- 05 Jul 2019
- Dave Higdon
- Engines - BizAv
What is Jet Engine Maintenance?
Aircraft engine specialist Chris Kjelgaard provides answers to all the key questions on jet engine maintenance, highlighting all you should know about the needs of your business jet's powerplants.
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Chris Kjelgaard
Chris Kjelgaard has been an aviation journalist for more than 40 years and has written on multiple topics...
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What should you know about business jet engine maintenance? Detailing how turbofan engines work, the type of maintenance required and the cost, Chris Kjelgaard also considers who can provide the best maintenance for your jet’s powerplants…
Why do jet engines need maintenance?
Even though business jet engines have fewer moving parts than internal-combustion engines, their parts are subject to wear. This is the result of vibration, friction, the very high engine-interior temperatures, corrosion (especially when an aircraft operates in salt-laden or polluted environments), and the physical damage caused by accidental ingestion of foreign object debris.
In addition, the huge physical stresses which rapidly rotating jet engine parts such as compressor blades and turbine blades are subject to causes wear, which can cause metal fatigue, cracking and eventual failure of the parts.
As a result, the replacement of rotating parts (called life limited parts) is mandatory after certain periods of use.
The specified operating lives of life limited parts (LLPs) depend on the useful life limit the manufacturer determines for those parts during the process of certifying the engine as being safe for flight. Typically, the lifespan of an LLP is expressed in thousands of hours of operation, but may also be defined in thousands of take-off, climb, cruise, descent and landing cycles.
All of this makes periodic inspections and overhaul of all the parts in a jet engine vital and mandatory for the preservation of flight safety.
A secondary, but important reason for performing maintenance on jet engines at specified periodic intervals also exists. This is to ensure that all the rotating stages in each engine are completely balanced, so that vibration levels are kept to a minimum for the comfort of the occupants of the aircraft when flying.
Each engine on a business jet can be operating within the vibration limit allowed by its type certificate, but if the vibration levels in each engine is mismatched the occupants of the aircraft will quickly feel that something isn’t right.
Moreover, reducing vibration also improves the aircraft’s overall fuel efficiency, and increases each engine’s reliability and durability.
How do jet engines work?
The turbojet engine was the first-developed of the three major categories of gas turbine engine used in Business Aviation today. The other categories are the turboprop and the turboshaft (the turboprop being used to power thousands of small or medium-sized, fixed-wing aircraft and the turboshaft powering most helicopters in service today).
All three engine categories make use of gearboxes. However, unlike the turboprop and the turboshaft, the gearboxes in most jet engines are used only to produce electrical power. They do so by driving accessories such as generators mounted on the outside of each engine.
Turbojet Engines: A turbojet (in which all the air that enters the engine is compressed, mixed with fuel, combusted and expelled in a high-speed flow of hot gas, producing thrust) is the simplest form of gas turbine aviation engine.
However, for the past six decades, virtually all jet engines made for civil aviation use have been turbofans, not turbojets.
Turbofan Engines: In a turbofan, only a fraction of the air which enters the core of the engine is used to produce thrust.
Most of the air entering the engine inlet is accelerated by a large fan just inside the inlet and propelled backward as cold ‘bypass’ air, which doesn’t enter the engine core but instead travels outside the engine case to produce a high proportion of the engine’s overall thrust.
In modern turbofans such bypass air can provide up to 90% of the engine’s rated take-off thrust. The two- or three-spool, bypass air-producing turbofan arrangement is useful to aircraft designers for two reasons.
- First, it greatly improves the propulsive efficiency of the engine making the turbofan more fuel-efficient than a turbojet engine.
- Second, in the exhaust nozzle the cold bypass air both shields and mixes with the fast-moving, noisy jet of exhaust gas exiting the low-pressure turbine, reducing the engine’s overall noise signature.
Jet (turbofan) engines powering business jets come in a wide variety of sizes and thrust capabilities. Nevertheless, turbofan engines are generally designed to power aircraft which fly higher, faster and go further than those powered by turboprops or turboshafts.
Moreover, jet engines are generally more powerful than the other two turbine engine categories to enable them to fulfill the missions for which they are designed.
To develop the power required, modern turbofan engines make extensive use of advanced metal alloys, composite materials, proprietary coatings and parts made using state-of-the-art processes such as additive manufacturing.
All of these factors make maintaining and overhauling jet engines a complex, time-consuming process that should only be performed by organizations with the technical and physical resources, the engineering expertise, the employee skill-base and the ability to understand and comply with the regulatory requirements that the process requires.
When is jet engine maintenance needed?
Modern business jet engines often have longer mandated Times Between Overhauls (TBOs) than turboprop and turboshaft engines, simply because the different engine categories are designed for different missions.
While older or smaller business jet engines (such as the Williams FJ44) tend to have TBOs of 5,000hrs or less, most modern business jet engines usually have TBOs of 6,000hrs, or sometimes more. Considering that many business jets accumulate less than 500hrs flying time annually, it’s increasingly common for operators not to need to schedule full engine overhauls for periods of 12 years or more.
Older turbofan engines such as Honeywell’s TFE731 in its various models have mandated Major Periodic Inspections (MPI) and Hot Section Inspections (HSI) – also known as Compressor or Core Zone Inspection (CZI) intervals.
These must be performed after set numbers of flying hours, but MPI and CZI intervals can be extended by having the engine maintenance provider complete modification work set out in various supplemental type certificates (STCs) covering each model.
Example: The Honeywell TFE731-60 series has a standard MPI interval of 2,500hrs and a standard CZI interval of 5,000hrs.
Performing STC work on a TFE731-60 jet engine extends its MPI interval to 3,000hrs and its CZI interval to 6,000hrs, or perhaps even 3,500hrs and 7,000hrs, respectively, depending on the STC used. A 7,000hrs interval would correspond with an entire TBO interval for most business jet engine types.
While modern engines still need their LLPs replaced at set times, operators are increasingly able to extend the time their engines spend on-wing by taking advantage of the digital engine trend monitoring/engine health monitoring services the engine manufacturers now provide.
Such services allow operators to adopt ‘on condition’ jet engine maintenance programs for their engines, which lets them operate their engines longer than in the past because maintenance only needs to be scheduled when trend monitoring indicates a part will soon need replacing.
Example: The average TBO of Pratt & Whitney Canada-powered aircraft equipped with the engine manufacturer’s Full-flight data Acquisition, Storage & Transmission (FAST) engine-data wireless transmission system is 6,000hrs. The average for aircraft without the system is 3,600hrs.
What does jet engine maintenance cost?
Although turboprop and turboshaft engines are expensive to buy and maintain, modern business jet (turbofan) engines usually cost far more to acquire, requiring more financial outlay per maintenance inspection or overhaul.
Simply put, jet engines cost more to acquire because they’re bigger and the missions they’re designed to perform are different. They cost more to maintain because the processes of rebuilding and rebalancing their rotor stages are more complex.
Typically, jet engine overhauls may cost 150% as much as overhauls of reasonably equivalent turboprops, according to one leading maintenance provider.
For some particularly complex jet engine parts or modules (such as the turbine module), repairs might require longer lead times. Additionally, overall maintenance turnaround time is usually longer than for turboprops and turboshafts.
A rule of thumb is that a full overhaul will cost from one-third to one-fifth of the engine’s price when new. Small jet engines tend to cost in the high six-figure to low seven-figure bracket, so a per-engine overhaul cost is often in the $200k-$300k range. Large, modern turbofans used in Business Aviation can cost within the eight-figure dollar range, so eventual overhaul of such engines might cost between $1m-$3m.
Nowadays all the major jet engine OEMs offer ‘power by the hour’ engine-maintenance plans that usually represent good value for money. For a fixed cost per operating hour, such plans can cover the cost of all scheduled maintenance required (but not always unscheduled maintenance); the costs of parts and labor; and often the cost of transporting the engine to and from the designated repair shop.
The plan may even cover the cost of loaning a replacement engine while an engine is away for unscheduled maintenance.
Moreover, as turbofans become more computer-controlled and produce ever-larger amounts of sensor data, their manufacturers are increasingly offering digital engine monitoring services.
Along with other, specifically designed technical services such as Pratt & Whitney Canada’s powerful new engine oil analysis technology, these services are designed to defer the need for engines to undergo scheduled maintenance when their condition and performance indicate the engines don’t need to do so.
Where can I find jet engine maintenance?
As technologies in jet (turbofan) engines become more advanced, the engines themselves become more complex and the digital condition and performance data available from them provides a veritable flood. The jet engine maintenance landscape is certainly changing.
Smaller independent maintenance shops still competently overhaul thousands of business jet engines. But increasingly large companies which are independent in themselves but have a global presence are winning larger shares of the work on modern engines.
Regardless, when trying to find the best maintenance provider for your business jet, it pays to consult the engine’s manufacturer first. Because the manufacturers are increasingly focused on developing their advanced new engines and getting those to market, each has created a global network of high-quality maintenance companies which it licenses to support specific legacy engine models.
Those licensed companies might not offer the lowest bids for engine work, but the quality of the work they perform and their committed use of OEM-made parts in the repairs they make are invariably reflected in higher residual values of overhauled or repaired engines.
Industry insiders advise against engine-maintenance buyers promptly accepting any low-ball bids by maintenance shops. In an increasingly competitive maintenance industry, such bids often come with contracts that have many exclusions, so that a bid of $1m might well end up with a final invoice of $2m.
You should always read engine-maintenance contracts carefully and if you don’t understand the full import of a contract, seek professional advice from one of the many experienced Business Aviation technical consultants.
- For more articles like this, visit our Aircraft Engine Maintenance Hub
- Read more on Business Aircraft MRO on our new hub.
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