Common maintenance events on the Allison 250 C20

With the fuel nozzle removed, hook up the P3 fuel line again and hang the nozzle so it will spray into a large mason jar. Make sure the ignition is off, then motor the engine with the throttle open. Observe the spray pattern. If you sea any streaks or voids, replace the fuel nozzle.

First Stage Nozzle Shield — The shield must be removed from the turbine. This is essential not only to better inspect the nozzle shield it self, but it also allows a much better view of the first stage nozzle and the first stage wheel and blade path.

It's normal to see wavy edges on the deflector which is welded on to the dome. What you don't want to see however are cracks and burnt edges. CEB-A-1341 applies to the first stage nozzle shield, and advises operators of the possibility of having an incorrectly manufactured nozzle shield made with a dome of the stainless 310 material — the wrong material. Most Allison Maintenance Centers (AMC) have an acid test kit available to confirm which type of material of the dome on your nozzle shield.

First Stage Nozzle — With the first stage nozzle shield removed, you can now have a good look at the first stage nozzle. Use good lighting and a 6-inch steel rule to help measure any of the cracks you might see. Inspect each vane and measure and record on a piece of paper the number and length of any cracks.

Check also for erosion, convergent cracks, V-type notching, and pieces missing from the both the leading and trailing edges. The two rings of material that hold the vanes in place are called the inner and outer bands. These require a detailed inspection as well.

Pay particular attention to cracks that go into the "saddle areas" which are the vanes that wrap around the support struts of the G/P ( gas producer ) support. Axial inner band cracks can sometimes extend into the diaphragm and then run circumferential. Circumferential cracks in the diaphragm can cause excessive power loss. Again, take good notes, so that when you check the limits in the maintenance manual you can make an intelligent decision on whether to change the nozzle.

Special tools required to change the 1st stage nozzle:
1) Fixture - holds No. 8 sump nut socket,
2) No. 8 bearing puller,
3) Slide hammer for oil jet,
4) No. 8 press and adapters,
5) Scrap pea shooter and sags — to hold N1,
6) No. 8 sump socket,
7) No. 8 bearing nut socket align tool,
8) No. 8 bearing nut socket

Blade Path Area — The first stage blade path area is an integral part of the second nozzle. In performing a good hot end inspection, this area must receive a thorough inspection for blade tip rub and evidence of metal transfer from the blade tips of the first stage turbine wheel.

Rubs in the blade path area are not allowed and are an indication that a serious overtemp has occurred or that there is misalignment between the GP & PT supports of the turbine — both of which are serious enough to justify removing the turbine and sending it to an Allison AMC for repair or overhaul.

Turbine Blades — The turbine blade tips are another important area to inspect. Blade tips that are in good shape have square sharp corners and the tops are flat, smooth, and square to the blade path. Blade tips that have a ragged edge may have rubbed as a result of an overtemp.

Blade tips that have rounded corners or a taper along the tops of the blades are showing signs of erosion and wear. These conditions will usually be accompanied with low power and maybe even compressor surge.

After completing a hot end inspection and reviewing your data with the limits in the maintenance manual, you have three options: if there is no damage found, and if the overtemp didn't exceed prescribed limits, the turbine can be returned to service. If there's an excessive amount of damage, the turbine has to be sent to an AMC for repair or overhaul. If there's damage, and it's limited to the first stage nozzle, you can change the nozzle.