FLUG REVUE April 2004: APUs are important

APUS – THE INDISPENSABLE POWER PACKS

By Patrick Hoevler

Over at the edge of the apron, there are still mounds of cleared snow. Typical winter weather at Munich airport. A crew boards its Airbus A320 for a flight to Hamburg. In the cockpit, the pilot starts the auxiliary power unit (APU). The digitally controlled APU in the rear of the aircraft gets going completely automatically by a battery powered starter. It consists of a gas generator (compressor, turbine, annular combustor), which serves as a drive unit, and a compressor on a shaft, which generates compressed air. Usually this is a single-stage centrifugal compressor with high stage pressure ratio. The air for both compressors flows through a common intake. All rotating parts are connected by a single shaft. The flow of air to the compressor used to generate compressed air is controlled by variable inlet guide vanes.

So it is on the A320 on this winter morning. The compressed air from the APU compressor in the rear flows into the air-conditioning system of the airliner. In approximately 20 minutes it has raised the cabin temperature to a pleasant level. Once the Airbus is airborne the bleed air from the engine compressors take over this task.

When the plane is on the ground, its power supply also comes from the small power plant in the tail cone. It drives a generator that runs with a constant rotational speed and is identical to those on the engines. On larger APU's, such as the Pratt & Whitney PW901A on the Boeing 747-400, the auxiliary gas turbine serves as a backup for parts of the hydraulic system as well.

Today the temperature is just above zero in Munich. To prevent the build-up of ice, hot air (150ºC) generated by the APU blows onto the wing leading-edges and air intakes. This is not witnessed by the passengers in the warm cabin. Finally the boarding process is complete. The crew now turn to the task of starting the A320 engines. It is during the engine start phase that the APU experiences its highest loading. On each engine, the compressed air fed into the aircraft's own system enters a free-running starter turbine, which uses a planetary gearbox (gear ratio approx. 22:1) to generate a large torque; this in turn starts the engine. Fuel now flows into the engine combustor and is ignited: the engine is running. However, it only works like this on the ground. If a powerplant should fail in the air, it is restarted with the assistance of the fan, which the incoming air flow causes to rotate (windmilling). The power of the APU turbines is not used.

Generally there are several different types of turbine. Radial turbines such as the Hamilton Sundstrand APS2000 have the advantage of smaller size and simplicity, but on the down side they are less powerful. Two-or three-stage axial turbines are more complex, but draw more power out of the hot gases. On our A320, this had heavy demands placed on it. Both engines are running. The pilot switches the APU off. It has performed its purpose.

Despite their affinity with larger aero engines, auxiliary power units are not booster powerplants, but purely turboshaft units. They generate torque, which drives shaft-mounted devices through a gearbox. Thus they do not provide any thrust. Despite this, like their big brothers, they have to satisfy stringent requirements. They have to fit into the smallest possible amount of space, which, like the weight, is specified by the aircraft manufacturers.

On top of this, since the introduction of Extended-range Twin engine Operations (ETOPS) restrictions, they are no longer required just to assist on the ground. “Twin-engined long-haul aircraft have given APU's a new significance,” explains Oliver Gillmann, product engineer APU Maintenance and Repair at Lufthansa Technik. The pilots have a “minimum equipment list” in the cockpit, which lists all the equipment which has to be fully operational for an ETOPS flight, for example, an Atlantic crossing. This includes the auxiliary power unit which in an emergency must be capable of starting at altitude, that is, of starting up at temperatures of up to -40ºC. When there are engine problems, the APU relieves the engines of the task of supplying generator power.

“APU's have become a safety item,” says Gillmann. If the APU is not working before take-off, then an aircraft will not be allowed on an ETOPS flight. This makes reliability of the equipment critically important. Moreover, a failure will cause discomfort to the passengers, especially at high outside temperatures. On a cargo aircraft, any perishable goods on board would be endangered. “This makes the APU is doubly important,” says Ole Gosau, who has headed the newly formed APU business unit at Lufthansa Technik since 1 January of this year.

The stages in an overhaul that these mini-power plants undergo are very similar to those to which the main engines are subjected, even if some parts are loaded more heavily. Thus the rotational speeds in APU's, 45,000 to 60,000 revolutions per minute, are significantly higher than on large engines. Modern types offer the advantages of diagnostics capabilities. For example, a controller in the Full Authority Digital Engine Control (FADEC) system records parameters such as oil temperatures over a period of months. This makes it possible to monitor an APU over time and detect any problems.

The intervals between maintenance inputs depend on the particular customer's operational profile. In northern latitudes, the APU:engine ratio of operating hours is approximately 0.7 (APU) to 1 (main engines), whereas in more southerly regions it is roughly equal. The gas turbines can go between 2,000 and 9,000 hours before visiting the workshop. APU's on the A320 or 737 last out for around 5,000 hours, while on the 747-400 maintenance intervals are as high as 8,000 hours. Their maintenance is therefore more expensive and can cost up to $500,000.

Replacing an APU normally takes one to one-and-a-half hours, an overhaul lasts 30 days. Here the problems engineers encounter are the same as on the main engines, and the same preventive measures are employed. Thus, the two-stage axial turbine of the APS3200 for the A320, which is exposed to turbine inlet temperatures of over 1,000ºC, is protected against hot gas oxidation and corrosion by platinum/aluminium. Few organisations around the world are capable of carrying out the full range of APU maintenance work. In this area, Lufthansa Technik's new department, with a workforce of 30 and some 500 APU's under contract, is seeking to expand its position. The pool of manufacturers is very small as well. Essentially, three companies share the market between them. Apart from Honeywell, which offers about 40 models, some of them taken over from Garrett, two other companies which are both members of the United Technologies Group, Hamilton Sundstrand and Pratt & Whitney Canada, are vying for orders. On some aircraft types, customers have several products to choose from, whereas on types like the 747-400, 777 and A380, only one model is available.

In this market, the Canadians have established themselves at the higher power end. The PW901A for the 747-400 produces 1,145kW, as Pw980

much as some turboprop engines and incorporates some parts from the JT15D-5 jet engine. Pratt & Whitney Canada's PW980A, intended for the Airbus A380 mega-liner, needs to be even more powerful. The APU possesses oil-cooled generators, turbine blades from single-crystal alloys and an effusion-cooled combustor. A derivative of the PW901A, this unit, which ran for the first time in June 2003, has the same architecture. Altogether, four prototypes have completed 575 hours of testing to date. Certification is planned for March 2005.

Between now and then, demand for these power packs is likely to increase further. Aviation analysts from Forecast International predict that 25,436 new APU's will be built between now and 2012. Including other small engines, they predict demand to the tune of over $7 billion. According to analyst Rich Henderson, Honeywell is the market leader, with a market share of 69.7 percent. “The APU market is at a crucial juncture, as increased demands for onboard power inspire design innovations among the market leaders. The next generation of APU's will very likely set new standards in terms of output, economy of operation and in-service durability.”

As an example, he mentions the Honeywell RE50, which weighs only 25 kilograms. This mini-engine intended for smaller aircraft is supposed to need maintenance only every five years and employs an air-bearing rotor so that it can dispense with oil altogether. In this respect, the Americans are following the trend. Engineer Oliver Gillmann also believes it is feasible for APU's of the future to manage without lubricants. On the other hand, he cannot imagine fuel cells being used in the near future. “Aviation is traditionally conservative,” he says. The hydrogen required for this process must be capable of being transported economically. At present, kerosene still offers advantages as a standard fuel used both by engines and APU's.

Among the manufacturers, reduction of emissions and costs are therefore a priority at the moment. The research departments of P&W Canada and Hamilton Sundstrand are already working on new combustors which could reduce emissions of nitrogen oxide by 40 percent. New designs for intakes and exhaust outlets should reduce the noise to below 80 dB.

The first representative of a new generation could be the auxiliary power unit for the Boeing 7E7, which Hamilton Sundstrand is developing. According to Karl Johanson, Director Business Development at HS, it will contain parts from existing products, but the outcome will be a new system. Details are not yet allowed to be published, but one can assume that the new APU will manage without engine bleed air and will start the engines with electrical motors. In any case, it could set new standards in the areas of reliability and life-cycle costs.

From FLUG REVUE 4/2004, page 80

Last updated 12 March 2004

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