If the pressure exceeds a certain limit, a pressure relief door  opens and relieves the pressure. Zone 2 is cooled by fan air  from the upper part of the fan duct and is exhausted at the  lower end back into the fan air stream. This area has both  fuel and oil lines, so removing any unwanted vapors would  be important. flowing around it. The engine exterior and the engine nacelle  are cooled by passing fan air around the engine and the  nacelle. The engine compartment frequently is divided into  two sections. The forward section is referred to as the cold  section and the aft section (turbine) is referred to as the hot  section. Case drains drain almost potential leaks overboard  to prevent fluids from building up in the nacelle. Zone 3 is the area around the high-pressure compressor to  the turbine cases. This zone also contains fuel and oil lines  and other accessories. Air enters from the exhaust of the  precooler and other areas and is exhausted from the zone  through the aft edge of the thrust reverser inner wall and the  turbine exhaust sleeve. Turbine powerplants can be divided into primary zones  that are isolated from each other by fireproof bulkheads  and seals. The zones are the fan case compartment,  intermediate compressor case compartment, and the core  engine compartment.  [Figure 6-61]  Calibrated airflows are  supplied to the zones to keep the temperatures around the  engine at levels that are acceptable. The airflow provides for  proper ventilation to prevent a buildup of any harmful vapors.  Zone 1, for example, is around the fan case that contains  the accessory case and the electronic engine control (EEC).  This area is vented by using ram air through an inlet in the  nose cowl and is exhausted through a louvered vent in the  right fan cowling. To reduce the temperature of the structure in the vicinity  of the exhaust duct or thrust augmentor (afterburner) and  to eliminate the possibility of fuel or oil coming in contact  with the hot parts of the engine, it is sometimes necessary  to provide insulation on the exhaust duct of gas turbine  engines. The exhaust duct surface temperature runs quite  high. A typical insulation blanket and the temperatures  obtained at various locations are shown in  Figure 6-62 . This  6-40 Figure 6-61.  Accessory zone cooling.  Turbine Engine Insulation Blankets Accessory Zone Cooling Turbine case cooling Zone 2 drainage pipe Zone 1 ventilation exit Ram air Zone 2 ventilation exit Zone 1 pressure relief door Precooler Fancase compartment (zone 1) Thrust reverser seal leaks (zone 3 inlet) Zone 1 ventilation inlet Zone 2 ventilation inlet IP compressor compartment (zone 2)