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...


Now you can push the torquemeter shaft support through the roller bearing and into the torquemeter insert in the gearbox cover. Install the torquemeter nut and lockcup and just snug it up at this time. This will help in the assembly of the gearbox cover onto the housing — it's a little more comfortable torquing up the nut when there is a whole gearbox to hang on to.

Mating the cover and housing — It's advisable to use one of the approved sealants on the splitline to help avoid oil leaks from this area. However use it sparingly, as the excess will be squeezed into the gearbox where it will get into the oil system and eventually end up in either the scavenge oil filter or the main engine oil filter.

After reviewing your work, making sure all the safety wire has been done, and that all the pieces are in place. Place the housing in front of you so that the oil pump side is closest to you.

This is important because you'll want to use the three long studs located beside the oil pump on the splitline as guides for lining up the cover.

Lift the cover up over the housing and while looking through the holes for the three long studs, line up the studs and lower the cover on to the housing. You will have to do a bit of jiggling, however, none of the gear trains will be in a position to mesh until the cover and housing are less than a 1/4-inch apart. Also, before the gears mesh, all of the oil transfer tubes have to be engaged. So don't try turning any of the gear trains, until the tubes are in place.

Quite often you must use a hooked pick or a small screwdriver to help line up the three oil transfer tubes located beside the oil pump. Once the tubes are fully engaged, and if your packings have been well lubricated with petrolatum and oil, the cover can be pushed on the remainder of the way — be sure to do this while turning the geartrains to ensure they mesh.

Very little pounding with a soft face hammer should be required at this point. Once all of the splitline hardware has been reinstalled and torqued, proceed to installing the garlock seals.

Use only engine oil to lubricate the lip surface of a garlock seal. During installation of a garlock seal it is normal that a small ring or orange seal material be scraped off of the outer portion of the seal. This is not a cause to reject the seal.

As a final note remember that this information is meant to assist the reader in performing certain maintenance tasks on the Allison Model 250 C20 series engines. It is not intended to replace the Operation and Maintenance manual. When performing any maintenance on Allison 250 series engines always an up-to-date current manual. Compare the revision status of the manual you intend to use with the revision status of the manuals listed in every CEB index.

Should you find your manual out of date contact the Allison Access Center at 1-888-255-4766 and order a new up-to-date manual or the required revisions. Also should you discover a contradiction between this article and the Operation and Maintenance manual, use the info in the manual.

We Recommend

  • Article

    Troubleshooting the Rolls-Royce Model 250

    Troubleshooting the Rolls-Royce Model 250 By David Marone May 2000 Helicopter operators would agree that the Model 250 is a tried and true powerplant. Still, as is the case with most...

  • Article

    When the unit stops, the confusion shouldn't start

    When the unit stops, the confusion shouldn't start AlliedSignal GTCP 36-100 Series APU troubleshooting By Tim Coggin April 1998 he AlliedSignal GTCP 36-100 series APU can appear as...

  • Article

    PT-6 Hot Section Inspection

    PT-6 Hot Section Inspection Tips for keeping this workhorse in top flight condition By Joe Escobar July 2001 On May 30, 1961, the Pratt & Whitney PT6 engine took flight for the...

  • Article

    Rolls-Royce 535

    Rolls-Royce 535 Operation and troubleshooting By Greg Napert March 1999 The Rolls-Royce RB211-535 is a very popular engine used on around 70 percent of the Boeing 757 aircraft in...