How Are Aircraft Engines Maintained?

Aircraft engines are the most expensive parts of an aircraft, and it is in airlines’ interest to keep their powerplants in tip-top condition. Apart from the fact that they are rather crucial to actual flight and safety, unscheduled service interruptions due to engine problems can quickly become costly affairs. The aero-engine maintenance industry is worth billions of dollars. But what actually goes on behind the scenes?

Airbus engine at maintenance
What goes into aircraft engine maintenance? Photo: Getty Images

A multi-billion-dollar industry

The aero-engine maintenance, repair, and overhaul (MRO) market is worth over $2.5 billion. Despite the hiccup in growth prognosis caused by the current crisis, it is bound to increase even more once commercial aviation and air traffic demand hit their stride again post-pandemic.

Jet engines themselves are the most expensive items on an aircraft. In turn, the most expensive parts of an engine are the turbine blades, bearings, and engine control systems. Aircraft powerplants can cost anywhere between 12 million and upwards of 45 million dollars.

While built from the absolute strongest materials their function will allow, jet engines are still subject to wear and tear. This results from friction, vibrations, extremely high temperatures, and potential corrosion due to external environments. Now and then, there are also damages from debris or other foreign objects, such as on the occasion of a bird-strike. 

Qantas Boeing 747 engine overhaul 1979
Engines are overhauled in special facilities – here; a Qantas 747 is in for its scheduled maintenance in 1979. Photo: Getty Images

On-wing vs. off-wing

Aircraft engine maintenance regulations are key to protecting lives in the sky and having a huge impact on the revenue of airlines globally. While MRO may be expensive, it is nothing compared to what an unreliable fleet with regular engine issues could end up costing carriers.

On-demand on-wing maintenance is essential for airlines in case anything goes wrong with an engine when the plane is far from the carrier’s own maintenance base. For instance, GE Aviation provides more than 4,500 rapid repair solutions every year to its engine customers globally. Its promise is to have an engine maintenance team on the flight-line anywhere in the world within 24 hours.

Such on-wing rapid maintenance is often completed quickly, and the aircraft can be back flying within a few hours to a couple of days. Light maintenance off-wing, on the other hand, takes anywhere from 15 to 35 days. Heavy maintenance, known as an overhaul, where all components associated with the engine are refurbished, can take over two months to complete.

Engine open
On-wing maintenance can often be completed quickly. Photo: Jay Singh – Simple Flying

Both maintenance and complete engine overhauls are usually performed at regular intervals, meticulously scheduled so as not to disrupt airline operations. MRO scheduling is one of the areas where artificial intelligence is predicted to have a massive impact on carriers’ cost savings in the years to come.

Time Between Overhaul

Apart from immediate interventions following problems such as engine failures and routine lighter maintenance, there is mandated Time Between Overhaul (TBO). This is the manufacturer’s recommended number of running hours before an aircraft engine, or one of its components, requires a complete overhaul.

For modern jetliner powerplants, these are usually around 15,000 hours. However, in 2019, a Rolls-Royce Trent 700 engine completed more than 50,000 flying hours on-wing on an Aeroflot Airbus A330 without needing an overhaul, setting a world record.

So what happens during an overhaul? A simplified chronology of events is that the engine is stripped down and checked thoroughly. Any parts that need to be replaced are exchanged for new ones. It is reassembled, tested, and, hopefully, sent on its way in mint condition. But let’s take a closer look at what actually goes on.

Chengdu engine maintenance facility
Asia’s largest aero-engine maintenance facility is located in Chengdu, China. It receives over 600 engines for MRO per year. Photo: Getty Images

The overhaul

An engine can be said to be overhauled when it has been inspected, disassembled, inspected, cleaned, inspected, repaired according to the manufacturer’s instructions, inspected again, and then tested using civil aviation regulatory body approved procedures.

After initial diagnostics and inspections, the engine and all its parts are taken apart completely. They are then studied again before being treated to a chemical bath to remove any pollutants or contaminants. This is to assist efficiency and airflow once the engine is back in the air.

All cleaned up, they head to a maintenance bay where licensed technicians refurbish all of the components. That is some 2,000 parts that need to be individually inspected and maintained, each worth thousands of dollars.

GE Airbus new narrowbody engine
Engineers take apart and inspect every single minute part of an engine during an overhaul. Photo: Getty Images

Balancing of rotating parts

Engineers make sure that the rotating stages in each engine are entirely balanced. If not, this can cause additional and unnecessary vibrations. Not only is this uncomfortable for the inhabitants of the seats on the plane, but it reduces fuel-efficiency and the engine’s durability.

The stresses placed on the rotating parts of an engine are enormous. Hence, the replacement of such parts, also known as life limited parts (LLPs), is mandatory after a certain period of use. Then commences the meticulous and crucial task of reassembly.

After the engine has been reassembled with all of its parts, it is sent off to a test-bed. There it is rigged with the testing equipment and run while engineers monitor its performance. When they are satisfied and sign off on the overhauled engine, it is returned with the “time since overhaul” reset to zero and the “time since new” unchanged.

How Are Aircraft Engines Maintained?
The GE9X weighs close to 10,000 kg. Photo: Dan Nevill via Wikimedia Commons

Weighing in at up to 21,000 lbs

While an unserviced engine is unsafe, maintaining these giant powerplants is also not without its perils. The GE9X, the world’s largest turbofan engine developed especially for the Boeing 777X, weighs a whopping 21,230 lbs (9,630 kg). It is also wider than the fuselage of a Boeing 737.

But engines such as the Rolls-Royce Trent 900 are also no joke at 13,770 lbs (6,246 kg). Even parts of such a construction are not something to be handled lightly around the shop floor.

Workers at locations such a Delta’s TechOps in Atlanta, the largest aircraft maintenance facility in North America, or at Asia’s largest aero-engine maintenance base in Chengdu, China, must exercise extreme caution and good judgment at all times. Both for their own safety and that of others.

Next time you are sitting in that window seat looking out over the humming engine under the cowling, take a moment to appreciate all those hours of meticulous inspections that are keeping you flying safely.