"One of the biggest problems related to rigging or performing a rigging check," says Jeche, "is that technicians don't really know when the pins are bottoming out in the rigging holes. Unfortunately, the Ôfeel' of the pins entering the rig holes is the same whether you hit the side of the rigging hole or hit the bottom. Hitting the side means that your not in alignment. To verify that you're in the bottom of the rigging hole with any of the pins, you need to look on the side of pin with a flashlight, and visually verify that the pins are in the rigging hole. This is a particular problem with the front unison ring on the variable inlet guide vanes (VIGV's). This pin is difficult to visually verify — you have to pull off the ignition exciter box in order to get a good look at this pin."
Follow procedures in the maintenance manual to complete your adjustments and make all necessary safety wiring.
P2.5 bleed rigging
The P2.5 bleed valve is a ring assembly positioned on the fan case assembly at the discharge end of the low pressure compressor. Its purpose is to open and close as needed for engine start and performance adjustments. Improper operation or adjustment of this bleed band will result in poor performance and poor starting characteristics. When the engine is shut down, the 2.5 bleed should be found in the open position.
The bleed valve is actuated by a hydraulic actuator at the 7 o'clock position, which is controlled by the Electronic Engine Control (EEC).
In order for the EEC to properly control this 2.5 bleed actuator, it must receive feedback telling it the exact position the actuator is in at all times. This is done with the LVDTs (Linear Voltage Directional Transducers).
There are actually two feedback channels on this "position sensor" (referred to as Primary and Secondary) for redundancy. There are several cases where you will need to check, and possibly adjust, the rigging on the 2.5 actuator. The first is if the actuator doesn't move the distance that is commanded due to damaged linkage or an actuator that is hanging up. When this happens, the Electronic Engine Control (EEC) will produce an error message to the aircraft and the mechanic will receive an error code on the EICAS (Engine Indicating and Crew Alert System). This can happen as a result of linkage wearing, or from bent or disconnected linkage, or from a problem with the actuator.
Another reason to check the P2.5 actuator and linkage is if you have unexplained performance problems. There have been cases where the linkage has become disconnected or the clevis pin sheared on the 2.5 bleed. In this case, you will have a situation where the actuator is moving, but the bleed ring is stationary. The result will be that everything appears to be working, yet the engine exhibits performance problems.
Jeche says, "Some people don't understand why you can have the P2.5 bleed disconnected from the actuator and not have it produce an error code. The reason being that the LVDTs are only measuring the movement of the actuator, not the bleed valve itself. So, you can have the actuator working perfectly, but if it isn't connected to the bleed valve, you will have performance problems."
"Customers have come to me with data that shows a really marginal low compressor that is just freshly overhauled," Jeche explains. "I tell them to check for a 2.5 bleed leak. They then ask how they would know this. The only way is to physically go to the engine and close the 2.5 bleed actuator to the EEC commanded maximum travel and then look in the air exit slots where the 2.5 bleed ring is and make sure that the bleed ring is in the closed position. You can see the bleed ring through the slots with a good flashlight and mirror, or by actually positioning your head inside the fan air discharge area and looking into the bleed holes. Most times they observe that the ring isn't closed all the way."
Keep in mind that errors in the P2.5 system are not always mechanical, the error codes can also be due to electrical faults.
Chris Clements, Line Maintenance & Troubleshooting representative for Pratt & Whitney says, "Many engine component problems including the 2.5 bleed actuator experienced out in the field originate from a damaged electrical harness. These harnesses, when damaged, have a potential to short and generate a fault. Incorrect installation and improper maintenance practices of the engine harnesses can cause harness damage. If the harness is pulled too tight, it may rub on a engine case and short out — sending messages to the EEC.
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