Honeywell by the Letters - APU

June 8, 2016 | Author: Carrie Fan


In this second installment we'll take a look at one of Honeywell's hallmark technologies—a product line that traces its heritage almost 70 years: the auxiliary power unit or APU. (Note that while most people just call it an APU, if you write it out you'll only need one "el".)

Essentially, the APU is a small gas turbine engine that provides electrical and "bleed air" power to an aircraft for main engine starting, air conditioning and electrical power on the ground or in-flight. Since it generally doesn't have to generate thrust, it burns considerably less fuel than the main engines do, and so is intended primarily for ground use, although most APUs can also be operated in flight for things like a second source of electrical power if an engine generator has failed.

A good example of this was the 2009 US Airways flight, known as the "miracle on the Hudson," that lost both engines during takeoff due to bird ingestion. The pilot used APU power to perform a controlled, safe landing into the Hudson river.

But river landings aside, the APU is normally used during ground operations prior to main engine start, powering the critical electronic systems and providing air to the environmental control to condition the cabin for passenger comfort. Today, integrated environmental control systems and components provide advanced cabin air conditioning and temperature control, cabin pressure control and ventilation for improved passenger comfort and safety.

For you history buffs, the development of the APU stands as one of the key aviation milestones in our History of Firsts.

Here's how it happened.

The GTC 43/44 engine had its genesis in sketches made by pioneering Garrett AiResearch engineer Homer J. Wood in 1947. In 1948, the GTC 43/44 became the first small gas turbine engine to pass a 200-hour Navy endurance test and soon gained commercial success with more than 500 units manufactured through the early 1950s. (Wood, and colleague Frederick Dallenbach, were honored with the Wright Brothers Medal in 1949 for their effort.)

Commercial air travel was forever changed in 1954 thanks to the 85 series APU as flights could now access the most remote regions of the world. What this meant was that ground crews no longer had to rely on ground-based electrical power and compressed air to start engines.

The 85 series APU was inducted into the Smithsonian Institution for being the world’s most widely used aircraft APU.

More recently, the 131-9 series APU is known for reliability, maintainability and performance on narrow body commercial aircraft. In July 2013, the stalwart power system achieved 100 million hours in service─an impressive number by any standard.

131-APUOver the years, Honeywell continues to be the market leader for these evolving products, producing more than 95,000 APUs for over 150 applications of commercial, business and military aircraft and ground carts…but who's counting?

Well, maybe just the satisfied aircraft customers.
For more information, please check out the Honeywell APU website.

Carrie Fan

Carrie Fan

Carrie Fan is the Content Producer for Honeywell Aerospace. Previously, she worked for Honeywell Aerospace Asia Pacific as the Digital Media Manager.


  • Madan Mohan

    Pls see my write..up in FB on APU for the sake of public interest..Received lot of good comments..Best wishes..Madan

  • G K Chetty

    This is a wonderful small machine (Engine) attached to the tail of the plane. Most people do not keep this in mind that when the a/c is on ground, how is the environment system kept active to provide comfort to the passengers. Starting of the main engine is always a problem. As the altitude of the airbase goes high, the starting of the engines become more difficult. Its this machine which keeps the main engines cranking till it starts up. A very respected machine indeed.

  • Brian Cottrell

    I enjoyed your article Carrie. APUs are indeed an interesting product area. I retired a year ago as Aerospace Chief Engineer for Structures and had lots of engagement in the APU design/field support tasks. Back in the eighties, before the current advanced computer modeling capability, the task of designing for "containment" was a big challenge. This design requirement is that the APU has to contain the fragments resulting from the rare event of a ruptured turbine or compressor rotor. I very much enjoyed wrestling varied physics principles into useful analysis tools, enabling one to characterize such a high energy and chaotic event. Good memories.