Engine offtakes – drive for accessory units

[Ref AircraftSystems: Mechanical,electrical,andavionicssubsystemsintegration,Third Edition ISBN: 978-0-470-05996-8]

Engine’s primary function is to power the aircraft (either by providing thrust or power), there are various engine offtakes from accessory gearbox

In turbine engine, accessory gearbox – that enables accessories to be connected to the engine high pressure shaft for various applications:

  1. power engine-driven generators to provide electricity
  2. power engine-driven hydraulic pumps to provide hydraulic power
  3. provide engine bleed air for : 1) cabin and equipment conditioning system ; 2) anti-icing and de-icing system
  4. provide pneumatic power
  5. Oil breather

[Ref ISBN 0 902121 2 35]

A rotating engine shaft, via distinguish types of gearboxes which provides a mount for accessory units

Accessory units provide the power for some of the above discussed system, such as hydraulic, pneumatic and electrical pumps and systems.

Taking multi-shaft engines as an instance, in practice the high pressure system should rotate in order to generate airflow through the engine and the high pressure compressor shaft is therefore coupled to the internal gearbox

bevel gear

Internal gearbox – design

  • To minimize unwanted movement between compressor shaft bevel gear and radial driveshaft bevel gear, there are three basic methods.
  • To spread the load of driving accessory units, some engines take a second drive from the slower rotating low pressure shaft to a second external gearbox – the advantage is that accessory units divided into two groups: 1) high pressure system and 2) low pressure system


Radial driveshaft is design to transmit the drive from the internal gearbox directly, sometimes via intermediate gearbox, to an accessory unit or the external gearbox. 

To minimize disrupting the airflow, the radial driveshaft is housed within the compressor support structure. Also, the driveshaft diameter is as smallest as possible – the physics tells “the smallest the diameter, the faster the shaft”

Although the radial driveshaft provides the same power as the compressor shaft, it raises the internal stress and gives result in vibration

Intermediate gearbox

Intermediate gearbox is used because in some arrangement that the radial driveshaft is not possible to directly align with external gearbox.

External gearbox

To reduce drag, it should avoid from presenting frontal area to the airflow. In general, it is wrapping around the engine like a banana in shape. Also, for maintenance purposes, the gearbox is usually located on the underside of the engine to allow ground crew to gain access

In addition, a design called ‘shear-neck’ is deployed to prevent secondary failure occurring in event of most of the accessory unit fails. Albeit, the “protection” is not including certain primary engine accessory units – oil pump for example – because these units are vital to the running of the engine and any failure would necessitate immediate shutdown of the engine


To reduce weight is important, therefore:

Internal gearbox casing – aluminum (casting by this material because somehow the location is more near engine hot area)

External gearbox casing – magnesium (it could be cast by even lighter materials)



Leave a Reply

Your email address will not be published. Required fields are marked *