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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, and the gearbox.
Compressor Maintenance
Evidence of oil spraying out of the bleed valve of an Allison 250 C20 engine should lead you to suspect an oil leak from the carbon seal for
the No. 1 bearing at the front of the compressor.
To access the carbon seal area, first the compressor front support must be removed. Remove the nut in the front of the bullet nose, along with the split line bolts and nuts, that go around the aft flange. To help reduce the grip the case halves have on the pilot diameter of the front support, loosen up at least two of the split line bolts that run axially along the case halves. To lift off the support, gently tap the bullet nose with a plastic mallet to over come the o-ring friction. Do this while applying a steady pulling force and maintaining a tight grip on the support. This will prevent the support from injuring you or falling on the floor.
Once the front support is off, it's a good practice to pressure check the oil transfer tubes. Remember that these tubes will give you the same symptoms as a leaking No. 1 carbon seal. Many people don't perform this step until after they've changed the carbon seal 2 or 3 times.
You can do a quick check quite easily by supplying about 15 lbs. of shop air to one of the oil fittings on the front support. Then block the corresponding hole on the inside of the bullet nose with your finger.
Submerge the whole apparatus in a bucket of water and look for air bubbles coming out of the anti-ice air slots found on the trailing edges of the inlet guide vanes of the front support. (Hint: make sure the air nozzle is not on a fitting for the anti-ice air.) A broken transfer tube inside the front support is cause to reject the front support.
At that point you should be looking for another front support. Contact your preferred supplier and perhaps they can sell you an overhauled or used serviceable front support. If not, you'll have to buy a new one.
