05/30/16
Overview
The bypass section is a space around the engine which takes the bypass air from the inlet to the rear of the engine and out into the atmosphere. The air flows between the core engine fairings and the bypass duct. The bypass duct is a structural engine component which makes a smooth annular duct for the bypass airflow. Fairings installed on the core engine also help to make the airflow smooth. The inner surface of the bypass section is the core engine fairings and the outer surface is the bypass duct.
A tray that is resistant to fire, on the top of the bypass duct, holds the engine electronic control (EEC). Panels on the bypass duct give access to components on the core engine.
05/30/16
Bypass Duct
The bypass duct is a structural engine component which has the shape of a cone that is open at the two ends. The bypass duct is installed between the intermediate case and the thrust reverser/exhaust unit. At the bottom of the bypass duct, there is a bypass services fairing with a sealing plate. Tubes, hoses (air, oil and fuel) and electrical wires go through the bypass services fairing which also divides the airflow around them.
The bypass duct is made of composite material honeycomb and skin sections with holes bonded together. At the front end (smaller diameter), it attaches to the intermediate case and at the rear end to the rear mount ring. It has six large rectangular openings, three on each side of the engine which give access to most of the components that are on the core and three smaller square openings which give access to the inter-turbine temperature (ITT) thermocouples. All nine openings are made stronger at the edges with laminates and C-section stiffeners and are sealed with access panels. Near the horizontal center line at the front, there are two small openings which give access to borescope bosses. These also are sealed with panels.
At the 6 o'clock position, the duct has an opening for the bypass services fairing and the services sealing plate. One of the functions of the bypass services fairing is to divide the airflow around the services from the core engine. Four brackets installed between its walls divide it into five compartments. These contain the fuel, oil and air tubes, the electrical harnesses and the air ducts. The tubes and harnesses are held in piston seal housings which are in the services sealing plate (to prevent the leakage of bypass air where the service tube and harness go through). Holes in the plate align with the holes in the fairing flange and the two are installed with bolts.
On the outer surface of the bypass duct are some of the fuel and oil system components. The fuel-cooled oil cooler (FCOC) is on the left side. The main generators oil-cooler and the fuel flow meter are on the right side. The starter air duct and the anti-icing duct are along the lower right side of the engine. These ducts attach to support brackets installed on the bypass duct. There are also many clamp points to hold the tubes and harnesses to the outer surface of the bypass duct.
Bypass Duct Access Panels
The structural bypass duct (SBPD) is equipped with ten (10) access panels which enable access to the core engine mounted line replacement units (LRUs). The larger ones are installed with ten fasteners and the smaller ones with eight.
The SBPD engine access panels allow access (after the removal of the appropriate core engine fairing), to various core mounted components, such as the VSV actuator, buffer air valve, bleed valve solenoid unit, overspeed and splitter unit, turbine overheat detector, fuel spray nozzles, igniters, ITT thermocouples, T30 thermocouples, borescope plugs etc.
The following points should be noted:
- The fixed cowl prevents access to the inboard forward center access panel on each engine
- If the forward lower right access panel is to be removed, the HP5 bleed air duct should be removed first
- If the rear lower right access panel is to be removed, the rear starter air duct should be removed first
The access panels give access to the core components that follow:
| ACCESS PANEL | COMPONENT |
|---|---|
| Forward top left (451AL/461AL) | Stage-8 bleed valve |
| Forward center left (451CL/461CL) | Compressor borescope bosses and T30 thermocouple |
| Forward bottom left (451DL/461DL) | Variable stator vane (VSV) actuator, buffer air valve and igniter borescope access |
| Forward top right (452JR/462JR) | Stage-5 bleed valve |
| Forward center right (452HR/462HR) | Compressor borescope bosses and T30 thermocouple |
| Forward bottom right (452FR/462FR) | Bleed valve solenoid control block, fuel-manifold splitter unit and igniter borescope access |
| Rear top left (451BL/461BL) | Inter-turbine temperature (ITT) thermocouples |
| Rear top right (452KR/462KR) | ITT thermocouples |
| Rear bottom right (452GR/462GR) | ITT thermocouples and turbine overheat thermocouple |
| Rear bottom left (451EL/461EL) | Permits turbine borescope inspection |
