APU Arithmetic

Dec 9, 2008
By Elyse Moody/Overhaul & Maintenance

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. This distortion of the cycles-to-hours ratio will affect the mean time between removals (MTBR) for APUs and the power by the hour type agreements that cover their maintenance. As MROs see APUs requiring the same type of maintenance at the same number of cycles, but at a lower hour mark, some industry executives say they will consider reshaping hourly based contracts to take cycles into account.

Cycles-To-Hours Ratio

While their calculations took slightly different shapes, Elgin and Tim Fischer, VP of StandardAero in Maryville, Tenn., came to similar conclusions in describing how decreased usage will impact hourly based maintenance contracts. The Maryville facility Fischer oversees gained approval in November from Hamilton Sundstrand as an approved repair facility for the APS 2300 APU found on Embraer 170s and 190s. Hamilton Sundstrand also repairs units at its facilities in San Diego, Phoenix, Singapore, and soon Shanghai. And, it finished buying EADS Sogerma's shares in what had been the 50/50 joint venture Revima APU, in France, in July. Both men noted that APU usage and practices vary from one operator to another, from one airport to another, and from one engine type to another.

Assume that, in the past, an airline had a cycles-to-hours ratio of 1:1. Every turn at the gate equaled one start-stop cycle and one hour of APU use. (Note: these numbers serve an illustrative purpose and are not realistic figures.) Today, that airline might experience 15 minutes of APU run time with the same set start-stop cycle, for a ratio of 1:0.25. Elgin summed this change. "An event that used to occur at 5,000 hours and 5,000 cycles, today might appear on paper to be occurring at 5,000 cycles but only 1,250 hours."

The impact decreased APU use has had on maintenance "has taken a lot of us by surprise," Fischer said. He likened these APUs to cars that travel more stop-and-go city miles than highway miles, in terms of resulting wear and long-term performance. Elgin pointed out that "in reality the same event is taking place," mainly impacting the bearing system, rotable components and lubricated parts. The kicker is that the MTBR has been distorted.

This distortion does not add to the types of repairs required, as Elgin said, but it may affect the degree of severity. The same performance factors, such as heat, thermal variation, added vibration, ramp-up and starter activity, affect APU components' performance, Fischer said. The difference a change in the cycles-to-hours ratio will make is that components may see more wear-and-tear between maintenance visits, since visits often coincide with an hours-based--rather than cycle-based--schedule. "What would impact us," explained Rich Barlow, director of engineering for another primary APU OEM and service provider, Honeywell Aerospace, "would be the fact that on some of those particular shop visits, some of the hardware such as the life-limited components might have actually received more cycles for the same amount of time frame than they previously would have achieved--and that could drive our costs up, where we're dictated by certain replacement requirements."

It also could impact operators' costs. Fischer explained how using one of his facility's major products, the Honeywell RE220 on Bombardier CRJ700/900 aircraft. When this engine comes in with higher cycles, he said it will have more hot section distress, more accessory wear-and-tear and more lube system damage--this could raise man-hours per event by as much as 20%. "The RE220 is still a young engine in comparison, so we have not seen this impact yet," he noted. He said that operators' might not be aware of how these changes will affect their maintenance costs.

"In addition, I don't think the operator totally understands that the higher cycle ratio is going to drive a change in their hourly rate," he said. "In their mind they are getting a double bonus if they are on a pay-per-hour program: less fuel consumption, and less APU hours to pay on." The industry has not yet discovered the "magic number" to optimize fuel savings and APU maintenance hours, Fischer added.

MRO Impact

What do these changes mean for maintenance, repair and overhaul providers? To adapt to the market, some industry executives say MROs will need to take a look at asset distribution and work scopes, make the most of technologies to forecast events, and reevaluate the way they write contracts to better reflect hourly and cycle-based needs.

First, changes in APU usage may mean adjustments to repair providers' supply chain operations. Fischer cited his facility's operational efficiency as a particular strength here, saying he does not anticipate any impact to turnaround times or spares. For Honeywell, Barlow said changes in APUs' MTBR will mean taking a look at planning and asset management. The company will need to ensure it has more available spare rotating hardware components, he said.

Honeywell will leverage the size of its APU fleet and its worldwide service network for an advantage here. Its 131-9A APU for the Airbus A320, for example, is in use on 1,400 aircraft in operation by 90 customers, and it repairs various APUs in Phoenix, Ariz., in Raunheim, Germany; and in Xiamen, China. In Kuala Lumpur, Malaysia, it repairs the 85 series APU. In forecasting usage trends, a large, diverse data pool benefits analysis. "One of the things we've done here of late that has really helped us with our maintenance costs is we've compared usages on many of these fleets," Barlow said. "We've been able to check to see the replacement or the reusage rates per each one of these different sites, which is a real advantage." Working in concert with technology, this can help repair providers like Honeywell have the right assets in the right place at the right time.

Health monitoring sensors keep an eye on APU engine performance and notify the OEM and, in turn, the airline customer, of changes in performance. Both Honeywell and Hamilton Sundstrand offer health monitoring services for many of their APU models. For Honeywell, these include the 131-9 series on the Boeing 737NG and Airbus A320; the 331-200 and the 331-350 on the A330; and the 331-500 on the 777; and it is adding the 331-600 on the A340-500 and A340-600, as well. According to Jim Balonis, senior manager of engine programs for Honeywell, about 30 operators have signed up to use Honeywell's APU health monitoring service.

Honeywell's Barlow pointed out that APU health monitoring "has the potential to prevent catastrophic failures by allowing us to service the APUs when they are just beginning to deteriorate in terms of performance, opposed to once they have actually had significant damage." Using technology to monitor and detect APUs before they reach considerable damage levels will become more important for performance as airlines use them less frequently. APU health monitoring also allows MROs to better tailor their work scopes and align necessary assets for upcoming maintenance activities, noted Balonis.

Health monitoring "also [can] help to avoid some of the failure modes that some engines might have experienced in the past, which can result in heavier maintenance overhaul costs when the engine does come in," Hamilton Sundstrand's Elgin said. He expects to see the trend of using health monitoring systems accelerate with the next generation of aircraft. His firm's APUs for the A380 and the 787, for example, both take advantage of health monitoring technologies among other advanced features.

Barlow and Elgin both expressed a necessity to revisit hourly maintenance agreements for APU services. Elgin noted that the low-cost carriers Hamilton Sundstrand serves, particularly those operating A320 and Embraer regional aircraft, often request power by the hour type agreements. He estimated that more than half and maybe even more than 60% of his firm's fleet today is under some kind of power by the hour agreement. He explained that hours-based contracts reflect airlines' lower APU run times, and that, from a revenue standpoint, the cost remains essentially fixed. "We are trying to deal with that," he said. As a result, Hamilton Sundstrand's hourly maintenance agreements are beginning to take decreased use into account, "to try to reflect a cycles-based accounting as well as an hours-based accounting."

Honeywell's Barlow echoed Elgin's concern, nothing that right now, most of the company's life-limited hardware is set up on a life time frame, but a number are on a cycles time frame. "If we're getting a lot more cycles per time frame," he said, "we are going to have to make some adjustments there."

A North America and Europe Phenomenon

These adjustments may be limited to agreements with North American and European operators for the time being. In the U.S. in particular, rising fuel costs made APU reduction imperative. As StandardAero's Fischer pointed out, an APU, unlike an engine, can be shut off for periods of time without affecting airline operations. This presents a promising area for potential cost cuts.

Emissions and noise also drive APU reduction in Europe, a goal that took off as fuel prices escalated. "We started seeing some airports in Europe restricting APU use just because they are in fairly densely populated areas, and they wanted to cut the noise. Also, emissions from the operating APUs became an issue in some parts of Europe," said Elgin.

"It was kind of spotty, and it was hard to say it was a growing trend," he said. "When you add to that the run-up in fuel costs over the last couple of years, we've definitely seen the U.S. carriers cut back on [APU] operation driven by the fuel cost."

Barlow agreed with Elgin's interpretation of the pattern. "I think it's probably a leading trend in the U.S., but I would anticipate that it will be occurring worldwide as all of the airlines make an effort to try to drive their costs down." Higher fuel costs and environmental concerns prompted North American and European airlines to evaluate the dynamics of their operating expenses, he noted. These factors also help shape emerging APU design and repair technologies, including all-electric APUs for next-generation commercial and business aircraft.

Still, the cost of fuel, emissions mandates and noise limits do not affect operators everywhere. Elgin noted that the trend to reduce APU usage has not emerged yet in places where aviation infrastructure still experiences rapid expansion, such as in the Middle East or in China. The trend also may affect commercial air transport more than business aviation: Pratt & Whitney Canada representative Pierre Boisseau said that since it has always been the case to switch to electrical power at the gate when it is available, "we haven't seen any changes in terms of scheduled maintenance programs."

This article appeared in the December 2008 issue of Overhaul & Maintenance.

Photo: Honeywell Aerospace

Aviation Week / Overhaul & Maintenance

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