No need for alarm in the air after reports of engine troubles
A recent spate of incidents involving aircraft jet engines began in June 2016 when Rolls Royce, after finding accelerated rates of wear and corrosion in its Trent-1000 engines, announced it was replacing all of the 100 engines operated by ANA (All Nippon Airways), a major Japanese airline.
Exactly one year later, an Air Asia Airbus A330 flying from Perth to Kuala Lumpur reported one of its Trent engines had to be shut down when the engine began to shake violently after a substantial piece of the turbine fan blade had sheared off.
The aircraft returned safely to Perth.
In July 2017 Virgin Atlantic after discovering faster than expected wear/corrosion in their Trent engines, grounded most of their fleet of Boeing 787 Dreamliners.
In December 2017 an Air New Zealand Dreamliner flight to Tokyo had to turn back to Auckland after the pilots reported vibrations in one of the aircraft’s engines.
Soon after an engine problem was experienced on a second Air New Zealand Dreamliner.
The engines involved were Trent 1000s.
The Financial Times in London reported earlier this year that over half of the 434 Trent 1000 engines in service had turbines affected by wear and corrosion.
Why is this happening? Rolls Royce have not issued any statement to date on the reasons for the wear/corrosion but a brief look at the current evolution of modern jet engines might give some clues.
The principle behind a jet engine is to accelerate air and achieve high fluid speed at the exit.
Force is mass times acceleration, so the rate of change in velocity (acceleration) of a mass of air delivers a force.
The reaction to that force thrusts the aircraft forward.
In a jet engine air is pulled through a wide inlet fan assembly.
To achieve high exhaust gas speeds the inlet air is compressed, usually using a two-stage compressor.
This dense air then has atomised jet fuel sprayed on it and is ignited.
The very hot gas expands rapidly and is sent through a narrow exhaust nozzle which dramatically increases its speed.
After the combustor but before the exhaust nozzle there is a turbine.
This is on the same shaft as the inlet fan. The turbine extracts some of the energy from the outlet gas stream, spins rapidly and spins the inlet fan. This is called a turbo jet.
All civil jet airliners have a refinement to this.
If all the air is put through the hot section the fuel consumption is high, so only a modest amount of the air drawn in by the inlet fan is sent through the hot core of the jet engine, most by-passes the hot core.
This cool by-pass air is accelerated when it passes through the same narrow exhaust nozzle as the hot exhaust gas.
The name for this type of engine is a turbofan.
By limiting the amount of air through the hot core but still achieving high flow rates via the by-pass, the efficiency of the turbofan engine is much higher than the turbojet.
The revolution in jet engine technology has been in producing light-weight materials that can withstand very high temperatures.
Many of these materials are not metals, they are Ceramic Matrix Composites (CMCs).
These are very thin fibres of silicon carbide bound together in a matrix and wrapped in an envelope made of a secret material.
All the big engine manufacturers now use CMCs.
The fan blades of the many jet engines are not solid, they have a honeycomb structure, this makes them even lighter.
This honeycomb structure has been used reliably for many years, but it may be that the competition to chase exotic light-weight, temperature-resistant materials has led to faster than expected material stress issues.
Rolls Royce cannot afford not to solve this problem.
For this reason, one can feel assured that any Trent engines now operating will be some of the best maintained of any aeroengines.
It is also comforting to know that, as demonstrated by the recent incidents, a giant aircraft can lose one of its engines and still fly perfectly safely.
Reportedly, some airlines are now asking for each of the two engines on an aircraft to come from different production batches.