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ALERT

SUBJECT: POWER TURBINE OUTER COUPLING NUT INSPECTION

The Model 250 Series Engines have accumulated approximately 23million flight hours since 1966. During this time there havebeen instances wherein corrosion has attacked the power turbinecoupling nut (P/N 6846278 or 6871338). This condition usual lyoccurs in extremely humid areas which have abnormal amounts ofsulphur in the atmosphere. Failure of this nut would result indisengagement of the power turbine curvic coupling, resultingin possible N2 overspeed and turbine disk failure.To alleviate this condition of corrosion, an aluminum corrosionresistant paint has been released to the nut, (Ref.P/N 6846278, Change G) as shown in Figure 1. Production andoverhauled engines now incorporate the painted nut; however,corrosion has attacked the coated nut.Therefore, a mandatory inspection of coated and uncoated nutsis required as follows:

a)

b)

c)

For engines with turbine sections having more than 400hours or one calendar year time in service sinceinstallation in an aircraft, initial compliance must beaccomplished within the next 100 hours or 60 days timein service, whichever occurs first.For engines with turbine sections having less than 400hours and less than one calendar year time in servicesince installation in an aircraft, initial compliancemust be accomplished within 500 hours or one year timein service since installation in an aircraft, whicheveroccurs first.For each turbine section incorporating an acceptableuncoated or coated coupling nut, repatitive complianceis required at intervals not to exceed 500 hours or onecalendar year time since last inspection, ,whicheveroccurs first.

October 27, 1976Revision No. 5July 22, 1994

250-Cl8250-C2O/C2DB250-B15G250-81718176

CSL-88CSL-1060TP CSL-25

TP CSL-1030Page 1 of 6

Rolls-Royce Proprietary Data Uncontrolled Printed Copy

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ALLISON ENGINE COMPANYCommercial Service Letter

NOTE: The following turbine assemblies incorporate P/N6890531 or later power turbine coupling nuts andare exempt from this inspection:MODELCl8c20C2OBB15G8178178

TURBINE SERIAL NO.CAT 23649BCCAT 38617PCAT 33892CAT 50100CAT 60063PCAT 80204

Compliance with the following Commercial EngineBullet ins which install PIN 6890531 or P/N 23001801Power Turbine Coupling Nuts removes the inspectionrequirement:

250-Cl8 CEB-193/207250-C2O/C2OB CEB-112011158250-815G250-81718178 ;;

CEB-53/76CEB-108811134

The nut is located approximately one and one half inches,forward ofthe fourth-stage turbine wheel inside the center section of theexhaust collector support. (See Figure 2.1 By using a borescope,some type of fiberoptic equipment, or a small mirror inserted betweenthe forward face of the fourth-stage turbine wheel and the innersection of the exhaust collector support, it is possible to examinethe condition of the nut.NOTE: The following vendors can supply acceptable borescopes;however, any acceptable alternate device can be selected by

the operator.American Cystoscope Makers, Inc.300 Stillwater AvenueStamford, Connecticut 06902


250-Cl8 CSL-88250-C2O/C2OB CSL-1060250-B15G TP CSL-25250-B17/817B TP CSL-1030Page 2 of 6


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Dolan-Jenner Industries, Inc.68 IngallsMelbrose, Massachusetts 02176Dyonics. Inc.69 Pine StreetWolburn. Massachusetts 01801

An alternative way to inspect the condition of the nut consistsusing a high intensity light source and a small non-magnifyingmirror. It is possible to examine the condition of the nut by

of

inserting the mirror approximately halfway inside the exhaustcollector, holding the light source in a position such that thelight falls upon the nut, and positioning the mirror until the nutis in view.With the equipment inserted, slowly rotate the fourth-stage turbinewheel by hand so that the full 3600 area of the nut can be viewed.Uncoated or coated nuts wil l show a grainy textured riurface createdby the corrosion attack. (See Figure 3.)

1) If the corrosion is continuous around the full 3600 ofrotation, the nut must be replaced. (See Figure 3.)

2) If there is only local, intermittent spots of corrosionaround the full 360 rotation, the nut should be continuedin service. (See Figure 4.) Reinspect acceptable coatedand uncoated nuts per step (Cl.

If you experience difficulty in complying with the inspection, it issuggested you contact the Distributor for your area for assistance.Aooroval:Technical aspects are FAA approved. The accomplishment instructionsof Revision 3. Revision 4, or Revision 5 to this bulletin areapproved by the Manager, Chicago Aircraft Certification Office forcompliance with AD 77-15-12 as indicated.Compliance with this inspection should be entered in the applicablepages of the engine log book.

CUSTOMER SUPPORTALLISON ENGINE COMPANY


25D-ClB CSL-BB-- _~-250-C2O/C2OB CSL-1060250-B15G TP CSL-25250-B17/B17B TP CSL-1030Page 3 of 6


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FOURTH STAGE TURBINE WHEELI--PART NO. 6846278 CHANGE G NUT

POWER TURBINE ROTOR ASSY. SHOWING P/N 6846278CHANGE "G" NUT

FIGURE 1October 27, 1976Revision No. 5July 22, 1994

250-08250-C20/C20B250-8156~250-B17/817B

CSL-88CSL-1060TP CSL-25TP CSL-1030Page 4 of 6


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INSERT BORESCOPE, FIBEROPTICS,OR MIRROR THIS LOCATION.

TURBINE TO COMPRESSOR COUPLING

250-C20 TURBINE SECTIONFIGURE 2


250-Cl8250-C2O/C2OB250-B15G250-B17/817B



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gjj?&~. NOTE IRREGULAR, GRAINYTEXTURE WHICH 1s CONTINUOUSAROUND FULL 360.

TYPICAL UNACCEPTABLE CORRODED NUTFIGURE 3

TYPICAL ACCEPTABLE CORRODED NUTSFIGURE 4

October 27, 1976Revision No. 5July 22. 1994

250-Cl8250-C20/C20B250-B15G250-B17/817B



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