How, when & why aircraft have their engines replaced

A lot goes on behind the scenes to keep aircraft safe and airworthy, something that many of us take for granted.

One of the major considerations for an airline when it comes to maintaining their aircraft is the engines, which undergo extreme stress conditions every time the aircraft takes to the skies.

Despite recent advances in technology, engines will not generally last the entire lifespan of the aircraft.

Some commercial aircraft still in service are more than 40 years old, such as Nolinor Aviation's Boeing 737-200, registered as C-GNLN, which is the oldest aircraft currently in commercial service.

This means that throughout the aircraft's lifetime, it is likely to undergo several complete engine repairs or replacements.

When it comes to determining when an engine overhaul is needed, many factors come into play, but the most important one is the number of flight cycles.

Put simply, a flight cycle is the number of take-offs and landings that the aircraft has completed, irrespective of the length of each flight (go-arounds are also included as a flight cycle).

This is because it is during these two phases of flight that the engines are put under the most strain.

The lifespans of modern aircraft, as measured in flight cycles, can vary significantly, depending on the size, weight, and age of the aircraft. Typical examples include:

Aircraft	Flight cycles
Airbus A320	50,000
Boeing 737	60,000
Airbus A350	45,000
Boeing 787	44,000

During its lifetime, a typical aircraft engine will undergo two or three complete overhaul cycles, during which engine parts are either replaced or repaired.

The first complete overhaul cycle typically takes place after around 12,000 flight cycles, but the gap shortens to between every 5,000 and 10,000 cycles thereafter.

Narrobwody, short-haul aircraft tend to complete more flight cycles than their long-range counterparts, so it is to be expected that their engines would need to undergo a complete overhaul cycle more frequently.

Data from ch-aviation tells us how many flight cycles each aircraft has completed to date. Taking a number of examples from airline fleets across the world, we can estimate how many complete overhaul cycles their engines may have gone through:

Airline	Aircraft	Registration	Age	Flight cycles	Estimated overhauls
Delta Air Lines	Boeing 767-300ER	N1603	26 years old	15,822	1
Qantas	Airbus A330-300	VH-QPB	21 years old	13,448	1
Nolinor Aviation	Boeing 737-200	C-GNLN	41 years old	30,473	3
American Airlines	Airbus A320	N103US	26 years old	38,349	4

The frequency of engine maintenance work varies very little between engine manufacturers. Among the largest manufacturers supplying engines around the world today are:

Rolls-Royce
Pratt & Whitney
General Electric (GE)
Safran.

With the help of technological progress, engine manufacturers are constantly developing more advanced aircraft engines that are more efficient and have a longer lifespan:

Operating conditions, such as temperature and pressure, can also affect the lifespan of an engine. For example, the hot section of an engine experiences extremely high temperatures on a regular basis.

Such extreme conditions gradually degrade the section's components, meaning that they need replacing or repairing more regularly than other parts of the engine.

Humid conditions can also cause the oxidation of metals within the aircraft engine. This is particularly the case if the aircraft is parked near to the sea, where humid air contains corrosive salt.

For this reason, when aircraft need to be stored longer-term, they are sent to parts of the world with dry conditions, such as Mojave Air and Space Port (MHV) in California.

Each complete overhaul cycle can cost hundreds of thousands of dollars, both in terms of the specialist materials and labor required to complete the maintenance work, and the loss of revenue while the aircraft is on the ground.

This is often still significantly cheaper than a brand-new engine, the cost of which can run into millions of dollars.

With this in mind, it is important that airlines are able to identify the most effective time to complete any non-urgent engine work.

This may be in the low season, when taking an aircraft out of service will not have a major impact on flying schedules, and therefore reduce any potential losses in revenue.

It goes without saying that safety is and always will be of paramount importance to the aviation industry.

With their power providing the thrust necessary for take-off, and reverse thrust that brings an aircraft to a complete stop on landing, aircraft engines are put under significant stress on a regular basis.

Engines are also critical to the safe operation of an aircraft, which is why such regular checks and maintenance are necessary.