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Common maintenance events on the Allison 250 C20
Before going to the Allison maintenance manual to look up the specific temperature limits, be sure to obtain the most accurate information available for the highest temperatures observed and for how long the temperature was above the suspected limit.
Vague data should force you to assume a worst case scenario. Once you've reviewed your data, check it out with the limits in the Allison maintenance manual.
In the event a hot end is required, the first items to remove for a turbine inspection are the outer combustion case and the discharge tubes. These items are "on condition." They should be given a close inspection every time they're removed.
Some of the key items to look for during the hot end inspection are:
Combustion Liner — Check it for cracks, localized bulges and warpage. Check for the incorporation of CEB1299, which calls out for the addition of weld beads all around the fuel nozzle and ignitor ferrules (which are located on the aft dome section of the combustion liner). This CEB has been successful about 98 percent of the time in stopping the formation of "carbon clinkers" which can form in the dome section of the liner. If you discover or know that you have these "carbon clinkers" or "carbon fingers" forming in your liner, do yourself a favor — take steps to stop them from forming. These carbon deposits form and break off quickly and continuously. As the deposits fly into the turbine, the impact erodes the first stage wheel causing loss of power, compressor surge, and expensive repair bills. Bulges and warpage are usually indications that there is streaking in the fuel nozzle.
Fuel Nozzle — Check it by first removing it from the combustion can. It's much easier to remove the fuel nozzle when the outer combustion can is still attached to the turbine module.
As the wrenching flats are relatively thin on the fuel nozzle, it's a good idea to grind off the lead-in chamfers from your socket or wrench so there will be less chance of rounding off the corners of the fuel nozzle.
