The accessory section of a gas turbine engine performs several essential functions. The primary function is to provide space for the mounting of accessories necessary for operation and control of the engine. Generally, it also includes accessories concerned with the aircraft, such as electric generators and hydraulic pumps. Secondary functions include acting as an oil reservoir and/or oil sump and housing the accessory drive gears and reduction gears.
The arrangement and driving of accessories has always been a major problem on gas turbine engines. On most turbofan engines, driven accessories are mounted on the accessory gearbox (AGB), typically located on the lower portion of the engine. The location of the accessory gear box varies somewhat, but most turboprops and turboshafts have the 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.
The basic elements of the accessory section are:
- The accessory case, which has machined mounting pads for the engine-driven accessories, and
- The gear train, which is housed within the accessory case.
When a separate oil tank is used, a sump is typically provided below the front bearing support to collect and scavenge lubricating oil from the bearings and drive gears. The accessory case is also provided with adequate tubing or cored passages for spraying, lubricating oil on the gear train and supporting bearings.
The gear train is driven by the engine high-pressure compressor through an accessory drive shaft (tower shaft) gear coupling, which splines with a gear box gear and the high-pressure compressor. The reduction gearing within the case provides suitable drive speeds for each engine accessory or component. Because the high-pressure rotor operates at very high rpm, significant reduction gearing is required to provide suitable operating speeds for engine-driven accessories. [Figure]
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| Typical turboprop accessory case |
⚙️ Key Takeaways
- Primary Purpose Provides mounting locations and mechanical drive for engine- and aircraft-related accessories.
- Drive Train Typically driven by the high-pressure (N2) rotor system through a tower shaft and bevel gears.
- Speed Reduction Internal reduction gearing lowers engine core RPM to suitable operating speeds for accessories.
- Oil System Role The AGB case often functions as an oil sump/reservoir and houses oil pressure and scavenge pumps.