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 rotor operating rpm is so high,  the accessory reduction gear ratios are relatively high. The  accessory drives are supported by ball bearings assembled in  the mounting pad bores of the accessory case.  [Figure 1-45] The compressor section of the gas turbine engine has many  functions. Its primary function is to supply air in sufficient  quantity to satisfy the requirements of the combustion  burners. Specifically, to fulfill its purpose, the compressor  must increase the pressure of the mass of air received from  the air inlet duct, and then discharge it to the burners in the  quantity and at the pressures required. A secondary function of the compressor is to supply bleed- air for various purposes in the engine and aircraft. The  bleed-air is taken from any of the various pressure stages of  the compressor. The exact location of the bleed ports is, of  course, dependent on the pressure or temperature required  for a particular job. The ports are small openings in the  compressor case adjacent to the particular stage from which  the air is to be bled; thus, varying degrees of pressure are  available simply by tapping into the appropriate stage. Air  is often bled from the final or highest pressure stage since,  at this point, pressure and air temperature are at a maximum.  At times it may be necessary to cool this high-pressure air. If  it is used for cabin pressurization or other purposes to which  excess heat would be uncomfortable or detrimental, the air is  sent through an air conditioning unit before it enters the cabin. 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. Driven accessories  on turbofans are usually mounted on the accessory gear box,  which is on the bottom 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 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  provided with adequate tubing or cored passages for spraying  lubricating oil on the gear train and supporting bearings. Figure 1-45. Typical turboprop accessory case. 1. The accessory case, which has machined mounting  pads for the engine-driven accessories, and  2. The gear train, which is housed within the accessory  case.  1-39 Figure 1-44. Typical turbofan inlet. Compressor Section