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
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 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 is verified by using the variable
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 oclock
split line, aft looking forward.
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 protractors 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
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
without regular attention to corrosion