours with only a regular inspection.
But jet engines still need constant maintenance actions because they deteriorate with time due to wear, erosion, distortion, fatigue, and other forms of stress.
The severe pressure and extreme temperature experienced during operation have significant implications on the engine’s components and its maintenance cost.
The maintenance cost for jet engines is the highest maintenance cost per item for airlines making it crucial to develop efficient management techniques.
How Reliable Are Jet Engines Nowadays?
In the early days of the Jet Age, an engine was removed and overhauled after 2,000 flight hours.
Nowadays, it takes about 20,000-30,000 flight hours for any major overhaul of the engine. Today, wide-body jets equipped with just two engines fly to all corners of the globe.
Also, early jet engines had an In-Flight Shutdown (IFSD) rate of 0.9 shutdowns per 1,000 engine hours, making them unreliable. Today, jet engines such as the giant GE90, boast a massive IFSD rate of 0.001 per 1,000 hours (that is, one engine IFSD per one-million engine flight-hours).
The IFDS rate to a large degree dictates the EDTO(Extended Diversion Time Operations). In essence, this is the same measure as the ETOPS, with the difference that it applies to not only twin-engine airplanes:
Here are the requirements:
- 0.05/1000 engine hours for Extended Diversion Time Operations (EDTO) of 120 minutes
- 0.02/1000 engine hours for EDTO of 180 minutes
- 0.01/1000 engine hours for EDTO beyond 180 minutes
- Rate computed based on world fleet data, on a 12-month rolling average
The huge gains in jet engine reliability over the decades made this remarkable evolution in the engine industry possible.
Jet Engines Rarely Have Incidents!
According to International Safety statistics, there are about 25 incidents a year involving a jet engine failing either in flight or on the ground.
That translates into less than one for every million flights worldwide.
Despite the low engine failure rate, they can be disastrous once they occur.
Therefore, financiers allocate enough resources for the proper and regular maintenance of jet engines. There can be no room for cutting corners to save maintenance costs if a disaster can be prevented.
Benefits of Jet Engine Reliability
The extraordinary increase in reliability has had far enormous improvements in aircraft design, airline flexibility, and costs, all contributing to a reduction in airfares.
As discussed earlier, the increased reliability of the jet engine allowed regulators to ease restrictions allowing twin-engine aircraft to fly more direct and economical routes as against the earlier days when some aircraft were not allowed to fly more than 60 minutes away from an airport in case of engine failure.
Today, Boeing 777, a twin-engine aircraft can fly up to 330 minutes away from an airport.
Similarly, an Airbus A330 would have to fly for 17,880 years before both engines suffered a loss of power from unrelated causes, statistically speaking.
Results from reliability models can be used to better predict spare parts requirements for the purchasing department of an airline.
Conclusion
In your next flight trip, remember that you are sitting on a decade and million-dollars worth engine fabricated by enormous human brain-power and sweat.
Companies like Pratt & Whitney, General Electric, and Rolls Royce invest decades of human and non-human resources ensuring that the aircraft and its engines are reliable and perfectly suited to your flight.
The good part is that the process of ensuring the reliability of the jet engine for human and cargo safety goes on. It never stops!
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