Mechanical The key to a successful mechanical rigging is a proper alignment. Remember, although the PLA actuator arm is mechanically linked to the MFC lever arm, the PLA is electrically driven. The slightest mechanical restriction (binding) may cause incorrect PLA movement during engine operation. PLA movement is most sensitive to a restriction when in either engine speed or torque, and/ or shaft torque limiting condition. If a possible restriction is suspected, advance and retard the PLA electrically and check for any hesitation or jerking during travel. If hesitation exists, there may be a mechanical restriction.

Electrical The normal process for this rigging will be checking dc voltages at idle and full throttle positions. However, the moment the throttle is moved out of idle, indicated torque will go to midrange and oscillate. For example, on the DD-963/ DDG-993 class ships, if mid-torque oscillations are accompanied by an overtorque indication and a PLA failure indication, then another problem exists.

Why? When the PLA is at idle, there is a PT5.4 bias that assures PT5.4 is greater than PT2 for engine start purposes. During PLA electrical rigging, the bias drops out when the throttle is advanced.

If PT5.4 is several tenths of a pound less than PT2, the torque computer goes berserk. But, PLA rigging may be continued by pressing the BATTLE OVERRIDE button.

However, suppose you have just been informed that the PT5.4 transducer requires calibration When the torque goes berserk as described, immediately dial up PT5.4 and PT2 on the respective DDIs. PT5.4 will be lower than PT2 , thus requiring the activation of BATTLE OVERRIDE to continue. This lower reading tells you the PT5.4 transducer requires calibration.

VSV FEEDBACK CABLE RIGGING Why is it accomplished? Why is it important? VSV feedback cable rigging is necessary because we are "timing" a pilot valve inside the MFC so the correct vane angle is obtained for a given CIT/ gas generator speed day. Actually, we are assuring that the pilot valve is timed to close off high pressure fuel flow to both the ROD END and the HEAD END pressure ports inside the MFC. These pressure ports direct the fuel flow to the vane actuators via tubing.

This timing is accomplished by pumping the vanes to the full open position and maintaining 100 to 200 psig on the system. At the same time, be sure you check to see if the bottom of the rig plates are parallel with each other. NEVER relax the pressure on the system while performing the rig plate check. If the pressure is relaxed, the feedback lever may drift and the rig will be incorrect.

When the system is full open, and 100 psig is applied, try to move the bolt back and forth on the forward end of the cable at the bellcrank. If the bolt cannot be moved, it most likely indicates a binding at the front section of the cable assembly. Of course, this "assumes" that the bolt is of the correct diameter and is normally free to move. If the bolt cannot be moved back and forth, reverse the pump selector handle to the RE (rod end) position. You then need to apply a few strokes to move the vanes slightly towards the closed position. If the bolt becomes free to move, the forward section of the cable assembly is definitely binding. Binding is easily connected by loosening the jamnut on the forward rod end bearing and rotating the rod end bearing a half turn at a time. This adjustment shortens the distance (right-hand rotation) and usually solves the problem. Reinstall and recheck the rigging. If the rigging is still slightly off, use the trimmer bracket adjustment to make the correction.

The bottom line

What are you really trying to accomplish? Look at it this way. The system is pumped full open, and the piston inside each actuator is physically bottomed out. This in turn fixes the position of the bellcrank bolt hole when a minimum of 100 psig is maintained. If you manually hold the feedback lever arm in order to match the bottom of the rig plates, the plates become parallel to each other. The two fixed ends now have a fixed distance between them. By adjusting the length of the cable assembly, you will maintain a proper fit between the two fixed points.

You have now successfully accomplished a VSV feedback cable rigging. You were able to do so by a combination of adjusting the fore and aft rod end bearings within the limitations and following the published trimmer bracket instructions.

VSV SCHEDULING VSV scheduling is verified by using the variable vane protractor (1C5714). You must check the protractor for accuracy before every use on the engine. The check will detect any inaccuracies due to damage, wear, and so forth. Use the protractor setmaster (9441M67G01) to accomplish this test. The protractor is installed on the master vane located at the 9 o'clock split line, aft looking forward.

CAUTION Do not check the vane angle if idle speed is less than 4,900 rpm, or greater than 5,000 rpm. Before adjusting the idle rpm screw on the MFC, assure PLA mechanical and electrical rigging is connect.

Once the protractor's accuracy has been verified, you are now ready to install the protractor on the engine. Install the protractor locator on the engine and ensure it mounts correctly (it will only goon one way correctly).

If installed incorrectly, the shaft cannot be threaded on the vane stud. If the locator is wiggled around so that the shaft can be threaded on the vane stud, it will be cocked, but the protractor can still be installed over the locator. However, the vane angle is now 4 too far open.

If the protractor is left in this position, the angle will be 4 more open when checked at idle rpm than when compared to the table. This means a perfectly serviceable engine could be rejected because it failed the angle check. When installed correctly, the locator is flush to the lever and, at the same time, the shaft can be threaded on the vane stud without wiggling it around.

GAS TURBINE PRESERVATION AND CORROSION CONTROL

Modern gas turbines and their support equipment are dependent upon the structural soundness of the metals from which they are fabricated. The greatest threat to the structural integrity of this equipment is metal corrosion. With the higher demands being made on these metals, both in strength and in closer tolerances, this equipment would rapidly deteriorate and become inoperative control. without regular attention to corrosion

Corrosion endangers the gas turbine and its support equipment by reducing the strength and changing the structural characteristics of the materials used in their construction. All such materials are designed to carry certain loads and withstand given stresses and temperatures, as well as to provide an extra margin of strength for safety. Corrosion can weaken the structure, thereby reducing or eliminating this safety factor. Replacement or repair operations are costly, time consuming, and restrict the usage of the equipment. Corrosion in electronic and electrical components can cause serious malfunctions. These malfunctions reduce the effectiveness and reliability of the engineering plant and can often completely destroy these components.

A thorough comprehension of the dangers of corrosion and the ability to recognize and cope with the various types of corrosion should be included in the objectives of any maintenance training program. As a work center supervisor, you may find that corrosion prevention and control frequently turn out to be an all-hands evolution. To some extent you can avoid this situation through frequent inspections, effective use of available manpower, and proper training of your subordinates.

CORROSION The problem of gas turbine engines and support equipment protection is threefold: (1) prevention of corrosion of the metal parts; (2) control of deterioration of nonmetallic materials; and (3) elimination of physical damage during replacement, repair, and maintenance. Of the three basic problems, corrosion of metals is the most difficult to control.

Metal corrosion is the deterioration of a metal. When the metal is combined with oxygen, it forms metallic oxides. This combining is a chemical process that is essentially the reverse of the process of smelting metal from ore. Very few metals occur in nature in the pure state. For the most part, they occur as metallic oxides. The refining process involves the extraction of relatively pure metal from its ore and the addition of other elements (both metallic and nonmetallic) to form alloys.

After refining, regardless of whether or not they are alloyed, base metals possess a potential or tendency to return to their natural state. However, this potential is not enough in itself to initiate and promote this reversion. There must also exist a corrosive environment in which the significant element is oxygen.

It is the process of oxidation that causes metals to corrode.

It is a well-known fact that the tendency to corrode varies widely between various metals. For example, magnesium alloys are very difficult to protect and have a very low corrosion resistance. Copper alloys have relatively good corrosion resistance and are very easy to protect.

Corrosion may take place over the entire surface of a metal by having a chemical reaction with the surrounding environment. Or corrosion may be electrochemical in nature between two different metallic materials or two points on the surface of the same alloy that differ in chemical activity. The presence of some type of moisture is usually essential for corrosion to exist.