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EPCOR repairs six types of APUs at its shop in the Netherlands.
Built in the tail end of an aircraft to provide electric and pneumatic power for onboard subsystems, an auxiliary power unit is typically maintained over average cycles with maintenance driven by factors such as fleet size, utilization and operating environment.
Auxiliary power unit (APU) repairs are carried out whenever condition necessitates removal—owing to factors such as high exhaust gas temperatures, oil consumption and cabin vibration, for example—and usually during an overnight stop rather than as part of a scheduled base maintenance check.
“Different airlines have different approaches to APU maintenance, but in general, most elect ‘on-condition maintenance,’ where modern monitoring capabilities exist,” says Peter Gille, CEO of U.S.-based APU repair specialist TurbineAero, which has APU repair shops in Chandler, Arizona, and Chonburi, Thailand. The company also operates APU and engine component (piece part) repair and component manufacturing and coatings facilities near Phoenix.
“Depending on the age and condition of the APU, the maintenance will either be repaired to correct the symptoms and findings found on the APU, or a refurbishment of the APU power section and load compressor plus modifications to a higher standard to improve the APU reliability,” adds Rob van de Graaf, commercial director at Netherlands-based APU specialist EPCOR.
There are relatively few APU specialists like TurbineAero and EPCOR operating today, and the number of designated APU MRO providers, particularly in-house at airline shops, has declined in the past few decades. Several larger multiservice MRO providers, such as Lufthansa Technik, operate APU repair businesses, while OEMs have always been traditionally strong in the aftermarket.
Among the largest commercial aircraft APU manufacturers is Honeywell, which provides APU models for multiple Boeing and Airbus aircraft programs. The company often enters into long-term APU aftermarket service agreements with operators and also has a network of approved ser-vice centers to maintain its products.
Another OEM with a sizeable market share is Pratt & Whitney, which provides the APS3200 for the Airbus A320, APS5000 for the Boeing 787, PW980 for the Airbus A380 and APS2300 for Embraer 170/195 aircraft. The engine manufacturer also has a global network of its own shops and designated repair facilities.
CYCLES AND CONTRACTS
Reflecting other maintenance segments, much of the repair demand in the commercial APU segment is related to Boeing 737 and Airbus A320 aircraft both classic and new. This is due to narrowbody aircraft typically having higher cycle utilization, resulting in more frequent APU shop visits. The 737NG and MAX models use the Honeywell 131-9B APU, while the A320 family predominantly is equipped with the Honeywell 131-9A, while some in the family use the Pratt & Whitney APS3200, says Dominic Ryan, general manager of base maintenance and APU at Dublin Aerospace. He also notes that some widebody APU repair markets have experienced an upturn in recent times, “such as the Honeywell GTCP 331-350C for the A330 and the GTCP 331-200ER for Boeing 767 freighters.”
Gille says TurbineAero also sees strong demand for the Honeywell GTCP331-500B model found on the Boeing 777. It also services mature wide-body APU models for which OEMs have either diminished or discontinued support, such as APUs on Boeing 747, 757 and 767 aircraft and Airbus A330s and A340s.
MAINTENANCE SCHEDULES
Dublin Aerospace operates one APU repair shop at its facility at the Irish capital’s airport and maintains relationships with four other APU repair shops in its network. Additionally, it collaborates with several specialized component and piece-part repair shops for APU parts.
In terms of repair cycles, the APU is removed from aircraft at different periods, depending on the model.
“For narrowbody APUs like the Honeywell 131-9A and 131-9B, the time between overhauls typically ranges from 6,000 to 8,000 hr.,” Ryan says. He explains that these intervals can vary depending on operating conditions, with harsher environments potentially leading to shorter time on wing due to increased erosion and contamination. “High utilization rates can also reduce the time between overhauls,” Ryan says.
TurbineAero’s Gille says typical mean times between removals vary from 4,500 APU operating hours on the GTCP331-200/-250 models to 6,500 APU operating hours on the GTCP331-500 or even exceeding 10,000 APU operating hours in the case of the newer GTCP131-9B and PW901A/C APUs. In terms of cycles, Roger Willis, APU program director at StandardAero’s Maryville repair shop, says while this varies according to the specific model of APU as life limits are different, the average time of removal is typically between 20,000 and 35,000 cycles. “There are some [life-limited part] items such as the compressor that can see up to 65,000 cycles,” Willis adds.
In Maryville, Willis says a typical overhaul of an APU is 25-30 days, but these repair times have extended recently. “We have seen this increase due to material constraints on parts such as life-limited part items, bearings and seals,” Willis explains. “Most of the delays are due to raw material shortages and the quicker-than-expected recovery in flight volumes after COVID.”
A similar estimate is made by EPCOR’s van de Graaf, who cites generally 30 days for narrowbody APUs and 40 days for widebody models. The process involves teams working in logistics, inspection, disassembly, cleaning, repair, assembly and testing.
EPCOR, a wholly owned subsidiary of Air France Industries KLM Engineering & Maintenance, services six APU models under license as well as pneumatic components from its shop at Amsterdam Schiphol Airport. It plans to add its seventh license, for the HGT1700 installed on the A350, by early 2025.
The wider market for APU repairs is seeing some capacity constraints felt by other repair segments—such as the wheels and brakes repair market—-albeit to varying degrees.
“Currently, there are available slots for APU repairs at our Dublin facility, although there are some delays in the supply chain and with turnaround times for component and piece-part repairs. These delays are manageable within the overall APU repair cycle,” Ryan says. Typically, there is a push toward more power-by-the-hour (PBH) contracts for newer operators over time and material agreements, he says. “Time and material customers typically prefer minimal repairs to reduce shop visit costs, which often depends on fleet utilization and age,” Ryan notes. “In contrast, PBH customers aim to maximize the benefits of a shop visit by extending the APU’s time on wing until the next scheduled visit. Generally, operators of newer fleets are more likely to choose PBH agreements.”
The preference for PBH agreements for APU repairs is also supply chain-related, van de Graaf says. “Given the supply chain issues and current geopolitical situation, more and more operators tend to spread their risk on receiving units back in time,” he says. Van de Graaf adds that time and material contracts are normally used by single shop visits and short-term contracts while PBH is for long-term agreements as well as on newer aircraft types. “This provides financial stability for both the operators and the MROs,” he says.
EFFICIENCY IMPROVEMENTS
APU models have evolved considerably over the decades to become more efficient. “Earlier generations had more mechanical control with minimal electronics and took bleed air from the power section, reducing efficiency,” Ryan says. “Over time, APUs incorporated more electronic controls and data collection capabilities, with load compressors added to enhance efficiency.” He adds that the latest APU models have eliminated the need for bleed air and moved from fixed-speed to variable-speed designs, improving efficiency under different load conditions.
StandardAero’s Willis cites fuel efficiency and emissions as the current drivers of advancements in new-gen APU models, with the latest APUs delivering 10-15% improvements over older modules. “We have also seen the development of all-electric APUs, such as the Pratt & Whitney APS5000 on the 787 and the Safran eAPU60,” he adds.
Repairs for new-generation APUs are also still in development as the market continues to mature. However, many new-gen aircraft APUs are based on similar older-generation models, which is an advantage to repair shops with older-gen capabilities.
“Newer APU models are often a derivative of previous models with common components and parts and so the type of required repairs is common,” says EPCOR’s van de Graaf, who cites the biggest difference being that new APUs there undergo fewer modifications.
“They use similar core components and line replaceable units, although advanced alloys and machining techniques tend to make new-generation APUs more performant and better able to endure higher pressures and temperatures,” Gille adds. TurbineAero has invested in improved machining and rework capabilities and modernized engine test cell equipment in recent years to meet new requirements, he notes.
APU maintenance on newer models also involves predictive tools to determine the APU’s health and performance in between maintenance cycles. Gille says TurbineAero, like other APU providers, is developing an in-house trends monitoring program, influenced partly by the shift toward PBH programs, and determining when to remove the APU before it fails. This could also help teams identify the APU issue and remove the unit before it becomes unrepairable and is subsequently scrapped. “When you let it run until there’s a problem and wait until then to repair it, the cost will be high,” Gille says.
