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What Happens when Jet Engines Age?

Facing the future with older power sources...

Dave Higdon   |   23rd February 2017
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Dave Higdon Dave Higdon

Dave Higdon writes about aviation from his base in Wichita Kansas. During three decades in...
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It’s hardly difficult to draw the correlation between an jet engine's age and increased maintenance costs, notes Dave Higdon. But how can operators keep those costs predictable and controllable?

Research puts the rate of maintenance cost-growth for aircraft between 5-15 years of age at about 3.5% per year. After that, allowing for jet engine and overhead costs, the total maintenance expenses grow relatively little. But surprises are possible.

Of course, Part 135 charter operators may fly their aircraft more than operators flying privately under Part 91, but the point remains: Older business aircraft stop becoming progressively more expensive to maintain overall after factoring in engine-related costs and aircraft overhead costs.

The engine need not be a major factor with the appropriate planning and investment. Handled at its best, the worst aspect of engine maintenance will be the required downtime for installation of a Loaner engine, enabling the aircraft owner to keep flying while the regular powerplant is overhauled. Even that period can be put to good use when major avionics upgrades or airframe maintenance are done at the same time.

With detailed planning, control and scheduling, an engine issue need not become a major disruption to your travel opportunities.

Time, Cycles, Rotables & Recurrents

A detailed examination of logs, maintenance records, and airframe and powerplant checks are standard procedure when considering the purchase of a used aircraft.

For the powerplant, that process should include an examination of internal components – particularly parts subject to high rotation speeds and higher temperatures.

Every engine comes with its own upkeep instructions and periodic maintenance times. Some approved maintenance cycles include instructions on replacing parts well before they can fail from fatigue.

Depending on the powerplant, those replacement cycles may be hours-based, cycle-based, calendar-based or even condition-based (meaning if no flaws are detected the part can stay in service indefinitely).

Modern turbine engines for business aircraft boast remarkable inspection periods, most upward of 2,000 hours, but some beyond 4,000 hours. That 2,000-hour inspection cycle covers 5.7 years of flying 350 hours a year with nothing more than routine maintenance.

Oil changes, oil-level maintenance, clean heat exchangers and exceedance avoidance all contribute to long engine life at minimal maintenance costs or downtime.

Assurance of Minimum Downtime

Some operators – particularly owner/operators – put aside an overhaul reserve based on a current-time estimate of an overhaul at some future date, divided by the hours remaining before the required inspection or overhaul.

Other operators simply plan to write a big check – confident that they'll have the liquid funds to cover what can be a six-figure undertaking. Some operators refinance the aircraft in question, paying interest on what they have to spend on the overhaul.

In between these extremes many operators pay for powerplant insurance through a program proscribed to meet their particular needs... theirs and the aircraft. Depending on the operator’s contract, the fees may cover only catastrophic maintenance needs or everything from preventive maintenance to overhaul or replacement.

The most complete programs leave nothing uncovered in return for a flat fee per-engine-hour, assessed monthly. Those fees work similarly to insurance policies, and are based on fleet numbers, known issues and maintenance costs. As the aircraft engine ages, the per-hour fee may be adjusted over time to allow for changes in costs and fleet numbers.

Programs exist to help operators with various levels of financial strength match their resources to a program suitable for their needs, and typically provide the subscriber with a single point of contact designated to act on their behalf – backed by staff experts in the hardware involved.

Cessna Citation Jet Engine

From Inlet to Exhaust

The fundamental components of turbine engines spend their lives in a spin. Up front the inlet fan of a high-bypass turbine engine feeds the majority of its atmospheric consumption through chambers in the engine cowl, with a smaller proportion routed into the engine core. In the core an axial compressor consisting of many stages progressively squeezes the air into ever-smaller spaces.

As the compressor performs its job, the act of compression heats the squeezed air to hundreds of degrees Celsius. Because of the extremes in temperature and extreme loads of the compressor blades spinning at thousands of revolutions per minute, routine maintenance requires periodic inspection of the compressor section. For some engines, that means removal from the aircraft. For others, the inspection occurs on-wing.

The compressor pushes scalding-hot air through an exit that opens into the combustion chamber – a metal can fitted with fuel nozzles that spray high-pressure fuel into the air where igniters light it. The combustor, or combustion chamber, endures the heated compressed air plus the extreme heat of a blue-flame jet-fuel fire. So it's another point of inspection on a cycle basis prescribed by the manufacturer.

The gases expand out of the combustion chamber through a series of nozzles, impinging on the blades of the various power turbines near the back end of the jet engine. Those turbines power the air compressor section and accessories. While turbofan engines use the hot gasses to generate jet propulsion, turboprops use the energy to drive a propeller.

Between the extreme heat (upward of 1,400°C) and high rotation speed of the turbine wheels – the turbine blades endure a torturous existence. Again, these parts face a rigid inspection schedule, and some operate under mandatory replacement limits set by the engine maker and approved by the FAA.

Other Engine Issues...

Turbofan engines employ accessories that also experience great swings in temperatures and high-speed rotation of parts (such as oil pumps, hydraulic pumps, starters, alternators and starter/generators). Most of these are components relatively easily removed and replaced – particularly compared to the engine’s internals.

Turboprop and turboshaft engines feature gearboxes that also experience high loads, temperature extremes and many rotating parts. In most turboprop engines the power turbine spinning the propeller turns the propeller shaft through a reduction gear, regardless of whether they are free or geared turbines.

In helicopters the high-speed rotational power of the powerplant runs through a gearbox to drive the main rotor and anti-torque mechanism, whether a tail rotor or a fan that provides air power to counter the main rotor’s torque.

Both types of reduction gears carry their own routine and preventative maintenance needs, and both can be sources of maintenance issues as they age.

The Inevitability of Down Time

Even the best-maintained machine occasionally suffers from an unanticipated failure – often something simple. Alas, no shop keeps spares for every powerplant component for every engine. And as the age of the powerplant advances, availability of spare parts can become more of a challenge.

While overhaul costs have been shown to remain relatively stable up through age 15, that’s less the case for turbine aircraft aged above 20 years (and for turboprops over 25).

In many cases new turboprop engines are available as replacements for original equipment engines under supplemental type certificates (STC).

And, in a bit of good news, the replacement costs can come in below the expense of overhauling the originals – as well as offering significantly higher overhaul periods and greater fuel efficiency with lower specific fuel consumption.

That makes the replacement option a win/win for many operators – and a path to reducing both maintenance and direct operating costs on older airframes.

Regardless of the aircraft, it pays to remember that maintenance and overhauls are part of the experience of aircraft ownership and operation.

The universal advice for avoiding most surprises and keeping maintenance costs at their lowest: Keep up with routine maintenance; don't defer unexpected needs; and have an alternative form of lift on stand-by because, inevitably, it will be needed.

Where your jet engines are concerned, plan ahead and be prepared!


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