Auxiliary Power Units

Auxiliary Power Units 1

On July 22, 2008, Honeywell announced that for the next 10 years, it will provide maintenance services for Honeywell avionics and mechanical products to Southwest's entire fleet of Boeing 737s ("Honeywell Signs Extension…,”2008). Honeywell reports that its maintenance program for avionics and mechanical equipment has saved Southwest money and time over the past years. Under the extended agreement signed on July 22, 2008, Honeywell confirmed its agreement to “provide aftermarket repair and overhaul services for Honeywell equipment on Southwest's entire fleet”. This contract includes work on up to 520 aircraft, and will include future aircraft deliveries. Maintenance/repairs will include Honeywell's hydro mechanical units (HMU), avionics, lighting, mechanical components, as well as wheels and brakes, and – it’s Auxiliary Power Units (APU).
Aircraft power systems generate, regulate and distribute power throughout an aircraft. These solutions can range from auxiliary power units that provide power when main engines are not running to complete electrical power generation and conversion systems that provide essential power for safe aircraft operation. Honeywell’s robust family of power systems led the industry by lowering cost of operations and increasing reliability.
In Air Quality and Comfort in Airliner Cabins, Niren Laxmich and Nagda defines an auxiliary power unit (APU). An APU constitutes: “an engine used to provide aircraft air and electrical power when not provided by the main engines” (Nagda, 2000, p. 261). The following relates information regarding categories of APUs:
Category 1 and Category 2 APU
The criticality of an APU relative to flight safety in any particular aircraft installation will determine if the APU system should be considered essential, or non-essential. Airplanes that rely on APUs for provision of back-up electrical power in flight in the event of a failure of the primary power sources are usually considered essential installations. The following definitions may be helpful. * Category 1 APUs are installed where in-flight auxiliary power operation is necessary. Category 1 APUs are usually required for essential APU installations. These APUs have been shown to meet all of the test and analysis requirements of the Minimum Performance Standard (MPS) of TSO C77b, Appendix 1. * Category 2 APUs are installed where in-flight APU operation is not necessary (non-essential installations).
Both Category 1 and Category 2 APUs are acceptable for non-essential APU installations. Category 2 APUs are not required to meet all of the test and analysis requirements that Category 1 APUs are subjected to. For particular categories of civilian airplanes, the FFA requires a fire extinguishing system for each engine nacelle and auxiliary power unit (APU) compartment. (“Minimum Performance Standards…,” 2006). Auxiliary Power Units 2 * For main engine starting power, pneumatic power and electrical power generation, the highly reliable 131-9A series APU delivers robust performance, increased engine life, and easy maintainability for a lower cost of operation and ownership. The 131-9A provides quiet operation from noise reduction features integrated into the inlet, compressor and hot section design. (Honeywell APU, 2009) * A two-stage axial turbine increases engine life and a single starter/generator that uses electrical power to start the APU, but then becomes a generator once the APU is running, the 131-9A provides a cost-effective APU solution for Airbus A318, A319, A320 or A321 aircraft (Honeywell APU, 2009)
Technical Features * Certified to start and operate at flight levels up to 41,000 feet. * Provides 154 lb/min of bleed air at 52 psi for main engine starting and environmental control.
Its 90 kW electrical power generator fully meets all system needs up to a flight level of 35,000 feet in the case of an emergency. For particular categories of civilian airplanes, the FFA requires a fire extinguishing system for each engine nacelle and auxiliary power unit (APU) compartment. (“Minimum Performance Standards…,” 2006). One concern relating to the APU, the potential to overheat could be resolved by the passive cooling system provided for an APU installation on an aircraft. The U.S. Patent & Trademark Office reports that: “A passive cooling system for an APU installation on an aircraft…is for an auxiliary power unit having at least a compressor portion of a gas turbine engine and oil cooler contained separately within a nacelle” ("Canadian Inventor Develops…,” 2008). The Auxiliary Power Unit Passive Cooling System, developed by Luc Dionne of Candiac, Canada includes: … the auxiliary power unit housed within the nacelle of the aircraft, an engine exhaust opening defined in the aft portion of the nacelle and communicating with the gas turbine engine, at least a first air inlet duct communicating with a second opening defined in said nacelle and with said compressor portion and the oil cooler is located within a second duct communicating with an opening other than the engine exhaust opening of said nacelle and with the engine exhaust opening. Exterior cooling air and engine exhaust ejected through said engine exhaust opening entrain cooling air through said second duct to said oil cooler, and thus provide engine oil cooling. An exhaust educator is also provided. ("Canadian Inventor Develops…,” 2008

Auxiliary Power Units 3 A significant early demonstration was of the first liquid-fueled, fuel cell APU on a vehicle. This was a methanol-fueled, proton exchange membrane (PEM) APU on a Freightliner® long-haul tractor. The system was developed and built by Ballard Power Systems and was first demonstrated publicly in March 2003. Collaborative R&D continued with several component development programs that sustained the development of fuel reformers for more complex and "JP-8-like" fuels.
The next major demonstration was a synthetic-jet-fuel-powered PEM APU on a Bradley M2A3 during February 2005 at the Association of the United States Army (AUSA) Winter Symposium in Fort Lauderdale, FL. This system was designed and assembled through collaborative efforts between United Defense, Battelle and Pacific Northwest National Laboratory. The PEM APU is significant because it is the first complex hydrocarbon-fueled APU and it is the first one installed in a military combat vehicle. In addition to reformer-based systems, TARDEC continues to develop regenerative fuel cell auxiliary power. In this type of system, an electrolyze uses vehicle power to decompose water into hydrogen and oxygen. The hydrogen is stored and is later used in a fuel cell for silent watch or standby electrical power generation. TARDEC has teamed with Hydrogenics to develop this concept, which was first demonstrated as a proof of concept in January 2004. The program's second phase began in early 2005 to integrate a regenerative fuel cell One concern relating to the APU, the potential to overheat could be resolved by the passive cooling system provided for an APU installation on an aircraft. The U.S. Patent & Trademark Office reports that: “A passive cooling system for an APU installation on an aircraft…is for an auxiliary power unit having at least a compressor portion of a gas turbine engine and oil cooler contained separately within a nacelle” ("Canadian Inventor Develops…,” 2008). The Auxiliary Power Unit Passive Cooling System , developed by Luc Dionne of Candiac, Canada includes: … the auxiliary power unit housed within the nacelle of the aircraft, an engine exhaust opening defined in the aft portion of the nacelle and communicating with the gas turbine engine, at least a first air inlet duct communicating with a second opening defined in said nacelle and with said compressor portion and the oil cooler is located within a second duct communicating with an opening other than the engine exhaust opening of said nacelle and with the engine exhaust opening. Exterior cooling air and engine exhaust ejected through said engine exhaust opening entrain cooling air through said second duct to said oil cooler, and thus provide engine oil cooling. An exhaust educator is also provided. ("Canadian Inventor Develops…,” 2008).

Auxiliary Power Units 4

References 1. www.honeywell.com/sites/aero/Power_Systems.htm 2. Honeywell Signs Extension With Southwest Airlines to Maintain Fleet for 10 Years. (2008) 3. Air Quality and Comfort in Airliner Cabins, Niren Laxmich Nagda, 2000, p. 261 4. Canadian Inventor Develops Airplane Auxiliary Power Unit Passive Cooling System. US Fed News Service, Including US State News. HT Media Ltd. 2008 5. Minimum performance standards for aircraft engine and APU compartment fire extinguishing agents/systems (Draft). (2006, Sept. 26).Minimum Performance Standards, 2006 6. Leathers, Edwin Jr; Berels, ; Bradford, ; Kallio. Army AL & T. Superintendent of Government Documents. 2005.