Jetrader - November/December 2011 - 26

Aging Aircraft Systems
Part 2 of 2: Engines On-condition maintenance takes the guesswork out of maintaining aging engines
By Patrick Doyle, Senior Manager, Aircraft Maintenance, FedEx Express In my years of maintenance, I have seen many humorous write-ups from pilots, such as “No. 3 engine is missing” or “No. 1 engine runs like it’s sick,” but the reality of engine maintenance and the costs involved are serious matters for any aircraft owner. Maintenance costs for aircraft and engines vary with age. When new, the costs associated with the upkeep of these systems are relatively low but rise steadily under manufacturer’s warranties, then leveling off as maturity is reached (around ﬁve years). A mature system has a steadier, more predictable cost, which begins to rise again as the systems age after around 15 years in operation. Older airframes and engines require considerably more nonroutine maintenance and remedial work due to Airworthiness Directives, corrosion, and environmental factors like the higher humidity, salt or volcanic particles of a trans-Paciﬁc operation.

Predictive Maintenance in

Jet Engine Stresses
A jet engine is a complex piece of machinery with numerous parts, many of which rotate on the shaft. Both the compressors and the turbines are composed of many airfoil shaped blades, which are constantly subject to high pressure gases at extreme temperatures. For example, as the air moves through the compressors, its pressure will increase by as much as 40 times and the temperature will rise dramatically. Combustion in the combustion chamber increases the temperatures and the gas velocities even further. The speeding gases that exit the combustion chamber then exert very high forces against the turbine blades. Turbine blades exist in a much more hostile environment than compressor blades because of these velocities and temperatures. Both compressor and turbine blades are parts that are replaced often in a jet engine because of the stresses of high pressure and temperatures.

Basic Jet Engine Aging Issue
Since problems can be both internal and external to the engine, to understand what we’re facing with age, some basic principles of engine time calculations need to be discussed. Stress on engines is measured by a metric called “cycles.” The number of cycles is the measure used to make many maintenance decisions. Cycles are computed based on the number of takeoffs, the number of landings and the accelerations an engine experiences. In commercial engines, the number of cycles is relatively easy to determine since it is directly related to the number of takeoffs and landings. Commercial engines rarely go through any unplanned acceleration, compared to military engines. This clearly has implications for spares planning decisions.

Parts and Planning
Internal jet engine parts can cost anywhere from a few dollars to several hundred thousand dollars. The parts that cost more than $100,000 tend to be one-off items such as casings. Highvolume items include bolts, nuts and specialized items such as compressor and turbine blades. Standard parts like bolts and nuts cost a dollar or so, whereas specialized items such as compressors and turbine blades can cost as little as $60 or as much as $2,000 or more each, depending on the type of blade and type of engine. Lead times for parts often range from 60 days to 270 days. With these long lead times,

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Jetrader - November/December 2011

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