Rigging the PW2000
Introduced in 1983 for the Boeing 757, the PW2000 is a two-spool, 37,000- to 43,000-pound engine, which has been also applied to military C-32 (757 military equivalent) and C-17 aircraft, as well as the Ilyushin wide-body IL-96M and T passenger and cargo aircraft. The PW2000 is adaptable to any 757 model of aircraft.
Pratt has made a few changes to the PW2000 over the years with a Reduced Temperature Configuration (RTC) offering in 1994, and the PW2043 43,000-pound-of-thrust version, most recently. The RTC version offers improved reliability and durability for long on-wing times and the PW2043 offers the ability to be used on 757 models from the -200 to the -300.
As with any modern jet engine, there is going to be the need to perform some rigging of engine bleeds and variable vane and stator controls.
These tasks may appear difficult, especially if mechanics don't understand the principles of rigging, or don't understand what it is they're trying to accomplish. One mistake can mean that the engine will run poorly or inefficiently. Careful planning and follow-through of all rigging procedures can produce positive results with minimal headaches.
Rigging is required infrequently, but is needed when actuators, brackets, linkage, or other components are changed or damaged or when engine performance problems are noted.
On a PW2000 engine, there are only three areas where rigging is required, explains Norm Jeche, Technical Instructor at Pratt & Whitney's East Hartford Customer Training facility, the Stator Vane Actuator (SVA) assembly, P2.5 bleed ring, and the TCA bleed valves.
VSGVs and VIGVs
The PW2000 engine uses both variable stator guide vanes (VSGVs) and variable inlet guide vanes (VIGVs) to help improve the efficiency of the high compressor. It does this by regulating the volume of air flowing through the engine. According to Jeche, of all the rigging required on this engine, the rigging procedure for these vanes is probably the most critical for satisfactory engine performance.
"Unfortunately this procedure is all to often misunderstood and not followed correctly," Jeche says. "Most of the rigging errors are a result of lack of understanding, using the wrong rigging pins, or using the pins in an incorrect manner."
He continues, "Some mechanics try to line up the rigging holes by sight, or substitute an improper tool for performing the alignment of the rig and check pins. Stator guide vane positioning is very critical and can affect engine performance by being only a few thousandths off. Even using damaged, bent, or worn rig pins can result in rigging errors. The tooling or rig pins used to perform the procedure need to be in good condition in order to conduct the procedure properly."
The first step in performing this procedure is to make sure you have the correct rigging pins and that they are in good condition. "If the rigging pins show signs of being bent or have ridges, scratches, or other damage," Jeche adds, "replace them with new pins immediately."
The stator vane actuator (SVA) is controlled by a torque motor located in the fuel control, which is in turn controlled by the electronic engine control (EEC). The torque motor basically sends fuel to either side of a hydraulic ram in the stator vane actuator system, which in turn moves the unison rings, which then changes the angle of the SGVs and VIGVs.
In order to perform any rigging, you must be able to manipulate the actuator and the bellcrank assembly. There are two methods of actuating the bellcrank. One is to use a hydraulic pump unit and apply hydraulic pressure to the actuator, and the other is to simply use a wrench on one of the flats of the bellcrank. If you use the wrench method, it's important to use a torque wrench and not to exceed 300 inch-pounds of force. Exceeding this value can damage the bellcrank or linkage. You also want to make sure the wrench is on the flat that is near the bellcrank center bearing. If you use other bellcrank surfaces to turn it, you may twist the actuator and damage may occur.
Remember also, not to actuate the bellcrank with the rig pins installed, otherwise the pins will bend and/or linkage, brackets, and mounts will bend or break.
To review, the objective is to first rig the actuator to the bellcrank by pinning the bellcrank to a pre-set position and then adjusting the rod end of the actuator so that it connects to the bellcrank while it is in its full open position. Once that is accomplished, the variable vanes are rigged to the bellcrank by pinning the vanes in a pre-set position on the front split compressor cases and adjusting the linkages connecting the unison rings to the pinned bellcrank.
There are really two different procedures related to stator vane actuator rigging. In the case of replacing or installing components, or if you find that the engine is misrigged, you will be performing the stator vane actuator rigging from scratch. Otherwise, you will simply be performing a check of the rigging. The important point here is that different pins are required for checking the rigging versus actually performing the rigging and you've got to use the correct pins.
In either case, before you begin, you need to first verify that full movement of the stator vanes correlates with full movement of the actuator. To do this, you need to move the actuator through its full stroke while observing the stator vane unison rings (these are the ring like assemblies that connect all the stator vanes together around the circumference of the engine).
With this verified, you will next move the bellcrank down towards close and then up towards open until the rigging pin in the bellcrank can be inserted to hold the bellcrank in a Vane-Open position. This up and down stroke is important because it removes any play (hysteresis) from the linkage assemblies. Next, verify that the actuator is in the full open position by disconnecting it from the bellcrank and extending it to its internal mechanical stop position.
Jeche says, "This can also be done by two methods, manually by using specialty PWA pliers that force the hydraulic actuator into the full open position or by the hydraulic pump method. In fact, many technicians don't use the hydraulic cart while they are on the line because the pliers are much simpler."
When this is accomplished, you can rig the actuator to the bellcrank. Next, insert the rig pins on each stage of vanes through the "unison" rings and verify they line up with the rigging holes in the split case flange.
When you verify that all of the rigging pins are in and are in a fairly neutral position (not binding), you have confirmed the rigging.
Jeche adds, "If you have to tap, hammer, or otherwise force the pins into position, they are not properly adjusted. Also, if you have any damaged pins that are grooved or "chewed up," you should throw the pins away. You might be able to get damaged pins into the holes, but you won't be able to tell if you're in a neutral position. The rig "check" pins for the stator vane acuator allow for up to .005 inches error in either direction. These pins can be identified by a small step in the pin where they are machined to provide the .005 play in either direction. If you're rigging the stators, however, the "rig" pins are machined to precisely fit into the rigging holes. Use the appropriate pins for the operation you're performing and verify the part number of the pins.
"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.
Rigging the PW2000
"A harness problem," he says, "typically shows up as an intermittent indication. If you have a hard fault or one which is always there, there is less of a chance that the problem is harness related."
Clements continues, "The Boeing recommended approach to component fault messages is to write down, or record your fault message from the EPCS, check the source of the message, perform an erase procedure to the engine, and then apply alternate EEC power to see if you have duplicated the problem. If the fault is still there, it is considered a "hard" fault, which is typically inside of the component in question. If it is not a "hard" fault and the item didn't come back, it is considered an "intermittent" fault and you need to take the time to go through the engine and move harnesses around, re-installing plugs, etc. to see if you can duplicate the problem. If you can't, it may have just been a nuisance fault in the computer system — a high-tech hiccup."
"The EICAS system can sometimes behave like your laptop computer, where just re-booting it will make the error go away and never come back. But for safety's sake, it's important to investigate the potential source of any problem to be sure there is nothing wrong," he explains. "Mechanics should always consult the Boeing 757 Fault Isolation Manual (FIM) when troubleshooting component fault messages."
To rig the 2.5 actuator, you first need to verify that you are getting full travel of the 2.5 bleed valve with full travel of the actuator. To do this, you first remove the hydraulic lines from the actuator and stroke the actuator to remove any fluid. Then, move the actuator through its full travel by hand while observing the movement of the 2.5 valve.
Before adjusting the rigging of the 2.5 bleed, you first have to check for excessive play in the linkage. To do this, you insert a locally fabricated tool into the 2.5 bleed ring to hold it into the full open position. With this accomplished, you then move the actuator back and forth and make sure that you have less than .250 inch movement in the linkage. Anything over .250 of movement, and you have to tear down the assembly and linkage and rework it. You should check the maintenance manual for reduced interval inspection criteria in relation to linkage wear.
With this established, you can now check the rigging on the actuator. You do this by placing a depth micrometer onto the top surface of the LVDT and zeroing it out. Next, you move your actuator all the way to the forward position. You then move your depth micrometer to the new location and it should read between 2.6 and 2.66 inches. If you don't get this reading, the actuator may be unserviceable. In this case the actuator must be removed and replaced. Once the travel is within these dimensions, proceed to rig the actuator to the bleed linkage according to the manual.
The objective of the rigging is to make adjustments so that when the actuator is in the EEC commanded closed position, the bleed ring is fully closed. To rig it properly, the manual describes a procedure whereby you apply a load of 75 lbf to the rod end. This pressure can be held on the rod end with a special PWA fixture or other acceptable means. With the actuator rod end loaded to 75 lbf, the rigging pin is inserted, and the linkage adjuster is backed out until it bumps against the rig pin. You then can move the adjuster up to 1/2 turn forward so it lines up with the clevis bolt. When you are done performing the rigging procedure, you were instructed in the original procedure to check, using hydraulic pressure on the actuator, the stroke of the actuator and measure at the LVDT to make sure that you are still getting the 2.6 to 2.66 dimension.
Jeche explains, "One problem we have experienced in the field is that if mechanics don't get this 2.6 to 2.66 measurement after connecting the linkage, they are increasing hydraulic pressure above that specified in the manual until they get the measurement they desire. Many times they bend linkage or damage the actuator by doing this. Unfortunately, some of the airlines were writing into their job card's instructions for increasing pressure from the hydraulic cart until they achieve full travel. This is incorrect and can damage your equipment."
"You don't need to get full travel with the linkage connected," he continues. "Some mechanics are even backing off the adjuster a bit after the rigging was performed just to achieve the correct measurement. This is also wrong because the bleed ring doesn't go to the full closed position and you will unrig the system by doing this. Pratt has removed this step from the procedure, but many technicians who have been doing this for years are not reading the latest manual revision close enough, or they have not adjusted their job cards at the airlines to reflect the change in procedure."
"The thing you've got to understand," explains Jeche, "is that once the linkage is connected, make sure the bleed is closed all the way when the actuator is in the EEC commanded closed position. Being off a few thousandths in the full open position really has no effect on engine operation."
Pratt has since revised this procedure to eliminate this final check of LVDT travel. "Pratt & Whitney is at all times reviewing ways to improve the rigging procedures and increasing the reliability of their engines," says Jeche.
Rigging the TCA (Turbine Cooling Air) system
Rigging of the modulating TCA system is straightforward. The TCA system routes cooling air from the high compressor 14th stage to the high pressure turbine 2nd stage vanes for cooling. Engines incorporating the modulating TCA system will be noted by having two valves in the 11 o'clock and 4 o'clock TCA tube assemblies, an actuator and necessary linkage and plumbing. Precise rigging in this area is not as sensitive to error as the 2.5 bleed or the SVA/VIGV rigging, but it is important nonetheless. Misrigging of this system could result in 2nd stage turbine vane failure associated with possible engine deterioration.
The modulated TCA rigging procedure requires disconnecting two adjusters and inserting two rig pins to be placed on either side of the engine — one on the upper left valve, the other on the lower right valve. With the pins installed, the push rods (adjusters) are adjusted so that the system is connected together evenly. The pins are then removed, and you're back in business.
The key to performing any rigging on this engine is to have a good understanding of what it is you're trying to accomplish. Properly armed with knowledge of how the system works, you can apply common sense to insure that you rig the engine the proper way every time.