AD 69-18-04

69-18-04 ALLISON: Amdt. 39-860. Applies to Allison Models 250-C10 and 250-C18 Series Engines having Gearbox Serial Numbers CAG 20183; CAG 20185; CAG 20193 through CAG 20265; CAG 20508; CAG 21001 through CAG 21395; CAG 21600 through CAG 21706; CAG 21800 through CAG 21803 installed, except those engines modified in accordance with Allison Commercial Engine Bulletin 250 CEB-90, dated August 21, 1969, or later FAA approved revision, or having P/N 6852085 nitrided helical torquemeter shaftgears. COMPLIANCE: Amendment 39-833 effective September 10, 1969. To prevent torquemeter shaftgear failure, accomplish the following: A) Adjust the N1 speed at ground idle so that N2 speed does not exceed 71% RPM. Allison Commercial Service Letter No. 250 CSL-34, dated August 4, 1969, or later FAA approved revision relates to this subject. B) Avoid continuous operation at any flight or ground condition, other than during ground idle or practice autorotation, at engine power turbine speeds (N2) below 98% or above 102% RPM. Allison Commercial Service Letter No. 250 CSL-34, dated August 4, 1969, or later FAA approved revision relates to this subject. *See footnotes. This amendment becomes effective October 18, 1969. *As a result of these operating restrictions, the Rotorcraft Flight Manual and certain powerplant instrument markings for the Bell Model 206A and Hughes Model 369 Series Helicopters in which the affected engines are installed may require revision or modification. The necessary information for such changes is available from the manufacturers of these helicopters. Revision No. 1 dated September 24, 1969, to Allison Commercial Service Letter 250 CSL-34 and Allison Commercial Engine Bulletin 250 CEB-90 is a later FAA approved revision which relates to the subject matter of this airworthiness directive.