74-10-11\tAIRESEARCH: Amendment 39-1837 as amended by Amendment 39-1869. Applies to Model TSCP 700-4B Auxiliary Power Units (APU) installed in McDonnell Douglas Model DC-10 aircraft. \n\n\t"Pursuant to the authority of the Federal Aviation Act of 1958, delegated to me by the Administrator the following airworthiness directive applicable to AiResearch Model TSCP 700- 4B Auxiliary Power Units (APU) (installed in McDonnell-Douglas Model DC-10 Series Aircraft) is effective immediately upon receipt of this telegram because of numerous reports of fatigue cracks in the fuel control differential pressure regulator body. The presence of crack(s) can allow fuel to leak into the APU compartment. Compliance required as indicated, unless already accomplished. \n\n\t(a)\tWithin 25 hours additional time in service after the effective date of this AD, as amended, unless already accomplished, install a placard in view of the flight crew to prohibit all in-flight operation of the APU. Thereafter the APU may not be used in flight but may be used in ground operations." \n\n\t(b)\tThe operating restriction prescribed in (a), above, may be removed when the inspections and modifications described in paragraph 2.B. and C. of AiResearch Service Bulletin 969900-49-3635, Revision 1, dated May 14, 1974, or later FAA-approved revisions, are accomplished. Differential pressure regulator body assembly P/N 977320-1 or -2 which does not meet the wall thickness and fillet radius requirements specified in the referenced Service Bulletin must be rendered unserviceable. Those found satisfactory or that can be reworked per the instructions contained in paragraph 2.B.(4) of the referenced Service Bulletin must be re- identified as P/N 977320-4 and may be continued in service. Re-identify the fuel control assembly per paragraph 2.D. of the referenced service bulletin." \n\n\t(c)\tWithin 3000 APU operating cycles in service after accomplishment of paragraph (b), above, and at intervals not to exceed 3000 APU operating cycles in service thereafter, inspect the fuel control differential pressure regulator body, P/N 977320-4, for cracks in accordance with paragraph 2.B.(2) of AiResearch Service Bulletin 969900-49-3635. If cracks are found, replace the housing with a serviceable housing conforming to P/N 977320-4, prior to further in-flight operation. \n\n\tNote: For the purpose of this AD, an APU operating cycle is any operation consisting of a start and shut-down. The number of cycles may be determined by actual count, or, subject to acceptance by the assigned FAA Maintenance Inspector, may be calculated by dividing the fuel control assembly time in service by the operator's fleet average APU operating time per APU operating cycle. \n\n\t(d)\tThe inspections prescribed in (c) above may be discontinued when the fuel control differential pressure regulator body assembly P/N 977320-4 is replaced with a new part conforming to P/N 977656-1. \n\n\t(e)\tEquivalent procedures may be approved by the Chief, Aircraft Engineering Division, FAA Western Region, upon submission of adequate substantiation data. \n\n\tAmendment 39-1837 was effective May 13, 1974, for all persons except those to whom it was made effective immediately by telegram dated April 17, 1974. \n\n\tThis Amendment 39-1869 becomes effective June 14, 1974.