Replacing the first stage nozzle — Replacement of the first stage nozzle shouldn't be attempted unless you've had specialized training or the assistance of someone who is very familiar with the procedure. Many AMCs provide on-site technical support for this very reason. Give your preferred AMC a call, and perhaps they could send out a service representative to help. Also, the AMC should lend you the special tools required to change the first stage nozzle.
Here are some points to remember while performing a nozzle change:
• AlliedSignal fuel controls have to be removed before taking off the turbine module from the engine assembly. However, CECO fuel controls can remain in place.
•As the turbine comes off the engine, take care not to damage or drop the N1 and N2 coupling shafts.
•After the turbine is removed, check the gearbox for the following:
— If the torquemeter nut on the gearbox cover is leaking, the gearbox should be removed and repaired.
— Surface "G" or turbine mounting surface shouldn't be showing any signs of lifting or missing material.
— Take a good look at the No. 4 ball bearing for nicked or spalled balls. With a flashlight look inside the pinion gear and check the No. 2 1/2 bearing to make sure all the rollers are still round, and turning smoothly when the N1 gear train is turned.
For dismantling the turbine itself here are few more pointers: if the oil pressure tube that delivers oil to the No. 8 bearing oil jet is removed, it must be reinstalled prior to installing the No. 8 bearing oil jet.
Before the No. 8 bearing oil jet is installed, the oil tube must be properly in place and then blown out before installing the oil jet. Once the oil jet is installed, pump some turbine oil through the pressure tube and oil jet to make sure it's not plugged and that it's targeted to supply a stream of oil right into the balls of the No. 8 bearing.
Following these steps will help to make sure there is no carbon blocking the entrance to the oil jet. This is very important because if this oil jet gets plugged up, the No. 8 bearing will fail shortly afterwards. Remember the nut for the No. 8 bearing has a left-hand thread.
With the aft side of the first stage wheel exposed, inspect the rim for cracks. Also, give the first stage blade path a close look, checking for lifting and or missing pieces of blade path material. Once you have the first stage nozzle in your hands, review your inspection notes and the repairable limits in the maintenance manual.
As a general rule pieces missing from more than seven vanes or cracks through the saddle area are not repairable. To avoid getting an expensive bill for a "core charge," talk to the AMC your buying the replacement nozzle from and make sure your nozzle core will accepted as an exchange unit.
Assembly Tips — The turbine goes back together much the same way it came apart. Here are a few more tips to help you out:
• Use petrolatum on the fiberglass packing to help keep them in place during assembly • Remember to safety the No. 8 bearing nut by deforming the locking ring into one of the notches of the stub shaft in the first stage wheel, (not the notches in the tie-bolt nut). Use a pin punch with the end rounded off to perform this task.
• Follow the steps in the maintenance manual for torquing the No. 8 bearing sump nut.
• To help get a good seal between the sump nut and the GP support, make sure the u-ring groove is spotlessly clean.
• Lap the sealing surface of the sump nut and use a lot of oil on the threads and sealing surfaces.
Common Maintenance Events on the Allison 250 C20
By Jim Taylor
Gearbox maintenance tips
The purpose of this next portion is to give the reader some tips and advice on dismantling, inspecting, and reassembling an Allison 250 C20B gearbox. Again, if you don't have a maintenance manual, please call the Allison Access Center (phone number located at the end of this article) to arrange the purchase of an up-to-date manual.
Some of the reasons why you would want to split an Allison 250 C20B gearbox include:
— The torquemeter insert in the gearbox cover could be leaking.
— The threads for the oil fittings could be stripped.
— A suspect oil pump is causing low oil pressure.
— Through troubleshooting, the gearbox has been determined to be the source of metal generation.
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