79-18-12 AIRESEARCH MANUFACTURING COMPANY of ARIZONA: Amendment 39-3550. Applies to the following groups of Model GTCP85 series Auxiliary Power Units (APUs) which have cast turbine wheel Part Number 968095-1, -3, -5 or 3604604-1 installed: \n\n\tNote: These APUs are installed in but not limited to Boeing 727-100/-200, Lockheed L-100, Boeing 707 (conversion), Lockheed 382-Series, Douglas DC-9, Avions Marcel Dassault -Breguet Aviation Mercure and British Aerospace Corporation BAC 1-11, Trident 3B, Trident 1, and Trident 2E aircraft. \n\n\tGroup A: GTCP85-98CK, GTCP85-98CK(A), or GTCP85-98CK(B) with exhaust pipe assembly Part Number 379529 or 976987-1. \n\n\tNote: Part Number 976987-1 may be either a new production part or part reworked from Part Number 379529 per AiResearch Service Bulletin 49-1790, Revision 1, dated March 21, 1971, or original issue of this service bulletin. \n\n\tGroup B: GTCP85-98DCK, GTCP85-115CK, GTCP85-115H, GTCP84-139H, or GTCP85-163CK with exhaust pipe assemblyPart Number 379529 or 976987-1 reworked from Part Number 379529 per AiResearch Service Bulletin 49-1790, Revision 1, dated March 21, 1971, or original issue of this service bulletin. \n\n\tGroup C: GTCP85-185L with exhaust pipe assembly Part Number 899607-1. \n\n\tNote: Operators of airplanes incorporating the APUs affected by this airworthiness directive are advised to examine the applicable sections of AiResearch Alert Service Bulletin GTCP85-49-A5078, Revision 1, dated March 29, 1979, and AiResearch Service Bulletin GTCP85-49-3688, Revision 5, dated February 9, 1979, to identify production configuration of these units as well as the methods of reworking the exhaust pipe assemblies to the approved replacement configuration. \n\n\tCompliance required as indicated. \n\n\tTo prevent the release of high energy turbine wheel blade fragments, accomplish the following, unless already accomplished: \n\n\t(a)\tFor APUs with turbine wheels Part Number 968095-1, -3, -5, or 3604604-1 installed, remove the exhaust pipe assemblies listed for Group A, B, and C above and install a strengthened exhaust pipe assembly as defined below at next APU overhaul, or within the next 3,000 operating cycles after the effective date of this AD, whichever occurs first, or in any case, not later than three years from the effective date of this AD. \n\n\t\t(1)\tAPUs in Group A are to have a Part Number 3607748-1, 966580-1, or 966580-3 exhaust pipe assembly installed. \n\n\t\t(2)\tAPUs in Group B are to have an exhaust pipe assembly Part Number 3607748-1, 966580-3, or an exhaust pipe assembly originally manufactured as Part Number 976987-1 installed. \n\n\t\t(3)\tAPUs in Group C are to have a Part Number 3607748-1 exhaust pipe assembly installed. \n\n\tNote: For the purposes of this AD, one cycle equals one start/shutdown of the APU. If the operator does not have a record of operating cycles on individual turbine wheels, he may assume two starts have occurred for each recorded APU operating hour of service, or any other cycle per hour ratio approved by the operator's assigned FAA maintenance inspector, provided the request contains substantiating data to justify the alternative ratio. Operators who have not kept a record of APU operating hours of service shall estimate hours of APU operation by equating APU operation to airplane hours time in service using a ratio approved by the operator's assigned FAA maintenance inspector and justified by substantiating data. \n\n\t(b)\tSpecial flight permits may be issued in accordance with FAR 21.197 and 21.199 to operate airplanes to a base for the accomplishment of modifications required by this AD. \n\n\t(c)\tAlternative inspections, modifications or other actions which provide an equivalent level of safety may be used when approved by the Chief, Aircraft Engineering Division, FAA Western Region. \n\n\tThis amendment becomes effective October 8, 1979.