Common maintenance events on the Allison 250 C20

Common Maintenance Events on the Allison 250 C20 By Jim Taylor February 1998 his article discusses some of the most common maintenance events on each of the three major components that make up an Allison 250 C20. The compressor, the turbine...

Rubs between the stator vanes and the compressor rotor, however, are not allowed. If rubs are noticed in this area, pull the compressor from service immediately. Rubs on the rotor are most commonly caused either by corrosion between the stator vane band assemblies and the case halves skins which results in the stator band lifting into the rotor. They can also be caused from misalignment of the case halves during installation. Any signs of blueing associated with rubbing is quite serious.

Blueing indicates that the material affected has been stressed by the heat of the friction. The effects of this friction usually penetrates quite deeply. The slightest tint of blue on the surface can be seen on the inside of the wheel as well, and if left unchanged, can quickly develop into a hole. The result will be an under-powered engine.

Any time you have a compressor module off, give it a spin and listen to it for rubs. Any ticking, or rub sounds should be investigated by removing a case halve and having a look to see what's going on. To install the compressor, the turbine must be off. This allows you to use the 2 1/2 bearing guide. Also with the turbine off, this gives you an important opportunity to have a better look at the 2 1/2 bearing. Check each roller for spalling, dents or flat spots. If there are any, reject the bearing.

Also, carefully inspect the roller retainer. If the rollers are loose and they look as if they could fall out, replace the bearing. Prior to installing the compressor, pack the bearing with petrolatum.

Before removing the compressor it's a good idea to count the number of shims under each of the compressor mount legs. This will be a great help when it comes time to reinstall the compressor. When mating the compressor with the gearbox, keep the compressor rotor turning, and seat the assembly onto the gearbox by hand.

Don't draw it up with the mounting bolts. It's possible to bend the first stage reduction gear. This happens when the teeth of the spur adapter are stacked, instead of engaged with the spur teeth of the fuel control, oil pump, and idler gear.

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Turbine Module Maintenance
Tips A hot end inspection could be required if the turbine outlet temperature (TOT) limits are exceeded during a start, afterfire, power transient, or a one-engine inoperative event.

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.

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