99-26-20 MD HELICOPTERS INC.: Docket No. 99-SW-89-AD. \n\n\tApplicability: Model MD-900 helicopters, with Main Rotor Upper Hub (hub) Assembly, part number (P/N) 900R2101006-105 or 900R2101006-107, installed, certificated in any category. \n\n\tNOTE 1: This AD applies to each helicopter identified in the preceding applicability provision, regardless of whether it has been otherwise modified, altered, or repaired in the area subject to the requirements of this AD. For helicopters that have been modified, altered, or repaired so that the performance of the requirements of this AD is affected, the owner/operator must request approval for an alternative method of compliance in accordance with paragraph (c) of this AD. The request should include an assessment of the effect of the modification, alteration, or repair on the unsafe condition addressed by this AD; and if the unsafe condition has not been eliminated, the request should include specific proposed actions to address it. \n\n\tCompliance: Required as indicated, unless accomplished previously. \n\n\tTo prevent failure of the hub assembly, loss of drive to the main rotor, and subsequent loss of control of the helicopter, accomplish the following: \n\n\t(a)\tFor the hub assembly, P/N 900R2101006-107, \n\n\t\t(1)\tWithin 6 hours time-in-service (TIS) or before flight after January 31, 2000, whichever occurs first, visually inspect the hub assembly drive plate attach flange (flange) for a crack and determine the torque of each flange attach nut (nut) in accordance with the Accomplishment Instructions, Part I, paragraph 2.A., steps (1) through (7) of MD Helicopter Inc. Service Bulletin SB900-072, dated December 10, 1999 (SB). If a crack is found, before further flight, remove and replace the hub assembly with an airworthy hub assembly. \n\n\t\t(2)\tWithin 25 hours TIS or before flight after January 31, 2000, whichever occurs first, conduct the Accomplishment Instructions, Part II, paragraph 2.B., steps (1) through (6), (8),and (9) of the SB. If a crack is found, before further flight, remove and replace the hub assembly with an airworthy hub assembly. \n\n\t(b)\tFor the hub assembly, P/N 900R2101006-105, \n\n\t\t(1)\tWithin 6 hours TIS or before flight after January 31, 2000, whichever occurs first, visually inspect the flange for a crack and determine the torque of each nut in accordance with the Accomplishment Instructions, Part I, paragraph 2.A., steps (1) through (7) of the SB. \n\n\tNOTE 2: The SB effectivity does not include hub assembly, P/N 900R2101006-105; however, for the requirements of this AD, certain provisions of the SB do apply to this P/N . \n\n\t\t(2)\tIf any nut has less than 180 inch pounds (20.34 Nm) of torque, before further flight, remove the hub assembly, disassemble, inspect, and reassemble in accordance with the procedures in paragraph (b)(4) of this AD. If a crack is detected, before further flight, remove and replace the hub assembly with an airworthy hub assembly. \n\n\t\t(3)\tWithin25 hours TIS or before flight after January 31, 2000, whichever occurs first, remove and visually inspect the hub assembly as follows: \n\n\t\t\t(i)\tIf present, remove sealant from the drive plate attachment to the hub assembly. \n\n\t\t\t(ii)\tMark the main rotor hub holes, bolts, and nuts to correspond with the drive plate hole numbers. (See Figure 1.) \n\n\t\t\t(iii)\tRemove the main rotor drive plate (drive plate) assembly and anti-fretting ring (fretting buffer). \n\n\t\t\t(iv)\tInspect drive plate to rotor hub assembly mating surfaces and the fretting buffer for fretting. \n\n\t\t\t(v)\tUsing paint stripper (C313 or equivalent) and cleaning solvent (C420 or equivalent), remove the paint from the upper mating surface of the hub assembly to enable an accurate visual inspection of the drive plate attachment bolt hole (bolt hole) area for cracking (Figure 1). Ensure the paint stripper and solvent DO NOT contaminate the upper bearing and upper grease seal areas. \n\n\t\t\t(vi)\tUsing a 10-power or highermagnifying glass and bright light, inspect the mating surface area and the area around and inside the 10 bolt holes of the hub assembly for a crack. If a crack is found, prior to further flight, replace the hub assembly with an airworthy hub assembly. \n\n\t\t\t(vii)\tIf no crack is found, remove fretting from the mating surfaces of the hub assembly and the drive plate assembly, reassemble, fillet seal (C211 or equivalent) the surface of the drive plate to fretting buffer to hub assembly mating lines, and seal all exposed unpainted upper surfaces of the hub assembly. \n\n\t\t\t(viii)\tReinstall the main rotor drive plate using 10 new sets of replacement attachment hardware. Torque the nuts to 160 inch pounds above locknut locking/run-on torque in the sequence shown (Figure 1). Record in the rotorcraft logbook, or equivalent record, the locknut locking/run-on torque for each nut. \n\n\t\t\t(ix)\tAfter the next flight, verify that the torque on each of the 10 nuts is at least 160 inch pounds above the locknut locking/run-on torque (minimum torque). Retorque as required without loosening nuts \n\n\t\t\t(x)\tThereafter, at intervals of at least 4 hours TIS, not to exceed 6 hours TIS, verify that the torque of each of the 10 nuts is at least the minimum torque. Retorque as required without loosening nuts. This torque verification is no longer required after the torque on each of the 10 nuts has stabilized at the minimum torque 160 inch pounds for each nut during two successive torque verifications. \n\n\t(c)\tAn alternative method of compliance or adjustment of the compliance time that provides an acceptable level of safety may be used if approved by the Manager, Los Angeles Aircraft Certification Office, FAA. Operators shall submit their requests through an FAA Principal Maintenance Inspector, who may concur or comment and then send it to the Manager, Los Angeles Aircraft Certification Office. \n\n\tNOTE 3: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Los Angeles Aircraft Certification Office. \n\n\t(d)\tIf any nut torque is below minimum torque and no hub assembly crack is found before disassembly inspection, after retorque in accordance with the applicable Maintenance Manual, a special flight permit for one flight below 100 knots indicated airspeed may be issued in accordance with sections 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 21.199) to operate the helicopter to a location where the requirements of this AD can be accomplished. \n\n\t(e)\tCopies of the applicable service information may be obtained from MD Helicopters Inc., Attn: Customer Support Division, 5000 E. McDowell Rd., Mail Stop M615-GO48, Mesa, Arizona 85215-9797, telephone 1-800-388-3378 or 480-891-6342, datafax 480-891-6782. This information may be examined at the FAA, Office of the Regional Counsel, Southwest Region, 2601 Meacham Blvd., Room 663, Fort Worth, Texas. \n\n\t(f)\tEmergency Priority Letter AD 99-26-20, issued December 17, 1999, becomes effective upon receipt. \n\n\n\t\t\t\t\tFIGURE 1\n \t\t\t\t\t 99-26-20