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APU Arithmetic
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Airlines today see how reduced auxiliary power unit (APU) use reflects positively on fuel costs. A Delta Air Lines APU reduction initiative, discussed in last month's "Fueling Force" feature (O&M, Nov. 2008, p.35), for example, aims to save the carrier $30 million this year through working with other airlines to share ground power and heating units at various hubs and stations. Few contest that such initiatives present environmentally responsible and cost-effective measures in terms of fuel. However, the pattern of reducing APU use that has emerged over the last several years impacts the APU cycles-to-hours ratio and may upset the equilibrium of some hourly maintenance agreements.
Just how the decline in usage affects this ratio is a matter of mathematics. Several MRO executives cite a trend of APUs running for the same number of cycles--but for fewer hours--under fuel-saving initiatives. This trend has emerged over the last several years as fuel costs increased. "Of course, most airplanes today still require either a ground cart or the APU in order to start the main engine," said Rick Elgin, director of business development for Hamilton Sundstrand. "So, we're still seeing the APUs used, pretty much as often when you count start-stop cycles, but with less operating time." Some MROs say they have not felt the full impact of these changes yet, but they see a data shift in the long-term. Others say they have not yet noticed a change, but acknowledge its potential impact. Table of Contents
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