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Gas Turbine Engine – Accessory Section and APU


The accessory section of the gas turbine engine is usually mounted beneath the compressor section.


●      The primary function is to provide space for the mounting of accessories necessary for the operation and control of the engine.

●      Secondary functions include acting as an oil reservoir, oil sump, and housing the accessory drive and reduction gears.


This section contains the following:

  •  Accessory drive gearbox
  •  A housing (case)
  •  Provisions for mounting the engine-driven accessories
    • Constant speed drive transmission
    • Fuel
    • Oil pumps
    • Tachometer generators
    • Etc.

In gas turbine engines with air turbine starters, the starter is mounted on the forward face of the accessory gearbox. The accessory gearbox also includes many of the gas turbine engine’s internal lubrication system components.

Location of the Components

Driven accessories on turbofans are usually mounted on the accessory gearbox, which is on the bottom of the engine. The location of the accessory gearbox varies somewhat, but most turboprops and turboshafts have accessory cases mounted to the back section of the engine.

The components of the accessory section of all gas turbine engines have essentially the same purpose, even though they often differ quite extensively in construction details and nomenclature.

Basic Elements

The essential elements of the accessory section are:

  1. Accessory case
    1. The case has machined mounting pads for engine-driven accessories.
    2. The accessory case may be designed to act as an oil reservoir. If an oil tank is utilized, a sump is usually provided below the front bearing support for the drainage and scavenging of oil used to lubricate bearings and drive gears. The accessory case is also equipped with adequate tubing or cored passages for spraying, lubricating oil on the gear train, and support bearings.
  2. Gear train
    1. The gear train is housed within the accessory case.
    2. The engine’s high-pressure compressor drives the gear train through an accessory drive shaft (tower shaft) gear coupling, which combines with gearbox gear and the high-pressure compressor. The reduction gearing within the case provides suitable drive speeds for each engine accessory or component.
    3. The accessory reduction gear ratios are relatively high because the rotor operating rpm is so high. The accessory drives are supported by ball bearings assembled in the mounting pad bores of the accessory case.


Auxiliary Power Units (APU) are devices that supply essential electrical power, pneumatics, or hydraulics to a vehicle when the regular operational sources are not available. They are being used on aircrafts, boats, trucks, and trains, and they can be powered by diesel, electricity, or a turbine. The aviation industry is by far the most prevalent application of the auxiliary power unit.

It is a small gas turbine engine mounted in the tail cone of an aircraft to provide autonomous electrical and mechanical power for the following:

  • Starting power for the main engines.
  • Pneumatic power for cabin air conditioning systems.
  • Shaft power for other pneumatic and hydraulic systems.
  • Backup electrical and pneumatic power for in-flight operations and emergencies.
  • Electric and pneumatic power for ground operations with the engines shut down.

APUs supply electric power, compressed air, and hydraulic pressure to the aircraft systems on most commercial turboprop and jet engine aircraft. These are required supply important resources when the aircraft is undergoing repair or being prepped for a flight before the main engines are activated.

On commercial aircraft, generators and pumps that run through auxiliary gearboxes on the main engines are the primary source of in-flight electric power, pneumatic and hydraulic pressure, and air conditioning. The main engines are rarely powered during ground operations, despite the fact that power to the aircraft systems is often needed for testing, maintenance, pre-flight cabin and flight deck preparation, and air conditioning.

A small jet engine is typically located in the tail section of the fuselage behind the rear cabin bulkhead as the aircraft’s auxiliary power unit. This engine powers an electric generator as well as pneumatic and hydraulic pumps that provide sufficient power, air, and oil pressure to run the aircraft’s key systems. These engines are self-contained electric start engines that draw fuel from the aircraft’s main fuel tanks.


The three main sections of a typical gas-turbine APU for commercial transport aircraft are:

  • Power Section

– This section is the engine’s gas-generator section, which generates all of the APU’s shaft power. Air and gasoline are combined, compressed, and ignited in this portion of the engine to create hot, expanding gases. This gas is extremely energetic, and it is utilized to spin the turbine, which then powers other engine components including auxiliary gearboxes, pumps, electrical generators, and, in the case of a turbo fan engine, the main fan.

  • Load Compressor Section

– The load compressor is typically a shaft-mounted compressor that delivers pneumatic power to the aircraft, while some APUs draw bleed air from the power section compressor. The input guiding vanes, which regulate airflow to the load compressor, and the surge control valve, which keeps the turbo machine steady or surge-free, are both actuated devices that help control the flow of air.

  • Gearbox Section

– For electrical power, the gearbox transfers power from the engine’s primary shaft to an oil-cooled generator. Power is also supplied to engine accessories such as the fuel control unit, lubrication module, and cooling fan through the gearbox. A starter motor is also linked to the gear train to conduct the APU’s starting function. To simplify complexity, some APU designs use a combined starter/generator for APU startup and electrical power generation.

One disadvantage of APUs is the noise they make when operating on the ground. When the bleed air is turned on to heat or cool the cabin, the noise of the APU’s turbine engine might grow greater in some older aircraft. APUs are still restricted at a number of airports around the world, particularly at night, to avoid bothering residents in neighboring communities.

On the contrary, APU’s greatest advantage is that it uses much lesser fuel than a standard engine and can be shut down during flight, reducing the amount of time they are in use and requiring less maintenance. 


(n.d.). ACCESSORY SECTION. Retrieved April 11, 2022, from

(n.d.). Gear Boxes & Accessory Drives. Retrieved April 11, 2022, from–accessory-drives.html

Scott, P. (2022, April 11). What is an Auxiliary Power Unit? Wiki Motors.

Sforza, P. (2014). Commercial Airplane Design Principles. Science Direct.

Collantes, C. (2022, March 15). What is the Purpose of an Auxiliary Power Unit? Aeroclass.Org.

Mark, R. (2021b, November 9). How an Auxiliary Power Unit Works. FLYING Magazine.

Wikipedia contributors. (2022, March 25). Auxiliary Power Unit. Wikipedia.

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