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Common maintenance events on the Allison 250 C20
When locating the front support to the case halves, put the scavenge fitting at or about the 6 o'clock position. The final seating of the front support to the case halves is done using hand pressure. Don't draw the front support up to the case halves with the splitline bolts. Push the support into place by hand, hold it there and then tighten up two opposite split line bolts until they are almost snug. Pulling the front support in the bolts can damage the pilot diameter on the aft side of the front support.
Once the pilot is damaged, the support won't sit right. The misalignment will cause vibrations, rubs, and maybe even a bearing failure.
Common Maintenance Events on the Allison 250 C20
By Jim Taylor
February 1998
Inspection and replacement of the case halves
Refer to the maintenance manual for specifics on the inspection criteria and for the proper sequencing of tightening the splitline bolts when
removing or replacing the case halves. The torquing procedure can be confusing, and it helps if you get another person to call out the sequence while you do the tightening and loosening of the split line bolts.
The intent of the inspection criteria for the case halves is to make sure they are removed from service when certain conditions exist. If the plastic looks like it is lifting, replace the case halves. A good place to look for lifting plastic is along the split line cross section. Here you can observe how the plastic is maintaining its grip on the parent steel material of the case halves. The plastic usually starts lifting from the bleed band slot. Sometimes little pieces of plastic break out. That's OK provided the pieces aren't coming from the rotor blade path area, and as long as the hole left in the plastic is acceptable per the limits in the maintenance manual.
The key to analyzing eroded plastic is to look for any exposed base metal — if you see any metal exposed around the base or root of the vane, remove the case halves from service.
The plastic that covers the root of the stator vanes serves two purposes: one is to smooth the airflow through the compressor, the other is to protect the root of the stator vane from erosion.
The root of the vane is the most highly stressed area of the vane. Any material missing at the base of the vane could lead to a vane failure. Certain types of rubs in the plastic are permitted as long as they are light and there is no evidence that the plastic is lifting.
