Accessory drive

Last updated
Klimov TV3-117 turboshaft engine. The accessory drive is the large casting on the top. Klimow TW-3-117.jpg
Klimov TV3-117 turboshaft engine. The accessory drive is the large casting on the top.

The accessory drive is a gearbox that forms part of a gas turbine engine. [1] Although not part of the engine's core, it drives the accessories, fuel pumps, etc., that are otherwise essential for the operation of the engine or the aircraft on which it is mounted. Accessory drives on large engines handle between 400500 hp. [2]



Turbomeca Adour, with the accessory drive mounted beneath. The accessories are removed and their mounting flanges covered by bright red blanking plugs. Rolls-Royce Turbomeca Adour Mk811 at HAL Museum 7889.JPG
Turbomeca Adour, with the accessory drive mounted beneath. The accessories are removed and their mounting flanges covered by bright red blanking plugs.

Power for the accessory drive is taken from the central shaft linking the turbine and compressor sections of the engine. This requires an internal gearbox that couples the drive to a radial driveshaft [3] or towershaft [4] that drives an external gearbox.

Internal gearbox

Internal gearbox of the Rolls-Royce Pegasus Rolls-Royce Pegasus - 04.JPG
Internal gearbox of the Rolls-Royce Pegasus

The design of the internal gearbox is complicated by the heat and small space available in which to connect the driveshaft. It is usually placed between the compressor outlet and the combustor. In turboprops or designs with centrifugal compressors, it may be placed ahead of the compressor.

For two-shaft designs, an accessory drive will be taken from the high-pressure shaft, [4] i.e. the outer and shorter of the two concentric shafts. This shaft comes up to speed more quickly when the engine is started. The drive and accessory gearboxes may also be split in two, one driven from each engine shaft, so as to distribute their loads. The engine-critical systems, including the starter drive, are arranged on the high-pressure shaft, with aircraft systems on the low-pressure shaft. [3] The high-pressure shaft also rotates faster than the low-pressure shaft, which may influence the distribution of accessories.

To allow for thermal expansion, the drive from the main shaft may be taken by one of three means: [3]

Radial driveshaft

de Havilland Goblin
Separate accessory drives are taken from the nose cone, above and below the engine core. Rolls Royce Goblin II cutaway.jpg
de Havilland Goblin
Separate accessory drives are taken from the nose cone, above and below the engine core.

To make the best use of the limited space for the driveshaft and internal gearbox, the driveshaft runs at high speed, thus allowing it to be of small diameter. [3] This reduces the disruption to the airflow and the size of the hollow fairing that encloses it.

If it is not possible to arrange a single straight path for the driveshaft, it may be arranged in two sections and linked by an intermediate gearbox. [6] This is most commonly required for high-bypass turbofans with large diameter fans.

External gearbox

Sectioned accessory drive on top of a Rolls-Royce Pegasus Rolls-Royce Pegasus - 05.JPG
Sectioned accessory drive on top of a Rolls-Royce Pegasus

The packaging of an engine within its nacelle is a complicated task. The accessory drive is usually arranged as a curved casing, so that the various accessories are mounted close to the engine. The casing is a pair of light alloy castings. Separate machined mounting pads are provided for each accessory.

The drive within the casing is provided by a train of spur gears. Accessories are arranged on both sides of the driveshaft entry, in reducing order of their speed. The gears are usually plain spur gears, running in roller bearings. Idler gears are commonly used between them, to increase the spacing between accessories. Helical gears are sometimes used for the high-torque drives, typically the starter, as these give smoother running. However helical gears also generate an end-thrust, which then requires a more complicated thrust bearing to support them. [7]

The complexity of an accessory drive and its gears is so great that they were used as a theme by the anthropomorphic illustrator Boris Artzybasheff in advertising for the Avco Lycoming company, who were making drive gearboxes for the Westinghouse J40 engine. [8]

Bleed air

In some engines, bleed air is also tapped to provide power for accessories, as well as a mechanical shaft drive. [5] Bleed air is particularly useful when a source of compressed air is specifically needed, either to pressurise cabin air, or as a supply of cooling air to other components (to avoid excess heat, this is taken for a low-pressure tapping, or from the LP compressor of a two-shaft engine). One important use for bleed air is for cross-starting of other engines in a multi-engine aircraft. [9]


Some of the accessories that may be driven include: [10]

There may be a number of fuel pumps: low pressure, high pressure, afterburner pump and also a speed-sensitive governor

Additional facilities are provided for a centrifugal oil breather, to separate the drive lubricating oil from the overboard breather air vent. Also access for hand-turning the engine, during ground maintenance.

Related Research Articles

Turboprop turbine engine driving an aircraft propeller

A turboprop engine is a turbine engine that drives an aircraft propeller.

Turbofan Airbreathing jet engine designed to provide thrust by driving a fan

The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the turbo portion refers to a gas turbine engine which achieves mechanical energy from combustion, and the fan, a ducted fan that uses the mechanical energy from the gas turbine to accelerate air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.

Turbojet Airbreathing jet engine, typically used in aircraft

The turbojet is an airbreathing jet engine, typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor, a combustion chamber, and a turbine. The compressed air from the compressor is heated by burning fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s.

Rolls-Royce Trent Family of turbofan aircraft engines

The Rolls-Royce Trent is a family of high-bypass turbofans produced by Rolls-Royce. It continues the three spool architecture of the RB211 with a maximum thrust ranging from 61,900 to 97,000 lbf . Launched as the RB-211-524L in June 1988, the prototype first ran in August 1990. Its first variant is the Trent 700 introduced on the Airbus A330 in March 1995, then the Trent 800 for the Boeing 777 (1996), the Trent 500 for the A340 (2002), the Trent 900 for the A380 (2007), the Trent 1000 for the Boeing 787 (2011), the Trent XWB for the A350 (2015), and the Trent 7000 for the A330neo (2018). It has also marine and industrial variants.

Bypass ratio

The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core.

Rolls-Royce Conway 1950s British turbofan aircraft engine family

The Rolls-Royce RB.80 Conway was the first turbofan engine to enter service. Development started at Rolls-Royce in the 1940s, but the design was used only briefly, in the late 1950s and early 1960s, before other turbofan designs replaced it. However, the Conway engine was used in versions of the Handley Page Victor, Vickers VC10, Boeing 707-420 and Douglas DC-8-40. The name "Conway" is the English spelling of the River Conwy, in Wales, in keeping with Rolls' use of river names for gas turbine engines.

Rolls-Royce Clyde 1940s British turboprop aircraft engine

The Rolls-Royce RB.39 Clyde was Rolls-Royce's first purpose-designed turboprop engine and the first turboprop engine to pass its civil and military type-tests.

Rolls-Royce Griffon 1930s British piston aircraft engine

The Rolls-Royce Griffon is a British 37-litre capacity, 60-degree V-12, liquid-cooled aero engine designed and built by Rolls-Royce Limited. In keeping with company convention, the Griffon was named after a bird of prey, in this case the griffon vulture.

Rolls-Royce Trent 900 2000s British turbofan aircraft engine

The Rolls-Royce Trent 900 is a high-bypass turbofan produced by Rolls-Royce plc to power the Airbus A380, competing with the Engine Alliance GP7000. Initially proposed for the Boeing 747-500/600X in July 1996, this first application was later abandoned but it was offered for the A3XX, launched as the A380 in December 2000. It first ran on 18 March 2003, made its maiden flight on 17 May 2004 on an A340 testbed, and was certified by the EASA on 29 October 2004. Producing up to 374 kN (84,000 lbf), the Trent 900 has the three shaft architecture of the Rolls-Royce Trent family with a 2.95 m (116 in) fan. It has a 8.5-8.7:1 bypass ratio and a 37–39:1 overall pressure ratio.

A compressor map is a chart which shows the performance of a turbomachinery compressor. This type of compressor is used in gas turbine engines, for supercharging reciprocating engines and for industrial processes, where it is known as a dynamic compressor. A map is created from compressor rig test results or predicted by a special computer program. Alternatively the map of a similar compressor can be suitably scaled. This article is an overview of compressor maps and their different applications and also has detailed explanations of maps for a fan and intermediate and high-pressure compressors from a three-shaft aero-engine as specific examples.

Constant speed drive

A constant speed drive (CSD) is a type of transmission that takes an input shaft rotating at a wide range of speeds, delivering this power to an output shaft that rotates at a constant speed, despite the varying input. They are used to drive mechanisms, typically electrical generators, that require a constant input speed.

General Electric CJ805

The General Electric CJ805 is a jet engine which was developed by GE Aviation in the late 1950s. It was a civilian version of the J79 and differed only in detail. It was developed in two versions. The basic CJ805-3 was a turbojet and powered the Convair 880, while CJ805-23, a turbofan derivative, powered the Convair 990 airliners.

Bristol Siddeley Nimbus 1950s British turboshaft aircraft engine

The Bristol Siddeley Nimbus, later known as the Rolls-Royce Nimbus, was a British turboshaft engine developed under license by Blackburn Aircraft Ltd. from the Turbomeca Turmo in the late 1950s. It was used on the Westland Scout and Westland Wasp helicopters.

Allison Model 250 Turboshaft aircraft engine

The Allison Model 250, now known as the Rolls-Royce M250, is a highly successful turboshaft engine family, originally developed by the Allison Engine Company in the early 1960s. The Model 250 has been produced by Rolls-Royce since it acquired Allison in 1995.

Geared turbofan Turbofan engine with a gearbox used to drive its fan

The geared turbofan is a type of turbofan aircraft engine, with a gearbox between the fan and the low pressure shaft to spin each at optimum angular velocities.

Aircraft systems Overview article of aircraft systems

Aircraft systems are those required to operate an aircraft efficiently and safely, their complexity varies with the type of aircraft.

General Electric T31

The General Electric T31 was the first turboprop engine designed and built in the United States.

Components of jet engines Brief description of components needed for jet engines

This article briefly describes the components and systems found in jet engines.

Aircraft engine starting

Many variations of aircraft engine starting have been used since the Wright brothers made their first powered flight in 1903. The methods used have been designed for weight saving, simplicity of operation and reliability. Early piston engines were started by hand, with geared hand starting, electrical and cartridge-operated systems for larger engines being developed between the wars.

Free-turbine turboshaft

A free-turbine turboshaft is a form of turboshaft or turboprop gas turbine engine where the power is extracted from the exhaust stream of a gas turbine by an independent turbine, downstream of the gas turbine and is not connected to the gas turbine. This is opposed to the power being extracted from the power spool via a gear box.


  1. "Engine Vocabulary: A". GE Aviation. Archived from the original on 2012-02-21.CS1 maint: discouraged parameter (link)
  2. Rolls-Royce, Jet Engine, p. 65.
  3. 1 2 3 4 Rolls-Royce, Jet Engine, p. 67.
  4. 1 2 Pratt & Whitney, Aircraft Gas Turbine Engine , pp. 3–45
  5. 1 2 "Accessory Systems". Introduction to Aircraft Power Plants. Archived from the original on 2015-03-26.CS1 maint: discouraged parameter (link)
  6. Rolls-Royce, Jet Engine, p. 69.
  7. Rolls-Royce, Jet Engine, p. 70.
  8. "How a jet engine runs on its "nerves"". Feb 1953.
  9. Rolls-Royce, Jet Engine, p. 124, 126.
  10. Rolls-Royce, Jet Engine , pp. 70–71